CA1142629A - Chopper type propulsion system with low speed electrical braking capability for traction vehicles - Google Patents

Abstract

CHOPPER TYPE, PROPULSION SYSTEM WITH
LOW SPEED ELECTRICAL BRAKING CAPABILITY
FOR TRACTION VEHICLES
Abstract of the Disclosure A propulsion system for an electrically driven traction vehicle includes a chopper, a d-c traction motor, and means effective when the system is operating in a motoring mode for connecting the chopper in series with the armature and the field windings of the motor to a d-c electric power source that includes a filter capacitor. Cyclically operative means provides periodic gating signals for alternately turning on and turning off the chopper, and it can be smoothly changed from a constant frequency, variable pulse width mode to a variable frequency, constant (minimum) off time mode so as to vary the "duty factor" of the chopper over a wide range extending up to 100 per cent on time. Brake set up means is operative in response to a motoring-to braking command for reconnecting the chopper in parallel with the motor and the filter capacitor and for reversing the polarity of the con-nection of the series field winding relative to the armature. In response to this operation of the brake set up means, current in the field of the motor is momentarily boosted, and an extended chopper turn on signal having a duration substantially longer than that of the aforesaid periodic gating signals is supplied to the chopper to ensure that it turns on and conducts armature current to begin the braking mode of operation even if the command happens to take place at low speeds when the electromotive force of the motor is corres-pondingly low.

Description

~1--CHOPPER TYPE PROPULSION SYSTEM WI'rH
_ LOW SPEED ELECTRICAL BRAKING CAPABILITY
FOR TRACTION VEHICLES
_ Back~round of the Inventlon The present invention relates generally to electrical propulslon systems for traction vehicles, and it relates more particularly to means for providing improved electrica~ braking of a system using electric power choppers to control the magnitude of current -in sel~ excited d-c traction motors.
Large electrically driven traction vehicles such as locomotives or transit cars are propelled by a plurality o~ traction motors mechanically coupled to the respective wheel sets of the vehicle. Such motors are usually of the direct current (d-c) type. A d-c traction motor comprises a stator, a rotor, armature windings on the rotor, and rield windings (either con-nected in series with the armature or separately ex-cited) on the stator. In order to control its tractive effort, there is associated with the motor suitable ~eans for regulating the magnitude Or direct current in the motor arm~ture. Electric power apparatus commonly ~G known as a chopper is an energy conservlng means for regulating arn!ature current.
A chopper ls essentiall~ a controlled switch connected in circuit with the motor armature to meter current from a ~ource Or relatively con~tant ~-c elec-,

-2-tric power to the motor. The swltch is cyclically op~
erated between open and closed states, and by appropr1-ately controlling the timlng of the successive tran-sitions between these alternate states the magnitude of armature current can be varied or maintalned substan~
tially constant as desired. Assuming the chopper ls ln series with the motor and the propulsion system is op-erating ln its motoring mode, during closed periods of the chopper the motor armature windings will be con-nected to the d-c power source through a path of negli-gible resistance, whereby virtually the full magni~ude of the source voltage is applied to the motor armature and the current tends to increase. During the open periods of the chopper, the motor is disconnected from the power source and armature current, clrculating through a free wheeling pathg decays ~rom the magni-tude previously attained. In this manner, pulses o~
voltage are periodically applied to the motor, and an average magnitude of motor current (and hence torque) is establîshed. The rate of change of current is limited by the circuit inductance.
The ratio o~ the closed time (toN) of the chopper to the sum of the closed and open times (toN + toFF) during each cycle of operation is the 2~ duty ~actor of the chopper. ~or a 0.5 duty ~actor, the repetitive closed and open periods of the chopper are equal to each other, and the width of each voltage~
puise has the same duration as the space between suc-cessive pulses. In practice, so long as the chopper fre-quency is relatively high (such as, ~or example, 300Hz) the circuit inductance (lr.cluding the inductance provided by the armature windi.ngs of the traction motor itself) wlll smooth the undulating current in the motor armature sufficiently to preven~ untoward torque pul-~3-sations, whereby the vehlcle is propelled without any uncomfortable amount of ~erking or lurchlng. ~y varying the duty ~actor of the chopper~ the average chopper output voltage ~as a percentage of the d-c source voltage) and consequently the average magnitude o~ current can be increased or decreased as desired.
This is popularly known as tlme ratio control or pulse control.
A propulsion system using choppers can be adapted for electrical braking by reconnecting the power circuits so that each chopper is connected to the d-c power source in parallel rather than ln series with its associated motor. In the braking mode of opera-tion, a traction motor behaves as a generator, and the magnitude of its generated voltage (electromotive force) i3 ~roportional to speed and field excltation.
The excitation of a series field machine is a function of the magnitude o~ armature current. With the chopper reconnected in parallel w~th the motor, during its closed periods the chopper provides a low resis-tance path for armature current which therefore tends to increase, whereas during its open periods the arma-ture current path includes the power source and the free wheeling path, whereby current tends to decrease.
The electric power output of the motor is either fed back to the source (regenerated), or dissipated in a dynamic braking resistor grid that can be connected in parallel with the chopper, or a comblnatlon of both.
In either case, the average magnitude of armature cur rent (and hence braking efrort) can be controlled as desired by varying the duty factor of the chopper.
In the present state of the art, choppers for traction vehicle applications use high-power, solid-state controllable switchlng devices known as thy-35 ristors or sllicon controlled rectifiers (SCRs), A

z~

- thyristor i~ typically a three-electrode de~ice haYing an anode, a cathode, and a control or gate terminal.
When lts anode and cathode are externally connected ln series with an electrlc power load and a source of ~or-ward anode voltage (i.e., anode potential i5 positive wlth respect to cathode), a thyristor will ordlnarlly block appreciable load current until a ~iring signal is applied to the control terminal, whereupon lt switches from its blocking or "off" state to a con~
ducting or "on" state in which the ohmic value of the anode-to-cathode resistance is very low. Once trig-gered in this manner and latched in by conducting load current of at least a predetermined minimum magnitude prior to removal of the firing signal, the thyrlstor can be turned off only by reducing the current through the device~to zero and then applying a reverse voltage across the anode and cathode for a time perlod suffi-cient to allow the thyristor to regain its forward blocking ability. Such a device forms the main load-current-carrying switching element of the chopper, and suitable means is provided ~or perlodically turning it on and off.
In practical applications the main thyristor of the chopper is periodically turned off by means of a "commutation" circuit connected in parallel there-with. A typical commutation circuit is a "ringing"
circuit, i.e., the circuit contains inductive and capa-citive components that develop an oscillatlng or ringlng current. A chopper commutation circuit may in-clude, for example, a precharged capacitorg an in-ductor, a diode, and the inverse parallel combination of another diode and an auxiliary thyristor. In a voltage turn-off type of chopper, these components o~
the commutatlon circuit are so interconnected and , ~L4Z~Z~

arranged as to divert load current from the maln thy-ristor in response to turning on the auxiliary thy-ri3tor, and the main thyristor current is soon reduced to zero. The ringing action of the commutation clrcuit temporarily reverse biases the main thyristor which ls consequently turned off, and during the reverse bias interYal the current in the auxiliary thyristor 05-cillates to zero so that the latter component will also be turned off. For an ensuing brief lnterval, load current will contlnue to flow through the capacltor and a series diode in the commutation circuit of the chopper, thereby recharging the capacltor from the d-c source to complete the commu~ation process. Now the chopper is in an open or non conducting state, and it cannot return to lts closed or conducting state until the main thyristor is subsequer,tly turned on by ap-plying another firlng slgnal.
The duty factor or percentage on time of the chopper is determined by the time delay between firlng the auxlliary thyristor and subsequently firing the main thyri3tor during any full cycle of operation.
The shorter this delay, the higher the duty factor~
whereas the longer this delay, the lower the duty factor. Practical limits are imposed by the nature of the switching devices used in the chopper. For example, the maximum duty factor is approxlmately .91 ~or a chopper using a main thyristor rated 1100 amps (average) and 2000 volts (peak forward voltage) and operating at a constant frequency of approximately 300 Hz. A higher duty factor cannot be safely obtained at that chopping ~requency because the aforementioned time delay must be at least 300 microseconds to make sure that the ~.ain thyristor ls not re~lred prematurely, l.e., beYore the auxiliary thyristor has time to be completely turned off during the commutation process.

, ~6--For the same assumed parameters, the minimum duty ~actor would be approximately ~09. Thls is because the mlnimum pulse width per cycle is determ~ned by the recharging tlme of the capacitor ln the o~cillatory commutation circult. Con~equently, so long as lt is belng operated in a constant rrequency ~ariable pulse width mode, the chopper is effective to control motor current only in a limited range between its predeter-mlned minimum and maximum duty factors.
It is generally desirable to be able to vary the chopper duty factor over substantially the ~ull range between 0 and 1Ø In U.S. patent No. 3 1 944 9 856, a constant frequency oscillator ordinarily determines the ~ree running frequency of the chopper, but at high motor speeds a pair of frequency dividers are comblned with appropriate logic components to effect a two-step reduced frequency, maximum pulse width mode of opera-tlon, thereby extending the range of duty factor vari-ations above the maximum attainable when the chopper is operated in lts constant high ~requency pulse width mode. At the lower chopping frequencies the minimum delay time required a~ter turning on the auxlliary thyrlstor before refiring the main thyristor iB a smaller fraction of the whole period of each cycle.
By thus increasing the duty factor, the percentage of the available d-c source voltage that the chopper can apply to the motor armature is desirably increased. In the referenced patent the chopper frequency is reduced in two discrete steps that are ~ust equal, respec-

3 tively, to one-third and one-half o~ the constant high frequency, and this technique ls not optimum for con-trolling armature current during low speed electrlcal braklng o~ a chopper type propulsion system on a large traction vehicle.

~14Z~i~g Smooth continuous varla~ions o~ ~he duty factor up to 1.0 are desirable during the braking mode o~ operation to obtain high, constant braking effort when the vehicle is traveling at low speeds. The higher the duty factor, the lower the minimum speed at which the maximum magnitude of armature current can be sustained during braking. Once the vehicle decel erates below this minimum speed~ braklng ef~ort wlll decrease or fade out, The lowest posslble minimum brake fade out speed ls generally desirable.
Before changing from motoring to braklng modes of operation9 it is good practice to reduce the chopper duty factor to zero so that there is no current in the armature of the motor at the time the propulslon syste~.n is reconnected for braking operation. If the vehicle were moving slowly when the motoring-to-braking transition is desired, it ~rould be di~ficult to turn on the chopper after the transition. This is because at low speeds the voltage generated by the motor is low, especially ln a series field motor with zero cur-rent. The low voltage may be insufficient to ~orward bias the main thyristor ln the chopper. Even if the main thyristor were successfully trlggered, a rela-tively long time is required for current to build up to an appreciable level in the armature current path, and there is a possibllity that latching current will not be attained during the period of the firing signal that is normally applied. Raising the voltage o~ the motor by using the prior art technique of boosting its field at the beginning of a braking mode of operation is helpful but does not completely solve the problem Lengthening the period of the normal ~iring signals is not a desirable solution because o~ the attendant energy loss and lsolation problems. Shunting the ~ree wheeling path with an inversely poled auxiliary thy-ristor that temporarily conducts current rrom the d-c ~ ~4~i~D

source for au~menting current flowing through the main thyristor when initially trlggered, a~ suggested in prlor art U.S, patent No 3,748,560, is not a practical solution.
Summary Or the_ Invention Accordingly, lt is a general ob~ective of the present inventlon to provide a chopper type of d-c traction motor propulsion system wherein the duty ~actor of the chopper is smoothly variable over a .10 wide range extending to a maximum of 1Ø
Another ob~ectlve of this invention is the provision of a traction vehicle propulsion system that is characterized, when operating in its braking mode, by an unusua~ly low minimum brake ~ade out speed.
A further ob~ect of the invention is to pro~
vide a propulsion system characterized, when operating in its braking mode, by smsoth, constarlt, relatively high braking effort as the traction motors decelerate to a low brake fade out speed.
Yet another object.ive is the provis~on of lm-proved means for effecting electrical braking of a chopper type of d-c traction motor propulsion system wherein initial turn on of the chopper at the be- .
ginning of the braking mode of operation can be ob-tained at relatively low speeds.
In carrying out our invention in one form3 a propulsion system having motoring and braking modes of operation includes a chopper and means e~fective when the system is opera.ting in its mo~oring mode for connecting the chopper, in series with the armature and field windings of a d-c traction motor, to a d-c electric power source that includes a filter capacitor.
In normal operation, the chopper is alternately turned on and off in response to periodic gating signals o~
short duration that are produced by cyclically operative means. During the off lntervals of the chopper, current .~

in the armature o~ the motor circulates through free ~heellng rectifier means that i~ connected in circult t~erewith~ Upon commanding a motoring-to-~raking transition, ~rake set up means i3 operat1ve for re-connecting the propulsion system to establish an arma-ture current path comprising the field winding in series ~ith first and ~econd parallel branches, the first ~ranch including the chopper and the second branch including the fil~er capacitor ~n series with the free lO wheellng rectl~ier means. At the same time the po-larity of the connection of the series field winding relative to the armature i5 reversed.
In one aspect of k~le invention, we provide burst firing means effective in response to the re-15 connecting operation of the brake set up means for~upplying an extended chopper turn on signal having a duration substantially longer than the aforesaid short duration of the normal gating signals, thereby ensuring that the chopper turns on and conducts arma-20 ture current to begin the braking mode of operation ofthe propulsion system. In another aspect of the inven-tion, the gating signal produclng means is controlled by a variable control signal whose value determines the duty factor of the chopper, and it is arranged so that 25 for control signal variations within a predetermined range, the gating signals are produced at a predeter-mined constant frequency while the off time of the chopper is gradually decreased toward a predetermined minimum as the value of the control signal approaches 3 a high end of that range, whereas for control signal variations beyond the high end of the afore~aid range the gating signals are produced at an average ~requency that decreases from the predetermlned constant frequency to zero as the value of the control signal increases ' ' 20-TX~1309 ~hlle maintalning the predetermlned minlmum of~ time, A~ a result, smooth variation of the chopper duty factor up to l,0 is obtained.
T~e invention will be better understood and its various ob~ects and ad~antages will ke more fully appreciated ~rom the following description taken ln con~unction with the accompanying drawings.
Brie~ ~es-cri~ion of the ~ra~ings Fig, l ls a ~unctional block diagram of a traction vehicle propulsion sys~em having a plurality of chopper/motor units connected in parallel to a d-c bus;
Fig. 2 is a schematic clrcuit diagram of the filter and dynamic brake shown aæ single blocks in Fig. l;
~ ig, 3 is a schematic circuit diagram of a chopper/motor unit shown symbolically in Fig. l;
Fig, 4 is a graph showing the armature current vs. speed characteristic of the Fig. 1 propulsion system;
Fig. 5 is a functional block diagram o~ the master controls and the No. l chopper control shown as single blocks in Fig. l;
Fig, 6 shows the interrelationship of Figs, 6A
and 6B;
Figs, 6A and 6B are schematic diagrams of the logic and contactor actuating mechanisms in the brake control block of Fig, 5;
Fig, 7 is a chart showing the sequence in which the various components of the brake control are operated during a motor-to-braking transition;
Flg, 8 is a schematic diagram of the burst firing block of Fig, 5;

.; , .
.

. i , . . . . . .

~l~4Z~i~

20-~R-1303 Fig, ~ ig a ~chematic dlagram of the chopper reference block of Fig, 5;
Fig~ 10 ~s a schematic dlagram Or the chopper pulses block of Fig, 5, Fig, 11 Is a graph showing the relationshlp of t~e output frequency to the ~nput voltage of the V~F
converter ~lock of Fig. 10;
Fig. 12 is a simplified schematic circuit diagram of the armature current path of the illustrated system after being reconnected ~or electrical braking operation;
Fig, 13 is a diagram similar to Fig, 12 but showing an alternative propulsion system;
Flgs, 14A, 14B, and 14C are time charts of three different duty factors; and -Fig. 15 is a graph showing the manner in which armature current and electromotive force of the traction motor vary with speed as the vehicle is elec trically braked from 45 to 2.7 miles per hour.
Description of the Preferred Embodime_t Fig. 1 Fig. 1 depicts a propulsion system comprising at least two d-c traction motors 11 and 21 suitable for propelling or retardlng a large traction vehicle such as a locomotive or transit car. The motor~ 11 and 21 are shown symbolically in Fig, 1 and are res-pectively labeled "Ml" and "M2", It wlll be understood that each motor has conventional armature and series field wlndings (see Fig. 3), The motor rotors a~e mechanically coupled by speed reducing gears to sepa-rate wneel sets or the vehicle (not shown), and the armature windings of the motors Ml and M2 are elec-trically connected via duplicate electric power choppers 12 and 229 respectively, to a common d-¢

2~-rrR-l30s power hu~ 31~ ~Person~ ~killed ln the ~rt ~111 h~
awar~e that additional chopper~motor units can he readlly connected to the bus 31 in parallel w1th the two units that are illustrated ln Fig, 1~ The d-c ~us 31 ls coupled to a suita~le source of d-c electric power. Conventional filtering means 32, including a s~unt capacitor ~see Fig, 2), is connected between bu~
and source for isolation purposes and to provide a by-pass of the source for high-frequency, chopper gen-erated currents.
Preferably the d-c power source for the pro- -pulsion system includes a controllable electric power converter 33, means lncluding a contactor 34 for con-necting the input o~ the converter 33 to a source 35 of relatively constant voltage, and regulating means 36 effective when the propulsion system is operating in a motoring mode for controlling the converter 33 so as to limit the average magnitude of voltage across the shunt capacitor in the filter 32 to a predetermined level (e,g,, 1750 volts) during light load conditions when the capacitor voltage would otherwise tend to rise higher. In the illustrated embodiment of the invention, the voltage source 35 is stationary and feeds alternating voltage of relatively high magni-tude and commercial power frequency to an alternatingcurrent (a-c) line 37 comprising a catenary or third rail located along the wayside of the traction vehicle, The magnitude of the a-c line voltage may be, for example, 25,000 volts rms, and the frequency may be 3 60, 50 or 25 Hz. Onboard the vehicle there is a power transrormer 38 to step down this voltage~ The primary winding of the power transformer 38 is connected by way of a high voltage circuit breaker 39 to a current collector 40 (e,g,, a pantograph) that makes sllding 26z~3 contact with the wayside line 37. The secondary winding of the transformer 38 is connected by way of separable contacts of the contactor 34 to a set of a-c input terminals of the converter 33.
Preferably the converter 33 is a phase-controlled rectifier circuit utilizing controllable solid state electric ~alves such as thyristors or silicon controlled recitifers in selected leys of a full-wave bridge rectifier configuration, and the associated regulating means 36 is constructed and arranged in accordance with ~he teachins of United States Patent Number 4,152,758 issued May 1, 1979 to R. B. Bailey, T. D. Stitt~ and D. F. Wil~iamson and assigned to the General Electric Company. As is indicated in Fig. 1, a capacitor voltage feedback signal is supplied from the d-c bus 31 to the regulating means 36 on a line 41, and a~n alternating voltage feedback signal is supplied to the regulating means 36 on a line 42 which is coupled through a potential transformer 43 to the input terminals of the controlled rectiier circuit 33.
In order to meter the current in the arma-tures of the motors Ml and M2 that are connected in parallel array to the d-c bus 31, each of the respec-tive choppers 12 and 22 is alternately turned on (closed) and turned off ~opened). For the first chopper 12 this pulsing type of operation is controlled by an associated No. 1 control means 13 which normally supplies chopper No. 1 with alternate turn on and turn off signals on lines 14 and 15, respectively, and the second chopper 22 is controlled by a similar No. 2 control means 23 which normally supplies it with alter-.

nate turn on and turn off signals on ll,~es 24 and 25, respectl~ely, The chopper turn on and turn off signals are s~nchronized with a train of' discrete clock pul~es that are generated at a constant high frequenc~ Ce,g "
300 H~) 'oy a master clock 44. The clock 44 is con-nec~ed to the control means 13 and 23 by lines 45 and 46J respectively. The clock pulses supplied on line 46 to the No. 2 control means 23 are phase shifted or staggered with respect to the clock pulses that are supplied on line 45 to the l~o. 1 control means 133 here~y the two or more choppers used ~n the illus-trated propulsion system have their respective turned-off periods sequentially initiated at substantially equally spaced intervals during each cycle of opera-tion. By operating the choppers in sequence rather than in unison, the amplitude of' ripple current in the filtering means 32 and the rms current in the filter capacitor are desirably reduced, thereby minimizing the size of the flltering components that are required to provide a desired degree of electrical isolation be-tween the choppers and the wayside power line 37.
In each of' the motors Ml and M2 the average magnitude of armature current (and hence motor torque~
will depend on the duty factor of the associated chopper. As will soon be explained in more detail, each of the control means 13 and 23 is arranged to vary the duty factor as necessary to minimize any dif-ference between a current feedback signal and a current reference signai. To provide current feedback signals, conventional current transducers 17 and 27 in the arma-ture current paths of the respective motors Ml and M2 are connected vla lines 16 and 26 to the control means 13 and 23, respectively. The current reference s~gnal in each control means is derlved from a current call Z~

~ignal recei~ed on llne 47 from a master controls block 5Q, In accordance with one aspect of the present in-vention, the chopper control means 13 ~and 23) has the capability o~ smoothly varyi~g the duty factor of the chopper 12 (and 22) over a continuum that extends all the way between zero at one extreme (chopper turned o~ continuously~ and 1.0 at the opposite extreme (chopper turned on continuously). This will be more apparent hereinafter when Fig, 10 is descri~ed.
rrhe master controls 5O, shown as only a single ~lock in Fig. 1, perform several funckions that will now be briefly summari~ed~ The construction and operation of these controls will hereinafter be ex-plained in more detail in connection with the descrip-tion of Figs. 5 and 6. One function of khe master con-trols is to provide the aforesaid current call signal on output line 47. The value of this signal is varied as a function of the setting of either a manually op-erated throttle 51 or a manually operated brake con-troller 52~ which is mechanically interlocked with thethrottle, and it is also a function of the speed of the vehlcle. The vehicle speed is indlcated by speed sensing means 18 and 28 which are respectively coupled to the wheel sets of the vehicle or to the armatures of the motors Ml and M2. These speed senslng means typically are tachometer generators, and they feed back to the master controls 50 on lines 19 and 29 sig-nals representative of the angular velocities of the armatures of the respective motors.
26 Another function of the master controls 50 is to provide a voltage reference signal for the regu-lating means 36 that controls the phase-conkrolled rectifier circuit 33 in the d~c electi~ic power ~ource of the illustrated propulsion system~ rrhis signal i~
27 supplied over a line 53 from the rnaster controls ko the regulator 36. Its value, which is set in the masker .
., controls~ determines the limit level of voltage across the ~hunt capacitor in the fllter 32.
A thlrd function of the master controls 50 is to carry ou~ an orderly transition of the propulsion system between its motoring and braking modes on com-mand. This entails actuating the contactor 34 that connects the input term~nals of the controlled recti~
fier circuit 33 to the secondary windings of the power transformer 38, and accordingly the master controls are shown connected to the contactor 34 by a line 54, It also entails actuating certain additional contactors and a reverser in the armature and field circults of the motors 11 and 21. These additional contactors and the reverser for the first motor 11 are shown in Fig, 3 which will soon be described, ln accordance with a second'aspect of the presen~, invention, at the start of a braking mode of operation the master controls 50 will momentarily boost the motor fields and will supply a burst firing signal 20' on line 55 to the chopper control means 13 and 23.
The burst firing signal causes each of the control means 13 and 23 to supply an extended turn on signal to its associated chopper~ thereby ensuring that the chGpper in fact turns on while the field ls being boosted.
When the illustrated propulsion system is operating in its braking mode, electric energy from the motors Ml and M2 (now behaving as generators) is dissipated in a resistor grid that needs to be con-nected to the d-c power bus 31 for this purpose. The braking resistor grid is represented in Fig, l by a -' block 56 labeled "Dynamic Brake," and the master con-trols 50 are connected to this block by a line 57 in order to act-late a contactor that will connect certain resistors in the grid in paral,lel circuit relatlonship .

.

62~

wlth the shunt capacitor in the ~ilter 32 in response to a transition ~rom motoring to ~raking modes or operation, It should ~e noted that a single dynamic brake 56 is shared by all of the chopper/motor unit~
that are connected in parallel to the d-c bus 31, ~here is also provided ln the master controls mean3 effective during braking for actuating additional "staging" contactors in the dynamic brake ~lock 56 for changing, in three discrete steps, the amount of resistance connected to the d~c bus as necessary to prevent the generated energy from.charging the filter capacitor to an unacceptably high level of voltage~
Fig, 2 The dynamic brake block 56 and the ~iltering means 32 of the propulsion system have been shown in more detail in Fig, 2 which will now be descri~ed.
The d-c power bus 31, shown as one line in Fi~, 1, is actually a pair of conductors 31p and 31n which are respectively connected via the filtering means 32 to a pair of d-c output terminals 33p and 33n of the phase-controlled recti~ier circuit 33, The terminal 33n and the conductor 31n are both at ground potential, and a potential that is positive with respect to ground ls developed on the terminal 33p and on the conductor 31p, The positive conductor of the d-c bus is connected to ungrounded power terminals 12a and 22a of the res-pective choppers 12 and 22.
' As is shown in Fig. 2~ the filtering means 32 comprises a voltage smoothing capacitor 60 connected between the conductors 31p and 31n and a current limiting inductor 61 connected between the positive conductor 31p and the corresponding source terminal 33p, Although the filter capacitor 60 is illustrated and referred to in the singular, ln practlce this com-ponent will usually comprise a bank of parallel capa-citor elements. ~he capacitor 60 provides a current ~Z~2~
20~TR-1309 path for any instantaneous dlfference between source current and total load current during the motoring operation of the propulsion system, thereby attenu-atlng both line-frequency ripple and chopper-generated harmonics. For thlS purpose a fllter capacitor having a capacitance value of 6000 mlcrofarads iB contem-plated in one practical application of the invention.
The inductor 61 can have an inductance value of the order of 6 millihenrys. The capacitor voltage feed-back signal line 41 is connected to the positiveconductor 31p of the d c bus 31. As can be seen in Fig. 2, the dynamic brake resistor grid 56 preferably comprises two reslstors 62 and 63. These resistors have nearly equal ohmic values, with resistor 62 being divided into two serial elements 62a and 62~. One pole of a normally open two-pole con-tactor BB is connected between the upper end of the resistance means 62 and positive conductor 31p of the d-c bus, a conducting path 64 is connected between the lower end of the resistance means 62 and the upper end of the resistor 63, and the second pole of the con-tactor BB is connected between the lower end of the resistor 63 and the grounded conductor 31n, whereby all of the resistor elements 62a, 62b and 63 are con-nected in series with one another across the d-c bus 31 when the contactor BB is aetuated to lts closed position A d-c motor 65 that drives a blower ~not shown) for forcing cooling air across the resistor grid is connected in parallel with the element 62 of the resistance means 62 for energization b~y the voltage drop across this element when conducting current. If and when an approxlmately 50 per cent reduction in the amount of resistance across the d-c bus is desired, a flrst sta~ing contactor Bl can be 2~ ~

closed to connect the lower end of the reslstance means 62 to the grounded conductor 31n of the d-c bus 9 thereby short circuitin~ resistor 63~ Thereafter, 1 and when ano~her similar decrement in resistance is desired, a second staging contactor B2 can be closed to connect the upper end of resistor 63 to the positive conductor 31p. A diode 66 in the path 64 is reverse biased when both of the sta~ing contactors are closed so that no current can flow in the path 64, and now the resistors 62 and 63 are effectively connected in parallel with each other across the d-c bus. The con~
tactor BB, Bl, and B2 are coupled by broken lines 57a, 57b and 57c, respectively, to brake control means shown in Fig. 5. Their opened and closed positions are res-pectively determined by the brake control means, andthe mechanisms for activating these contactors are shown in more detail in Fig. 6B. The operation of the staging contactors Bl and B2 to prevent the braking energy from overcharging the filter capacitor 60 will be explained in connection with the description of Fig. 6.
~ig. 3 Turning next to Fig. 3, a preferred embodi-ment of the first chopper 12 will now be described.
The illustrated chopper is of the type disclosed in U.S. patent No. 4,017,777 issued on April 12, 1977, to R. B. Bailey and assigned to the General Electric Company. In brief, it comprises a main thyristor 70, an oscillatory commutation circuit 71 connected across the main thyristor, and an auxiliary or commutating thyristor 72 in the commutation circuit. The main thyristor 70 is connected between the power terminals 12a and 12b of the chopper, with a commutating inductor 73 belng dis~osed between its anode and the terminal 12a, AB ~as previously mentloned, the anode terminal 12a o~ the chopper 12 is connected directly to the positive conductor 31p of the d-c power bus 31~
The commutation clrcuit 71 of the chopper in-cludes, in addition to the thyristor 7~, a commutatingcapacitor 74, an inductor 75, and a diode 76. The positive plate of the capacitor 74 is connected di-rectly to the terminal 12a) and the negative plate of this capacitor is connected to the terminal 12b through a diode 77 that is pole~ to block capacitor dIscharge current when the main thyristor 70 iæ turned on. The auxillary thyristor 72 is connected across the commutating capacitor 74, with the inductor 75 being connected between its anode and the positive plate of the capacitor. The commutating thyristor 72 is shunted by the inversely poled diode 76, and its cathode is connected through a resistor 78 to ground~ The gate or control electrode and the cathode of each of the thyristors 70 and 72 are connected to gate and cathode terminals G and C, respectively. While each of the thyristors 70 and 72 and each of the diodes 76 and 77 has been shown in Fig. 3 as a single element, it will be understood that in practice, if required in choppers having high voltage and/or current ratings, additional elements of l~ke kind could be connected in series and/or parallel with the illustrated elements and operated in unison therewith.
Normally the chopper 12 îs turned on by firing the main thyristor 70. This is done by applying 3 a discrete signal of appropriate magnitude and duration across its gate and cathode terminals, With the main thyristor 70 turned on and the commutating capacitor 74 charged, the diode 77 is reverse bias~d and there is no current in the commutation ci,rcuit 71. Subse-quently the commutating thyristor 72 is fired by applylng acro~s its gate and cathode terminals a di~-crete chopper turn off signal of appropriate magnitude and duration, Now the commutating capacitor 74 wlll discharge through the inductor 75, The resulting ringing action of the commutation clrcuit 71 soon forward biases the diode 77, whereupon current ln the main thyristor 70 is reduced to ~ero and the main thyristor is temporarily reverse blased. This turns off the main thyristor 70. During the reverse bias interval the current in the commutation circuit 71 oscillates to zero and reverses direction, While current is flo~ing through the diode 76, the commu-tating thyristor 72 is reverse biased and consequently turned o~f. For an ensuing brief interval, current continues to flow through the commutating capacitor 74 and the diode 77, thereby recharKing the capacitor from the d-c source to complete the commutation pro-cess. Now the chopper is turned off, and it wlll re~
main in this state until the main thyristor 70 is re-fired by the next turn on signal, So long as the propulsion system is operatingin its motoring mode, the chopper 12 is periodically turned on and off to regulate the average magnitude of current flowing from the d-c power source to the armature and series field windings of the associated motor Ml. In Fig. 3 the armature of this motor is shown at 80, and the series field winding is shown at 81. The chopper 12, the armature 80 9 and the field 81 are connected in series with one another between the terminal 12a and ground, and thiæ series combi-nation of components is therefore connected across the filter capacitor 60 (Fig, 2). As is shown in ~i~. 3, the means for serially lnterconnecting these components includes a current smoothing reactor 82 and the current transducer 17, both of which are connected between the cathode terminal 12b of the chopper and the armature 80, and a contactor M which connects the series ~ield 81 to ground, The contactor M is closed Cas shown) during the motoring mode of operation and ls open during the braking mode of operation. The lnterconnecting means also lncludes a reverser RR that determines the po-larity of the connection o~ the series field winding 81 relative to the armature 80.
The reverser RR is illustrated as a double-pole double-throw contactor. When this reverser is in a first position, the movable contact comprising one of its poles engages a stationary contact Fl and the movable contact comprising its other pole engages a stationary contact F2, whereas when the reverser is in a second, alternative position, the first-mentioned movable contact engages a stationary contact Rl which is connected to contact F2, and the other movable con~
tact engages a stationary contact R2 which is connected to contact Fl, Either the armature 80 or the series field winding 81 can be connected between the contacts Fl and F2. In the illustrated embodiment of the in-vention, it is the field winding 81 that is so con-nected.
During intervals when the chopper 12 is turned off, armature current IA in the motor Ml is conducted by free wheeling rectif'ier means FWR which is connected in circuit with the armature 80 and ~ield 81. In Fig, 3 the f'ree wheeling rectifier means is shown as a simple diode having its anode connected to ground and its cathode connected to the cathode termi-nal 12b of the chopper 12. Whenever thls element is conducti.ng current, terminal 12b is at nearly ground potential, If' desired, the free ~Iheeli.ng rectifier means FWR can comprise a thyristor instead of the 11-- lustrated diode. If a thyrlstor were used, its f'iring .
`

~ ~Z~i2~

can be controlled by the improved gate means described and claimed in United S-tates Paten~ No. 4,284,938, R.B. Bailey, dated August 18, 1981, and assigned to the General Electric Company.
To change from motoring to braking modes of operation, the contactor M is opened and a companion contactor B is closed. As is shown in Fiy. 3, the con-tactor B, when closed, connects the last-mentioned movable contact of the reverser RR to the anode terminal 12a of the chopper 12 (and hence to the positive conductor 31p of the d-c bus 31). Consequently, when the contactor ~1 is actuated to its open position and the contactor B is actuated to its closed position, the propulsion system is reconnected to establish an armature current path comprising the field winding 81 and the contactor B in series with at least two parallel branches. A first one of these parallel branches is provided by the chopper 12, and the second parallel branch is provided by the filter capacitor 60 (Fig. 2) in series with the free wheeling rectifier means FWR.
The conducting direction of the free wheeling rectifier means in the second parallel branch enables armature current to charge the filter capacitor 60 when the chopper 12 is turned off but blocks discharge of this capacitor through the chopper when turned on. A third branch paralleling the first and second branches of the armature current path i5 provided by the resistor grid 62, 63 (Fig. 2) whenever the dynamic brake contactor BB is closed.
During the transition from motoring to braking modes of operation, the reverser RR is actuated so as to reverse the polarity of the connection o~ the series field winding 81 relative to the armature 80 of the motor Ml. With the field 81 connected to the reverser RR as shown in Fig. 3, actuation of the reverser wlll reverse the direction of current ln the fleld 81 and thereby reverse the polarity of the electromotlve ~orce generated in the armature windings 80 during the braking mode Or the operatlon (when the motor Ml is behaving as a generator). As a result~ the electro-motive force will be applied across t~e chopper 12 with the proper polarity to forward bias the main t~yristor 70, Alternatively, if the reverser RR were connected across the armature 80 instead of the field 81, the polarity of the generated electromotive force would be the same during braking as during motoring but the pOSitiVe motor brush would be reconnected through the reverser and the contactor B to the anode terminal 12a of the chopper.
The opened and closed positions Or the res-pective contactors M and B and reverser RR in the arma-ture and field circuits of the motor Ml are determined by brake control means (shown in Fig 53 to ~hich they are coupled by broken lines 12~ and 153, and the mecha-nisms for actuating these components are shown in more detail in Fig. 6B. The same mechanisms can also be coupled, respectively, to similar contactors and to a similar reverser that are connected in the armature and field circuits of the second chopper/motor unit 22/M2, whereby the second unit of the propulsion system is reconnected for braking operation and the polarity of its field is reversed with respect to lts armature connection simultaneously with the occurrence of these events in the Fig. 3 chopper motor unit.
Fig 3 also illustrates means for boosting the fields of the traction motors Ml and M2 This means comprlses a transformer having a primary windlng 83 and multiple secondary windings 84 and 85, a suitable source 86 of alternating voltage, and a normally open double-pole field boost switch FS con-nected between the source 86 and the primary winding 83.

s3 20~TR-1309 -25~
The voltage across the transformer secondary windlng 84 when energized is rectifiecl by a pair of diodes 87 and applied to relatively positive and negative output terminals 88p and 88n. As ls shown in Fig, 3J opposlte ends of the secondary winding 84 are connected through the respective diodes 87 to the relatively positlve terminal 88p, and a center tap of the secondary winding is connected directly to the relatively negative output terminal 88n, A similar rectifier circuit connects the other secondary winding 85 to a second pair of output terminals 89p and 89n, The first pa~r of output terminals 88p and 88n of the field boost means are respectively connected to the upper and lower movable contacts of the reverser RR
and hence to the associated field winding 81 of the motor Ml. -A current limiting inductor is serially in-serted in this connection~ and a resistor 91 is con-nected in parallel with the field to minimize the ef'fect of chopper-induced ripple on motor commutation. When-ever the switch FS is closed, the transformer primarywinding 83 is energized by the source 86 and the field boost means then supplies current of desired magnitude (e.g,, 60 amperes) from its output terminals 88p and 8~n to the field winding 81. This will increase the field excitation of motor M, The movable contacts of the field ~oost switch FS are actuated by a suitable mechanism (shown in Fig, 6B) to which they are coupled via broken line 185. As will be more fully explained hereinafter in connection ~rith the description of Fig, 6, this actuating mechanism is automatically operative to close the switch FS temporarily, for a predetermined period of time (e,g,, approximately one se~oncl), in response to a motoring-to-kraking transition of the propulsion system, whereby a momentary lncreas~ Or field current _26-is obtained at the beginning of the braking mode of operation, At the same time, the ~ield boost means will also momentarily increase current in the field wlnding ~not shown) Or the second motor M2 to which the second pair of output terminals 89p, 89n are connected, The additional excitation during the period of field boost results in more electromotive force being generated by the traction motors, whereby the voltage across the armature of each motor is desirably increased, This armature voltage increase is signifi-cant if the braking mode of operation is initiated with the vehicle moving at a relatively low speed, At low speeds without field boost the electromotive force might be insufficient to force current to build up ln the armature and series field of the motor.
During the above mentioned period of time that the field boost means is operative to increase field excitation at the beginning of the braking mode of operation, a burst firing signal from the master controls causes the No. 1 chopper control means 13 tQ
apply to the gate terminal of the main thyristor 70 in the chopper 12 an extended firing signal having a dura-tion substantially longer than that of the discrete turn on signals that are periodically supplied by the control means in normal operation, This solves a prob-lem of effecting initial turn on of chopper 12 when electrical braking is initiated at low speeds. Even with the previously described operation of the field boost means, the increased voltage across the armature 80 of the motor Ml is not high enough at low speed ~e,g,, 3 miles per hour) to force current in the arma-ture current path to increase abruptly as soon as the leadlng edge of the first firing signal is applied to the gate terminal of the main thyristor 70, The in-ductance o~ this path and the appreciable voltage drop , .

z~

-~7~
across the main thyristGr when conducting less than lt~ 'tlatching" current can result ln delaylng current building up to the minlmum latching level for an inter-val much longer than the duratlon of one of the normal periodlc flr~ng signals~ The extended firing signal that is applied during the field boost period in accordance with the present invention has a duration longer than the maximum anticipated time that will be required for armature current to attain the latc~llng level of the thyristor 70, and therefore successful turn on of the chopper is ensured when braking is initiated at low speed.
Flg. 4 In Fig. 4 the solld-line trace 94 depicts the relationship between the magnitude of armature current IA and the-vehicle speed (MPH) during the braking mode of operakion of an electrlc locomotive embodying the present invention, with the brake control means being set at its maximum ~1.0) rate, At speeds above the corner point 95 (approximately 21 MPH), the prcpulsion system is operating at constant horsepower~ whereas at speeds between the corner point 95 and a predeter-mined minimum point 96 the propulsion system is operating at a constant current and hence constant brakin~ effort. A m~nimum speed as low as 3 MPH can be obtained in practice. Electrical braking can be successfully initiated at any speed above this low mlnimum, and the maximum braking rate can be sustained without fadeout as the vehicle decelerates to the minimum point 96. The improved electrical braking per-formance of the present invention can be appreciated by comparing it with the armature current vs. speed characteristic of a typical prior art electric locomotive during the bralcing mode of operation. The latter 3y characteristic is shown in Fig, 4 by the broken-line trace 97. Below the corner point speed 95, armature current in the prior art propulsi,on system tends to fall o~f linearly with decreasing speed, but the ~raking range is extended to a minlmum point 98 (e.g., 8 MPH) by using staging contactors to reduce the ohmic ~alue of the braking grid resistors in four discrete 3teps.
Below the mi~imum speed point g8 the braking effort of the prior art propulsion system fades out.
Fig, 5 (Master Controls) The master cont,rols ~0 of the propulsion system as well as the No. 1 control means 13 that periodically turns on and of~ the first chopper 12 have been illustrated in Fig. 5 in functional block diagram form. As i6 shown in ~lig~ 5, the master controls com-prise a reference generator 100 that receives inputs from both~the throttle 51 and the brake controller 52 and that also receives a th~rd input, representative Of vehicle speed, on a line 101 from an averaging circuit 102 which is coupled by lines 19 and 29 to the speed sensors (tachometer generators 18 and 28) associated with the repective motors Ml and M2. The re~erence generator 100 is operative to produce a current call signal I~ which is fed via the output line 47 to both the No. 1 and No. 2 control means. In practice, the current call signal is passed through a conventional power limit circuit ~not shown) which proportionately reduces its value in response to either lo~ voltage on ~he fiiter capacitor or overcurrent or overtemperature in the po~er transformer. The master controls further comprise a brake control block 103 coupled by llnes 104 and 105 to block 106 labeled "Reference Level &
Ramp Up" and coupled by the line 104 and another line 107 to a block 108 labeled "Chop Enable," The brake control block 103 receives lnputs on line 110 from the throttle 51 and on lines 111 and 112 from the bralce controller 52.

2~ciZ9 20-T~ 1309 In the illustrated embodlment of the present invention, the throttle 51 and the brake controller 52 are mechanically interlocked wlth each other and with a manually operated, two-position forward/reverse controller 113. Together with the reference generator 100, these components form conventional command means ~or the propulsion system of an electric locomotive.
The command means has alternative motoring and braking states. In its motori.ng state, the brake controller 52 is locked in an "off" posltion and a manually operated hand].e (not shown) of the throttle 51 can be selectively moved in eight steps or "notches" be-tween a low power position (notch 1) and a maximum power position (notch ~). The reference generator 100 ls suitably constructed and arranged to vary the current ca;.l signal I* as a function of both the selected power notch of throttle 51 and the average speed of the traction motors of the propulsion system.
The graph shown in Fig 5 in the upper half of the reference generator blocX 100 indicates the manner in which the value of I* varies with speed for various different power notches of the throttle 51.
In the braking stat.e of the command means, the throttle 51 is locked in an "idle" position and a manually operated handle (not shown) of the brake con-troller 52 can be freely moved through a "brake on"
sector between a predetermined low limit or 0 brake (minimum braking rate) and a predetermined maximum limit 1.0 (maximum braking rate). The reference 3 generator 100 will now vary the current call signal I* as a function of both the setting of the brake con-troller 52 (in its brake on sector) and the average -speed of the +raction motors. Brake position 1.0 re-! sults in I* being maintai.ne~d at a c~nstant maximum ~; 35 value (whicll corresponds, e g., to armature current o~

L4'~ 9 20-TR~1309 ~30-approximate1~ 880 amps) throughout a low speed range and being varled inversely ~rith speed above the cor~er polnt, This maximum characterist~c is indlcated by the graph shown in Fig, 5 in the lower half of the reference generator block 100. 'At other positions of the brake handle, I* is reduced proportionately to the setting of the brake controller 52 (e.g., at 0.5 brake, the highest I~ is approximately 440 amps).
The interlockin~ of the throttle 51 and the brake control 52 make it necessary for the operator to follow a prearranged sequence of operations in order to change from motoring to braking states. The throttle handle must first be move,d to its notch 1 position from any higher notch (which therefore reduces the current call signal I~ to its lowest value for the given speed at which the vehicle happens to be movin~), and it can then be moved from notch 1 to "idle." In the idle position of the throttle 51 the brake handle is no longer locked in its "off" position. Now the brake controller 52 can be moved to a brake "set up" positlon which locks the throttle in its idle position. In this state, referred to hereinafter as the transition state, the illustrated command means is in between its motoring and braking states. Subsequently the operator can advance the brake handle from its set up position to the brake on sector, starting with the aforesaid low limit. When a return to the motoring state is desired', this sequence is reversed, The interlocks, also prevent the forward/reverse controller 113 from chan~ing position except when the throttle ls in idle and the brake is off.
The throttle 51 and the brake 52 are coupled -to the brake control block 103 by means Or lines 110, 111, and 112 in order to supply the latter component with indications of their respective positions.

.

, .

2~ ~

Expressed in digital terms, the throttle 51 provides on line llO a logic signal that is low or tlOII whenever the throttle is in its idle position and high or "1"
when the throttle is in any one of its power notches, Similarly, the brake controller 52 provides on line lll a logic signal that is low or "0" whenever the brake is off and hlgh or "l" when the brake ls in either its set up position or its brake on sector.
The brake controller also provides on the line 112 a logic signal that is low or "0" when the ~rake is in either its off or set up position and that is high or "1" whenever the brake is in the brake on sector (0 brake to 1,0 brake). The ~orward/reverse controller 113 is also coupled to the brake control block 103 by a line 114 on which is provided a logic signal that is low or "O".whenever thls controller is in a reverse position and high or "l" whenever a forward position is selected.
The brake control block 103 responds to a state change of the command means by appropriately actuating the various contactors shown in Figs, l, 2 and 3, More details of a practical embodiment of the brake control block are shown in Figo 6 which will soon be described. From the subsequent description of Fig, 6 it will be apparent that the contactor 34 ~Fig, l) connecting the propulsion system to the power transformer 38, which contactor is coupled via the line 54 to the brake control block 103 in Fig, 5, ls temporarily opened and then reclosed during a state 3 change of the command means. The opening of the contactor 34 is indicated by a reset signal on the line 104 from the block 103, In dlgital terms, whenever the contactor 34 is open the reset signal on line 104 is low or "0", ~ut when the contactor 34 is later re-closed and certain other inputs are normal, the signalon the line 104 changes to a high or "1" level, The reset signal line 104 1B connected to the reference level and ramp up block I06. This com-ponent of the master controls ls designed to produce and to set the value of a voltage reference slgnal VREF that is supplied over the line 53 to the pre-viously described regulating means 36 ~Fig. 11~ The block 106 is reset in response to a 0 signal on the line 104, whereby VREF is reduced to a low value Ce.g., zero~ whenever the contactor 34 opens to disconnect the controllable converter 33 from its voltage source.
Upon reclosing the contactor 34, the signal on the reset line 104 changes from 0 to 1. A 1 signal on this line enables the block 106 to increase or ramp up VR~F
to a predetermined value which depends on whether the command means has been changed to its motoring state or to its braking state. The reference level and ramp up block 106 is suitably constructed and arranged so that VREF increases at a predetermined rate (e.g., 2500 volts per second) until it attains either a first level corresponding to approximately 850 volts if there is a low or "0" signal on the line 105 (indicating a braking mode of operation) or a second higher level corresponding to approximately 1750 volts if the signal on line 105 is high or "1" (indicating a motoring mode of operation~. After VREF reaches its present level3 the regulating means 36 is effective to control the converter 33 so as to prevent the voltage across the filter capacitor 60 from falling below the aforesaid first level when the propulsion system is operating in its braking mode or from rising above the aforesaid second level when the propulslon system ls operating in its motoring mode.
As ls shown in Fig, 5, the reset signal line 104 from the brake control block 103 iæ also connected aZ~ji,~
20~TR-1309 ~33-to one lnput of the chopper enable block 108~ Another input of the block 108 is connected to the line 41 for recelving the capacitor voltage feed~ack signal ~rom the filter 32, The latter signal ls representati~e o~
the voltage across the filker capacitor 60 ~Fig. 2), which voltage is identl~ied by the reference character VF in Fig. 5 and hereinafter. The output of the block 108 is supplied on the line 107 to the brake control block 103 and also to the No. l (and No, 2) chopper control means 13 ~and 23).
The chopper enable block 108 comprises bistable means that is suitably constructed and arranged to detect the magnitude of the input on line 41 and to change states as this magnitude traverses predetermined "pickup" and "dropout" levels. It is in a dropped ollt state when VF is relatively low (e.g " below 500 volts) and ~n a picked up stake when VF is relatively h~gh Ce.g., above 725 volts). So long as the bistable means is in its dropped out state3 a low or "0" signal is provided on the output line 107 of the chopper enable block 108. But when the bistable means is in its picked up state and in res-ponse to the presence of a 1 signal on the reset line 104, the chopper enable block is effective to provide a high oOr l signal (hereina~ter referred to as the chop enable signal) on the output line 107. ~us a chop enable signal indicates that the contactor 34 is closed, the filter capacitor 60 is charged to an ap-preciable voltage level, and certain other inputs are normal.
In the brake control block 103, operation of the means that temporarily closes the field boost swi~ch FS (Fig. 3) is prevented ~nd no burst firing signal can be supplied on line 55 in the absence Or the chop enable signal on line 107. Upon receipt Or this signal, .

, , i-, .. .

the field boost period commences, pro~iding that the command means ls then In it~ braking state and the propulsion s~stem has been reconnected by opening the contactor M and closing t~e contactor B, and provlding further that the speed feedback signal on the llne 101 indicates that the average motor speed is not below a predetermined low magnitude (e.g., an angular velocity corresponding to a vehicle speed of appror.i-mately 3 MPH). In delayed response to the commencement of field boos~, the brake control block 103 supplies the burst firing signal on line 55 to the No, 1 (and No, 2) chopper control means 13 ~and 23). The brake control block 103 is also effective during the braking mode of operation to supply to these control means a signal designated B' on a line 115.
- Figs. 6A and 6B
Details of a preferred embodiment of the brake control block 103 are shown in Fig,x 6 which will now be described. This figure is a combination of two contiguous figures 6A and 6B, Fig. 6B is lo-cated on the same sheet as Fig, 6 and shows the actu-ating mechanisms of the various contactors and the re-verser. Fig. 6A is on a separate sheet~ and it shows a schematic diagram o~ logic circuits and other com-25 ponents that are used to control these actuatingmechanisms. In Fig. 6A each of the encircled dots symbolizes a conventlonal and logic function (output is high or 1 only when all of its inputs are 1, other-wise output is low or 0), and each of the encircled plus 30 signs symbolizes a conventional OR logic function ~output is 1 if any one or more of lts inputs is 1, and output is O only when all inputs are 0~. Known elec-tromechanical hardware or solid state electronic equi-valents can be used to lmplement these logic functlons.
35 Typically, cam actuated interlock contacts on the ~l~Z6~5~

throttle and brake controller and conventional inter-locks of the contactors and reverser are appropriately interconnected for this purpose.
The ~rake controls shown in Figs, 6A and 6B
comprise three main parts: means for opening and re-closing the contactor 34, brake setup means, and field boost means. The first-mentioned means includes an AND logic circuit 120 connected by a line 121 to an OR logic circuit 122 whose output is coupled via a line 123, a time delay component 124, and a line 125 to a mechanism 126 (Fig. 6B) that actuates the movable main contacts o~ the contactor 34 ~Fig. 1). The cir~
cuit 120 has two inputs: one is received on the line 110 and indicates the position of the throttle 51, and the other is received on a line 127 from the output of an AND logic circuit 130. The l~tter circult in turn res-ponds to two inputs. The first input of circuit 130 is coupled via a line 131 to an interlock contact (not shown) of a mechanism 132 (Fig. 6B) that actuates the contactor M (Fig. 3) to which it is coupled via broken line 129, and a 1 signal on the line 131 ln~icates that the contactor M is closed. (Note in Fig. 6B that the same mechanism 132 that actuates the contactor M
is also coupled by line 129 to the contactor B This mechanism has a flip flop type of control which is set in a first stable state in response to a 1 signal on a first input line 132a and which is reset to an alternative stable state in response to a 1 signal on a second input line 132b. In the first state the mechanism 132 will maintain M closed and B open, where-as ln the alternative state the mechanism maintains B
closed and M open. The first input line 132a Or this mechanism is connected directly to the 1 ne 110 so that M is closed and B is open when the throttle 51 is -~lfl2~2~

positioned in one o~ its power notche In some appll-cations of the invention it may be de~irable to o~tain the same results by using separate mechanlsms to actu-ate the respective contactors M and B ) ~he second input to the AND logic circult 130 of the contactor opening and reclosing means is re~
ceived on a line 133 from the output of an OR logic circuit 134. As will soon be explained, the output ~rom the circuit 134 is 1 whenever the reverser RR
~Fig, 3~ i5 in the correct one of its two positions.
Consequently a 1 signal on the output line 127 of the logic circuit 130 indicates that the propulsion system is properly connected for motoring operation. So long as there are 1 signals on both lines 110 and 127, there is a 1 signal on the output line 121 of the AND
logic circuit 120, and this signal is passed through the OR logic circuit 122, the time delay component 124, and the line 125 to the contactor mechanism 126. In response to a 1 signal on line 125, the mechanism 126 maintains the contactor 311 closed. The 1 signal on the output line 121 of the logic circuit 120 is also passed through an OR logic circuit 135 to line 105 which is coupled to the reference level and ramp up block 106 (Fig. 5) so as to set VREF at the value desired during the motoring mode Or operation.
To start a motoring-to-braking state change of the command means, the handle of the throttle 51 i8 moved from power notch 1 to idle This event is indi-cated by a signal change frolr. 1 to O on the line 110, and as a result the signal on the output line 121 of the logic circuit 120 changes from 1 to 0. This causes a corresponding signal change on the output line 123 of` t~le logic circuit 12?, providing that there is then a O signal on a second input line 136 of this circuit:. (As will soon be explained, the state .

~4Z¢i~
20-T~-1309 ~37-o~ the signal on line 136 is determined by the brake set up means. This signal is 0 during the motorin~
mode of operation but changes to 1 in response to the brake controller 52 being moved to i~s set up or on position and t~e propulsion system being properly re-connected for braking operation,~ A short time after the signal on llne 123 changes from 1 to 0, the signal on the output line 125 of the delay component 124 goes to 0, and the actuating mechanism 126 opens the con-tactor 34 in response to a 0 signal on line 125. ~he de].ay introduced by component 124 allows time for the choppers 12 and 22 to reduce armature currents in the respective traction moto~s at a controlled rate to a low magnitude or zero before the contactor 34 discon-nects the propulsion system from the power source.
Such armature current reduction occurs at the end of the motoring mode of operation in response to the current call signal I~ (Fig, 5) being reduced by move-ment of the throttle to its power notch 1 and to its idle position.
As soon as the contactor 34 opens, there i8 a signal change from 1 tc 0 on a line 137 that is coupled to an interlock contact (not shown) of the contactor actuatlng mechanism 126. This line is connected as an input to an AND logic circuit 138 whose output supplies the aforesaid reset signal to the line 104. Thus the signal on line 104 changes from 1 to 0 in response to the opening of the con-tactor 34, and this will reset the reference leYel and ramp up block 106 as previously described. When the contactor 34 is su~sequently reclosed, the signal on line 137 changes from 0 to 1. Hence the reset signal on line 104 returns to a 1 state~ providing that at the same time a 1 signal is being supplied from a terminal 140 to the second input Or the AND

20 TR-l3og lo~ic circuit 138. The terminal 140 i8 adapted to ~e connected to means ~not shown~ for supplying an input signal that is 1 so long as certain condi~ions are normal. Such conditions can compromise, e.g., the powèr transformer 38 being energized, proper con~
trol power being present, and a master shutdown swltch being in a run or on state.
Upon operation of the brake set up means ~described below~, the si~nal on the llne 136 changes from 0 to 1, and this 1 signal is passed through the OR logic circuit 122, the delay component 124, and the line 125 to the actuating mechanism 126 which auto matically recloses the contactor 34 in response thereto.
The delay component 124 is suitably designed so that it does not delay this 0 to 1 signal change. The 1 signal on ~he line 136 is also supplied to inverting means 141 whose output is connected to the OR logic circuit 1359 and there~ore the reference level signal on the line 105 changes from 1 to 0 at the same time reclosure of the contactor 34 is initiated by the brake set up means~ A zero 0 signal on line 105 sets VREF at a value that is desired during the braking mode of operation.
In the brake set up means of the brake con-trol block, the signal on the line 111 is fed through a time delay component 142 to a line 143 having three branches, One branch of the line 143 extends to Fig, 6B where it is connected not only tp the second `' input line 132b of the mechanism 132 that actuates 3 the contactors M and B (Fig. 3) but also to the input of an actuating mechanism 144 that is coupled by broken line 57a to the two-pole dynamic brake con-tactor BB (Fig, 2), When the signal on the line 143 changes from 0 to 1, the mechanism 132 actuates the contactor M to its open position and actuates th~
-contactor B to its closed position, there~y recon-necting the propulsion system for braking operation~
and at the same time the mechanism 144 closes the con-tactor BB to connect the dynamic brake resistor grid across the filter capacitor. This signal change on the line 143 takes place in delayed response to move-ment of the brake controller 52 from off to set up positions, The delay introduced by the component 142 ensures that armature current in the contactor M has time to decay to a low magnltud~ or zero befor~ the contactor M opens, Another branch of the line 143 provides inputs to two AND logic circuits 145 and 146 that are respectively labeled R and F in Fig. 6A, The second input to the first circuit 145 is received on the line 1-14 and indicates the position of the forward/reverse controller 113 (Fig. 5), whereas the second input to the second circuit 146 is received on a line 147 from the output of inverting means 148 whose input is connected to the line 114, Conse-quently, if the controller 113 is in a forward position when a motoring-to-braklng state change of the command means ta~es place, the output signal of the first cir-cuit 145 will reflect the O to 1 change on the line ~5 143, but if the controller 113 were ln a reverse posltion at this time, the output signal of the second circuit 146 would reflect such change, The output of the circuiks 145 and 146 are fed, respec-tiveIy, through an OR logic circuit 150 to a line 152b and through an OR logic circuit 151 to a line 152a~ The second input to the loglc circuit 150 is supplied by an AND logic circuit 155 whose flrst -input is received on the line 110 and ~!hose second input is received on the line 147, and a second input to the logic circuit 151 i8 supplied ~y another AND

-IJO-loglc circuit 156 whose ~irst input i5 also received on the line 110 and whose second input is received on the line 114.
The llne 152b from the output of the OR logic circuit 150 and the line 152a from the output of the OR logic circuit 151 are both connected to an actuating mechanism 152 (Fig. 6B) that ls coupled by broken line 153 to the movable contacts of the reverser RR (Fig. 3) associated with motor Ml and to the contacts of a cor-10 responding reverser (not shown) associated with motorM2. This mechanism has a flip flop type control; it maintains the mova~le contacts of the reverser in their first position (engaglng stationary contacts Fl and F2 in response to a 1 signal on the line 152a, and it 15 maintains the movable contacts of the reverser in their second position (engaging stationary contacts Rl and R2) in response to a 1 signal on the line 152b, The reverser RR is shown in its ~irst position in Fig, 3.
Hereinafter this will be called position F, and the 20 second position of the reverser will be called position R.
In operation, assuming that the vehicle is being propelled in the forward direction, the mechanism 152 will actuate the reverser RR from position F to 25 position R in response to a motoring-to-braking trans-ition of the command means, and it wlll actuate the reverser from position R to position F in response to a braking-to-motorin~ transition. Assuming lnstead that the vehicle is being propelled in the reverse 30 dlrection, the mechanism 152 will actuate the re-verser RR fro~ position R to position F in response to a motoring-to-braking transition or the command means, and it Yill actuate the reverser from posltion F to posltion R in response to a braking-to-motoring ~, 35 transltion. In each case, this operation of the brake ~.

2Ei29 ~41-set up means reverses the polarity of the connection Or the series field winding 81 relatlve to the motor armature 80~ Whlle not shown in Figs. 6A and 6B, the mechanism for opening and closing the Rl,R2 con-tacts of the reverser could be separate from themechanism for opening and closing the Fl,F2 conkacts~
and in some applications of the invention it will ~e desira~le to provide additional. lnterlocking to ensure that the reverser changes positlons only when both o~
the contactors M and B are open.
To indicate the postiion o~ the reverser RR, interlock contacts (not shown) associated with the actuating mechanism 152 are coupled to lines 157 and 158. A 1 signal on the line 157 indicates that khe reverser is in position F, and this signal provides an input to an ANn logi.c circuit 160 whose other input is ta~en from the input line 152a of the mechanism 152.
As is shown in Fig.-6A, the output of the circuit 160 serves as one input to the OR logic circuit 134 whose output is connected to the line 133, and it is 1 whenever a 1 signal on line 152a is directing the mechanism 152 to actuate the reverser RR to its posi-tion F and there is a 1 signal on line 157 to indicate that the reverser in fact is in this position. On the other hand, a 1 signal on the line 158 l~dicates that the reverser is in position R, and this signal provides an input to another AND logic circuit 161 whose second input is taken from the input line 152~
of the mechanism 152. The output of the circuit 161 serves as another input to the OR logic circuit 134, and it is 1 whenever a 1 signal on line 152b is di-recting the mechanism 152 to actuate the reverser to its position R and there is a 1 signal on line 158 to indlcate that the reverser in f'act ls i.n this posikion, Thus, the slgnal on the output llne 133 of the clrcuit 2~ ~
20~TR~1309 _42-134 is in a l state whenever there is proper corres-pondence between the directed position and the actual position of the reverser RR. This signal i~ red to an input o~ the previously described AND logic circuit 130, and it also is fed to an input of another AND
logic circuit 162. The latter circuit responds to two additional inputs received on lines 163 and 164, res-pectively. Line 163 is coupl~d to an interlock contact (not shown) of ~he M/B actuating mechanism 132, and a l signal on this line-indicates that the contactor B
ls closed. The other line 164 is coupled to an inter-lock contact (not shown) o~ the mechanism 144 that actuates the dynamic brake contactor BB, and a 1 slgnal on this line indicates that the contactor BB ls closed.
The signal on line 164 is also employed to activate staging means 165 for controlling a pair o~
actuating mechanisms 166 and 167 that are respectively coupled by lines 57b and 57c to the staging contactors Bl'and B2 in the dynamic brake circuit 56 (Fig, 2).
As is indicated in Fig. 6B, there are three lines 41, 168, and 169 connected to the staging means 165 for respectively supplying it wikh the capacitor voltage feedback signal, a f'irst reference signal represent~r.g a maximum level of voltage (e.g.g 1650 volts~ that is permissible on the filter capac~tor during the braking mode of operation, and a second reference signal representing a desired minimum capaci-tor voltage (e.g., 1200 volts), The staging means 165 is contentionally constructed to close and open the contactors Bl and B2 as necessary to minimize excur-sions of VF above the maximum level or below the minl-mum level.
Witil a l signal on each of` the lines 133, 163, and i6~J, all three inputs of' the AN~ logic circuit ~-43-162 are 1 and consequently there ls a 1 signal on the output line 170 of this circuit. A 1 signal on the output llne 170 lndicates that the propulsion system has been properly reconnected for ~raking operation.
5 In other words, the system is ready to begin a braking mode of operation. The line 170 is connected to one input of an AND logic circuit 171, and the third branch of the line 143 i5 connected to the other input of this circuit, The output of the circuit ]71 is connected to 10 the line 136, and consequently the signal on the output line 136 is in a 1 state whenever the system ls set up for braking and the brake controller is eitner in its set up position or in its brake on sector. As was pre-viously mentioned, a 1 signal on the line 136 is 15 passed throug'n the OR logic circuit 122, the delay component i24, and the line 125 to the mechanism 126 which responds thereto by actuating the contactor 34 to its closed position.
The field boost means of the brake control 20 block includes an AND logic circuit 172 having three inputs. As can be seen in Fig. 6A, the first input of the logic clrcuit 172 is received on the line 112, and it changes from O to 1 at the start of the braking state o f the command means when the brake control]er 25 52 is moved from lts set up position to its brake on sector. The second input af the circuit 172 is re-ceived on the line 170, and it changes from O to 1 in response to operation of the brake set up means to reconnect the propulsion system for braking~ The 30 third input of the circuit 172 is received on the line 107, and it changes from O to 1 when the chop enable signal is provlded by the chopper enable block 108 (Fig. 5) in response to a 1 slgnal on the reset line 104 and a high filter capacitor volta~e VF-. The 35 output of the circuit 172 is cr:nnected to .,he line 115, ;~ 20-TR-1309 ~411--and it will be in a 1 state whenever there are 1 s1g-nals on all three inputs of thls circult, as is true in the braking mode of operation. The 1 ~ignal on the line 115 is the aforesaid B' signal w~lich is supplied 5 to the chopper control means 13 and 23, This Rignal is also supplied as one input to an AND logic circuit 174 whose other input is received on a line 175 from level detecting means 176, The level detecting means 176 is connected via the line 101 to the speed aver-10 aging circuit 102 (Fig, 5), and ~its input is a speedfeedbacI; signal ~ representative of the average angular velocity of the armatures of' the respective traction mlotors. So long as this reedback signal exceeds a particular value corresponding to a pre-15 determined low vehicle speed (e.g,g approximately3 MPH), the means 176 wlll supply a 1 signal on the line 175 to the logic circuit 174, but the signal on line 175 is in a 0 state when the signal ~ is below this value. Thus a 1 output from the logic circuit 20 174 is prevented whenever speed is below the pre-determined low level.
The output of the AND logic circuit 174 læ
connected via a line 177 to the input of a one shot block 178, The component 178 can be a conventional 25 monostable multivibrator which produces a 1 output signal having a predetermined f'ixed duration (e.g"
0,5 second) once triggered by the s~gnal on line 177 changing from 0 to 1. The output of the component 178 is connected on a line 179 to a "B" input of a second 30 one shot block 180 similar to the component 178 except that it is triggered by the signal on line 179 changing from 1 to 0, whereby the 1 signal on the output line 181 o~ the componer,t 180 begins at the end of the 1 output signal on line 179. The output signals on the 35 lines 179 and 181 are respectively noted ~y the reference letters X and Y in ~ig. 6A, Both are con-.

20-T~-1309 nected through an OR logic circuit 182 to a line 183 w~ich is connected to a mechanism 184 (Fi~o 6B~ for actuatlng the field boost switch FS (Flg, 3), 1~e mechanism 1849 which is coupled to the movable contacts of` the switch FS ~y a ~roken llne 185, ls operative to close the switch FS only when there is a 1 signal on the line 183, as is true for a limlted period of' time (e.g., approximately one second~ after the one shot component 178 is triggered.
The one shot block 178 is initially triggered at the start of the braking modé of operation (as soon as the brake controller is moved to its brake on sector and the brake set up means produces a l signal on the line 170 and a chop enable signal is supplied on line 107). So long as the system remains in its braking mode, the component 178 will be automatically retrlg-gered anytime the vehicle slows down to a speed lower than the aforesaid predetermined low speed (e.g., 3 MPH) and subsequently accelerates to a speed above this threshold, whereupon the signals on the lines 175 and 177 change states from 0 to 1 and the field boost meanæ
responds by temporarlly reclosing the switch FS whicb momentarily increases field excitation of the traction motors (now behaving as generators). Such triggering is desirable because the electromotive force of these machines may have been too low to sustain armature current (and hence braking effort) when the vehicle was traveling slcwer than 3 MPH.
The output of the second one shot component 180 is connected on the line 181 to the input of yet another one shot block ]86 similar to the f'lrst com ponent 178 eY.cept that the fixed duratlon of lts 1 - output signal ~F is much shorter ~e.g~, 2 milli-seconds). Thls output signal is the af'oresaid burst Z~
20-TR~130 ~4~-flring signal which i~ supplied on the line 55 to the chopper contr~l means 13 and 23. The third component 186 produces it~ output signal when triggered ln re3-ponse to a 0 to l change of signal Y from the second one shot block 180, wh~ch event is delayed with res-pect to the initial triggering of the first one shot block 178 by an interval equal to the duration of the signal X on line 179. ~herefore the signal BF com-mences approximately midway through the period of time O that the field boost switch FS is clo~ed.
Fig. 7 The operation of the brake control means during a motoring-to-braking state change of the command means will now be summarized with the aid o~ Fig. 7, The motoring-to-braking sequence is begun by reducing the throttle setting to its lowest power notch (which reduces the current call signal I* to a low magnitude) and then moving the throttle handle to idle. This step of the transition process causes the signal on the line 110 to change from l to 0, and in response thereto the contactor 34 is opened, the signal on the voltage reference reset line 104 is changed from l to 0, and the chop enable signal on line ln7 is termi-nated (l.e., changed from l to 0). The first step also unlocks the brake controller 52 which can now be moved from its off position to the brake set up position.
The command means is now in its transition state, In Fig. 7 the beginning of the transition state is noted as time to~ At this time the signal on the line lll changes from 0 to 1, and in delayed response thereto a 1 signal on the line 143 causes four e~ents to take place. Two of' these events are implemented by the M/B actuating mechanism 132 which is directed by the l signal or. line 143 (and hence on line 132b) to open the contactor 1~7_ M and to close the corttactor B, The third e~ent ls implemented ~y the RR actuating mech-anism 152 which is directed by the concurrence Or the 1 signal on line 143 and a 1 sigrtal on elther line 114 or line 147 to reverse the position o~ the reverser RR. Assuming that there is a 1 signal on the llne 114, thls event will open the Fl,F2 contacts and close the Rl~R2 contacts of the reverser RR. The fourth event ls implemented by the BB actuatlng mechanism 144 which is directed by the 1 signal on line 143 to close the dynamic brake contactor BB. These four events do not necessarily happen simultaneously~ and whichever one that takes place last will cause the brake ready signal on line 170 to chan~e ~rom 0 to 1, as iB indl-cated at time tl in Fig, 7. The resulting 1 signal online 136 (and hence on line 125) directs the contactor actuating mechanism 126 to reclose the contactor 34, whereupon the signal on the reset line 104 changes from 0 to 1 (a 1 signal on terminal 140 is assumed). Now the voltage reference signal VREF ramps up to the de-sired level ~as set by a 0 signal on the reference level llne 105), and as soon as the capacltor filter voltage VF attains a sufficiently high magnitude (at time t3 in Fig. 7), a chop enable signal is supplied to line 107.
Anytime after to the operator can move the brake controller 52 to its brake on sector, thereby terminating the transition state and starting the braking state as indicated ~y the signal on the line 112 changing from 0 to 1, conld take place earlier or later than t3. In Fig, 7 it i5 shown at a time t2 that ---is earlier than t3. As soon as the signal on line 112 is 1 and both the ~ralce read~ signal on line 170 and the chop enable signal on llne 107 are 1, the B' signal on line 115 changes from 0 to 1. This mark~ the start 2~
20-TR-130g o~ the braking mode of operation of the propulsion 3ystem3 and it also triggers the first one shot block 178 of the field ~oost means (assuming that speed is not below 3 MPH), Therefore the signal X on line 179 changes from 0 to l concurrently with the signal B'.
When the signal X changes from 0 to 1, the resulting l signal on line 183 directs the FS mechanlsm 184 to close the field boost switch FS, and at time t4 the movable contacts of this switch reach their closed circuit poSitiOn to start a brief period of increased current ln the series field windings of' the kraction motors, Subsequently, at time t5, the signal X auto-matically reverts to its 0 state, thereby triggering the second one shot block 180. The signal Y on the output line 181 of block 180 now changes from 0 to 1.
This maintains a 1 si~nal on line 183 until the signal Y3 at time t6, automatically reverts to its 0 state, whereupon the FS mechanism is directed to open the field boost switch FS. The switch FS returns to lts open position at time t7, thereby terminating opera-tion of the field boost means. Signal Y changing from 0 to 1 at time t5 also triggers the third one shot block 186 which is then effective to produce on line 55 the burst firing signal BF that is shown by the bottom trace of Fig. 7.
In the illustrated embodiment of the present inventior., the period of time that the field boost means is operative to close the switch FS (from tLI
to t7 in Fig. 7) is approximately one secondg and the the time (t5) at which the one shot block 186 becomes effective to produce the signal BF is delayed until approximately 0.5 second after t4. Thls delay allows time for the increased field currerlt to overcome re-sidual excitation in the f'ield poles of each motor and to develop therein an appreciable reverse magnetic Z~i~

_1~9_ ~ield be~ore the associated chopper is turned on ln the armature current path to begin the braking mode o~ operation. Alternatively, lf the reverser RR
were connected across the armature rat~er than the 5 ~ield windings of the motor, less time ~ould be needed because the direction o~ fle]d ~oost agrees with the direction of residual exci~ation, and therefore the t4 to t5 interval could be made much shorter by corres-pondingly shortening the fixed duration of the signal X. In some applications o~ the invention it mlght even be desirable to start operation of the field boost means before the armature reversing aspect of the oper-ation of the brake set up means is completed.
Fig. 5 ~Chopper Controls) .4s is shown in Fig. 5, lines 55 and 115 from the brake control means shown in Flg. 6A are connected to the No. 1 (and No. 2) chopper control means 13 (and 23). The No. 1 control means 13 comprises a block 191 labeled 'IBurst Firing," a block 192 labeled "Chop.
Ref," a block 193 labeled 'IChop. Pulses," and a pair of blocks 194 and 195 each labeled "Gate Drive.~' The line 55 conveys the burst firing signal BF to an input of the burst firing block lgl. Another input o~ this block receives on the line 16 the current.feedback signal representative of armature current IA in motor Ml. The burst firlng block 191 has two output lines 196 and 197 connected to the chop pulses block 193;
and it is suitably constructed and arranged (see Fig~
8) to supply on the line 196 a d~c gate signal that ls 3 contemporaneous with the burst firing signal on llne 55 and to supply on line 197 a commutation suppressing signai that is initiated by the burst firin~ signal and terminated when ~he magnitude o~ IA increases to at least a predetermined threshold.
The line 115 conveys the B' signal to an lnput of the chopper reference block 192 in the No. 1 :.,i:,, i ~

:
.

i2~

control means 13, Other inputs of this block receive, respectively, the chop enable signal on the llne 107 ~rom t~e chopper enable means 108, the current call signal I* on line 47 from the reference generator in the master controls, the current feedback signal on line 16 ~rom the current transduçer 17 in the armature cur-rent pakh of the motor M1, and certaln additional slgnals from a terminal 198. ~he chopper reference block 192 is suitably constructed and arranged Csee Fig, 9) to process these inputs and produce therefrom 1~ a ~ariable control signa] Vc representative of the de-sired duty factor of the associated chopper 12~ This control signal is supplied on a l~ne 199 to the chspper pulses block 193.
The chopper pulses block 193 has five inputs lS that are respectively connected to lines 45, 1999 107, 1973 and 1~6, and it has two output llnes 201 and 202.
~etails of 2 preferred embodiment of this component are shown in Fig. 10 which will soor. be descri~ed. Nor-mally the chopper pulses block 193 is cyclically opera-ti~-e to produce on its output line 201 a train of first periodic gating signals of relatively short predeter-mined duration (e.g., 10 microseconds) and to produce on its second output line 202 a train of periodic second gating slgnals of the same short duration. The first gating signals are supplied on line 201 to the input of the ~ate driver l9lJ whose outpuk ls coupled via the lines 14 to the gate and cathode terminalæ G
and C of the main thyristor 70 in the No. 1 chopper 12, and the component 19l~ is suitably constructed and arranged to supply a firing signal to this maln thy-ristor in respGnse to each of the flrst gating signal~
received on line 201. The periodic s~?cond gating signals from the chopper pul~es ~lock 193 are supplied on the llne 202 to the input of the companion gate ~51-driver 195 who~e output i~ coupled vla lines 15 to the gate and cathode ~erminals G and C of the auxillary or commutatin~ thyristor 72 in the No, 1 chopper, and th~
component 195 is sultably cons~ructed and arranged to supply a firing signal to thls commutating thyristor in response to each of the second gating signals received on line 202, As will be apparent hereinafter from the description of Fig. 10, the first gating signal~ on line 201 are produced alternately with the second gating signals on llne 202, whereby the gate drivers are effective to alternately turn on and turn off the chopper. The chopper pulses block 193 includes means for synchronizing the ~eco~ld gatlng signals with the clock pulses on line 45 and means responsive to the value of the variable control signal Vc on line 199 for influencing the timing of the first and second gating signals so as to determine the duty factor of chopper No. 1.
ht the beginning of a braking mode of opera-20 tion, the d-c gate signal on line 196 is passed through the pulses block 193 to the output line 201 in the form of an extended chopper turn-on slgnal that effects firing of the main thyristor 70 throughout the period of the burst firing signal on line 55, which period 25 is substantially longer than the duration of a first gating signal that the block 193 periodically produces ln normal operation. At the same time the commutation suppressing signal received on line 197 is effective in the block 193 to prevent the production of any 30 second gating signal on the line 202 until the magni-tude of armature current increases to at least the afore-said predetermined threshold.
Fig, 8 With reference now to Fig, 8, a preferred 35 embodiment of the burst; firing ~lock 191 of the No~ 1 chopper control means 13 will be de~cr:Lbed~ This com-ponent comprises an AN~ logic circuit 2~4 havlng two inputs: one is the burst firing signal BF received on the line 55, and the other is received on an output line 205 of level detecting means 206. The level detector 206 ls supplied via the line 16 with the current feedback signal that indicates the actual magnitude of armature current IA in the motor Ml~ and ~t is suitably constructed and arranged to produce on its output line a signal that is 1 so long as IA is lesæ
than a predetermined threshold magnitude (e.g., 100 amperes) and that is 0 when the magnitude of lA in-creases to at least this threshold. As was previously explained, lA will be low or zero at the start of the field boost period, and thérefore both inputs of the logic circuit 204 wi]1 be 1 when the burst firing signal appears on llre 55. At this time the output signal o~
the circuit 204 changes from 0 to 1.
The output of the AND logic circuit 204 is connected by a line 207 to an input of another AND logic circuit 208 whose second input is received on a line 209 from a high frequency clock 210 that generates a train of discrete "1" pulses. By way of example, the frequency of the pulses on line 209 is 21,6 KHz, and each pulse can have a duration of 10 to 20 microseconds.
The output signal of the logic circuit 208 there~ore comprises a burst of high-frequency pulse~- that lasts for an interval equal to the duration of the 1 output signal from the logic circuit 204. The duration of the latter signal normally corresponds to the period of the burst firing signal on line 55, namely, approxl-mately 2 milllseconds. The output signal Or the circult 208 ls the d-c gate signal that is ~upplied over the line 196 to the cyclically operatlve chopper pulses block 193 in the No. I chopper control means 13 (~ig.

2~

20-TR~1309 ~53-5~, and during lts presence on line 196 the block 193 wlll be effective to supply an extended chopper turn on signal to the gate driver 194 of the main thyristor 70 in the chopper 12. The duration of the latter signal is substantially longer than the predet0rmined dura-tion of the per~odic first gating signals that are normally produced by the pulses bolck 193, Preferably the duration of the extended chopper turn on signal ls at least 100 times longer than that of the first gating signals, and in the exam~le glven herein lt is approximately 200 ti.mes longer. This ensures that if a motoring-to-braking transition were commanded while the vehicle is mo.ving at a relatively low speed, the initial firing signal for turning on the chcpper will not expire prematurely, ~efore armature current has time to attain the latching level of the main thy-rlstor, and it consequently ensures that the chopper is in fact turned on and conducts armature current to begin the braking mode of operation of the propul-sion system durlng the period of time that the seriesfield of the motor Ml is being boosted to increase the electromotive force that is being generated ln the armature of this machine. Once turned on, the chopper will freely conduct current in the armature current path of the motor Ml, and the rise af current in this path, including the series field, will augment the field boost so that the electromotive force rapidly increases, This further enhances the buildup ~f arma-ture current which soon attains its 100~amp threshold, ordinarily within less than one-half second of the time that the burst firing signal BF is produced, As can be seen in Fig, 8, the burst fi.ring signal BF on the line 55 in the burst ~iring means ].91 is also supplied to a cloc~ input of a conventlonal flip flop device 211 whose "D" input is connected to z~

a d-c control power terminal 112 which is positive Ce~g~, ~10 volts~ with respect to a predetermlned re~erence potential. The Q Outpllt of this device i3 COnnected to the line 197, and it changes from 0 to 1 when the cloc~ input signal changes from 0 to 1 on receipt Or the signal ~ Su~sequently this output ls chan~ed back to 0 by applying a 1 signal to the reset lnput of the device 211. The reset input i~
connected via a line 213 anrl inverting means 214 to the line 205, whereby its 0 to 1 change coincides with the output of t~e level det~cting means 206 changing from 1 to 0 as a result of armature current attaining the 100-amp t,hreshold, The Q output signal o~ the device 211 is the commutation suppressing signal that is supplied over the line lg7 to the pulses block 193 (Fig, 5), and in its 1 state this signal is effective to disable the puises block 193 and thereby prevent it from producing an~ gating signals that would otherwi~e turn off the chopper 12. The commutation suppressing signal on line 197 is in its 1 state for an interval that begins at the same time as the burst firing signal BF and that ends as soon as armature current attains its 100-amp threshold. During this interval the chopper is turned on in response to the d-c gate signal on line 196 and then remaln on continuously, but once the interval explres the pulses block lg3 can resume normally producing gating signals to alternately turn off and turn on the chopper.
~ig, 9 Turnin~ next to Fig, 9, a preferred embodi-ment of the chopper reference means 192 will now ~e described. Thls means, whlch was shown as a ~lngle block in ~ig. 5, includes a summing point 216 havlng a first input connected to a current reference signal llne 217, a second input connected to the current feedbacl{

.

20-TR~1309 ~55-slgnal line 16, and an output connected to an error signal line 218~ The current reference slgnal on line 217 1s representative of the desired magnitude IR~F f armature current in the motor M1, the current ~eedback Signal on line 16 is representative o~ the actual magnitude IA of this current, and the error signal on line 218 is therefore representative of the difference - between IREF and IA. Preferably the quiescent value o~
the error signal ~i.e., its value whenever bot~. the desired an~ actual magnitudes OL armature current are zero) is negative wit~ respect to the predetermined re~erence potential of the control power.
The error signal on line 218 is processed by a suitable gain network 219 having a proportional plus integral transfer characteristic, whereby a zero steady-sta~e error can be obtained. The galn of the -~
network 219 is varied as a function of speed ~indicated by the speed feedback signal on line 19) 3 current (in~
dicated by the current feedback signal on line 16) J
and whe~,her the system is in a motorlng or braking mode of operation (indicated by the B' signal on line 115).
In the braking mode, the transfer function of this component has a faster time constant and a higher gain than in the motoring mode. The output of the gain net-work 219 provides the variable control signal Vc whichis fed on line 199 to the cyclically operative chopper pulses block 193 in the No. 1 chopper control means 13 (Fig. 5), The value o~ Vc varies as a function of any to assume whatever value results in reducing this difference or error between IREF and IA and will tend difference to ~,ero. The value of Vc can vary between predetermined first and second extremesy and it is varied in a sense approaching the second or high extreme (e.g., ~10 volts on an analog ~asis) ~rom its rirst or low extreme (e.g., -1.5 volts) so long as IA is less 20~TR-1309 ~56-than IREF. The timing of the alternate firsk and second gating slgnals that are perlodically produced ~y t~e cyclically operative chopper pulses block 193, and consequently the duty factor Or the chopper 12, are determlned by the value of Vc on line 199.
~en the value of Vc is at lts low extreme, the duty factor is zero (chopper turned off contlnuously~, and ~hen Vc is at its high extreme the duty factor i8 1. O
Cchopper turned on continuously).
In order to provide the aforesaid current reference signal, the line 217 of the chopper reference means 192 is connected through three blocks 220, 221, and 222 in tandem to the line 47 on which the current ca]l si~nal I~ is received from the master controls.
The block 22Q is designed to be effective only ln a braking mode of operation, as indicated by a Bl signal on line 115~ to prevent the current rererence signal on line 217 from falling below a certain minimum value that corresponds to a predetermined magnitude of arma-ture current (e.g., 100 amperes). This is deælrableto maintain self excitation of the traction motor Ml and to ensure a minimum braking effort in the event the operator were to move the handle of the brake con-troller 52 to the lowest or zero position in its brake on sector The block 221 is labeled l'Jerk Limit," and it performs the conventional function of preventing the value of the current reference signal on the line 217 from being changed too fast. By way of example, the maximum rate of increase of the reference signal can be limited to a rate corresponding to 200 amps per second, and the maximum rate of decrease can be limited to a rate corresponding to 1000 amps per second.
The block 222 of the ch~pper reference ~eans 192 is suitably constructed and arranged to perform 6~

dual runctions. I~s first functlon is to proportion-ately reduce the current call signal either in the event Or a whee] slide involving the wheels that are coupled to the motor Ml or ln response to low voltage on the commutating capacitor of the chopper 12 compared to the magnitude of armature current that has to be commutated, In Flg 9, the two lines under the refer-ence number 198 respectively represent the wheel slide and the commutating capacitor voltage lnputs to the block 222. The second function of this block is to reset the current reference signal to a value corres-ponding to zero current whene~er there is no chop enable signal on the line 107. For this purpose the block 222 includes means for clamping its output to a low or zero value in response to the signal on line 107 changilig from 1 to 0, as happen.s at the beginning of a motorir,g-to~braking transition of the command - means when the contactor 34 is opened in response to the throttle handle being moved to its idle position.
If the current reference signal on line 217 were not already reduced in response to the handle of the throttle 51 being moved to power notch 1 and then to ldle, it would no~r be driven at its maximum rate (as limited by the block 221) to a reset level that ls slightly negative with respect to ground, thereby altering the value of the control signal Vc as neces-sar~ to ensure that this signal attains the aforesaid lo~r extreme. As a result, the chopper duty factor and hence I~ are rapidly reduced to zero. Subse-3o quently, when the chop enable signal returns to line107 ~at time t3 in Fig, 7), the output of the block 222 is unclamped and the current reference signal on line 217 can increase to whatever value is being called for by the signal I~ on line 47.

.:

.
, -5~-Figs, lO and ll Flg, 10 illustrates the preferred em~odiment o~ the chopper pulses block 193. In this component the ~arlable control signal ~C on line l9~ ls supplied as one input to a summing polnt 224 where it is compared with a saw-tooth reference signal produced ~y a ramp generator 225, The ramp generator 225 i8 connected to the master clock 44 ~y a line 45a, and lt ls period-ically reset ~y a phase l clock pulse on this line, The clock 44 generates a train o~f phase l pulses on the line 45a, each pulse being in a l state for a pre-determined duration (e.g., 3ao microseconds~ and successive pulses recurring at a constant frequency (eOg., 300 Hz).
The ramp generator 225 comprises integrating means for changing the value of the reference signal at a predetermined constant rate and means operative in synchronism with the phase l clock pulses for period-ically resetting the reference signal to a predeter-mined base value which is substantially equal to the aforesaid high extreme of the control signal Vc Ce.g "
~lO volts), After being reset, the reference signal changes in a sense approaching the aforesaid low ex~
treme value of Vc, and the rate of change is selected so that the reference signal excursion is approxi-mately lO volts during one period of the clock pulses.
This reference signal is subtracted from Vc in the summing point 224, and the difference is supplied on a line 226 to a zero crossing detector 227 whos~ output is fed on a line 228 to an AND logic circuit 230. In digital terms, the signal on the output line 228 is low or "0" so long as the value of the reference signal produced by the ramp generator i~ greater Ci.e,, more posltive) than the value of the control signal Vc, and it is high or "1" whenever the latter signal is greater than the former. When Vc is at its high extreme, the signal on line 228 is 1 continuously. When Vc has a negative value the signal on line 228 is 0 continuously.
When Vc is in a range between zero and its high extreme, the signal on line 228 will change states twice each cycle of the master clock; from 1 to 0 when reset by a phase 1 clock pulse, and from 0 to 1 concurrently with the value of the reference signal equalling the value of 10 Vc~
The variable control signal Vc on line 199 and the phase 1 clock pulses on line 45a are also supplied as inputs to a voltage-to-fr~quency converter 231. This component is suitably constructed and arranged to periodically produce at its output F a train of discrete 1 signals having an average frequency that is related to the value of Vc in accordance with the graph shown in Fig. 11. For variations of Vc between its low extreme (-1.5V) and a predetermined first intermediate value (e.g., -0.5 V), the frequency of the output signals F varies between zero and the clock frequency -(300 Hz) as a direct linear function of the value of Vc. For variations of Vc between its high extreme (+10 V) and a predetermined second intermediate value (e.g., +9.1 V), the frequency of the output signals F varies between zero and the clock frequency as an inverse linear function of the value of Vc. For variations of Vc in a predetermined range that is defined by the aforesaid first a~d second intermediate values, the frequency of F is constant and equal to the frequency of the master clock. A
V/F converter well suited for this purpose is disclosed and claimed in United States Patent No. ~,256,983 issued March 17, 1981, R.J. Griffith and R.D. Stitt and assigned to the General .
' 2 ~ ~ 20-TR-1309 Electric Company. Such a converter is so arranged that a 0 to 1 change of its output signal F always co-incldes with the leading edge of a phase 1 clock pulse on the line 45a. This converter receives addltional inputs via lines 45~ and 45c from the mas~er clock 44.
The clock is designed to generate on line 45~ a train of phase 2 pulses that are characterized ~y the same ~requency and duration as the phase 1 pulses on llne 45a but are displaced in time therefrom by a predeter-mined fraction of the period of the clock (e.g., by1/3 period, or 1/900 second), and to generate on line 45c a train of phase 3 pulses that are sim~lar to but further displaced in time from the phase 1 pulse ~e.g~, by 2/3 peri.od or 2/900 second). A 1 to 0 change of each output signal F produced by this converter coin~
cides with the leading edge of the phase 2 clock pulse that is next received after the output signal was ini-tiated. In addition, this converter is arranged to produce at a second output E a signal that is 0 only when the value of Vc is between its low extreme and the aforesaid first intermediate value and that other-wise is 1, The output signal F of the converter 231 ls connected by a line 232 to a first input of an AND
logic circuit 233. Another input of the latter cir-cuit is connected through a line 234 and inverting means 235 to the line.197 which receives the commutation suppressi.ng signal from the burst firing block 191 (see Flgs, 5 and 8), Thus there is a 1 signal on llne 234 except during intervals when the burst firing means is effective to supply a 1 signal on line 197. As is shown in Fig. 10, the third input of the logic circult 233 is connected via a line 236 to the Q bar output of a conventiorlal D type flip flop 237. The set input of the latter component is connect~d through inverting . ; , .
.. , .,,. , .,, > .,.,., . ,.. , . ~ .. ~

means 238 to the line lQ7 ~hich receives the chop ena~le slgnal from the chopper ena~le means 108 (Fig, 51, and the clock input ls connected tTlrough lnverting mean~
239 to t~e output line 232 of the V/F converter 231.
As will soon be explalned, the flip flop 237 serves a pulse steering purpose ~rhen the signal on line 107 changes from 1 to 0, Du-rlr.g normal operation the chop enable signal is 1 and the Q bar output of 237 is in a high or 1 state, 1~. With 1 signals on bothiof its input lines 234 and 236, the loglc circuit 233 will pass a 1 signal to its output line 240 concurrently with each of the perlodic 1 signal from the output F of the converter 231, The line 240 is connected through an OR logic circuit 241 to the input of a one shGt block 242 which produces a-l. OUtpllt signal having a relatively short predeterri~lned duraticn ~e,g " 10 microseconds) whenever it is trigggered by the signal on the line 240 changing from O to 1. I~he bl.ock 242 is connected by a line 243 to suitable amplifying and isolating means 244 which is effective ~Jhile the output signal on this line ls l to forward bias the base-to-emitter Junction of an NPN
trans:Lstor 245. The collect.or and emitter of the transistor 245 are coupled via terminals 202a and 202b 25 to the input of the gate drive block lg5 (Fig. 5), and when this transistor is forward biased its collector current is the aforesaid second gating signal that periodically causes the gate driver 195 to fire the commutating thyristor 72 in the No, 1 chopper 12, 30 This happens each tlme the output signal F of the con verter 231 changes from 3 to 1, providing that 1 slg-nals are then pl~eSent on both lines 23lJ and 236, Thus the frequency Or the second gating sig.n.als is t,he ~ro-quenc~ of th~e output si~nal F, , 6;Z~3 The output llne 243 of the one shot hlock 242 is also connected to a reset input of another D type flip ~lop 247, As can ~e seen in Fig, 10, the clock input of the latter component is connected to the out-put line 248 and the D input is connected directly tothe positlve control power terminal 212. The Q output of this flip flop is coupled over a line 250 and an OR
logic circuit 251 to the input of another one shot block 252 which produces a 1 output signal having a 10-microsecond duration each time it is triggered ~y the signal on the llne 250 changing from O to 1. ~he block 252 is connected by a line 253 to suitable ampli-rying and isolating means 254 which ls effective while the signal on this line is 1 to forward blas a tran-sistor 255 whose collector and emitter are coupled via terminals 201a and 201b to the lnput of the gate drive block 194 (Fig. 5~. When the transistor 255 is forward biased, its collector current is the aforesaid first gating signal that periodically activates the gate driver 194 which then fires the main thyristor 70 in the No. 1 chopper. This happens each time the signal on line 250 from the Q output o~ the flip flop 247 changes from O to 1.
The output o~ the flip ~lop 247 is reset to zero by the signal on line 243 each time a secondgating signal is produced, and it thereafter is re-turned to a 1 state upon receipt of a 1 signal on the line 248 connected to the clock input. Once returned to 1, the Q output remains in this state until reset 3 by the next 1 signal on line 243. As a result, in normal operation the signal on line 250 periodlcally changes from O to 1 at a frequency that is the same as the frequency Or the second gating signals, and the I'irst gating signals will alternate with the s~cond æating signals~

- , ~

.

lZ6~
20-TR~1309 The clock input of the ~lip flop 247 la connected by the line 248 to the output of the AND
loglc circuit 230. This circuit has four inputs: one ls received on the llne 228 ~rom the output of the pre-~iously described zero crosslng detector 227; anotherinput is received on the line 236 from the Q bar output of the flip flop 237; the third ls received on a line 256 from the E output of the V/F converter 231; and the fo~lrth is received on a llne 257 which is connected through inverting means 258 to the line 45a. The sig-nal on line 257 serves a lockout function; it prevents a 1 signal on line 248 while each of the phase 1 pulses on line 45a is 1~ which is the case for an interval of approximately 300 microseconds following the lnitlation f each oP the second gating signals. This interval, referred t~ hereinafter as the lockout interval, is required to make sure that the first gating signal is not produced prematurely, i.e., before the commutating thyristor has time to be completely turned off during the commutation process of the chopper.
So long as there is no phase 1 ~ulse on the line 45a, the signal on line 257 is 1, and assuming 1 signals on both of the lines 236 and 256, the signal on the output line 248 of circuit 230 will now reflect the state of the signal on line 228. As was previously explained, the signal on line 228 changes from 0 to 1 whenever the saw-tooth reference signal produced by the ramp generator 225 decreases to the value of the con-trol signal Vc on line lg9, Consequently, so long as 3 ~C has a value in a range between 0 and ~9.lV, the gating signals are produced at the constan~ frequency of the master clock (300 Hz) and the time interval from the production of one of the second gatlng signals for firing the commutation thyristor to the production o~
the succeeding rirst gating æignal for flring the main z~

20~TR 1309 thyristor varies lnversely with the value f ~C~ Thl~
interva] ls referred to as the off tlme (toF~ o~ the chopper 12. It decrease~ toward a predetermined mlni-mum as the value of Vc approaches 9.1 V. The minimum turn off time i.s the same as the aforesaid lockout in-terval (e.g., 300 microseconds~.
The duty factor of the first and second gating signals is equal to l -f x toFF, where f i~
the frequency of the output signal F of the V/F con-verter 231. So long as this converter is operating inits constant 300 Hz mode, the minimum of~ time of 300 microseconds restricts the maximum duty factor to approximately .91. As Vc i.ncreases from 9.1 V to its high extreme of +lO V, the duty factor is increased from .91 to nearly l.0 by reducing the average frequency of the periodlc output signals F of the converter 231 whl.le maintaining the off time substantially equal to the aforesaid minimum.
The minimum duty factor of the chopper is also restricted in the constant ~requency operating mode of the converter 231, even when Vc ls reduced to zero or to a negative value. This is because each time the commutating thyristor is fired it will conduct a pulse of load current having a minimum duration or width which is determined by the recharging time of the commutating~capacitor 74 in the oscillatory commutation circuit 71. This minimum 'ton" time therefore depends on the parameters of the commutation circuit~ and in a practical embodiment of the invention it resul.ts in a minimum duty factor of approximakely ,09 at a chopping frequency of 300 Hz, For variations of Vc from -0,5 to its lo~ extreme of -1.5 V, the duty ~actor is de-creased to nearl~ ~ero by reduci.ng the average frequency of the periodic output slgnal.s F Or the converter 231, During this va.riable frequency, mi.n.l.mum pu].se width mode of operation, the first gating signals for flring the main thyrlstor are lnhibited by the 0 signal on the line 256 which disables the ~ND logic circuit 230 and prevents it from supplying a 1 signal on line 248 to the clock input of the flip flop 247. Consequently no gating signals are supplied by the chopper pulses block 193 to the main gate driver 194, but the chopper is alternately turned on by firing its commutating thy-ristor and turned off by self commutation. The commutating thyrlstor 7 S periodically fired in response to the second gating signals which the block 193 is now supplying to the gate driver 195 at a reduced frequency th~t varies wlth the value of VC and that is zero when Vc is at its low extreme, and each time the commutating thyrlstor is fired it con-ducts armature current for a mi.nimum on time (toN) un~il automatica].ly extinguished by the rlnglng action of its oscillatory commutation circuit. The duty fac tor, which can be expressed as r x toN, ls proportional to the frequency of the output signals F of the V/F
converter 231. It will now be apparent that the chop-per pulses block 193 has the capabiltiy of smoothly varying the duty factor of the chopper over a con-tinuum that exter,ds from 1.0 when V~ is at its hlgh 5 extreme (~10 V~ to zero when Vc is at its low extreme i V).
As was previously explained, normally the signal on the chop enable line 107 is 1, but during a motoring-to-braking transition lt is temporarily 0.
3 Whenever this signal changes from 1 to 0, a 1 signal is applled to the set input of the flip flop 237, thereby changing the Q output o~ this component from 0 to 1 and the Q bar output from 1 to 0. ~he Q output is connected on a line 2~0 to a first input of an AND

~66-logic circuit 261 whose other input i8 connected to the line 250 and whose output is connected vla a llne 26 and the OR logic circuit 241 to the input of the one shot block 242. Conse~uently, if the chopper were in a turned on state (as lndicated by a 1 signal on li.ne 250) at the time the flip flop 237 ls set, the n to 1 change of the Q output on line 260 would trigger the one shot 242 and steer one last gating ~ignal to the gate ~river 195 of the commutating thyristor, thereby turning ofl the chopper 12. At the same tlme, the 1 to O chan~e of the Q bar output on line 236 disables the AND logic circuits 230 and 233, and no further gating signals can be produced by the chopper pulses block 193 so long as ~here is no chop enable signal on line 107. Later, after the chop enable signal is restored to i.ts 1 state, the flip flop 237 will return it~ Q
output to the 1 state and its Q bar output to the O
state upon receipt of a 1 signal at its clock input ~indicating that the F output of the V/F converter 231 is 0), and no~ the chopper pulses block 193 can re-sume normally producing gating signals to alternately turn on and turn off the chopper with a duty ~actor determined by the value of Vc To ensure initlal turn on of the chopper 12 during the period of time that the field of motor Ml is being boosted at the beginning of a braking mode of operation, t.he d-c gate signal on line 196 is connected through t.he OR logic circuit 251 to the one shot block 252. Preferably, as was pointed out above in connec-tion wlth the description of Fig. 8, this d-c gate signal is actually a short (e.g., approximately 2 milli-seconds) burst of hlgh-freauency (e.g.~ 21 6 K~z) dis-crete 1 pulses. Such pulses wlll repetitively trigger the block 252, and conse~uently a corresponding burst Or gatlng signals is prod.uced at terminal.s 201a and ~ z~

201b of the chopper pulses block 193. This burst of gating signals has the same frequency as the pulses comprising the d-c gate signal, and it is re~erred to herein as the extended chopper turn on signal. When-ever the burst firing means is effective to supply the gate driver 194 with this extended chopper turn on signal, the gate driver responds by supplying a correspondingly extended initial firing signal to the main thyristor of the chopper 12. A gate driver well suited for this purpose is described and claimed in Canadian patent application S.N. 362,551 filed October 16, 1980, R.B. Bailey and assigned to the General Electric Company.
concurrently with the extended chopper turn on signal, and for whatever additional time is necessary in order for IA to attain the aforesaid 100-amp threshold, the commutation suppressing signal on line 197 is in a 1 state (and the signal on line 234 is 0), thereby disabling the AND logic circuit 233 and preventing the chopper pulses block 193 from producing any second gating signals that would otherwise cause the gate driver lg5 to fire the commutating thyristor and turn off the chopper.
With one exception~ the chopper pulses block for the No. 2 chopper control means 23 is the same as the block 193 shown in Fig. 10. The one exception involves the connections to the master clock 44.
Where Fig. 10 shows a line 45a supplied with phase 1 pulses from the master clock, the corresponding line of the No. 2 chopper pulses block should be supplied with phase 2 pulses, whereby the resetting of its ramp generator and the production of an output signal F by its V/F converter will be delayed one-third of the period of the master clock with respect to the occurance of these events in the No. 1 chopper pulses ,~

Z6~

-68_ bl~ck Similarly, the pulses block in the contrGls for a third chopper (not sho~n) should be synchronized with the phase 3 pulses of the master clock. In propulsion systems using six chopper/motor units, the master clock could be provided with a 6-phase out~
put. In this manner the respective choppers are turn~d off in sequence rather than in unison. By thus stag-gering the of r times of the respective choppers, the amplitude o~ ripple current in the filter 32 i3 de~ir-ably minimized.
Figs. 12 - 15 Having described the various power and con-trol components of the illustrated propulsion system, the operation of the system during electrical braking will no~r be summariæed with the aid of Figs. 12-15.
T'lig. 12 i6- a simpli~ied diagram of the filter capacltor 60, the d-c bus 31, and the flrst chopper/motor unit 12/M1 after reconnection for the braking mode of opera-tion. In this figure the dynamic brake resistor grid is shown at 264, the lumped reslstances of the motor arma-ture and field and of the cab]es in the armature cur-rent path are represented by a single resistor 265, and the lumped inductances of these components are represented by a single inductor 266. While not shown in Flg 12, other chopper/motor units of the propulsion system are of course connected across the d-c bus conductors 31p and 31n in parallel ~ith the unit 12~Ml. The practice of the present invention is not limited to the particular propulsion system shown in Fig. 12, and it is useful, for example, in propulsion systems Or the type disclosed in U.S. patent No 4,051,J-121 issued on September 27, 1977, to T R. Brinner and T. D. Stitt and assigned to the General Electric Compan~. ~ig. 13 illustrate6 such an alternative system in its, electrical braking con--69~
figuration, Switching means 267 connects a dynamic brake resistor 268 across the parallel branch Or the armature current path that includes the chopper 12, and additional resistance 269 shunted by second switching means 270 can be inserted in the armature current path by opening the latter switching means when desired.
In this embodiment the armature current path during braking includes a flyback diode 271 instead of the contactor B, A separate set of resistors 268, 269 and switches 267, 270 is required for each of the indlvidual chopper/motor units of the propulsion system, In electrlcal braking, the motor Ml behaves as a generator. Its armature is driven by mechanical inertia of the vehicle and exerts on the wheels to which it is coupled a negative (braking) torque that is a function of the generated current, Thus IA is a measure Or bra~ing ef~ort. The armature current path for the motor Ml in Fig, 12 and the corresponding paths for the other ~raction motors ~not shown) o~ the same propulsion system include the dynamic braking resis-tance 264. The electrical energy generated by each motor is disslpated in the form of heat by the IR
losses in the armature curr.ent path. While part of the power loss takes place ln the armature and ~ield windings of the individual motors, in the associated chopper (when turned on), and in the resistance 265, most of the braking energy o~ all of the motors is ln-tended to be dissipated in the resistor grid 264, 3 (Although the illustrated embodiment o~ the invention employs dynamic braking, the invention can be practiced equally well in regenerating systems where braktng energy is fed back to a receptive source,) The voltage across the fllter capacitor is . ~

-7o-~F. Wlth contactor BB closed3 the ~ame voltage 18 lm-pressed across the resistor grld, and its average magni-tude is equal to the square root of the product of the resistance (in ohms) o~ the grid times the power ~in watts) being dissipated thereln. When the chopper 12 is turned off and constant current is conducted by the free wheeling rectifler FWR, the voltage V~b across the lumped inductance 266 and the reactor ~2 in Fig, 12 ls equal to VF - VQ, where VA equals the electromotive for~e generated by the machlne Ml less the sum of the voltage drop across the lumped resistor 265 and the forward voltage drop across ~WR. ~he magnitude o~ the electromotive force varies directly with motor speed and also as a function of IA which excites the series field of the motor Ml. (Note that if the chopper 12 were never-turned on, IA and hence braking effort would be zero so long as VF exceeds VA, as would be true at low motor speeds.) In order to control IA as speed is reduced and therefore achieve relatively high and constant braking effort at low speed, the chopper 12 is period-ically turned on during the braking mode of operation, and its duty factor is varied as a function of the value of the control signal Vc. When the chopper is turned on and conducting constant armature current, Vab is positive and equal to VA (assuming the forward volt-age drop across the chopper is not materially different than the forward drop across FWR). If V~ is greater than VA~ during the off time of the chopper Vab is 3 negative. In order to regulate armature current IA
(and hence braking effort) to a preset constant average magnitude determined by the current call signal I*, the average ma~nitude of Vab must be zero. Otherwi~e, IA
would be either increaslng (if the average were posi-tive) or decreasing (if the average were negative). A

-71~

zero average requires that VF, be greater than VA, and it requires a chopper duty ~actor equal to 1 VA~VF.
Fig. 14 shows the required duty ~actor durlng two cycles of steady state operating of the chopper 12 for each of three di~ferent speeds in the braking mode of a propulsion system having six chopper/motor unlts.
High speed operation (e.g., 42 MPH) is illustrated by the trace 273 in Flg. 14A~ At this speed with typical traction motors and other practical parameters, VA will be approximately 820 volts and IA is approximately 490 amps. Assuming that the resistance of the resistor g`rid is approximateiy 1.1 ohms, VF will be approxi-mately 1640 volts and the duty factor is seen to be 0.5. In Fig. 14B the trace 274 illustrates Va~ during operation at the corner point speed (e.g., 21 MPH) 3 where VA is approximate].y 460 volts and IA is 880 amps.
The power (6 x VA x IA) di~ipated in the resistor grid at this speed is substantially the same as at t~e higher speed. Therefore VF remains nearly 1640 volts and the duty factor is approximately .72. In Fig. 14C
the trace 275 illustrates Vab during operation at a much lower speed (e.g., 4 MPH) with IA maintained at 880 amps. VA is now appro~imately 34 volts, while VF
has fallen to the minimum le~el of 850 volts that i5 maintained by the controllable converter 33 during braking. Therefore the duty factor is now .~6 which ls higher than the maximum obtainable when the chopper is in its constant frequency mode. The desired duty factor ls or~tained by reducing the chopping frequency 3 (to approximately 267 Hz) while maintaining the minimum off time.
Fig. 15 traces the arma~ure current IA and the electromotlve force Vemf generated by the motor Ml during an ~l~ctrica~ bra~clng process t~lat be~lns at 20-T~-1309 -72~

45 MPH and that continues at the maximum braki~g rate until the vehicle has decelerated to a low speed of 2.7 MPH. (A level track ls assumed.) Point 1 marks the initial turn on of the chopper 12 by the bur~t - 5 firing means of this invention, whlch takes place ap-proximately one-half second after the chop enable slg-nal on line 107 enables the current reference signal IREF in the chopper reference means 192 (Flgs, 5 and 9) to begin ramping up, at a rate of 200 amps per secondt from its negative reset level t~ a value correæponding to t~le desired magnitude of armature current, which value will ultimately be determined by the current call signal I* from the reference generator 100 in the master controls (Fig, 5), Prlor to this initial turn on of the chopper, the fleld boost means is operative to increase Vemf, but there is no current in the armature current path because the filter capacitor voltage VF
is greater than Vem~ and the free wheeling rectlfier FWR is reverse biased. ~hererore IA starts rising from 0 at point 1, and it quickly attains the 100 amp-threshold at point 2 in Fig, 15, whereupon the commu-tation suppressing signal on line 197 terminates and the chopper pulses block 193 (Figs. 5 and 10) ~s able to produce a second gating signal for firing the com-mutating thyristor and hence turning o~f the chopper 12.
Thereafter the chopper pulses block 193 alternately pro-duces its first and second gating signa]s at a duty factor d'etermined by Vc, whereby IA proceeds to track IP~EF
As IA increases, so does field excitation, and this causes the volts per RPM to increase along the field saturation curve of the motor M1, As the generated voltage increases, so does VA. The power to b~ dissipated ln the dynamic braking resistor grid ~73-ri3es with the product of IA and VA~ and this pumps up the filter capacitor voltage VF At high speeds, VA is su~ficiently high when IA traverses point 3 ln Fig. 15 to raise VF a~ove 1200 volts (assuming slx chopper/motor units and braking resistance o~ at least four ohms), and between points 3 and 10 the staging means 165 in the brake controls (Fig. 6B~ will switch the staging contactors Bl and B2 (Fig. 2) as necessary to maintain VF in a range between 1200 and 1650 volts.
For example, at point 4 V~ reaches 1650 volts and the staging means responds by c]osing contactor Bl to re-duce the dynamlc brake resistance. Again at polnt 5, VF reaches 1650 volts and the staging means responds by closing contactor B2 to further reduce the resist-ance to its minimum value (e.g., 1.1 ohms).
At point 6 in Fig. 15, armature current in-tersects the constant power segment of the current ca]l slgnal curve that is set by the reference generator 100, and from this point I~EF tracks I*. Maximum power is now being dissipated in the resistor grid, and VF i3 nearly 16~0 V. At point 6 the duty factor of the chopper is approximately 0.5 (see Fig. ll!A). The vehicle is being retarded by the braking effort that increases with IA, and as it decelerates (above corner point speed) I* is increased exponentially by the reference generator 100. Fig. 15 reveals that speed decreases, Vemf decreases, and power remains substan-tially constant as IA increases from point 6 to the corner point 7.
From point 7 to point 12 of Fig. 15~ the vehicle is braking at maximum, constant current. The chopper duty factor at the corner point is illustrated ln Fig. 14B , As speed decreases from the corner point 7, the generated voltage decreases and the duty factor will be increased in order to regulate IA to the 20-TR-130g constant magnitude called for. As vol~age decreases, sc does the power to be dissipated in the reslstor grid, and consequently VF decreases. At polnt 8 VF
reaches 1200 volts, and the staging means responds by opening the ccntactor B2 to increase the dynamic brake resistance. Although IA tends to decrease when B2 is opened because of the resulting step increase in VF, the chopper reference means 192 quickly responds by increasing Vc, and hence the duty factor of the chopper, as necessary to mainta~n the called for magni~
tude of current, and there is no noticeable torque bump in the braking process. At point 3 VF again falls to 1200 volts, and the staging means opens Bl to insert the maximum resistance of the grid. As speed decreaseæ
from point 10 to point 11, VF drops below 1200 volts and decreases to a minimum of 850 volts, whereupon the controll2ble converter 33 begins charging the filter capacitor 6n from the power source so as to maintain this minimum level. The minimum level of VF is se-lected to ensure sufficlent voltage for successful op-eration of the commutating circuit in the chopper 12, whereby proper operation of the chopper can continue during low speed electrical braking when the generated voltage is very low.
When speed has decreased to a minimum of approximately 2.7 MPH, at point 12 in Fig. 15, Vemf is Just equal to the sum of the voltage drops across the chopper 12 and the lumped resistance of the armature current path, and therefore VA ls 0. The duty ~actor is now 1.0, and the chopper can no longer control IAwhich rapidly decays from its called for magnitude.
Decreasing current in the motor field results in less electromotive force, and the system col~apses~ Thi.s is the lo~r speed brake fadeout point of the lllustrated propu]sion system. Additional braking at such low 20-T~-1309 ~75-speeds can easlly be erfected by conventional frictlon or air brakes. Note that the minimum fadeout speed of electrical braking ls e~en lower than 2.7 MPH i~ the brake controller ~s calling for less than maximum IA, Note also that the fade out speed of electrical braking would be appreciably higher than 2.7 MPH lf the duty factor were limited to a maximum of .9l.
While a preferred embodiment of the invention has been shown and described by way of example, many mGdifications wlll undoubtedly occur to persons skilled i.n the art. The concluding claims are therefore in-tended to cover all such modifications as fall within the true spirit and scope of the lnvention.

Claims (26)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Improved means for effecting electrical braking of a traction vehicle equipped with a propul-sion system having motoring and braking modes of operation, said propulsion system comprising a d-c traction motor that behaves as a generator during said braking mode of operation, said motor having an arma-ture and a series field winding and said propulsion system further comprising a filter capacitor adapted to be coupled to a source of d-c electric power, a chop-per, means for connecting said chopper in series with said armature and field windings across said capacitor when the propulsion system is operating in its motoring mode, cyclically operative means for normally producing periodic gating signals of relatively short predeter-mined duration, means responsive to said gating signals for alternatively turning on and turning off said chopper, and free wheeling rectifier means connected in circuit with said armature and series field winding to conduct motor current during intervals when said chopper is turned off, wherein the improvement com-prises:
a) command means having alternative motoring and braking states;
b) brake set up means operative in response to a motoring-to-braking state change of said command means for reconnecting the propulsion system to establish an armature current path comprising said field winding in series with first and second parallel branches, said first branch including said chopper and said second branch including said capacitor in series with said free wheeling rectifier means, and for reversing the polarity of the connection of said series field winding relative to said arma-ture, said brake set up means being effec-tive whenever said command means is in its braking state to maintain said armature current path and to maintain the reversed polarity connection of said field winding and armature; and c) burst firing means effective in response to the reconnecting operation of said brake set up means and the start of a braking state of said command means for supplying to said gating signal responsive means an extended chopper turn-on signal having a duration substantially longer than said predetermined duration of the gating sig-nals normally produced by said cyclically operative means, thereby ensuring that said chopper turns on and conducts armature current to begin the braking mode of opera-tion of the propulsion system.

2. The improvement as set forth in claim 1 wherein said burst firing means is also effective for disabling said cyclically operative means to prevent it from producing any gating signals that would otherwise turn off said chopper until the magnitude of armature current increases to at least a predetermined thres-hold, whereupon said cyclically operative means can re-sume normally producing gating signals to alternately turn off and turn on the chopper.

3. The improvement as set forth in claim 1 wherein the duration of said extended chopper turn-on signal is at least 100 times longer than said predeter-mined duration of the gating signals normally produced by said cyclically operative means.

4. The improvement as set forth in claim 1 wherein resistance means is provided for dissipating electrical braking energy and said brake set up means is additionally effective to connect said resistance means in parallel circuit relationship with said capa-citor when said command means changes from motoring to braking states and whenever said command means is in its braking state.

5. The improvement of claim 4 wherein said resistance means is connected across said capacitor in said second branch of said armature current path,

6. The improvement of claim 4 wherein said resistance means is connected across said second branch of said armature current path,

7. The improvement as set forth in claim 1 in-cluding field boost means temporarily operative in res-ponse to the reconnecting operation of said brake set up means and the start of a braking state of said command means for momentarily increasing current in said field winding to ensure an increase of voltage across the armature of said motor at the beginning of the braking mode of operation, said burst firing means being con-nected to said field boost means and being arranged to delay said extended chopper turn on signal until after the start of field current increase by said field boost means.

8. The improvement as set forth in claim 7 wherein speed sensing means is provided for sensing the angular velocity of the armature of said motor, and wherein said field boost means includes means connected to said speed sensing means for preventing operation of said field boost means whenever said angular velo-city is below a predetermined low magnitude and for causing operation of said boost means in response to said velocity increasing from below to above said pre-determined magnitude at any time after operation of said brake set up means and while said command means is in its braking state.

9. The improvement of claim 7 wherein said field boost means is operative to increase current in said field winding for a period of approximately one second.

10. The improvement as set forth in claim 9 wherein said burst firing means becomes effective ap-proximately midway through the period of time that said field boost means is operative.

11. The improvement as set forth in claim 1 including means for providing a current reference sig-nal representative of the desired magnitude of current in the armature of said motor, means for providing a feedback signal representative of the actual magnitude of armature current, and means for deriving a control signal having a value that varies as a function of any difference between said current reference and feedback signals, said cyclically operative means being con-nected to said control signal deriving means and being so constructed and arranged that the timing of said gating signals and consequently the duty factor of said chopper are determined by the value of said control signal.

12. The improvement of claim 11 and further including means connected to said reference signal pro-iding means and responsive to said command means changing states for resetting said current reference signal to a value corresponding to zero current and thereby altering the value of said control signal as necessary to ensure that the chopper duty factor is rapidly reduced to zero.

13. The improvement of claim 11 for effecting electrical braking of a traction vehicle equipped with a propulsion system supplied from a source of d-c electric power including a controllable electric power converter having a set of input terminals and a pair of d-c output terminals, means for connecting said capa-citor between said output terminals, means including a contactor for connecting said input terminals to a source of relatively constant voltage, and regulating means for controlling said converter so as to limit the average magnitude of capacitor voltage to a maximum level determined by a voltage reference signal when the propulsion system is operating in its motoring mode, wherein said command means is coupled to said current reference signal providing means and, prior to changing states, calls for a current reference signal value cor-responding to a low magnitude of armature current, and wherein the improvement further comprises means for opening said contactor and thereby disconnecting said converter from said voltage source in response to the start of a motoring-to-braking state change of said command means and means for reclosing said contactor in response to the operation of said brake set up means.

14. The improvement of claim 13 wherein means is provided for setting said voltage reference signal at a value that will prevent the capacitor voltage from falling below a predetermined minimum level when the propulsion system is operating in its braking mode.

15. The improvement of claim 13 wherein bi-stable chopper enable means is connected to said capacitor and to said field boost means, said chopper enable means being in one state when the capacitor voltage is rela-tively high and in another state when the capacitor voltage is relatively low and being effective only in said other state for disabling said burst firing means and thereby preventing it from supplying said extended chopper turn-on signal,

16. The improvement of claim 15 wherein means is provided for setting the value of said voltage reference signal, said last-mentioned means being reset in response to the opening of said contactor and being effective upon reclosing said contactor to vary said voltage reference signal at a predetermined rate until it attains a value corresponding to a predetermined level of capacitor voltage.

17. The improvement of claim 13 wherein bistable chopper enable means is connected to said capacitor and to said current reference signal pro-viding means, said chopper enable means being in one state when the capacitor voltage is relatively high and in another state when the capacitor voltage is relatively low and being effective only in said other state for resetting said current reference signal to a value corresponding to zero current, whereby an ap-propriate control signal value is attained to ensure that the chopper duty factor is zero until said chopper enable means changes from said other state to said one state.

18. Improved means for effecting electrical braking of a traction vehicle equipped with a propul-sion system having motoring and braking modes of operation, said propulsion system comprising a d-c traction motor that behaves as a generator during said braking mode of operation, said motor having an arma-ture and a series field winding and said propulsion system further comprising a filter capacitor adapted to be coupled to a source of d-c electric power, a chopper, means for periodically turning on and turning off said chopper in response, respectively, to alter-nate first and second gating signals, means for con-necting said chopper in series with said armature and field winding across said capacitor when the propulsion system is operating in its motoring mode, and free wheeling rectifier means connected in circuit with said armature and series field winding to conduct motor current during intervals when said chopper is turned off, wherein the improvement comprises:
a) means for supplying a variable control signal having a value that can vary between predetermined low and high ex-tremes and that can traverse a predeter-mined intermediate value in between said low and high extreme values;
b) cyclically operative means connected to said control signal supply means for nor-mally producing alternate first and second gating signals for respectively turning on and turning off said chopper, each of said gating signals having a relatively short predetermined duration, and the interval from the production of one of said second gating signals to the production of the succeeding first gating signal being re-ferred to as the off time of said chopper, said cyclically operative means being so constructed and arranged that (i) whenever the value of said control signal is at its low extreme no gating signals are produced and said chopper is continuously turned off, (ii) when said control signal is in a predetermined range between said low extreme value and said predetermined intermediate value said second gating signals are pro-duced at a predetermined constant frequency while said off time varies inversely with the value of said control signal and decreases toward a predetermined minimum as said control signal approaches said intermediate value, and (iii) whenever said control signal is be-tween said intermediate value and said high extreme value said second gating signals are produced at an average frequency that varies in-versely with the control signal value and decreases from said con-stant frequency toward zero as the control signal approaches said high extreme while said off time is maintained substantially equal to said predetermined minimum;
c) command means having alternative motoring and braking states, and d) brake set up means operative in response to a motoring-to-braking state change of said command means for reconnecting the propul-sion system to establish an armature cur-rent path comprising said field winding in series with first and second parallel branches, said first branch including said chopper and said second branch including said capacitor in series with said free wheeling rectifier means, and for re-versing the polarity of the connection of said series field winding relative to said armature, said brake set up means being effective whenever said command means is in its braking state to maintain said armature current path and to maintain the reversed polarity connection of said field winding and armature.

19. The improvement as set forth in claim 18 wherein said control signal supplying means comprises means for providing a current reference signal repre-sentative of the desired magnitude of current in the armature of said motor and means for providing a feed-back signal representative of the actual magnitude of armature current, the value of said control signal being varied as a function of the difference between said current reference and feedback signals in a sense approaching said high extreme from said low extreme so long as the actual current magnitude is less than the desired current magnitude.

20. The improvement of claim 19 and further including means connected to said reference signal pro-viding means and responsive to said command means changing states for resetting said current reference signal to a value corresponding to zero current and thereby altering the value of said control signal as necessary to ensure that it attains said low extreme, thereby ensuring that the chopper is turned off and that armature current decreases to zero.

21. The improvement as set forth in claim 18 and further including burst firing means effective in response to the reconnecting operation of said brake set up means and the start of a braking state of said command means for supplying to said chopper turn on and turn off means an extended chopper turn-on signal having a duration substantially longer than said pre-determined duration of the first and second gating sig-nals normally produced by said cyclically operative means, thereby ensuring that said chopper turns on and conducts armature current to begin the braking mode of operation of the propulsion system.

22. The improvement of claim 21 including field boost means temporarily operative in response to recon-necting operation of said brake set up means and the start of a braking state of said command means for momentarily increasing current in said field winding to ensure an increase of voltage across the armature of said motor at the beginning of the braking mode of operation, said burst firing means being connected to said field boost means and being arranged to delay said extended chopper turn on signal until after the start of field current increase by said field boost means.

23. The improvement of claim 22 wherein speed sensing means is provided for sensing the angular velocity of the armature of said motor, and wherein said field boost means includes means connected to said speed sensing means for preventing operation of said field boost means whenever said angular velocity is below a predetermined low magnitude

24. The improvement of claim 21 wherein said burst firing means is also effective for disabling said cyclically operative means to prevent it from pro-ducing any second gating signal until the magnitude of current in the armature of said motor increases to at least a predetermined threshold, whereupon said cyclically operative means can resume normally pro-ducing any second gating signal until the magnitude of current in the armature of said motor increases to at least a predetermined threshold, whereupon said cyclically operative means can resume normally producing second gating signals alternating with said first gating signals.

25. The improvement of claim 24 wherein said control signal supplying means is so constructed and arranged that the value of said control signal varies as a function of any difference between actual and desired magnitudes of armature current.

26. The combination as set forth in claim 18 wherein resistance means is provided for dissipating electrical braking energy and said brake set up means is additionally effective to connect said resistance means in parallel circuit relationship with said capacitor when said command means changes from motoring to braking states and whenever said command means is in its braking state.