CA1070785A - Propulsion system for tracked vehicle - Google Patents

Abstract

A B S T R A C T

This invention is a propulsion system for a rail car utilizing a chopper commutated inverter with an appropriate control unit and a linear induction motor, that does not rely upon adhesion for propulsion effort, is lighter and lower in volume than prior art equipment for equivalent rating, is in-herently more rugged the n alternative static inverter fed schemes and does not insignificantly deviate from the reliability and maintainability that can be expected from such systems.

Description

Q7~

This invention pertains to an electrical propulsion system for use in transportation applications which have dedi-cated guid~way, for example, a street ca~ on a trackl a subway system.
Presently, transportation systems in service supplied with electrical power utilize rotating elect:rical machines as the prime moti~e power source. The torque cleveloped by the ro-tating machine is transmitted, via a gearbox in normal circum-stances, to the vehicle driving wheels, which in normal circum-stances also serve to support the opexational vehicle from itstrack guideway. In addition to the motor, gearbox if required, and wheel, all of these types of propulsion systems have one further component which can be termed the power control unit, the type of which is dependant upon ~he type of elec~rical mo~or used in the application, i.e. direct current (DC) or alternating curre~t (AC).
Where DC motors are used the power contxol unit can be either a variable resistance and switch arrangement or a DC to DC solid state power converter ~chopper) or an AC to DC solid state power converter ~controlled rectifier). The firat two of these power control units are utilized when the power source i~
direct aurrent. The las~ type can be utilized when the power source is alternating current.
Where alternating current motors are used the power control unit can be either a DC ~ AC solid state po~ar converter (inverter~ or an AC to AC solid state converter (voltage regulator or cycloconverter).
The former type is utilized when the power souxce is DC. ~he latter type is utilized when the power source i~ AC~
3~ Systems of the first type, i.e. comprised of a solid state DC to DC power converter (chopper), DC machines, gearboxes ~r~

7~35 and drive~ wheels and systems of the second type, i~e. comprise~
of a solid state DC to AC power converter (inverter) AC electrical machines, gearboxes and driven wheels have c:ertain common charac-teristics.
These common characteristics include manual or auto-matic means which provide a demand signal to the propulsion system.
This demand which may be a signal corresponding to velocity, is normally compared with the appropriate state of the ~ehic:le in order to produce an error signal. This error signal is processed by the power conversion unit in such a manner that the electrical power rom the propulsion system power supply is conver~ed to a form which is suitable for the electxic motors and o the appro-priate magnitude. The power fed to the motoræ i5 convexted into mechanlcal power which is then tran~mitted via the gearbo~e~ and the wheels to the guideway in the appropriate manner. The action of this total power transferred is to reduce the operational error until the point is reached where the demand is equalled. ~;
The linear induction motor has de irable characteris-tics in transportation system applications that stem from the manner in which they apply thrust directly to the track. This a~oids reliance on adhesion characteristics between wheel and track/ eliminateG the need for a gearbox and generally offers a more reliable, lighter and easier to maintain system. However, to date a satisfactory method of supplying control~ed propulsion power to linear induction motors in a tra~sportation sy~tem has not been devised. Consideration has been given to po~er supply from a DC source through a pulse width modulated inverter but the inverter is too large and too heavy to be practical for most transportation systems. Con~ideration has also been given to 3Q power supply from an AC source through a cycloconvertex. In this case, the power fac or of the load is of proportions that make it S

objecti~nable or use in transportation systems~
This invention provides a compact, light-weight method for supplying power ~o a linear induction motor for a trackea vehicle that uses a DC power source. It avoids the problems of the pulse width modulated inverter and cycloconverter as power supply devices. The result is a practieal propulsion system that is a substantial improvement over the presently used rotaxy induction mot,.or and gsarbox.
The propulsion system described h~erein uses a l)C to AC power converter (inverter), including a control unit and a linear induction motor whose secondary member is laid in the guideway portion of the transportation system. The inverter is of a special type termed a chopper commutated inverter. The system described uses the combination of a chopper commutated inverter, an appropriate control unit and a linear induction motor. The advantages that the system provides over the priox art include:
i) Reliability and maintainability are improved by the use of this type o~ propulsion systemO
ii) There is no longer a reliance on adhesion character-istic~ between the wheel and the guideway because power is no longer transmitted via the ~eelq. This leads to signi~:icant advantages in the capability of the transit ~ystem as a whole.
iii) ~he combination of the chopper commutated inverter and the linear induction motor can provide the minimum weight to volume solution as a specified power level.
iv) The combi~ation of chopper commutated inverter and linear induction motor with appropriate controls provides a significant degree of ruggedness in terms of response to fault and overload conditions when compared with the alternate propul-sion system~, notably those which utilize DC to DC power converters ~choppers) and DC to AC powsr converter~ (inverters) of the voltage controlled,pulss width modulated types.
In accordance with this invention, a propulsion sy~tem for a vehicle having a dedicated guideway comprise~ a m~
phase linear induction motor having a stator mountable on said vehicle and a ~econdary incorporatable into said guidewayi a multipha~ gate controlled inverter connect:able to a source of DC power ~or supplying multiphase AC power ~o said stator of ~aid linear induction motor; each phase of ~aid inverter having means for controllably ~upplying DC current in termin~
able pulses ~o said inverter including commutating capaaitors, free wheel loop diode3 and gate controlled rectifiers~ means ox controlling the frequQncy of operation of said g,ate aon-txolled rectifiexs to control the magnitude of the DC current ~upplied by said terminable pulses, means or cyclically forcing current through said mean~ for supplying DC current to zero whereby to form it into terminable pulse~ as aforesaid whereby to control the output ~requency of each pha~e of said gate controlled inverter, and a control unit having a feedback related to vehicle velocity in use and sensitive to the dema~ds of the propul~ion 3y~tem adapted to operate the gates of ~aid gate controlled inver~er to supply power to said stator of said linear induction motor to propel the vehicle and to operate the gates of said gate controlled rectifiers by controlling the fre~uency of their operation in accordance with the demands to the propul~ion system. ~ '.
The inven~ion will be understood after reference to the following detailed specification read in conjunction with the drawings, wherein Fig~ a ~hematic illustra~on of a propulsion system;
Fi~. 2 is a ~chematic illustra~on of one phasle of the chopper commutated inverter; and Fig. 3 is a ~chematic illustration of ar appropriate :' ~ control unit.

~7~7~i Figure 1 illustrates schematically the basic parts of a propulsion system which include a control unit 1, a solid state chopper commutated inverter 2 and a linear induction mstor 3.
The stator of the motor 3 is carried by the vehicle and the secondary is incorporated into the vehicle g~ideway or trackO
Linear induction motors of themselves are well h~own but have lacked suitable control systems. The operation of this sytem as a whole is described as follows.
It i8 a well known principle of operation in the use of induction motors in propulsion systems generally to adjust the current supply to the motor in accordance with the demand for propulsion (say accelerate, decelerate or maintain speed) and, at the same time, adjust the frequency of the current supply on the ba~is of mainta~ning a constant slip requency. This involves measuring the velocity of the moving member,relating it to a corresponding synchronous frequency, adding the slip frequency to it and supplying the current to the stator at the frequency so devised. With such a system, one can achieve good torque or thrust characteristic from an induction motor and meet all requirements of a train or street car propulsion system in respect of acceleration, braking and maintaining speed.
The invention provides for the adjustment of stator frequency and supplies current in accordance with propulsion demand in accordance with this principle of operation from the output of a chopper commutated inverter. Figure 1 is a simple block diagram of a propulsion system in accordance with the invention wherein the numeral 3 refers to a linear induction motor, numeral 2 refers to a chopper commutated inverter and numeral 1 refers to a control unit for the chopper commutated 3~ inverter.

7~5 The stator of the motor 3 is caxried by a wheeled vehicle and the secondary of the motor 3 is associated with a track for the vehicle. Direct current power is supplied to the inverter 2 as at 9 and alternating current power flows from the inverter to the motor 3 through the link 6. Propulsion demand to the system (i.e. accelerate, brake or maintain speed) is ;~
transmitted to the control unit as at 4. The output of the control unit communicates with the converter as at 5. Feedback from the inver~er to the c:ontrol unit for inverter operation is through link 8 and a signal representative of the speed of the vehicle and coxresponding synchronous frequency i~ transmitted to the control un~t, through link lO as the means of controlling frequency of power supplied to the motor. A11 of the foregoing oomponents are mounted on the vehicle and, in use, one can by manually manipulating the propulsion demand in the case o a hand controlled vehicle, operate the motor to accelerate by increasing current, to maintain speed, or to decelerate by causing the motor to operate in a regenerative mode and feedback power into the line.
Figure 2 is a power circuit schematic of one phase of the chopper commutated inverter. Three single phase circuits, as shown, comprise a three phase inverter the ou~put of which is utilized with a linear induction motor load for propulsion of a vehicle on a track. Each sinyle phase inverter section is comprises of two chopper su~sections and one inverter subsection.
The chopper section commutates a chopped DC current which is subsequently converted to an AC output in the inverter section.
The chopper subsection in Figure 2 comprises a plurality of silicon controlled rectifiers hereafter identified as SCRI 5 11 to 18 and free wheel loop diodes 23 and 24. Normal commutation .` '"'- ~' 7~35 is achieved in the chopper stage by commutating capacitors 25 and 2~. SCR's 10, l4, 15 and 18 are triggered together during one half cycle o chopper operating frequency while SCR's 13, 12, 17 and 16 are triggered toge~her during the alternate half cycle.
The chopper is preferably operated at high frequency in the order of from 600 to 4000 Hertz to reduce the size and weight of conunutating capacitors 25 and 26. Power output is a direct function of frequency.
The capacitor current i8 monitored by sensors 30 and 31 to detect the in~tant of current zero. After z~ro curren~ flow through one group vf SCR' 8 terminates and the control unit initiates flow through the other groups, flow alternates be-tween the group 10, 14, 15 and 18 to 13, 12, 17 and 16.
During one half cycle of the chopper the current path i8 27-10-25-14-29-19-load-22-15-26-18-28, while dur:Lng the other half cycle the current ~lows through 27-13-25-12-29-20-load-21-17~26-16-28.
Capacitor~ 25 and 26 are charged in one directiDn during one half cycle and in opposite direc~ion during the;~iLternate half cycle. When fully charged the curren~ through capaci~or~
25 and 26 has allen to zero. At this instant, the inductive nature of the load maintains the current cons~ant which, there-fore, i9 allowed to flow through diode~ 23 and 24 back ints:~ the DC supply. Such flow o~ current amounts to instantaneous regen-eration of power during which the chopper SCR'~ are lcept off~
The pre~ens:e of capacitors in serie~ with the load enables nat-ural con~nutation to be achieved, the zero capa~:itor current turns off the conducting SCR's. Reactors 27 and 28 axe used to main-tain the rate o~ rise of current within the SCR rating.
rrhe frequency of chopper operation is controlled by a voltage controlled oscillator (VCO) in the control Ullit such S

that the peak DC output current is a direct function of this frequency.
The average output voltage of the chopper can be con-trolled between positive and negative supply magnitudes while the output current is always positive. The chopper, therefore, is a two quadrant regenerative type capable of passing power back intot~he DC supply. The chopper is designe~d for operation at a high frequency and, therefore, exhibits a fast response charac-teristic. To enable satisfactory operation at auch frequency fast switching SCR's and diodes are used.
Chopper operating frequency determines the size of the capacitors 25 and 26 and it is preferable to keep it high to minimize the size of the components. Frequency of between 600 and 4000 Hertz has been indicated preferable but other ~requencies are feasible.
As mentioned earlier, the inverter section comprises four steering SCR's l9, 20, 21 and 22 used to provide AC curxent in the load at controlled frequency usually in the range 0.1 to 100 Hertz depending on the m~tor. Whenever the polarity of the load current is to be changed the current is forced to zero in the chopper whereby the need for commutating components in the inverter i9 obviated. This result~ in signi~icant reduction in weight and volume for the overall system. Inverter SCR's 19 and 22 are triggered during one half cycle of the desired output frequency while SCR's 20 and 21 are triggered during the alternate half dycle.
It will be appreciated that the inverter section con-trols the frequency of current to the motor and operates ~t a very much xeduced frequency to the chopper sections.
The load thus experiences AC current drive. The in-verter SCR triggering and hence the inverter frequency is control-,~ .
- ', ." : ,, , :

s led by a voltage controlled oscillator (VCO) in unit 1 of Figure 1~ The VCO output responds to the sum ox difference of the slip frequency setting and the vehicle speed feedback signal through line 10 on Figure 1.
Reactors 27~ 28 and 29 are for protection purposes.
As there is no commutation requirement placed on the inverter, phase control SCR's are usedO These SCR's have the necessary power rating for propulsion systems.
It will be noted that there is no requirement or es-tablishing ~Iy initial conditions across any components which resul s in simple start up procedure.
Three single phase inverter units as described above are used in a three phase system. It will be appreciated that while re~erence herein is to a three phase system other multi-phase systems are possible depending on induction motor design.
The control unit 1 shown in Figure 1 is described in schematic form in Figure 3.
~ arious blocks shown comprise plurality of digital linear integrated circuit chips as well as discrete components 2n~ suitable for operational and environmental conditions specified for the system. The control unit c~nsists o~ the following main functional blocks.
One - Propulsion Controller - 40 Three - Chopper/Inverter Logic Units - 42,44,46 One - Inverter Logic Unit - 48 Three - Pulse Amplifier Units - 50,52,54 Three - Pulse Tran~fo~mer Units - 56,58,60 One - DC Power Supply 62 The propulsion controller 40 processes input signals to gi~e output signals to control inverter current magnitude, inverter frequency, direction o~ travel by phase sequence control 6;37~

and inhibition of the ir.verter SCR gating in the event of a faulty operation. The system thus has features of controllable cruise velocity, acceleration, deceleration. In the embodiment il-lustrated the input signals to the controller are velocity o vehicle, accelerate, decelerate, stop and start.
In case of faulty operation, the propulsion controLs initiate various inhibitsi resets and mode displays.
The chopper~inverter logic 4 for each phase 42, 44 and 46 comprises a plurality of inteyrated ciscuits -and discrete components. The VCO's as previously stated determine the fre-quency of operatio~ of the chopper which will vary according to the output current demand and the actual load curre~t feed-back from each phase. The VCO output is suitably processed and steexed to respective pulse amplifiers and transformers for sub-sequent triggeriny of chopper SCR's.
The common inverter logic 48 for the three inverter output phases consists of a VCO, a phase reversing mea~s for direction control and a ring counter for generation of si~nals 120 apart for balanced three-phase triggering of invertes-SCR's.
The VCO responds to an analogue sisnal which is the sum or dif-erence of slip frequency setting and the speed feedback signal fro~ the moving element of the motor, in this case a vehicle carrying the motor.
Each pair of inverter SCR's conducts for 120C of the output cycle in accordance with standard practice to remoYe the objectionable triplen harmonic.
Protection features incorporated in the control unit - illustrated include inhibition of the chopper VC0 ~Id, therefore, triggering of all inverters and chopper SCR's in the event of:
a) overcurrent condition b) overlap due to all chopper SCR's conducting.

il5 The following resets are provided in the control circuit:
a) overlap reset b) overcurrent reset c) turn off time reset.
The pulse amplifiers 50, 52 and 54 are used to interface digital outputs with the pulse transformers 56, 58 and 60. These pulse transformers isolate the high voltage power circuit from the voltage control circuit.
The DC power supply 62 isahigh noise immunity DC/DC
conver~er producing stabilized output voltages for various con~
trol wnit blocks.
The operation of the linear induction motor as an elec-trical machine i5 well known and needs no additional description.
In use the linear induction motor, lt~ chopper com-mutated inverter and propulsion system control unit are mounted on a tracked vehicle with the stator of the motor in operative relationship with the secondary which is mounted in the vehicle track. The vehicle is manually controlled as to propulsion by operation of the accelerate, decelerate, stop and start inputs to the propulsion controller 40. There are many practical variations. For example, a plurality of motors may be used, the manual inputs to the propulsion controller could be auto-matic. However, all variations use the chopper commutated inverter and would have the above noted advantage`s.
There ha~ been disclosed a propulsion system utilizing a chopper commutated inverter with an appropriate control unit and a linear induction motor, that does not rely upon adhesion for pxopulsion effort, is lighter and lower in volume than prior art equipment for equivalent rating, is inherently more rugged than alternative static inverter fed schemes and doe~s not insig-nificantly deviate from the reliability and maintainability that can be expected from such systems.

Claims (3)

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

1. A propulsion system for a vehicle having a dedicated guideway comprising:
a multiphase linear introduction motor having a stator mountable on said vehicle and a secondary incorporatable into said guideway;
a multiphase gate controlled inverter connectable to a source of DC power for supplying multiphase AC power to said stator of said linear induction motor;
each phase of said inverter having means for controll-ably supplying DC current in terminable pulses to said in-verter including commutating capacitors, free wheel loop diodes and gate controlled rectifiers, means for controlling the frequency of operation of said gate controlled rectifiers to control the magnitude of the DC
current supplied by said terminable pulses;
means for cyclically forcing current through said means for supplying DC current to zero whereby to form it into terminable pulses as aforesaid whereby to control the output frequency of each phase of said gate controlled inverter; and a control unit having a feedback related to vehicle velocity in use and sensitive to the demands of the propulsion system adapted to operate the gates of said gate controlled inverter to supply power to said stator of said linear induc-tion motor to propel the vehicle and to operate the gates of said gate controlled rectifiers by controlling the frequency of their operation in accordance with the demands to the propulsion system.

2. A propulsion system as claimed in Claim 1 wherein said gate controlled inverter has phase control SCR' s.

3. A propulsion system as claimed in Claim 1 and Claim 2 wherein said motor is a three phase motor.