USRE23312E - Control means for dynamic braking - Google Patents

Description

Dec. 26, 1950 A. C(DYER CONTROL MEANS FOR DYNAMIC BRAKING 3 Sheets-Sheet 1 Original Filed July 9, 1947 Mi I J4 Zla 22a -o| 0-41.-

INVENTOR. ALVl/V C DYE)? BY m f ar'roznreysQ Dec. 26, 1950 A. c. DYE-R CONTROL MEANS FOR mum-11c BRAKING 3 Sheets-Sheet 2 Original Filed July 9. 1947 INVENTOR. ALVIN c. DYE)? Dec. 26, 1950 A. c. DYER 23,312

CONTROL MEANS FOR DYNAMIC BRAKING Original Filed July e, 1947 a Sheets-Sheet 3 9770 always parallel connected series motors which matlc means for efiecting for each motor when braking "eight contacts dynamic braking circuits.

'larly important then the dynamic braking contacts are prefer- -tured, are single pole contactors.

Reissued Dec. 26, 1950 CONTROL MEANS FOR DYNAMIC BRAKING Alvin C. Dyer, Shaker Heights, Ohio, assignor to The Electric Controller & Manufacturing Co., Cleveland, Ohio, a corporation of Ohio Original No. 2,497,492, dated February 14, 1950,

Serial No. 759,817, July 9, 1947.

Application for reissue April 12, 1950, Serial No. 155,541

21 Claims.

Matter enclosed in heavy brackets reissue specification; matter printed in italics This invention relates to control systems for for dynamic braking automatically regardless of the direction of rotation of the motors, and more particularly to a motor control system which permits operation of less than all of a group of series motors while maintaining operative autodynamic braking of the motors being operated.

If a separate controller is used to control each motor of a group of series motors for operation in parallel with each other, four contacts are required to complete the dynamic braking circuits is to be produced by all of the motors from either direction of rotation. Consequently, when two separate controllers are used to control two series motors for operation in parallel with each other, a total of [are] is required to complete the close to complete the braking circuits after forward operation of the motors and the other four close to complete the braking circuits after reverse operation of the motors.

For reasons of economy, simplicity, and spacesaving it is desirable to use as few dynamic braking contacts as possible whether of the mechanical or electronic type. This is particufor automatic operation because ably contacts of spring-closing electromagnet contactors which, as most commonly manufac- Consequently, a material reduction in the number of dynamic braking contacts results in a much smaller and considerably less expensive controller.

Control systems are known, for example such as described in North Patent No. 1,699,743, for dynamically braking a pair of direct current series motors by completing two closed loops each including the field Winding of one motor and the armature winding of the other motor. As a result of such interconnection of the two mo- .tors, the number of contacts required to complete 'the dynamic braking circuits for braking of the -motors after operation in either direction is reduced from the usual eight to but four. If an 'even number of motors greater than two are to :be operated in parallel. with each other, the rhotors may be grouped in pairs to permit intercon .nection of the motors of each pair for braking purposes. required for each pair of motors so that, it 'four motors are used, a total ingcontacts is required. The controller of the Four dynamic braking contacts are of eight dynamic brakprovide V Four of these contacts appears in the original patent but forms no part of this indicates the additions made by reissue present invention may be used to control two or more motors and requires a maximum of only four contacts to complete the dynamic braking circuits for braking from either direction of rotation.

A further disadvantage of dynamic braking control systems .in which the motors are so interconnected that one motor supplies-excitation current for the other is that in the event of failure and consequent disconnection of one of the two motors of a pair, the remaining motor when operated alonecannot be stopped by dynamic braking. In many applications requiring the use of two or more series motors, such as for example the bridge drive of traveling cranes, it is desirable to have available dynamic braking action even though one or more of the motors is disconnected. The motors of the present invention are so connected that each motor supplies its own excitation during dynamic braking thereby permitting one or more of the motors to be disconnected without disturbing the dynamic braking action of the remaining motors.

It is an object of this invention to provide an improved dynamic braking control system for two or more parallel connected direct current motors having series field windings.

Another object is to provide an improved con trol system for a plurality of direct current motors having series field windings that is capable of rendering all or less than all of the motors effective for dynamic braking when operating in either direction.

Another obiect is to provide a control system for dynamically braking two or more direct cur-- rent series motors from either direction of rotation which requires onlv four contacts to complete the dynamic braking circuits.

Another object is to provide an improved motor control system which is operative regardless of the direction of motor rotation to complete a dynamic braking circuit for one of two series motors if the other motor is disconnected.

Another object is to provide a motor control system for dynamically braking two or more direct current series motors from either d rection of rotation which requires only four contacts to complete the dynamic braking circuits and which permits disconnection of one or more of the motors without disturbing the dynamic braking circuits for the remaining motors.

Another object is to provide an improved motor control system comprising means for disconnccting 9.9? QI':mD1B,11'l0tO1S of a. group oi-P allel connected direct current series motors arranged for dynamic braking without disturbing the dynamic braking circuit of the unconnected motors.

Another object is to provide cross-connections between like potential points in the circuits of two or more parallel connected direct current series motors which cross-connections permit a reduction in the number of contacts required to connect the motors for dynamic braking from either direction of rotation.

A further object is to provide an improved means for limiting the dynamic brakingtorque'of a direct current motor.

A further object is to provide a dynamic brak ing control system for a direct current series mo-- tially the same potential, and which includes ar mature cross-connections between the terminals of like polarity of the armature windings and field cross-connections between the terminals of like polarity of the field winding which crossconnections are selectively interconnected with each other for braking by contacts rendered operative selectively depending upon the direction of motor rotation.

A controller built in accordance with this invention includes means for connecting the several motors of a group of direct current" series motors in parallel acro s a source of power with each motor in series with its own accelerating and plugging resistor. The motor connections are such that like polarity terminals of the several armature windin s are at substantially the same potential and that like. polarity terminals of the several. field windings are at substantially the same potential. Cross-connections connect the like polarity terminals of the armature and field windings to commonjunction points. respectively. These cross-connections become parts of the dynamic. braking circuits which are completed by connections between selected pairs of the common junction points of the several cross-connections. The proper connections, between the several cross-connections may be. sel cted automatically. in dependence upon. the direction" of rotation of the motors by a plurality of springclosing. contactors responsive to. the counter volte age of one of the motors. Cut-out knife switches are. provided on the controller and are so arran ed that any motor may be disconnected without disturbing the dynamic braking circuitsfor the remaining motors.

Other objects andadvantages' of this invention willbecome apparent from. the following descriptionwhereinreference is made to the drawings, in which.

Fig.1 diagrammatically shows apair of. motors connected to a common load;

Fig. 2 is a wiring diagram illustratingthepower circuits-and some of the control circuits of a nreferred embodiment of the invention arrangedior controlling the two motors of. Fi 1;

Fig. 3' is a wiring diagram showing the remaining. control. circuits of the preferred embodiment;

' Fig. 4 is a simplified wiring diagram, showing the power circuits that are energized when both motors of Figs. 1 and 2 are operating as motors;

Figs. 5 and 6 are simplified wiring diagrams showing thev power circuits that are energized when both motors of Figs. 1 and 2 are being braked dynamically from forward and reverse directions, respectively, and;

Fig. 7- is a simplified wiring diagram showing how Figs. 2 and 3 may be modified for controlling more than. two motors.

Figs. 2 and3 when combined illustrate a com-- plete motor control system. Some of the contactors and relays-of Fig. 2 are shown incompletely in.Fig. but'allcontactors and relays shown in Fig; 3 are-shown completely in that figure, and

the contactors and relays that are shown incompl'etely in Fig. 2 are shown completely in Fig. 3.

As illustrated. in Figs. 1. to 6., a controlsystem in accordance with this invention is arranged'to control a pair of reversible direct current series motors. ii]. and M (Fig. 1-) to be operated in parallel with each: other for driving a common; load [-3. The motor in has an armature-winding 10a (Figs. 1 and 2) and a series field winding [0b and the motor H has an armature winding Ma and a-series field winding H b. Although the motors illustrated are series machines, it will be understood that the system can control compound motors as well.

Power may be supplied tothe'motors lfl and l I from the conductors l2 and I! (Fig. 2). which are arranged to be connected by atwo-pole knife switch l5to a suitable source of power represented by the conductors l6, To permit selective opera;- tion of either motor alone" or. both' motors to gether, suitable switching'means' such as knife switches I9 and 20 are provided; The knife switch I9 is associated with the motor ii!v and-has poles lilato I'9g inclusive; the poles Nd and [9f being double-throw. The knife switch 20' isassociated with the motor II' and haspoles 20a to 211i, inclusive, the poles 20d, 20f, 20h, and 201? being double-throw.

To permit the control" system to be arranged so that the motors I0 and l I may be: easily and automatically disconnectedfrom the'pow'err ource upon a decrease in the supply'voltagezor upon an overload, a plurality of el'ectromagneticcontactors 2!, 22, and 24 are provided ior'reversibly connecting the motors I0 and H inparall'elwith each other between the conductors l2 and. I 4. The contactor 2| has four normally open main contacts 2'la, 2Ib, Ho, and Md and the'contactor 22 has four normally open main contactsfla', 22b, 22c, and Md. The contacts 21a and. 21b when closed connect the armature winding I [la for forward rotation of the motor Dandthecontacts 2 lo and 2 Id when closed connectthelarmarture winding I la for forward rotation of the motor l I Similarly; the contacts 22a and 22b when closed connect the armature winding Illa for'reverse rotation of the motor Ill' and the contacts 220 and 22d when closed connect the armature winding Ila for reverse rotation of themotorr'l l. Although, as shown in Fig. 2; the current-in the armature windings Illa and Nb is reversed to effect reversal of the motors; it will be-apparent that the control system could also be arranged to reverse instead the current in thefield windings I 6b and I lb. The contactor 2'4 has a normally open main contact 2 5a which when closed con- :nects the motors l0 and II to the conductor II. The contactors 2|, 22, and 24 have operating h windings 2lw, 22w; and MW, respectively: As

shown ihFig; 3; the contactor?! has-normally open auxiliary contacts2le, 2lf, and2lg, and likewise normally open auxiliary contacts 22e, 221, and 22g are provided on the contactor 22. The contactor 24 has a normally open auxiliary con tact 24b.

Acceleration and plugging aswell as the speed of the motors l0 and Il may be controlled by suitable means such as the series resistors 25 and 26, respectively. Plugging sections 25a and 26a of the resistors 25 and 28, respectively, are arranged to be short circuited concurrently upon closure of main contacts 28a and 28b, respectively, of an electromagnetic plugging contactor 28 having an operating winding 28w. The contactor 28 has a normally open auxiliary contact 28c'and a normally closed auxiliary contact 28d. Accelerating sections 25b and 26b of the resistors 25 and 26, respectively, are arranged to be short circuited concurrently upon closure of main contacts 29a and 29b, respectively, of an electromagnetic accelerating contactor 29 having an operating winding 29w and a normally closed auxiliary contact 29c. Additional accelerating resistor sections and contactors may be provided if desired.

Operation of the contactor 28 is controlled by a suitable plugging relay 30 having a normally closed contact 30a and an operating winding 38w which is connected in parallel with the resistor section 26a. Operation of the contactor 29 is controlled by a suitable accelerating relay 3i having a normally closed contact 3ia and a series-type operating winding 3|w which is connected in the short-circuiting loop completed by the contact 28b. The relay 3| is preferably of the type described in Trofimov Patent No. 1,980,736 and has its contact 3la mounted on a conducting tube 3Ib that moves upwardly to open the contact 31a upon an increase in current in the winding 3lw and returns to the normally closed position shown after a time interval. Other types of plugging and accelerating relays than those shown may be used if desired.

The dynamic braking circuits to be described are controlled by suitable control means which includes the contacts 32a, 32b, 34a, and 34b, hereinafter described. Thus, in the illustrative example, the dynamic braking circuits are completed by selective closure of normally closed main contacts 32a and 32b of an electromagnetic forward braking contactor 32 and normally closed main contacts 34a and 34b of an electromagnetic reverse braking contactor 34. Any suitable means may be used to control the selective closure of the contactors 32 and 34 in accordance with the direction of motor rotation, but preferably for this purpose the contactors 32 and 34 are provided with polarizing windings 32p and 34p,- respectively, as described and claimed in a copending application of J. D. Leitch and P. G. White, :Ser. No. 736,146, filed March 21, 1947. The contactors 32 and 34 also have operating windings 32w and 34w, respectively, normally open auxiliary contacts 320 and 32d and 34c and 34d, respectively, and normally closed auxiliary contacts 32c and 34e, respectively. The windings 32p and 32w have a common magnetic circuit and thewindings 34p and 34w also have a common magnetic circuit as indicated. Although each of the contactors. 32 and 34 has been shown as a double-pole contactor, this hasgbeen done merely to simplify the drawing since usually two single-pole contactors would be used instead of one double-pole contactor.

A protective resistor 35 for the polarizing windings 32p and 34p is by-passed at slow motor speeds by a normally closed contact 36a of a counter-voltage relay 36 having an operating winding 36w, an additional normally closed contact 36b, and a normally open contact 36c. A protective resistor 38 for the windings 32w and 34w is by-passed before acceleration and during plugging by a normally open contact 39a of a time delay relay 39 having an operating winding 39w. Preferably the relay 39 is of the flux-decay type with the contact 392. being delayed in opening. Energization of the windings 32W and 34w is controlled by a normally open contact 40a of a braking control relay 43 having an additional normally open contact 40b and an operating winding 40w. x

The dynamic braking circuits include crossconnections 4| and 42 between like polarity terminals, respectively, of the armature windings Illa and Ha and cross-connections 44 and 45 between like polarity terminals respectively, of the field windings [0b and H b. Resistors 45 and 48 having respective center taps 45a and 48a are interposed in the cross-connections 4i and 42, respectively, and resistors 49 and 53 are interposed in series with each other in the crossconnection 44. Connections to be completed selectively for dynamic braking extend from the center-tap 46a through the contact 34b to the cross-connection 45 at a junction point 45a and through the contact 32a to the cross-connection 44 at a junction point 44a intermediate of the resistors 49 and 5!). Likewise dynamic braking connections extend from the center-tap 48a through the contact 32b to the cross-connection 45 at the point 45a and through the contact 34a to the cross-connection 44 at the point 44a.

It is to be noted that the mid-tap 45a is a common junction point for conductors leading from the left-hand armature terminals, the mid-tap 48a is a common junction point for conductors leading from the right-hand armature termi nals, and that junction points 44a and 45a are common to the like polarity terminals of the field windings ltlb and Nb, respectively. As shown in Fig. '7, more than two motors may be controlled by providing similar common junction points for like polarity armature and field terminals of all of the motors.

Portions 49a and 50a of the resistors 49 and V 50, respectively, are arranged to be short-circuited by normally closed contacts 51a and 5lb, respectively, of an electromagnetic contactor 5! having an operating winding 5Iw. Adjustable portions 49b and 58b of the resistors 49 and 50, respectively, are arranged to be short-circuited by normally closed contacts 52a and 54a, respectively, of electromagnetic contactors 52 and 54, respectively, which have respective series-type operating windings 52w and 54w interposed in the cross-connection 44 on opposite sides of the junction point 44a.

The direction of rotation and the speed of the motors l9 and il may be selected by a reversing master switch 55 (Fig. 3)" having electrically interconnected contact segments 55a through 55], inclusive, which are movable with respect to cooperating contact buttons 55n to 55t, inclusive. The master switch 55 has 'anoff position and three forward and three reverse positions as indicated. A low voltage protection relay 5% having a normally open contact 55a and an operating winding 55w is associated with the master switch 55 and maybe made responsive in the usual rrianner to the operation of overload relays :(not'shown); Dynamic braking 01 the motor-sit and H- may be. controlled by a master switch 58' having electrically interconnected contact segments 58a, 58b,-and 58c which are movable from an on position through two bra king positions with respect to contact buttons 58d, 58c, and 58f, respectively.

A contactor 59 having an operating winding 59w has normally open main contacts 59a and 59b which complete when closed connections through resistors 60 and 6|, respectively, to provide slight excitation for the field windings [b and Nb, respectively, while the motors are drifting" or: coasting. This slight excitation insures that the relay 36 remains'picked-up and the windings 32p and 34p remain energized during the coastingperiod and further insures that the field windings of the motors build up if the motors are braked after coasting.

Further understanding of the'preferred construction and arrangement of 'the component parts of the controller of Figs. 2 and 3"may be had from the following description of operation:

First it is assumed; that bothof the motors l'fl and H are to be operated together in the forward direction and then stopped by dynamic braking. With; the knife switch I5 closed to char-- 'gize the conductors l2 and I 4, both motors are arranged for operation together when the knife switch [9 is in its upper closed position and the knife switch is in its lower closed position.

With power supplied to the conductors l2 and H and both master switches 55 and'58 in their err positions as shown, energizing circuits (Fig. 3) are'completedfor the windings 39w, w, 51w, and 56w. The circuit for-the winding 56w is from the conductor I! through the contacts 36b, 55h, 55a, 55b, 550, a conductor 54, and the winding-Sfiw to the conductor l 4.' Energization of the winding 56w causes closure of the contact 56a to complete a'circuit from the conductor I; to the conductor 64 and the winding 55w which is independent of the positionof'the master switch 55'. The circuit for the winding 5lw is from the conductor 64' through the contacts 58d, 58a, 58b,

and 58e through the winding 51W to the conductor I 4. The contacts so and 5|]; upon energization of the winding 51w open and interrupt the short-circuiting paths around the resistor sections 59a and a, respectivelylFig. 2). The circuit for the winding 39w is from the conductor 64 through the contacts 3% and 34a in parallel, and the circuit for the winding 40w is from the conductor 64 through the master switch contacts 58d, 58a, 58c and 58f.

Energization of the winding 40w causes closure of the contacts lfla and 40b. Closure of the cont'act'flfla completes a circuit for the windings 32w and MW from the conductor 64 through the contact 39a which closed to by-pass the resistor 38 uponenergization of the winding 39w. Energization of the windings 32w and MW with the resistor 38 by-passed causes positive opening of the contacts 32a, 32b, 34a and 34b in the dynamic braking circuits. The contacts 3% and He also open to deenergize the winding 39w. After a short: time deay, the contact 39a opens to reinsert the resistor 38 in serieswith the windings 32w and MW, but the contactors 32 and 34 'remain-in their energized positions. The contacts 326. and 34d which closed'upon energization'of the windings 32w and 34w together with the now-closed contacts' lflb partially complete cit-'- cuits tor'the windings 2 iw-a'nd 22w: 5 I? I If the master switch is now movedto any of the'forward positions, energizing circuits from the conductor 61 to the conductor; l4 are completed for' the windings Zlw, 24w, and 59w. The circuit for the winding 2 [W includes the contacts 550, 515 b, 55d, 55q, 4flb[;], 32d-"and 34d, and the circuits for the windings MW and 59w include the contacts 55 [c] 0, 55b, 55c, and 55p. The contacts 290 are also in the circuit for the winding 59w.

Energization of the winding'2'4w causes closure of the contact 24a which connects the motors l0 and II to the conductor l4 and causes closure of the contact 25b in an energizing circuit for the Winding 28W.' With the contact 24a. Closed: closureof the contacts 59a, 59b, Zia, 2l b ,"2'lc and Zld upon ehergizati'oh of the windings 59w and 21w causes both motors t o erat in the forward direction with respective armature 'shunt circuits of relatively high resistance' Theeontacts 2le, 21f, and Zlg also close. The'eontact 2le insures that the windings 32w and am are energized when the winding z'rw' is energized, and the contact 2| g partially'completes theta: cuit for the winding 28w. Closure of the Contact 2lf completes a circuit for the winding saw, but the response of the'felay 39 is withoutopei'ative effectat this time. i As soon as power is supplied to the motor 1 I, the voltage drop 'ac'rossits armature'winding I la causes energization of the winding 32p through the contacts 320 "and 36a and energizatibn ofthe winding 34p through thecontacts 34c and 36a; The contacts 32c and'34c closed upon energiza' tion of the windings 32w and34 w. After the motorhas 'reaeheda predetermined low speed, the winding 36w which'is connected across the arma ture winding H a becomes sufiiciently energized by the counter-voltage of the motor H to open thecontact 35a thereby to insert the resisto'r35 in series with the windings 32p and 34p. Means may be providedin'a 'well known manner for pro tecting the winding 36w against overheating when subjected to the higher 'values of counter voltage to permit the relay 36 to have a're a'tively low pick-up current value. With the motors op-' erating in'the forward direction, the flu'xproduced by the winding 32p opposes that produced by the-winding 32w and the flux produced by the winding 34p assists that produced by the winding 34w. The flux 'producedby the winding 32w, however, is sufliciently in excess of that produced by the winding 32p even at'the maximum possible speed of the motor II that the contactor 32' remains in its picked-up or energized position.

It is assumed that the motors are accelerating from'rest' so that, when the master switch 55' reaches the second forward'position, a circuit is completed without time delay for the winding 28w through the contacts 55c, 55s, Zlg, 30a and 2416. Closure of the contacts 28a and 2810 upon energization of the winding- 28w short-circuits the resistor sections 25aand 26a, respectively, and" increases the voltage applied to the motorarma tures. 4 Completion of theshort-circuiting'loop including the contact 28bcausesenergization ofthe winding MW of the relay 3i which respondsto open its contact 3la. After a time delay-interval dependent upon the amount of current flowing' through the winding'3lw during the interval; the contact 3| a recloses. moved to the third forward position or if it fire; viously had been moved theraa'circui't isfco pleted for'the' winding 529w through the contacts. 55f, 55t, Ma and 28c. Response of the contac'toi If the master switch is new 2! to the energization of its winding 29w causes closure of the contacts 29a and 29b which exclude the remainder of the resistors 25 and 26 from the motor circuits, and causes opening of the contact 29c to deenergize the winding 59w. The contacts 59a and 59b thereupon open to interrupt the armature shunt circuits through the resistors 60 and SI.

The motors Ill and l l are now operating in the forward direction at their maximum speed for a given load and the motor circuits are as shown in Fig. 4. When so operating, the resistors 48, 49, and 50 serve to minimize any circulating currents in the cross-connections 42 and 44 resulting from possible differences in voltage drops across the armatures Illa and Ha. When the motors are reversed, the resistors 46, 49 and 50 operate to minimize such currents in the cross connections 4| and 44. Only the portions 49b and 50b of the resistors 49 and 56 are sh-ort-circuited during normal running of the motors. If care is taken to make the resistances of the two field circuits substantially equal to each other, the currents in the field windings lllb and llb are substantially equal, and the load is satisfactorily distributed between the two motors.

If at any time while the motors are operating in the forward direction, it is desired to effect a brakin operation, the master switch 58 may be moved to its first braking position. This effects deenergization of the winding 40w and consequent opening of the contacts Mia and 40b. Opening of the contact 40b deenergizes the winding 2lw and the contactor 2| returns to its normal position. Opening of the contacts 45a and Me deenergizes the windings 32w and 34w. The contactor 34 remains in its energized or pickedup position due to the flux produced by the winding 34p. Since the flux produced by the winding 32p is opposed to that produced by the winding 32w before deenergization of the latter winding, the flux in the contactor 32 reaches zero and the contactor 32 drops out. Opening of the contact 32c prevents further energization of the winding 32p and increases the voltage applied to the winding 34p.

Closure of the contacts 32a and 32b upon dropout of the contactor 32 completes the following dynamic braking circuit for the motor HI: from the left-hand terminal of the armature winding Illa, the knife switch pole l9b, the upper portion of the resistor 46 to the mid-tap 46a, the contact 322., the point 44a on the cross-connection 44, the winding 52w, the contact 52a, a portion of the resistor 49, the knife switch poles We and let, the field winding [b, the knife switch pole mg, the point 45a on the cross-connection 45, the contact 32b, the mid-tap 48a, the upper portion of the resistor 48, and the knife switch pole l9c to the right-hand terminal of the armature winding Ina. The following dynamic braking circuit for the motor I l is also completed: from the lefthand terminal of the armature winding Ila, the knife switch pole b, the lower portion of the resistor 45 to the mid-tap 46a, the contact 32a, the point 44a on the cross-connection 44, a portion of the resistor 55, the contact 54a, the winding 54w, the knife switch poles 20c and 29f, the field winding Hb, the knife switch pole g, the point 45a on the cross-connection 45, the contact 32b, the lower portion of the resistor 48, and the knife switch pole 20c to the right-hand terminal of the armature winding Ila. The motors are now connected as shown in Fig. 5.

Since the braking circuit for the motor l0 contains no part of the motor I l and the braking circuit for the motor ll contains no part of the motor If], an open circuit in one of the motors does not interfere with proper braking of the other motor.

With the master switch 58 in the first position, the contacts 5Ia and 5lb are open and the resistor sections 49a and 55a are efiective in the respective dynamic braking circuits. As soon as dynamic braking current flows in the cross-connection 44, the contacts 52a and 54a open due to current flowing through the windings 52w and 54w. When the contacts 52a and 54a open, the resistor sections 4% and 55b become eifective in the dynamic braking circuit. The resulting decrease in the dynamic braking current causes the contacts 52a and 54a to reclose. Reclosing of the contacts 52a and 542. results in an increase in current and the contacts reopen. The contacts 52a and 54a thus open and close in rapid succession until a low speed of the motors l0 and H is reached at which time the dynamic braking current is too low to cause further operation of the contactors 52 and 54. Final stoppin of the motors l0 and II is effected with the dynamic braking circuits including only the resistors 45 and 45 and the portions of the resistors 49 and 50 not bypassed by the .contacts 52a and 54a.

The contactors 52 and 54 are preferably designed with pick-up and drop-out current values so related to the resistance of the resistor portions 49a and 55a and the expected value of dynamic braking current that each contactor opens and closes several times during dynamic braking. By making the contactors 52 and 54 very rapid in operation, it is possible to limit the dynamic braking current to a predetermined value at the start of braking and to maintain the average value of the current but slightly below the predetermined value throughout a major portion of the braking cycle.

When the master switch 58 is moved to the second braking position, the winding 51w is deenergized and the contacts 5la and 5lb close. Stronger braking is thereby produced since the resistor sections 49a and 50a are excluded from the braking circuits. The contacts 52a and 54b open and close repeatedly as before to graduate the braking current and torque during the stopping interval.

When the motors IB and II reach a predetermined low speed, the contact 36a. of the relay 36 closes and short-circuits the resistor 35 so as to connect the winding 34p directly across the armature winding i la. Thus the contactor 34 remains in its energized position until a very low motor speed is reached. Preferably the contactors 32 and 34 have low drop-out current values so that the dynamic braking circuits remain completed as long as possible.

If the master switch 55 is moved to the reverse positions instead of the forward positions, operation during acceleration is the same as above described except that the winding 22w instead of the winding 2lw is energized. The circuit for the winding 22w includes. the contacts 55h, 55r, 45b, 32d, and 34d. Due to the reversal of counter- E. M. F. of the motors it and H for reverse operation, the flux produced by the winding 32p now assists that produced by the winding 32w whereas the flux produced by the winding 34p during reverse operation opposes but is less than that produced by the winding 34w. Consequently when the windings 32w and 34w are deenergized,

it the-eontactor 3% remains in its pickedeup. post. tions dueto the. flux produced. by thewinding 32p and the conta-ctor 34: drops out. because its .fluxis reduced to: zero. The contactors. 51, 52,. and. 54

respond during reverse braking in the manner described. above for-forward braking.

The-reverse braking. circuit for the motor L is through the upper portionof the; resistor; .48.

to the-mid-tap 48a, thecontact 34a, the. point 44a, the winding- 52w, theresistonfl, the fieldwinding Iflb, the point a, the contact 34b, and the upper portionof the resistor-45 tothe. leftehand ter-. min'al -of the armature winding. 1.9a. The. reverse brakingcircuitiorthe motor I.l. is'through .the. lower-portion of the resistor 4-8, thecontact-Ma, the point 44a, the resiston thewinding 54w, the field winding Mb, the pointMia, the contact 34b,-:and: the lower portionof. the; resistor-.45; to

the lefthandterminal:= of the armature.v winding lia. "l hemotorsare now connected as shown in I f-the'motors I0 and: H: should bepl-ugged from either directionof rotation, the. relay. 38". opens its contacts Z-ma'rto prevent energization of the contactor 2-3 until the motorsapproach standstill; Acceleration-then-proceeds as before. When the motor-1 t is plugged; the voltage across the. armature He becomes greater-thanthe voltage.

between the conductors l2 and It. To prevent the-flux of the. windings 342pv and- 34p from ex.- ceeding that produced by the-windingsu'izw and 3ii'w-dur-ing--plug ging, the winding 39w of. the relay 39 1s energized when the contacts2 ifsorZZ-f, andthe-contacts Z-Sdareclosed. Thecontact 39a thus;closesduring-plugging to exclude the. 116:. sister 38 fro'm;-th'e-- energizinge circuit for the. windings-- MW and BA-W thereby increasing thefluxproducedby those-windings. so as. to maintain the; necessary-excess-flux."

If it s desired tor-any reason to disconnect the motor l-iiiiromthecircum,theitnife switch 59 may be moved from its upper closed-position to its loiveneldsedposition. With the-knifeswitch l9 in-its-lowerclosed position, the poles [Sid and l9fconnect the---resistorv 25's inpara llel: Withthe resistor 26. Thisincreases the torque exerted by the; IliOtOP-lfl -Whll themasten switch is in thefirst two positions which; is desirable; since the motor l-l' -is now-driving alone thesame-load I101:- mall'ydriven b'y bothmotors IIi'CtOI l9 has-not altered the dynamic braking circu'it ior the-motor- H andthe nioton H may be stopped hy dynamic b ralging in the same manner as deserilid for 1 two-motor. operation.

= If the knife switch 2 9 is moved to itsupper closedp'o'sition'wliile the kniie switch I '9 is in its upperclosedposition themotor H- is disconnected from the circuitandtheresistors 25and25 are in parallel witheach otherthrough the knife switch poles 29d andZGf and both resistors are inlseries with the moton Hi.-- Thus the relays- 30: and 3| can-function-toc'ontrolplugging and acceleration of the motorlii; alone.

windings; 31w; 32p", and 34p across the armature ma throug l-i the knife switchpoles 2th and Zfii and does not alter the dynamic braking circuit of the motor [0.

If the knife switch poles We and 20eareoini tted; the resistors Miami 50- would be in parallel during braking if one of the motors were disconnected; from the circuit The resulting stronger braking torque for the single remaining motor may be desirable for some applications.

Asis clearfrom- Figs, 2, 5, and 6, opening of the circuit: to the 7 Moving of the knife switch 20 to itsgup'per closed'positionconnectsthe when OlOSfidlQQDHEQtSl the motorsto-lthe conduc;

" Fig. 7 respectively; areprovidedand .rcpewer. shculdifailat any.=.time.during: 1; tion; o't either one or of. thBimOIlOIZS; therelaw 56 opens its contacts. ifiaato deenergi-zethei con 3 ductor 64 With the. conductor. 64: deenergizedp alltof thelconta'ctors: and relays:- return t.o...thei-r;- normal positionsnexcept the. relay. 3,6. and: the. proper dynamic.brakingv contactor .321 or .3343 (16;; pending upon the direction of motor rotat i n Braking eactionith-usl. proceeds. as if the master switch 58.. had been-.movedto its. second position; During-.the short interval between. the time; that, power istremoved': from: the armature winding; .1; j; until: the. dynamic; bra-king. circuit ,for. the motor, 3' is established, the winding 3410; is energized bythecounter-voltage resulting. fromthe-residu magnetism. in. the. field of: the..mo.t01';.-.l:l= fact that the contactsiatb are opendurin rakin insures that. power cannotxbe reapplied" to; the. motors after a. power failure until the, motors; reacha1ow.speed;....

. The circuitscthrcugh the contacts 59a: and: Eli-b ofsthe contactor. 59:.and: the resistors; EB and 6 7 provide. a. small: excitation 01E the field; winding;

meand- Mb respectively, while.the. motors-arev coasting, that is,.w-hile. the..mot,ors, arerot t with power removed from the armatnresan 9. dynamic. braking circuits interrupted. s sures. build-up .ot. the; motonfieldistrength; W, 3, brakingcohnec-tions. are established; and; insures; that the; relay 36'. and; thescontactors: 32.; and 3 4 remain picked np while coasting. It; is be noted that since the field windings; lE-h, and-3U are. in parallel? during the build; up; of: onefield causes build up; of the; other. I 3 Fig. 7 shows. howv the control system; of; Eigs 2; and :.3 may; be. modified; to control. more; than two. motors without; an increase in; the number-10ft;- dyi'n'a 'micbraking-contacts; IntEig-i '7; a pl ur 'ty; ofi motors having armature windings; 'l la, lZ a, 13a and 14a. and respective fieldwindingsfl b, 12h, 13b, and- Nb are. arranged; to beconn ted in; parallel. with. each. othen and; to.-.be. supplie irogn h a. suitable. power source indieated.by. thecon ductors 15zand': iiiz. normalix open contact 18g tor 76, and a plurality of normally open tacte 19:. when closed: connectthe-several; armature windingsfortforward operationeithegnotorswhile; a plurality-.ofr normallyrvopen contacts 841; controlreverse;operation.ofgthemotors .EhQQo ta'cts Til-8. 1.8; and. Blhmaybe .cbxitactso electroe. magnetic:contactorszand4corlicspondzto he v contacts.- [to] oyiwthetcontactors12:1; 22 and; 2 Fig. 2'. Acceleratingresistors. 8,1 .for-themotors of; m

regulated in any suitable. manner.;

.The dynamic braking. conne'c-tione;w include; cross connections. 82 and: 58; inclusive. 'I' cross. connection. 8:2; is. between the. left-hand terminalr ofr the. armature. winding 'igla,.,andy;the. left-handaterminalr.ofrthe armature winding Maandzth'e. cross; connection =8 5:;is;betwcen. the righthand .term inaistof these. two. armaturelwindinge Likewise,- thecross connections. M-andfllare bee.

tween opposite terminals. of: the iarmatu're wind?- ing-JZa must. The cross. connection: 8&isb.e tween the left-hand: terminals of .the. field .win.-.. ings ll-b and: 14b andutht.crossconneetion.BJ-is; between-the left-hand terminals of.- thelfield-winde ings 12b and. 13h Theright-hand terminalspf; each of-the field windings. are-.interconnectedby the-cross conneotionhafl.

Resistors 89,- 90, 9|; aIIdtQZ areinterposedz-in. the cross connections 82,83",v 84. and respec.-;. tively. Mid-tapsof theresistors 89 and Smare 13 connected to a common junction paint and mid-taps of the resistors 9| and 92 are connected to a common junction point 95. Resistors 95 and 91 are interposed in the cross connections 86 and 81, respectively, on one side of a junction point 98 common to the cross connections 86 and 81, and resistors 99 and IE9 are interposed in the cross connections 86 and 81, respectively, on the opposite side of the junction point 98. The resistors 96 and 91 correspond'to the resistor 49 of Fig. 2 and the resistors 99 and I correspond toj'the resistor 50 of Fig. 2 and may be regulated in any suitable manner, but preferably in the manner disclosed in Fig. 2.

For controlling dynamic braking in dependence upon the direction of motor rotation, two normally-closed contacts [9| and two normally closed contacts I02 are provided. The contacts llll connect the motors for braking after a forward operation, and the contacts I02 connect the motors for braking after a reverse operation. The contacts I91 and H32 correspond to the main contacts of the contactors 32 and 34 of Fig. 2, respectively. Means may be added to Fig, '7 in a manner hereinabove explained to disconnect any one or more of the motors while permitting the remaining motors to operate.

' With the contacts [0| closed and the contacts I92 open the motors are connected for dynamic braking if they have been running in the forward direction. Current flows from the armature Ha through the upper portion of the resistor 89, the junction point 94, one of the contacts lfll, the resistor 96, the field winding Hb, the crossc'onnection 88, the remaining contact Hll, the junction point 95, and the upper portion of the resistor 92 to the armature Ha. Current supplied from the armature winding Ila thus excites the field winding 1 lb to produce dynamic braking action. The dynamic braking circuits for the remaining motors may be traced in a similar manner. If the motors are operating in the reverse direction the contacts IE2 are closed while the contacts ID! are open and similar dynamic braking circuits for each of the motors are completed. It is thus seen that, irrespective of the number of motors, only four dynamic braking contacts are required in order to complete the necessary dynamic braking circuits for braking from either direction of motor rotation.

' Thus, having described my invention, I claim:

1. A dynamic braking control system comprising a plurality ing an armature winding and a series field winding, switching means for connecting the windings of each motor in series with each other and said motors in parallel with each other across a source of power'for operation thereof as series machines with terminals of like polarity of said armature windings at substantially the same potential and terminals of like polarity of said field windings at substantially the same potential, said switching means including means for reversing said motors concurrently, armature cross connections connecting said armature terminals of like polarity to common armature junction points, respectively, field cross connections connecting said field terminals of like polarity to common field junction points, respectively, a first pair of dynamic braking connections respective to said common field junction points and independently connecting their associated field junction points with one of said common armature junction points, a second pair of dynamic braking connections respective to' said common field junction points and of direct current motors each hav- I 14 independently connecting their associatedfiela junction points with the other one of said common armature junction points, and control means for making and breaking said dynamic braking connections selectively.

2. A dynamic braking control system in accordance with claim 1 characterized in that said control means includes contact means interposed in said dynamic braking connections, respectively, and is operative to interrupt all of said dynamic braking connections when said motors are operating as motors.

3. A dynamic braking control system in accordance with claim 2 characterized in that said control means includes operating means operative to complete a selected pair of said dynamic braking connections upon cessation of power supply to said motors while maintaining the remaining pair of said dynamic braking connections interrupted, 5

4. A dynamic braking control system in accordance with claim 3 characterized in that said operating means is operativ to close some ofsaid contact means upon cessation of power supply to said motors while maintaining the remaining contact means open and includes meansre sponsive to the direction of rotation of said motors to select which of said contact means are to be closed and which are to remain open.

5. A dynamic braking control system in accordance with claim 1 characterized in that means are provided for disconnecting one of said motors from the source of power independently of the operation of said switching means while maintaining said dynamic braking connections connected with the windings of a remaining one of said motors through a portion of each of said armature cross connections, and a portion of each of said field cross-connections, thereby providing dynamic braking loop circuits for said remaining one of the motors.

6. A dynamic braking control system in accordance with claim 1 characterized in that said armature cross-connections define a closed armature loop circuit that is completed at all times while said motors are operating and which loop circuit includes two of said armature windings,

both of said armature junction points, and resistor means.

'7. A dynamic braking control system in accordance with claim 6 characterized in that said field cross-connections define a closed field loop circuit that is completed at all times while said motors are operating and which loop circuit includes two of said field windings, two of said field junction points, and resistor means.

8. A dynamic braking control system in accordance with claim 7 characterized in that means are provided for varying said resistor means.

9. A dynamic braking control system in accordance with claim 7 characterized in that means are provided for short circuiting at least a portion of one of said resistor means, and comprise a contact normally held closed to by-pass a. portion of said one resistor means, a coil in series with said one resistor means in the cross-connection containing said one resistor means between the common junction point and motor terminal of said cross-connection, said contact being responsive to current flowing in said winding to open said short-circuit.

10. A dynamic braking control system in accordance wtih claim 1 characterized in that resistors are interposed in said armature crossconnections between said common armature juncs connections, thereby providing dynamic braking loop circuits for said remaining one of the motors.

ALVIN C. DYER.

REFERENCES CITED The following references are of record in the file of this patent or the original patent:

UNITED STATES PATENTS Number I Name Date 1,231,605 Hellmund July 3, 1917 1,699,748 North Jan. 22, 1929 1,985,706 Wilson et a1. Dec. 25, 1934 2,046,970 Royer July 7, 1936 2,128,034 Austin Aug, 23, 1938 2,248,577 McNairy July 8, 1941

Images