US2448040A - Control system for variable-speed alternating-current motors - Google Patents

Description

1943- K MAHNKE ETAL 2,448,040

CONTROL S YSTEM FOR-VARIABLE-SPEED ALTERNATING-GURRENT MOTORS Filed Dec. 13, 1946 2 Sheets-Sheet 1 s WITNESSES: 1' K II/INXVENTORS 54 77% ur/ Q rMe and gY amesM ork vOCJ- TTORNEY A .1943. K MAHNKE ET AL 2,448,040

CONTROL SYSTEM FOR VARIABLE-SPEED ALTERNATING-CURRENT MOTORS Filed Dec. 13, 1946 2 Sheets-Sheet 2 \imi Z9 WITNESSES: Q? d N N INVENTOR5 @ZQW 3 E l u'rf Ma/Inke 00o l famea 1/. York. you I BY AT ORNEY speed exceeds a safe limit value.

Patented Aug. 31, 1948 CONTROL SYSTEM FOR VARIABLE-SPEED ALTERNATING-CURRENT MOTORS Kurt Mahnke, Forest Hills, and James W. York, Irwin, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application December 13, 1946, Serial No. 716,030

9 Claims. 1

Our invention relates to electric systems for controlling alternating-current multi-speed motors of the type in which several field windings and appertaining terminal groups are provided for producing diflerent respective numbers of field poles, in order to permit a selective operation of the motor at respectively different speeds depending upon which terminal group is energized at a time.

Such control systems, in general, are known, for instance, from United States Patent 2,393,998 of Kurt Mahnke, assigned to the assignee of our invention; and our invention, in some of its aspects, may be considered to involve improvements over systems according to the patent as regards circuit design and speed transfer performance of the control system.

It is among the objects of our invention to provide a proper timing of the accelerating or decelerating performance of an alternating-current multi-speed motor with the aid of timing relays of simpler design than those required in the known system. That is, in the known system the timing relays for determining the minimum period of time, which has to expire between the energization of one motor terminal group and the subsequent energization of the next terminal group for a higher or lower motor speed, had to be designed in such a manner that they operated with a delayed pick-up performance as well as with a delayed drop-out performance in order to render their timing function effective both when accelerating and when decelerating the motor. The invention, according to the justmentioned object, aims to permit the use of relays which are capable of timing the speed transfer for acceleration as well as for deceleration, although they have a substantially instantaneous pick-up performance and only a delay drop-out performance. Such relays may consist, forinstance, of the customary direct-current relays with a short-circuited winding on the magnet core of the relay. It thus becomes possible to equip the control system with these simple relays of customary type with the advantage of a considerable simplification of the system, reduced space requirements for the relays, and reduced cost of the relay equipment.

Another object of our invention concerns itself with the provision of a load-responsive relay in the motor control system in order to prevent the motor from operating at high speed when the load current fiowing through the motor at that In the known system containing such a load relay, this relay may prolong the time needed for accelerating the motor to its maximum speed. The last-mentioned object, therefore, aims at designing a motor control system in such a manner that the protective load-responsive relay does not interfere with a quick adjustment of the motor to high or highest speed but comes into efiect only when thereafter the load current persists unduly long at a value above the safe limit.

A further object of the invention relates to motor control systems for hoists and aims at the provision of a motor control system for alternating-current multi-speed motors in which a loadresponsive relay of the kind just mentioned is so controlled that its control effect occurs only when the motor is operated in the hoisting or heave-in direction but is eliminated or suppressed during lowering or pay-out performance.

These and more specific objects of the invention, as well as the means provided by the invention for achieving these objects, will be understood from the following description of the embodiments illustrated in the drawings, in which:

Figures 1 and 2 represent two modifications of a hoist control system designed in accordance with the invention and suitable especially for the control of hoisting equipment, such as marine winches.

In Fig. 1, the cable drum of a hoist or winch is denoted by D. The drum is driven from an alternating-current motor M of the squirrel-cage type, preferably through the customary reduction gears (not illustrated). The drum is equipped with a friction brake B, which is electrically connected to the motor control system so that the brake is automatically set whenever the motor M is deenergized. The design and control of the brake B is not a part of the invention proper and may correspond to known constructions, for instance, as shown in the above-mentioned patent. Therefore, details of the brake and the appertaining control connections are not illustrated and further described in this disclosure.

The motor M is of the squirrel-cage type. It has four groups of terminals Tl, T2, T3 and T4. The motor field windings (not shown) attached to these terminals are so wound and arranged that the motor operates with 32 field poles for slowest motor speed when the terminals Tl are energized, while the winding connections appertaining to the terminals T2 produce 16 field poles and hence, when energized, cause the motor to operate at twice the above-mentioned speed (second speed), the terminal group T3 energizes the motor field for 8 poles in order to operate the motor at four times the minimum speed (third speed); and the terminal group T4 is attached to field windings for l-pole operation to operate the motor at maximum speed (fourth speed), which is eight times as high as the minimum speed. The terminal groups are energized, one at a time and in sequential order, from line terminals Ll, L2 and L3 through a main switch S which remains closed during the performance of the system. The connection between the line terminals and the motor terminals is controlled by the contacts I and 2 of a hoist contactor H whose control coil is denoted by 3, and by the contacts 4 and 5 of a lowering contactor L whose control coil is denoted by 6. The function of contactors H and L is to reverse the phase sequence of the energizing voltage in order to permit operating the motor in the hoist or lowering (heave-in or pay-out) directions. The contactors H and L are so controlled that only one of them can close its contacts at a time.

The selection of the motor terminal group to be energized at a time is eflected by means of speed control contactors denoted by 1M, 2M, 3M and 4M, respectively. Contactor IM has a main contact device, which includes contacts i, 8 and 9, for connecting the terminal group TI to the line terminals. Contactor IM has also two interlock contacts III, II and a control coil l2. The contactor 2M has two contacts l3, ill for shorting the terminal group Ti when the main contacts l5, l6 and I! of the cont actor are closed for connection of terminal group T2 to the line terminal. These contacts, together with an interlock contact I8, are controlled by a coil 19. The contactor 3M has main contacts 20, 2| and 22 which, when closed, connect the mo tor terminals T3 to the line terminals, contacts 23 and 24 of cont actor 3M for short-circuiting the terminal group T4 when the main contacts are closed, and two interlock contacts 25 and 26, all contacts of contactor 3M being controlled by a coil 21. Contactor 4M has main contacts 28 29, 30 and an interlock contact 3! controlled by a coil 32. Contacts 28, 29 and 30, when closed, connect the motor terminals T4 to the line terminals.

The above-mentioned contactors form part of a control system whose primary control device consists of a master controller C, here exemplified as being of the drum type and illustrated in developed form. The master controller has a group of contact fingers, such as those denoted by 4|, for engagement by groups of contact segments denoted by 42, 43, 44 and 45. The controller is shown in its off position. It has 4 lowering positions and 4 hoisting positions for operating the motor in either direction at the 4 different speeds above mentioned. An emergency switch 46 is disposed close to the master controller, so that it can be operated by the attendant at any time during the actuation of the controller. The emergency switch 46 may consist of a. thumb contact in the handle for turning the contact drum of the master controller. The system is energized through the emergency switch 46 and becomes deenergized to stop the motor when the switch is opened.

The excitation of the control system is further dependent upon the operation of a low-voltage relay V whose coil 41 maintains its relay contacts 48 and 49 in closed position as long as the master controller is turned away from its off position. Upon occurrence of voltage failure at the line terminals, however, the relay V will drop out and then also deenergize the control system and stop the motor. Thereafter the relay V will pick up only when the master controller is first turned back to the off position.

The control system is equipped with two control relays 3GB and 40B in conjunction with respective timing relays 2'1 and 3T. This group of relays serves to secure a proper timing during accelerating and decelerating performance so that each following terminal group becomes energized only after the preceding terminal group has been energized a given period of time, thus limiting transitory overloads and securing a smooth speed transfer regardless of the speed at which the master controller is moved from one to the next position. The control relay 3CR has three contacts 50, 5| and 52 controlled by a coil 53 whose circuit extends through the normally closed contact 54 of the timing relay 2T. The coil 55 of relay 2T is connected to the output terminals of a rectifier 5'! and the relay magnet coil is equipped with a short-cirouited winding 58 so that after deenergization of coil 55, the relay will drop out only upon the elapse of a time interval, for instance, of about three-quarters second. The control relay 40R has three contacts 58, 59 and 60 controlled by a coil 6| whose circuit extends through the contact 62 of timing relay 3T. Relay 3T has a main coil 63 connected to a rectifier 64 and a short-circuited coil 65 to operate with a delayed drop-out. The timing period of relay 3T is preferably somewhat shorter than that of relay 2T, for instance, approximately half a second.

A load-responsive relay LB, with a contact 66 and a coil 6'1, is so arranged that the coil 61 is traversed by motor current when the terminal group T3 is energized. The coil 61 will then open the contact 66 if the motor current at the third speed exceeds a given value. This has the elTect of preventing a transfer of the motor to the highest motor speed as long as the overload current persists.

The system, as a whole, operates in the following manner.

When the master controller C is in the illustrated off position, the closing of switches S and 46 has the effect of energizing the voltage relay V in the circuit:

LL! 1- 1 8-3 l--l 2-44-5U-10-48-49-46L3 (2) Contactor IM picks up and prepares at contacts I, 8 and 9 an energizing circuit for motor terminals Tl. This circuit, however, remains open as long as neither contactor H nor L is energized.

' The contact In of contactor IM closes an energizing circuit for coil 55 of relay ET; and coil 63 of relay ST is also energized through the contact 25 of contactor 3M. Consequently, the two timing relays 2T and 3T pick up. Relay 2T opens its contact 54 and relay 3T opens its break contact and closes its make contact.

If now the master controller 0' is turned from off to first point hoist, the contactor H is energized in the circuit:

L2-I I-ZG-II-I 3-44-50-1 -48-49-4 6-L3 (4) The motor is now energized at terminals T2 for second speed. The interlock contact ll! of contactor IM opens and deenergizes coil 55 so that timing relay 2T drops out and closes its contact 54, after elapse of its timing period of about three-quarters second.

When the master controller is advanced to third point hoist and after the relay 2T has dropped out, coil 53 of control relay 3GB becomes energized in circuit:

L2-1l-53-54-42-48-49-46-L3 Contact 50 of relay R now interrupts the coil circuit (4) of contactor 2M so that the terminal group T2 becomes disconnected, while contact 5| of relay 30R closes for coil I'Zl of contactor 3M the circuit:

L2-! 1- l 8-3 [-2 1-58-5l-1 0-4 8-4 9-4 6-13 (6) Contactor 3M picks up and connects motor terminals T3 to the line for third motor speed. The

interlock contact25 of contactor 3M deenergizes the coil 63 so that the timing relay 3T drops out upon elapse of its timing interval and then prepares a circuit for the coil 6| of relay R.

When the master controller is advanced to the fourth point hoist and assuming that the relay 3T was given sufficient time to drop out, the control relay 40R becomes energized in circuit:

L2-I l-B 1-52-12-68-1 3-4 [-4 2-4 8-4 9-4 6-13 (7) Contact of relay 4CR closes and thereby completes a self-holding circuit in parallel to the timing relay contact 62. Consequently, relay 40R will remain picked up as long as the master controller remains in the fourth position hoist even if the timing relay 3T is subsequently caused to reopen its break contact at 62. Interlock contact 59 of relay 40R interrupts the coil circuit (6) of contactor 3M- so that terminals T3 become deenergized. Contact 58 of relay 40R. closes for coil 32 of contactor 4M the circuit:

L2-1 l-26-l [-32-58-10-48-49-46-13 (8) Contactor 4M connects the terminals T4 to the line terminals for operating the motor at maximum speed.

Interlock contact 3| of contactor 4M then opens the coil circuit (6) of contactor 3M. Contactor 3M drops out and, by closing its interlock contact 25, reenergizes the rectifier 64 so that timing relay 3T picks up and opens its brake contact at 62, This, as explained above, is now without effect on relay 40R and contactor 4M because relay 4CR remains sealed in at contact 60 as long as the master controller is kept on fourth position hoist. In this manner, the timing relay 3T is reset for a subsequent timing operation to take place when the master controller is turned back for deceleration of the motor.

It will be recognized from the foregoing that when the master controller is progressively ad- 6 vanlced from the "off position to the fourth point hoist, the timing relays 2T and 3T by virtue of their delayed drop-out performance determine the minimum interval of time that elapses between the initial energization of terminals T2 and that of terminals T3, and between the moments of initial energization of terminal groups T3 and T4. Consequently, when the master controller is moved quickly from off to fourth point hoist, the accelerating performance remains properly controlled by the timin relays and the associated other relay circuits. For instance, when the master controller is rapidly moved from off to fourth point hoist, contactor H picks up, contactor iM drops out and contactor 2M connects the motor for second speed. The interlock contact 10 then deenergizes the timing relay 2T, so that this timing relay drops out after about three-quarters second and then energizes the control relay 30R. This relay then energizes the contactor 3M for third speed and the interlock contact 25 deenerglzes the timing relay 3T. After one-half second, relay 3T drops out and energizes relay 40R which, in turn, trips the contactor 3M and picks up the contactor 4M for highest motor speed. With this performance the motor is caused to run first three-quarters second at second speed and thereafter one-half second at third speed before it is permitted to assume maximum speed.

When the master controller is suddenly move from fourth poin t hoist to first point hoist, the first action of the control system is to interrupt the coil circuit of relay 40R. This relay deenergizes contactor 4M and energizes the contactor 3M, thus transferring the motor from fourth speed to third speed. As the contactor 3M picks up, the time relay 3T is deenergized and drops out after elapse of its timing period of halfa second. Then the contact 62 interrupts the coil circuit of relay 30R. Relay 30R, at contact 5|, opens the coil circuit of contact-or 3M and energizes, at contact '50, the coil if! of contactor l M. As a result, the motor is transferred from third speed immediately to first speed, and then continu'es runin g at lowest speed.

When the master controller is suddenly moved from fourth *poinrt hoist to off, all cont actors drop out and the motor is brought to a stop.

The operation of the system in the lowering direction is similar to the above-described hoisting operations, except that the contactor L closes instead of the contactor H, causing the winch motor to run in the opposite direction.

In the event that the current in the load relay LR exceeds a .predetermined value when the motor is running at third speed hoist, the contact 66 opens and prevents relay 40R from transferring the motor from third to fourth speed. When loads are hoisted that are in excess of a given load, for instance somewhat above one-half rated maximum load, the motor will not come up to fourth speed. However, when the motor is opera-ted in the lowering direction, the contacts 66 of the load relay LR is bypassed in the master controller so that the system permits high speed lowering of all loads.

Reverting to the above-described functioning of the timing relays 2T and 3T, it will now be observed th-at the timing performance of these relays or either of them comes into effect not only during acceleration but also during deceleration of the motor and that the associated control circuits are so designed that in either case the delayed drop-out performance of the timing relays determines the maximum speed of transfer.

The embodiment of a motor control system for a marine winch shown in Fig. 2 is largely similar to that of Fig. 1 with the exception of the system elements specifically described hereinafter. In order to make the similarities apparent and to obvi'ate'the necessity of again describing them in detail, the contact-01's, relays, motor, brake, win-ch drum, and master controller in, Fig. 2 are denoted by the same unit characters as the corresponding system elements in. Fig. '1, namely, by H, L, IM, 2M, 3M, 4M, V, LR, 2T, 36R, 3'1, 401%, M, B, D and C. For the same reasons, the individual parts of these devices are designated in Fig. 2 by the same reference numerals as in Fig. 1, except that in Fig. 2 the prefix 1 or 10 has been added. For instance, the contacts IOI, I02 in Fig. 2 correspond to the contacts I, 2 in Fig. 1; and the contacts I I3, I M in Fig. 2 correspond to the contacts I3, I4 in Fig. 1.

The difierences of the system shown in Fig. 2 ascompared with the system of Fig. 1.1 are essentially as follows.

The hoist contactor I-Iv in Fig. 2 is provided with an additional interlock contact 8I, the lowering contactor L with additional interlock contacts 82, 83 and the relay 30R with an additional interlock contact 84. The function of these interlock contacts is to provide electric interlocking aside from the interlocking occurring in the master controller.

In further distinction from the system of Fig. 1, the timing relays shown in Fig. 2 are illustrated as being connected to a suitable source of direct current which is represented merely by its positive and negative terminals marked EB and G, respectively. It Will also be noted that the master controller in Fig. 2 is not equipped with a contact segment for bypassing the contact I66 of the load-responsive relay LR. Consequently, this relay is capable of performing its protective function during hoisting, as well as during lowering performances of the system.

The above-mentioned differences are of minor nature, and they do not essentially alter the performance of the system. Therefore, the operation. of the system when the master controller is moved, for instance from the off position, to

first, second and third points hoist is essentially in accordance with the above-given description of the corresponding operating sequence for the system of Fig. 1. That is, the foregoing description of the sequential occurrence of the circuits (1) through (6) is substantially applicable to the system of Fig. 2, provided the analogous reference numerals of Fig. 2 and the above-mentioned additional interlock contacts are considered. However, the system shown in Fig. 2 is also characterized by more essential differences over the system of Fig. 1, and these difierences account for modified performance when the master control is moved from third to fourth position hoist or lower. as will be explained presently.

In the system of Fig. 2, the coil I61 of the loadresponsive relay LR is so connected in the ener gizing circuit of motor M that the coil is traversed by load current only when the terminal group T6 of the motor is energized by the contactor AM. As a result, the relay LR cannot interfere with the transfer of the motor from third to fourth speed but becomes effective only after the motor has been switched over to maximum speed and only when at that time an excessive load condition exists. Associated in functional respects with the load relay LR is a timing relay IT with a main coil 81, a short-circuitedcoil 88 and a contact 89, and also a control relay 50R with two contacts 90, BI and a control coil 92. Due to the just-mentioned connection of coil I61 in relay-LR. and due to the operation of the additional relays IT. and 50R, the control system operates in the following manner when the master controller is moved, from third to the fourth position, for instance, in the hoisting direction. Relay 40R picks up because its coil I 6 I become energized in the circuit:

Contact, I58 of relay 40R closes for coil I32 of contactor 4M the circuit:

Contactor 4M now connects motor M for operation. at fourth speed, while contactor 3M drops out because its coil circuit, analogous to circuit (6), is opened at contact I31 of contactor 4M. Timing relay IT, previouslyenergized, drops out after elapse of its timing period because the circuit of its coil 8'! is interrupted at contact 8-6 of relay 40R. When at that moment the load current of motor M is below the safe limit, relay LR remains dropped out with contact I66 open, and nothing further happens. When, however, the load current is excessive, so that relay. LR. is picked up and contact I 66 closed when time relay IT closes its contact 89, the following circuit is completed for coil 92 of relay 50R:

Relay 50R picks up and, at contact 98,0pens the coil circuit (7a) of relay 4CR while sealing itself in at contact 9I. Relay lCR when dropping-out, opens at contact I58 the coil circuit (8a of contact 4M and closes, at contact I59, the circuit of coil I21 so that contactor 3M picks up. Thus, the motor M is automatically transferred back to third speed. When thereafter the load current of motor M drops to a permissible value, the contactor 4M can be made to pick up again for operating the motor at maximum speed by turning the master controller C first back tothird position hoist. 'This opens the holding circuit of relay 5GB and returns the system into the condition obtaining when the master controller C is moved from second to third point hoist. Then, the return of the master controller 0 to fourth point hoist will cause the contactor 4M to pick up.

The timing period of relay IT is so chosen that it exceeds the duration of the load peaks normally occurring when the motor is transferred from third to fourthspeed. Consequently,- if these load peaks should temporarily cause the relay LR to pick up while the timing relay IT holds its contact BIG open, the temporary closing of the load relay contact i615 is of no effect on the coil circuit (9) of relay 50R so that the motor M' is kept running at maximum speed.

It will be understood by those skilled in the art that control systems according to the invention can be altered and modified in various respects without departing from the essence of the invention and Within the scope of'the essential features of the invention, as set forth in the claims annexed hereto.

We claim as our invention:

1. A motor control system, comprising a multispeed alternating-current motor having a plurality of selectively energizable field terminal groups for correspondingly different operating speeds, circuit means for supplying energy to said motor, reversing contactor means disposed between said circuit means and said terminal groups for selectively controlling said motor to run in hoisting and lowering directions respectively, a plurality of speed-controlling contactor means disposed between said reversing contact means and said terminal groups for selectively connecting said respective terminal groups to said circuit means, motor control means having selective contact devices and being operator-adjustable between an ofi setting and a plurality of hoist settings and a plurality of lower settings, said master controller being connected with said reversing contactor means and said speed-controlling contactor means for sequentially operating said speed-controlling contactor means for either selection of said reversing contactor means, a load-responsive relay having a coil connected between said reversing contactor means and one of said terminal groups for a given lower speed so as to be responive to motor current flowing when said latter terminal group is energized and having a contact connected to the speed-controlling contactor means for the next higher speed so as to prevent said latter contactor means to energize said motor for said higher speed when said current exceeds a given maximum value, said master control means being connected with said contact so as to render it inoperative when said master control means are set for lowering operation.

2. A motor control system, comprising a multispeed alternating-current motor having a plurality of selectively energizable field terminal groups for correspondingly different operating speeds, circuit means for supplying energy to said motor, reversing contactors having respective contact devices series-disposed between said circuit means and said terminal groups for selectively controlling said motor to run' in hoisting and lowering directions respectively, a plurality of speed-controlling contactors having respective contact devices series-disposed between said contact devices of said reversing contactors and said terminal groups for selectively connecting said respective terminal groups to said circuit means, an operator-adjustable master controller having a plurality of selective hoist positions and a plurality of selective lower positions, all of said contactors having respective control coils connected with said master controller so that said speed-controlling contactors are sequentially operated together with either reversing contactor when said master controller is progressively adjusted through said hoist and lower positions respectively, a load-responsive relay having a coil circuit connected between said contact devices of said reversing contactors and the terminal group for second highest motor speed and having a break contact series-connected between said master controller and the coil of the speedcontrolling contactor for highest motor speed so as to prevent said latter contactor from energizing said motor for highest speed when the motor current flowing at the second highest speed exceeds a given value, said master controller having contact means disposed to short-circuit said break contact when said master controller is in lower positions.

3. A motor control system, comprising a multispeed alternating-current motor having a plurality of selectively energizable field terminal groups for correspondingly different operating speeds, circuit means for supplying energy to said motor,

reversing contactor means disposed between said circuit means and said terminal groups for selectively controlling said motor to run in hoisting and lowering directions respectively, a plurality of speed-controlling contactor means disposed between said reversing contact means and said terminal groups for selectively connecting said respective terminal groups to said circuit means, master control means having selective contact devices and being operator-adjustable between an oil setting and a plurality of hoist settings and a plurality of lower settings, said master controller being connected with said reversing contactor means and said speed-controlling contactor means for sequentially operating said speed-controlling contactor means for either selection of said reversing contactor means, a loadresponsive relay having a coil connected between said reversing contactor means and one of said terminal groups for a given lower speed so as to be responsive to motor current flowing when said latter terminal group is energized and having contact means'connected to the speed-controlling contactor means for the next higher speed so as to prevent said latter contactor means to energize said motor for said higher speed when said current exceeds a given maximum value, and circuit means connected with said relay and controlled in dependence upon the selected condition of said master controller so as to render said relay effective as regards the load-responsive control when said master control means are in hoist condition and inefiective when said master control means are in lower conditions.

4. A motor control system, comprising a multispeed alternating-current motor having a pluralit of selectively energizable field terminal groups for correspondingly difierent operating speeds, circuit means for supplying energy to said motor, a plurality of'contactors having respective contact devices for selectively connecting said terminal groups to said circuit means and having respective coil circuits for closing when energized said respective contact devices so that the motor speed is determined by the one contactor coil circuit energized at a time, an operator-actuable master-controller connected to said coil circuits for sequentially energizing them one at a time, relay means comprising a timing relay of substantially instantaneous pick-up and delayed dropout performance and being connected with two of said contactors for controlling their respective coil circuits so that when said master controller is actuated to successsively energize the coil circuits of said two contactors the minimum time elapsing between the successive energizing moments of said two respective contactor coil circuits is determined by the delay period of said relay drop-out performance regardless of whether said master controller is actuated in the accelerating or decelerating direction.

5. A motor control system, comprising a multispeed alternating-current motor having a plurality of selectively energizable field terminal groups for correspondingly different operating speeds, circuit means for supplying energy to said motor, a plurality of contactors having respective contact devices for selectively connecting said terminal groups to said circuit means and having respective coil circuits for closing when energized said respective contact devices so that the motor speed is determined by the one contactor coil circuit energized at a time, an operator-actuable master controller connected to said coil circuits for sequentially energizing'them one at a time, a

timing relay having-a coil circuit and a contact device and being of the'type having substantially instantaneous pick-up and delayed drop-out performance, said relay coil circuit being electrically interlocked with one ofsaid contactors so that 'said relay is picked uponly when the coil circuit of said one contactor is deenergized, and circuit means controlled by said relay contact device and connected to the coil circuit of said one contactor and to the coil circuit of another one of said contactors so that, when said master controller is actuated to successively energize the coil circuits bf said latter two contactors, said timing relay picks up before the energization .of the first energized contactor coil circuit and drops out before the energization of the subsequently energized contactor coil circuit, whereby the minimum time elapsing between the successive energizing moinents of said two respective contactor coil circuits is determined by the delay period of said relay drop-out performance regardless of whether said master controller is actuated in the accelerating or decelerating direction.

6. A motor control system, comprising a multispeed alternating-current motor having a plurality of selectively -energizable field terminal groups for correspondingly different operating speeds, circuit means for supplying energy to said motor, a plurality of contactors having respective contact devices for selectively connecting said terminal groups to said circuit means and having respective coil circuits for closing when energized said respective contact devices so that the motor speed is determined by the one contactor coil circuit energized at a time, an operator-actuable master controller connected to said coil circuits for sequentially energizing them one at a time, a timing relay of substan. tially instantaneous pick-up and delayed dropout. performance having a coil circuit and contact means, said relay coil circuit being interlocked with one of said contactors so as to be energized only when the coil circuit of said one contactor is deenergized, a control relay having a coil circuit which is controlled by said contact means and extends through said master controller so as to be energized when said, timing relay is dropped out while said master controller is in position to energize the coil circuit of another one of said contactors, said control relay having a normally open self-holding contact connected across said contact means and having a normally closed interlock contact disposed in the coil circuit of said one contactor and a normally open. interlock contact disposed in the coil circuit of said other contactor, whereby the minimum time elapsing between the successive energizing moments of said two respective contactor coil circuits is determined by the delay period of said relay drop-out performance regardless of whether said master controller is actuated in the accelerating or decelerating direction. I

7; A motor control system, comprising an alternating-current motor having four terminal groups for operating at four different respective speeds, line terminals, four speed-controlling contactors having respective contact devices for selectively connecting said terminal groups to said line terminals and having respective coil circuits for closing when energized said respective contact devices, a master controller con nected to, said coil circuits for sequentially energizing them one at a time, a first timing relay and a second timing relay both having substantially instantaneous pick' up and delayed drop-out performance, the delay period of said first timing relay being longer than that of said second timing relay and each timing relay having a coil. circuit and contact means the coil circuit of said first timing relay being interlocked with the contactor for lowest motor speed so as to be energized when the coil circuit of said contactor is energized, a first control relay having a coil circuit controlled by said contact means of said first timing relay so as to be energized when said first timing relay is dropped out and having contact means connected in the coil circuit of another one of said contactors for a higher motor speed to permit energization of said latter coil circuit through said master controller only after said first timing relay has dropped out, the coil circuit of said second timing relay being interlocked with said latter contactor so as to be energized only when said latter coil circuit is deenergized, a second control relay having a coil circuit which is controlled by said contact means of said second timing relay and extends through said master controller so as to be energized when said second timing relay is dropped out while said master controller is in position to energize the coil circuit of a third one of said contactors for a still higher motor speed, said second control relay having a normally open self-holding contact connected across said contact means of said second timin relay and having a normally closed interlock contact disposed in the coil circuit of said second contactor and a normally open interlock contact disposed in the coil circuit of said third contactor.

8. A motor control system, comprising an alternatingwurrent motor having terminal groups for operating at different respective speeds, line terminals, speed-controlling contactors having respective contact devices for selectively connecting said terminal groups to said line terminals, control means including a selectively operable master controller and being connected r to said contactors for sequentially closing them one at a time in order to thereby control the motor speed, a load-responsive relay having i contact means and a control coil, said coil being connected between one of said terminal groups for a high motor speed and the contactor appertaining tosaid one group so that said coil is energized to actuate said contact means when the current flowing through the motor at said higher motor speed exceeds a given value, said contact means being associated with said control means so as to open said one contactor and close another one of said contactors for a lower motor speed when said contact means are actuated by said coil, and timing means connected with said load-responsive relay so as to maintain said contact means inefiective a given interval of time after the closing of said one contactor.

9. A motor control system, comprising an alternating-current motor having terminal groups for operating at different respective speeds, line terminals, speed-controlling contactors having respective contact devices for selectively connecting said terminal groups to said line terminals and having respective coil circuits for closing when energized said respective contact devices, a master controller connected to. said coil circuits for sequentially energizing them'one at a time, a control relay having a normally closed contact series-disposed in the coil circuit 13 of the one contactor for highest motor speed, said control relay having a control coil and a normally open self-holding contact series-connected with each other and connected to said master controller so that said coil is energizable when said master controller is set for highest motor speed, a load-responsive relay having a coil connected between the one terminal group and the appertaining contactor for highest motor speed to be energized by motor current flowing when said motor is set to run at said highest speed, said load-responsive relay having a normally open contact controlled by said latter coil to close when said current exceeds a. given value, a timing relay having a coil circuit interlocked with said one contactor for highest motor speed so as to drop out upon elapse of a timing period after said one contactor has closed, said timing relay having a normally closed contact series-connected with said normally open contact of said load-responsive relay across said self-holding contact whereby said control relay causes opening of said one contactor when said load-responsive relay responds to excessive current after elapse of said timing interval, and circuit means linking said one contactor with the one other contactor for the next lower motor speed so as to cause closing of said other contactor when said one contactor is opened due to excessive current.

KURT MAI-INKE.

JAMES W. YORK.

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