US2298821A

US2298821A - Control system

Info

Publication number: US2298821A
Application number: US426735A
Authority: US
Inventors: James W Cooke
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1942-01-14
Filing date: 1942-01-14
Publication date: 1942-10-13
Anticipated expiration: 1959-10-13

Description

Oct. 13, 1942.

J. W. COOKE CONTROL SYSTEM Filed Jan. 14, 1942 Fig. 2.

2 Sheets-Sheet Inventor'- James VV. Cocke,

His Attorney.

Patent ed Oct. 13, 1942 CONTROL SYSTEM James W. Cooke, Schenectady, N. Y., assignmto General Electric Company, a corporation of New York Application January 14, 1942, Serial No. 426,735

14 Claims.

This invention relates to control systems, more particularly to systems for controlling the opera.- tion of electric motors, and it has for an object the provision of a simple, reliable, and improved control system of this character.

More specifically, the invention relates to motor control systems in which the motor is subjected to overhauling loads and in which a power type limit switch is employed to interrupt the power circuits of the motor. A crane hoist is a good example of an installation of this character. In a crane hoist, the hoist is usually driven by a series direct current motor, .and a power type limit and for establishing a loop dynamic braking circuit in which a dynamic braking resistor is included when the switch is tripped from one position to another by the crane hook block as it approaches its upper limit of travel. The

,characteristics when the hook is being lowered out of the limit switch zone at the time the motor is first energized for lowering until the limit switch is reset to its normal position as the hook reaches the limit end of the switch zone in the lowering direction.

In normal operation, crane hoist controllers used with direct current series motors provide for connecting the motor as a series machine in the hoist direction and as the equivalent of a shunt machine in the lowering direction. The shunt circuit employed for lowering develops either a motoring torque for driving down an empty hook or a dynamic braking torque for retarding an overhauling load. The change from one torque to the other is inherent in the system itself.

However, within the limit switch zone, the

contacts of the power type limit switch so change the connections between the motor armature and series field winding that a straight series. circuit with the armature and field winding limit switch is provided for interrupting thepower circuits of the motor at the overhoist,

loop circuitis established in the lowering direction as long as the limit switch is tripped, i. e.

while the hook block remains in the limit switch zone.

When both the armature and field winding of a series motor are shunted, only series operating characteristics are produced and no dynamic braking torque is available to retard the speed of an overhauling load. Since the pull of the load and the torque of the motor are in the same direction, the hook block and motor can attain an excessively high speed in coming out of the limit switch zone between the time that the motor is first connected to the power supply and the time that the limit switch resets.

Furthermore, the light hook hoisting speeds of modern cranes are being greatly increased. At the present time, light hook hoisting speeds of the order of 250 per cent of rated full load speed are quite common, and some cranes are being operated at speeds as high as 300 per cent of rated full load speed. These high speeds have made it necessary to increase the amount of dynamic braking resistance which isinserted in the loop dynamic braking circuit by the limit switch when the hook reaches the overhoist limit. Since this dynamic braking resistor is shunted around the motor armature and series field winding when the lowering connections are established by the master switch, a correspondingly increased amount of current is passed through the motor, a correspondingly increased motor torque is developed, and the speed of the motor in the lowering direction is correspondingly inshunted by the dynamic braking resistance of the 55 creased. When the descending load reaches the position at which the limit switch resets, the equivalent of a shunt motor connection is established. Consequently, it the motor has attained a relatively high speed, e. g. 250 per cent of rated full load speed,.or higher, in coming out oi the limit switch zone, its speed will suddenly he reduced when the limit switch is reset. This sudden change in speed subjects all the mechanical parts of the crane to a severe shock which is highly objectionable.

Accordingly, an additional object of this invention is the provision of means for limiting the speed of the motor to a safe value while lowering out of the limit zone.

The hoist motor, brake, contacts of the power type limit switch, and the loop dynamic braking resistor controlled by the limit switch are located on the trolley motion of the crane. The balance of the equipment such as contactors, relays, switches, and speed controlling resistors,

are moimted on a panel which is located on the bridge structure .of the crane. Consequently, trolley wires and cooperating collector shoes are required totransier powerbetween the control apparatus panel on the bridge and the motor, solenoid brake, and limit switch on the trolley motion structure. It is-desirable thatthe number of trolley wires bekept at a minimum. It is also desirable from the point of view of mainten ance and reliability to mount as much of the electrical equipment as posible on the panel which is located on the bridge where it is better protected and more accessible for inspection and. repair than would be the case if it were mounted on the trolley motion structure.

Another object of the invention, therefore, is the provision of means for limiting the speed while lowering out of the limit zone, which can be mounted on the control panel and which is connected in such a manner that the number of trolley wires required is kept at a minimum.

In carrying the invention into effect in one form thereof, means controlled by operation of the limit switch and responsive to a terminal electrical operating condition of the motor following operation of the limit switch are mounted on the control panel and serve to establish a dynamic braking circuit for the motor while the motor is lowering out of the limit zone and automatically to interrupt this dynamic braking circuit as a result of the resetting of the limit switch as the hoist lowers out of the limit switch zone.

Another aspect of this invention is the provision of means for preventing oscillation of the hook in the overhoist limit zone. Heretofore crane hoist controls have been provided with a spring closed contactor for establishing an emergency dynamic braking circuit in shunt with the motor armature and field winding to provide slow speed lowering in the event of power failure ;or failure of the solenoid brake to set. In order "to obtain improved speed regulation, it has been customary to maintain this emergency dynamic braking contactor closed on one or more low 'speed points of the master switch. This shunted "motor connection introduces an undesirable operating condition in that it establishes a by-pass or sneak circuit around the motor armature and series field winding which keeps the series brake released even though the limit switch has oper- Consequently, if the load is overhauling,

crane hook is set up which will continue until the operator returns the master switch to the off position in which the brake can set and stop the hoist. This condition is, of course, very undesirable because it is extremely disconcerting to an operator to observe that the load is being lowimproved speed regulation'on the low speed hoist positions of the master switch. In addition, it

. provides dynamic braking of the hook in the low speed hoisting positions of the master switch and thereby eliminates coasting of an empty or lightly loaded hook in the hoisting direction after the master switch has been notched back from a high speed position to a low speed position.

For a better and more complete understanding of the invention, reference shouldnow be had to the following specification and to the accompanying drawings in which Fig. l is a simple, diagrammatical illustration of the master switch and control circuits controlled thereby; Fig. 2 is a simple, diagrammatical sketch of the power circuits and of the contacts-therein controlled by the master switch and control circuits of Fig. l; and Fig. 3 is a simple, diagr-ammatical sketch of a crane hoist cable and power type limit switch operated thereby.

Referring now to the drawings, the hook It of a crane hoist is connected to a cable' H which is wound upon a drum I 2. The hoist cable drum two supply lines it and it. A pair of reversing contactors it and I? is provided for connecting the hoist motor to the power supply for rotation in either the hoisting or lowering direction. These contactors have operating coils lta and Na, respectively, as indicated in Fig. 2. The operating coils ita and We are shown included in their controlling circuits in Fig. 1. When contactor i6 is closed, the motor is is connected. to the source iii, for rotation in the hoisting direction, and when contactor H is closed, the motor is connected to the source for rotation in the lowering direction.

. Referring to Fig. 1, the operating coils Na and Na of the directional contactors are under the control of a multiposition, reversing type master switch it. The master switch it is illustrated as having fiv hoist positions and five "lower positions. A larger or smaller number of operating positions for each direction of rotation may be provided if desired.

ered although the master switch is in a hoisting position, and also because with heavy loads this oscillation produces high and unnecessary strains in the apparatus.

.; Heretofore, additional apparatus has been required to correct this condition. The present invention safely limits the speed of the motor while I lowering out of the limit zone'and also makes it possible to obtain during normal operation the improved speed regulation which is desired without maintaining the emergency dynamic braking The hoist motor 83 is provided with a solenoid brake (not shown) having its operating soleture and field winding ofmotor l3 through a resistor 25 in the first or lowest speed position of the master switch in the lowering direction for the purpose of providing a desired slow lowering speed. This contactor is also closed in the ofl position of the master switch. As shown, contactor 2:; is provided with an operating coil 24a which when energized serves to open the contactor against the tension of its closing spring A plurality of accelerating resistors 25 and 21 X are connected in series with the armature of motor I3. These resistors are controlled by means of accelerating

contractors

23 and 29 having operating'coils 23a and 23a, respectively. The control circuit for the operating coils 28a and 29a are shown in Fig. l.

A power type overhoist limit switch 30 is provided for disconnecting the motor l3 from its power supply at a predetermined position of the hook block in order to prevent overtravel in the hoisting direction. This limit switch is illusfrated in Fig. ,3 as comprising a plurality of pairs of stationary contacts 3|, 32, 33 and 34 and a movable arm 30a carrying a plurality of pairs of movable contacts which cooperate with the stationary contacts. A projecting member 35 secured to the cable I serves to actuate the movable contact carrying member 33a. The contacts 3| and 32 are normally closed and the

contacts

33 and 34 are normally open except when the hook block reaches its overhoist limit and the member 35 actuates the movable member 30a to open contacts 3| and 32 and to close

contacts

33 and 34. The hoist motor l3, solenoid brake I3, and limit switch 30 are mounted on the trolley motion structure, and the various resistors and contactors described in the foregoing are mounted on a main magnetic control panel which is usually located onthe bridge structure. The master switch I8 is of course also mounted on the bridge structure. Thus it becomes necessary to make the power connections between the motor and brake and the panel by means of trolley wires and collector shoes.

These trolley wires and collector shoes are indicated by

reference characters

35, 31, 33 and 33 in Fig. 2.

For the purpose of limiting the speed of the motor when lowering out of the limit switch zone after the limit switch has been tripped, means comprising a control relay 40, a contactor 4|, and a dynamic braking resistor 43 are provided.

These parts are preferably mounted on the con-.

trol panel on the bridge structure where they are accessible for inspection and repair. When the contacts of contactor 4| are closed, the dynamic braking resistor 43 is connected in a shunt circuit to the armature terminals I30 and |3d of the hoist motor |3. As shown in Fig. 2; one terminal of the dynamic braking resistor 43 is connected through conductor 44, trolley 36 to the armature terminal I30, and the other terminal of resistor 43 is connected through the contacts of contactor 4| to the opposite armature terminal l3d. One additional trolley wire 45 is required to complete the connections between the resistor 43 through contacts of contactor 4| to the armature terminal l3d. The operating coil of relay 40 is connected from the conductor 44 t the conductor 45. The

conductors

44 and 46 are on the control panel on the bridge. It will be observed in Fig. 2, that conductor 44 leads through trolley 36 and limit switch contact 32 to one terminal of the series field winding I34. and that the conductor 48 leads through trolley 31 to the opposite terminal of the series field winding |3a. Consequently, when the limit switch contact 32 is closed, the operating coil 40a of relay 40 is connected across the series field winding I311. The relay 40 is provided with two pairs of normally open contacts 40b and 400 and as shown in Fig. 1, the normally open contacts 4||c are connected in series with, the normally closed interlock contacts lib of the hoist contactor and the operating coil 4|a oi the dynamic braking contactor 4|. The relay 40 is so designed that the voltage drop across the field winding I30. is insufllcient to pick up relay 43.

Therefore when the operating coil 40a of relay 40 is connected across the field winding l3a, its contacts are open. Since the operating coil 4|a of contactor 4| is connected in series with the normally open contacts 400 of relay 40, the contactor 4| cannot be picked up when the operating coil 40a of relay 40 is connected across field winding |3a except in the first two positions hoist of the master switch. In these positions, the contactor 4| is picked up directly by a circuit completed through the master switch.

With the foregoing understanding of the elements and their organization in the control system, the operation of the system itself will read- I ily be understood from the following detailed description.

It is assumed that the hook is in its lower position and that the master switch I8 is in its central or off osition. The apparatus is all in the normalposition in which it is illustrated in .the drawings. and the hoist motor I3 is at standstill and the solenoid brake l9 set by its operating spring. In the off position of the master switch l8 an emergency dynamic braking circuit is established which is traced from the armature terminal |3d through limit switch contact 3|, dynamic braking resistor 25, normally closed contacts of emergency dynamic braking contactor 24, series field winding |3a, limit switch contact 32 to terminal |3c of the motor. Also,

in the off position of the master switch, the

op erating coils

41a, 48a, and 49a of

contactors

41, 43 and 49, respectively, are energized. The energizing circuits for operating coils 41a, 43a, and 49a are traced in Fig. 1 from the side 50 of a suitablesource of control voltage through conductor 5|, fingers I81, I87, and I8k, of master switch |'8, bridged by cooperating segments and

operating coils

41a, 48a, and 49a in parallel to the opposite side 52 of the source of control voltage. In this connection, it will be noted that the operating coil 48a of contactor" is energized and that the energizing circuit for coil 41a is completed when the contacts 48b of contactor 43 close.

As a result,

contactors

41, 48, and 49 are closed and resistors 20, 2|, and 22 are short circuited.

To hoist the hook and its load, the master switch is operated to its first position hoist.

In this position, the coil 54a of line contactor 54 is energized. The circuit is traced from the side 50 of the source of control voltage to power conductor 5|, fingers |8a of the master switch bridged by the cooperating segment on the hoist side of the master switch, conductor 55, coil 54a to the opposite side 52 of the control voltage source. Contactor 54 closes its contacts 54b in response to energization and connects one terminal of the hoist motor to the side |5 of the power supply.-

Simultaneously, the operating coil |6a of hoist contactor I6 is energized. The circuit for this coil is traced from power conductor 5| through fingers |8e of the master switch bridged by the cooperating segment, normally closed contacts 55b of a control contactor 56, operating coil |5a. oi the hoist contactor to the side 52 of the source. As a result, the hoist contactor closes its normally open contacts lib, I60, and ltd, and opens its closing complete the power circuit for the motor which is traced from the side H of the supply source through conductor 51, contacts lSb of the. hoist .contac'tor, accelerating resistors 2'! and 28, limit switch contacts 3|, armature of motor i3,'conductor 58, limit switch contacts 32, series field winding 13a, series brake operating solenoid l9, contacts 41b oficontactor ll, conductor 59, resistor 23, contacts 5 th of contactor 5% to the side It of the power supply. Contacts Ito of the hoist contactor in closing complete anenergizing circuit for the operating coil 26a of the emergency dynamic braking contactor. This energizing circuit is traced from the power con.-

' ductor 5i through fingers lBg of the master switch bridged by the cooperating segment on the, hoist side of the master switch, conductor 60, contacts ific of the hoist contactor, and through operating coil 2 3a to the side 52 of the control voltage source. In response to energization, contactor 26 opens its normally closed contacts 2% to interrupt the emergency dynamic braking circuit.

Also on the first point hoist of the master switch, an energizing circuit is completed for the operating coil i la of contactor il which is traced from the power conductor at through fingers of the master switch l8b bridged by the cooperating segment of the master switch, and through operating coil lla to the side 52 of the source.

Contactor ll closes its contacts ilb to complete a shunt circuit for the armaturev of motor it through resistor d3. As a result of this armature shunting circuti, the motor 53 rotates at a very stable, slow speed and hoists the load. It will be observed that the operating coil Ma of contactor ll remains energized on the second point hoist of the master switch, and consequently the armature shunting circuit through resistor (it remains intact on the second position hoist. It will thus be clear, that if the motor is do in the case of an empty or lightly loaded hook:

- To increase the speed, the master switch it is moved to its second position hoist. In this posiiton, a circuit is completed for the operating coil Sic of accelerating contactor 5!. This circult is traced from the power conductor til, through fingers i8h bridged by cooperating segments on the hoist side of the master switch, conductors t2 and 63, contacts ltd of the hoist contactor and through operating coil tic to the side 52 of the source. In response to energiza- Movement of the master switch to the fourth position hoist completes an energizing circuit for v the'operating coil 28:; of contactor 28. This cir'- cuitis traced from the. power conductor- '5! through fingers I81. of the master switch bridged by the cooperating segment 0n.'the hoist side, conductor 64, normally closed contacts b, op-

erating coil 28a of contactor'28 to the side 52 of the source. RelayBS is preferably a time delay relay which closes its contacts a predetermined interval of time after the 'deenergization of its operating coil 650 by the. opening of contacts Therefore the energizing circuit for the operating coil 28a is completed. at the expiration of this predetermined interval tic of contactor SI.

of time and the contactor closes its contacts 28b to short circuit resistor section 25 and further. Contactor 28 also closes its contacts 280 to prepare the ener-' increase the speed 01" the motor.

coil 29a of acthe projection 35 strikes the movable contact carrying am 3011 and moves it from the position shown to a position in which the contacts 3| and 32 are open and

contacts

33 and 36 are closed. Contacts 3i and 32 in opening disconnect the motor 53 from the source i6, i5 and contacts 33 and 3 3 in closing complete a dynamic braking circuit for the armature of motor it which is.

traced from the armatureterminal ltd through contacts 33, series field winding l3a, dynamic braking resistor 65, contacts 34 and conductor 53 through the left-hand terminal ltic of the motor armature. As a result of the establishment of this dynamic braking circuit, a large braking torque is set up which is eifective in rapidly re-' ducing the speed of the motor. In this connection it will be noted that the contacts 53 and 34 of the limit switch reconnect the series field winding ltia to the motor armature so that the current flows through the series field winding tion, contactor 6i closes its contacts bib to short circuit accelerating resistor-23. This results in increasing the speed of motor it.

In the third position hoist of the master switch, the energizing circuit for operating coil fiila is interrupted at the fingers i812. As a result of its deenergization, contactor M opens its contacts Mb to interrupt the armature shunting circuit through the resistor 43. This results in further increasing the speed of the motor it.

of the master switch, the opening of limit switch contacts 3i and 32 therefore interrupts the energization of the solenoid it of the solenoid brake and as a result, the spring of the brake sets the shoes against the drum andassists the dynamic braking torque of the motor in bringing the motor to rest.

Since the mechanical brake is set, the load will not descend, and consequently the undesirable oscillating condition described in the foregoing is completely corrected. Consequently, the special apparatus which was required to correct this condition in crane hoist controls used heretofore is entirely eliminated.

The opening of limit switch contacts 32 disconnectsthe left-hand terminal of the series field winding 83a from the motor armature terminal iSc. Conseqently, the operating coil etc of relay it is no longer connetced across the series field winding 53a but on the contrary, is

connected acrossthe armature and series field winding. The voltage drop across the armature and series field winding is suflicient to energize the operating coil 40a and consequently, the relay 40 picks up and closes its

contacts

40b and 40c. Contacts 400 in closing prepare an energizing circuit for the operating coil 4Ia of dynamic braking contactor 4| which is interrupted, however, at the interlock contacts lie of the hoist contactor.

To lower the hoist out'of the limit switch zone, the master switch must be moved to one of its lowering positions. The return of the master switch to the oil postiion restores the apparatus to the normal position shown in the drawings with the exception of the limit switch which remains in the tripped position.

Movement of the master switch to the first position lowering completes an energizing circuit for the operating coil 54a of contactor 54 which closes its contacts 54b to connect the junction point of the field winding Ila and dynamic braking resistor 65 to the side I of thev power supply. The

contactor

41, 48, and 49 remain picked up in the first position lowering. The energizing circuits for the

operating coils

41a, 48a, and 49a of these contactors are the same as traced for the hoisting operation. Consequently, resistors 20, 2| and 22 are short circuited on the first position" lowering. In response to the voltage drop across the armature of the motor I3 and the series field winding I30, the relay 40 is energized and closes its contacts 40b and 400. Also in the first position lowering of the master switch, the operating coil I1a of the lowering contactor is energized and the lowering contactor closes its normally open contacts I11) and H0. Contacts no in closing complete connections from the motor armature to the side H of the power supply. The motor circuit is traced from the side I4 01 the power supply through the line switch in the closed position thereof, contacts Nb of lowering contactor, trolley 36 and conductor 58 through commutating field winding |3b and armature of motor I3, limit switch contacts 33, conductor 65, series field winding I3a, brake solenoid I9, contacts 41b, conductor 59, and resistor 23 to the side I5 of the power supply. The operating coil 56a of the contactor 56 is also energized on the first position lowering of the master switch and the contactor closes its contacts 56b. However, this has very little effect on the operation of the mo- Ior at this point since the limit switch contacts 3| are still open.

On the second position lowering of the master switch, an energizing circuit is established for the operating coil 24a of emergency dynamic braking contactor 24, and in response to its energization, the contactor opens its contacts 24b. This, however, has substantially no effect on the speed of the motor when operating in the limit zone. As the master switch leaves its second position lowering, the energizing circuits for the relay 63 and contactor 41 are interrupted. In response to deenergizatlon, relay 53 opens its normally open contacts 3a and contactor 41 opens its normally open contacts 41b, thereby to insert resistance section 20 in the motor circuit. Contactor 41 in dropping out also closes its normally closed contacts 410 to prepare an energizing circuit for the operating .coil Sla of contactor 6|.

Between the third and fourthpositions lowering of the master switch, the fingers I8h are brid ed by the cooperating segments on the lowering side of the master switch further to prepare the energizing circuit tor the operating coil Ia of contactor 6|. However, this energizing circuit remains open at interlock contacts 48d of contactor 48.

Movement of the master switch to the fourth position lowering interrupts the connections 0! the operating coil 48a of contactor 48 through the fingers I87 of the master switch. However, the operating coil 48 a remains energized through the contacts b of relay 40. Consequently, movement of the master switch to the fourth and fifth positions lowering will have no further effect on the control as long as the relay 40 remains energized. Since the relay 40 will remain energized as long as the hook block is in the limit zone, movement of the master switch to the fourth and fifth positions lowering will have no effect on the control as long as the hook block remains in the limit zone.

When the master switch was moved to the first position lowering, the motor I3 was connected to the source with the armature in series with the field winding I3a. Although the loop circuit through the armature, field winding I311, resistor 55 and

limit switch contacts

33 and 34 provided a dynamic braking circuit when the motor was disconnected from the power supply by the tripping of the limit switch, dynamic braking of the motor is no longer available when the motor is reconnected to the power supply by the lowering contactor I1 because the motor armature and series field winding are connected in series to the source with the resistor 55 connected in parallel with the armature and field winding. Consequently, the only eiiect of the resistor 65 is to divert a portion of the current from the power supply from the armature and field winding. As a result of this loss of dynamic braking, the motoring torque and the overhauling torque produced by the load onthe hook are in the same direction. In the absence of any means to prevent it, it is possible for the moto I3 to attain an excessively high speed in the'lowering direction before the hook block reaches the limit and resets the limit switch 30. This is prevented, however, by the action of the relay 40, and the contactor 4| in conjunction with the normally closed interlocks of the hoisting contactor. when relay 40 in responding to the voltage across the armature and field winding |3a of the motor closed its contacts 400, it completed an energizing circuit ior the operating coil Me of contactor 4|. This circuit is traced from the power conductor 5| through fingers l8a. oi the master switch bridged by the cooperating segment on the lowering side, conductor 61, contacts 40c of relay 4D, conductor 68, normally closed interlock contacts Hie of hoist contactor I6, and thence through operating coil 41a of contactor 4| to the side 52 Of the source. I

Contact-or 4| in responding to energization closed its contacts 4Ib to complete a dynamic braking circuit throigh the resistor 43 in parallel withr the armature of the motor I3. The shunting of the armature of the motor I3 by the resistor 43 causes the motor to have shunt motor characteristics so that either motor torque or dynamic braking torque, within limits, is obtained depending upon the load conditions. If the load is overhauling, dynamic braking torque will be obtained and the descending speed of the motor in the limit zone will be limited to a reasonable value.

- Since the speed of the motor I3 is maintained at a reasonable value during the descent of the "engages the contact carrying arm 30:: of the limit switch, thereby resetting the limit switch to open the

contacts

33 and 34 and close the contacts 3| and 32. Contacts'33 and 34 in opening interrupt the loop circuit through the armature of motor "l3, the series field winding and resistor 65.

Con-

contacts 290 to interrupt the energizing circuit for the operating coil 49a of contactor 49. This contactor then drops out and opens its contacts 48b to insert the resistor 22 in the field circuit of the motor, thereby to weaken the field and to increase the speed of the motor to full speed.

Although in accordance with the provisions of the patent statutes this invention is described as embodied in concrete form and the principle thereof has been explained and the best mode in which it is now contemplated applying that principle, it will be understood that the apparatus tact 32, in closing, connects the field winding l3a across the source i through resistor 20 and contact 3! in closing connects the armature of the motor to the source through

resistors

26 and 21. The motor is now connected as a shunt motor with the current through the field winding l3a limited by the resistor 20.

The closing of contact 32 also connects the operating coil 50a of relay 40 directly across the field winding i3a. As previously pointed out, the voltage drop across the field winding I30. does not energize the operating coil 40a sufiiciently to maintain the relay picked up. Consequently, the relay drops out and opens its contacts 401) and 40c. Contacts 400 in opening interrupt the energizing circuit for the operating coil Ma of contactor 4|. This contactor in response to dener- .gization opens its contacts tlb to interrupt the armature shunt circuit through the resistor 43', thereby removing the dynamic braking effect of this circuit.

As explained in the foregoing, the progressive operation of the accelerating contactors was interrupted by the closing of the contacts 40b of the relay 40 so that even though the master switch were advanced to the fifth position. lowering, the effect on the control was the same as if the master switch had not been advanced beyond the third position lowering. The opening of 'contacts 4% therefore permits the control to resume its progressive operation to the point called for by the operating position of the master switch. Assuming that the master switch has beenadvanced to the fifth position lowering, the opening of contact 40b of relay 40 interrupts the energizing circuit for the operating coil 48a of contactor 48. In response to deenergization, contactor 48 opens its contacts 481) to insert resistor 2| in cirsuit with the field winding 13a, thereby weakening the field of. the motor and increasing its speed. In the dropped out position of contactor 48, the

' normally closed contacts tad complete the previouslyprepared energizing circuit for contactor 6|. In response to energization contactor 6| closes its contacts Bib to short circuit the resistor 23, thereby increasing the speed of the motor, and also opens its contacts Sic to interrupt the energizing circuit for the operating coil 65a. of time delay relay 65. This relay in responding to deenergization closes its contact a after a predetermined interval of time to complete an energizing circuit for the operating coil 28a of contactor 28. In response-to energization, contactor 2t closes its contacts 28b to short circuit the resistor 26 in the armature circuit and thereby further increase the speed of the motor.

Simultaneously, contactor 28 closes its contacts 280 to complete an energizing circuit for the operating coil 29a of contactor 29. Responsively to energization, contactor Z9 closes its contacts 29b to short circuit resistor 21, thereby further to increase the speed of the motor. In picking up, contactor 29 opens its normally closed predetermined limit of operation,

shown and described is merely illustrative and that the invention is not limited thereto, since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or from the scope of the annexed claims. I

What I claim as new and desire to secure by Letters Patent of the United States is:'

1. In combination, an electric motor having a series field winding, limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operationand preparing reverse power connections from said motor to said source, reversing switching means for completing said reverse power connections, and means responsive to an electrical operating condition of said motor upon completion of said reverse power connections for completing a dynamic braking circuit for said motor.

2. In combination, an electric motor having a series field winding, limit switch mechanism for disconnecting said motor from its source at a reversing switching means for completing reverse power connections from said motor to said source, means response to an electrical operating condition of said motor upon completion, of said re verse power connectionsfor completing a dynamic braking circuit for said motor, and means responsive to operation of said limit switch mechanism at said predetermined limit in the reverse direction of rotation of said motor for interrupting said dynamic braking circuit.

3. In combination, an electric motor having a series field winding, limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operation and means controlled by said limit switch and responsive to the voltage of said motor for completing a dynamic braking circuit for said motor and for in terrupting said dynamic braking circuit at said limit of operation in the reverse direction of rotation of said motor.

4. A control system for hoists in which the hoist is driven by an electric motor having a series field winding comprising in combination, an electromagnetic switching device for controlling the continuity of a dynamic braking circuit for said motor and having an operating coil normally connected across said field winding, and overhoist limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operation of said motor in one direction and connecting said operating coil across said motor to provide for completion of said dynamic braking circuit in response to the voltage across saidmotor and for deenergizing said coil at said limit of operation inthe reverse direction of rotation of said motor thereby to interrupt said dynamic braking circuit.

motor from its source at a predetermined limit of operation in one direction of rotation, reverstion, a dynamic braking resistor, limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operation in one direction of rotation, and completing a series dynamic braking circuit through said resistor and said field windingand the armature of said motor, reversing switching means for completing reverse power connections from a point in the circuit between said armature and resistor to said source so that said motor is'connected to said source with said dynamic braking resistor in parallel with said armature and series field winding, means responsive to the terminal voltage of said motor upon completion of said reverse power connections for completing a second dynamic braking circuit in parallel with the armature of said motor and responsive to operation of said limit switch at said predetermined limit in the reverse direction of rotation of said motor for interrupting said second dynamic braking circuit and'completing power connections from said motor to said source, with 'said field winding in a parallel circuit with said armature.

'7. A control system for an electric motor having an armature and a series field winding comprising in combination, limit switch mechanism having normally closed contacts between a source and a first terminal ofsaid armature and between the second terminal of said armature and a first terminal of said field windingand having normally open contacts between said first armature terminal and said first field terminal and between said second armature terminal and the second field terminal, said limit swi ch being actuated at a predetermined limit of peration or said motor to open said normally closed contacts and close said normally open contacts, a switching device having an operating coil connected from said second armature terminal to said second field terminal and contacts controlled by said coil for establishing a dynamic braking circuit in parallel with said armature following operation of said limit switch at said limit.

8. A control system for an electric motor having an armature and a series field winding comprising in combination, limit switch mechanism having norm-ally closed contacts between a source and the line terminal or said armature and between the field terminal of said armature and the armature terminal of said field winding and having normally open contacts between the line terminal of said armature and the armature terminal of said field winding and between said field terminal of said armature and the remaining terminal 01' said field winding, a switching device having an operating coil connected between said field terminal of said armature and said remaining terminal of said fieldwinding, said limit switch mechanism-being actuated at a predetermined limit of operation of said motor to open said normally closed contacts thereby to disconnect the motor from its source and to connect said coil across said armature and field winding and to close said normally open contacts to complete a first dynamic braking circuit'through sai armature and field winding, reversing switching means for connecting said armature to said source for reverse rotation with said dynamic braking circuit connected across said armature and field winding, and means responsive to the margintion of said coil for completing a second dynamic braking circuit in parallel with said armature.

9. A control system for an electric motor having an armature and a series field winding comprising in combination, limit switch mechanism having normally closed contacts between a source and the line terminal of said armature and between the field terminal of said armature and the armature terminal of said field winding and having normally open contacts between the line terminal of said armature and the armature terminal of said field winding and between said field terminal of said armature and the remaining terminal of said field winding, a switching device having an operating coil connected between said field terminal of said armature and said remaining terminal of said field winding, a switching device having its operating coil connected between said field terminal of said armature and said remaining terminal of said field winding,

said limit switch mechanism being actuated at a predetermined limit of operation of said motor to open said normally closed contacts thereby to disconnect the motor from its source and to connect said coil across said armature and field winding and to close said normally open contacts to complete a first dynamic braking circuit through said armature and field winding, a reversing type master switch and switching means controlled thereby for completing reverse power connections from said motor to said source, and means responsive to the energization of said coil only in the reverse position of said master switch for completing a second dynamic braking circuit in parallel with said armature.

10. A control system for an electric motor having an armature and a series field winding comprising in combination, limit switching mechanism actuated by said motor for disconnecting said motor from its source at a predetermined limit or operation in one direction of rotation, re-

versing switching means comprising a reversing type master switch and means controlled thereby for completing reverse power'connections from said motor to said source to eflect operation of said motor in the reverse direction, means jointly controlled by said limit switching mechanism and said reversing switching means and responsive to the voltage across said motor for completing a dynamic braking circuit in parallel with said armature. and for interrupting said dynamic braking circuit at a predetermined limit of operation of said motor in the reverse direction.

11. A control system for an electric motor having an armature and a series field winding comprising in combination, a dynamic braking resistor, limit switching mechanism for disconnecting said motor from its source at a predetermined limit of operation of said motor in one direction of rotation, completing a dynamic braking circuit through said armature, field winding and resistor, and preparing reverse power connections from said motor to said source, reversing switching means comprising a reversing type master switch and means controlled thereby for completing said reverse power connections from said motor to said source with said dynamic braking" 3 resistor connected in parallel with said armature and field winding, and means jointly controlled direction of rotation of said motor, said master switch having a plurality of speed controlling positions for the lowering direction, means responsive to progressive operation of saidmaster switch through said positions for progressively adjusting the speed of said motor, means controlled by the operation of said limit switch for completing a dynamic braking circuit for said motor in the lowering direction and interrupting the progressive operation ofjsaid speed adjusting means within said zone and responsive 12. A control system for hoists and the like comprising in combination, on electric motor having an armature and a series field winding, an overhoist limit switch for disconnecting said motor from its source at a predetermined limit of operation in the hoisting direction and completing adynamic braking circuit through said armature and field winding, reversing switching means comprising a master switch having ahoisting position and a lowering position and means controlled thereby for completing reverse power connections from said motor to said source to efiect rotation of said motor in the reverse direction, and switching means jointly controlled by said reversing switching means and said limit switch and responsive to the voltage of said motor for completing a'second dynamic braking circuit in parallel with said armature only in the lowering position of said master switch and for interrupting said second dynamic braking circuit in the lowering direction at said limit of operation.

13. A control system for hoists comprising in combination, a series hoist motor, limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operation of said hoist in the hoisting direction thereby to stop said hoist within a predetermined .zone, a

reversing type master switch for controlling the '40 to operation of said limit switch at said limit in the lowering direction for. interrupting said dynamic braking circuit and restoring the progressive operation of said speed adjusting means.

14. A control system for hoists comprising in combination, a series hoist motor, limit switch mechanism for disconnecting said motor from its source at a.predetermlned limit of operation of said hoist in the hoisting direction thereby to stop said hoist within a predetermined zone, a reversing type master, switch for controlling the direction of rotation of said motor, said master switch having a plurality of speed controlling positions for the lowering direction, a

plurality of electromagnetic switching devices in the lowering direction and for interrupting the sequential operation of said switching devices while said hoist is within said zone and respon-v slve to operation of said limit switch at said limit in the lowering direction for interrupting said dynamic braking circuit and resuming the sequential operation of said contactors.

JAMES W. COQKE.