US2351796A - Motor control system - Google Patents

Description

Patented June 20, 1944 MOTOR CONTROL SYSTEM David C. Wright, Euclid, Ohio, assignor to The Electric Controller 8; Manufacturing Company, Cleveland, Ohio, a corporation Ohio Application March 30, 1942, Serial No. 436,780

24 Claims.

This invention relates to a motor control system and more particularly to a .control system for a direct current hoist motor wherein the armature winding and field winding are connected in series with each other for hoisting and in parallel with each other for lowering, and wherein a limit switch is used for automatically setting up an emergency dynamic braking circuit for the motor in event the motor raises the load too high.

Although it obviously has other applications, the invention is described herein in connection with its application to a hoisting system.

In the operation of a hoist by a direct current motor, it is common practice to control the lowering operation 'by causing the motor either to drive the load as a shunt motor or to retard the load as a generator, the size of the load determining th action of the motor. Suitable resistors in series with the motor and also in the field and armature circuits are arranged to be regulated to control the lowering speed. The m0- tor is generally connected as a series motor for hoisting purposes because of the good speedtorque characteristics of such a connection. Suitable resistors in series and in parallel with the motor e ra ged to be regulated to control the hoisting speed.

It is also common practice in such hoist control systems to provide suitable control means such as a limit switch, or limit stop, which opcrates to establish an emergenc dynamic braking circuit in response to movement of the hoist beyond a predetermined point in the hoisting direction. Operation of the limit switch also disconnects the motor circuits from the power source which causes the usual series connected, electromagnetically released, friction brake to be applied. In the event of over-travel in the hoisting direction, therefore, the motor is brought to rest under the combined influence of dynamic and friction braking. In such prior systems, after the limit switch has operated to stop the motor, a lowering circuit may be established by moving the master switch to a lowering position to enable the hoist to be lowered out of the limit. While the limit switch remains in its operated or tripped position, the motor can be connected only I as a series motor withthe armature energized at ing circuit is immediately established causing the motor to slow down suddenly. As a result the entire hoisting mechanism is subjected to severe mechanical strains and the motor is apt to have commutation difficulties. The change of speed is especially great if the master switch is in the first lowering position which, if a heavy load is being lowered, is the preferred position, but even if the master switch is in thlast lowering position, the speed change upon resetting of the limit switch is also very large.

It is therefore the general object of this invention to provide a motor control system which does not have the aforementioned disadvantages but which retains all of the advantages of the usual dynamic braking hoist control systems and which eliminates any sudden changes in speed which would subject th hoist mechanism and motor to severe shocks and strains.

More specifically, a'principal object of this invention is temporarily to connect a direct current hoist motor as a. slow speed series machine for lowering operation after the limit switch has tripped to set up a dynamic braking circuit, and to maintain said connection onlyuntil the limit switch resets due to said lowering action.

A more specific object is to obtain the slow speed series motor connection by completing an armature shunt circuit while lowering the hoist out of a limit stop which has operated to con nect the motor in a dynamic braking circuit.

Another object is to effect lowering of a direct current motor driven hoist, after the hoist limit switch has operated to connect the motor in a dynamic braking circuit, by connecting the motor to the source of power as a series machin with an armature shunt and maintaining said connection until the limit switch operates to reconnect the motor to the source of power as a shunt motor for dynamic lowering.

Many cranes include a trolley structure on which the hoist motor, electromagnetic brake, limit switch and certain other parts of the hoisting mechanism are mounted. The electromagnetic control panel or manual controller is usually mounted on the crane bridge instead of the trolley structure which moves along the bridge. This requires connections from the panel or controller to the apparatus on the trolley structure by means of trolley bars and collector shoes. Heretofore, most of such control systems have required four electrically independent trolley bars. Trolley bars are expensive to install, take up valuable space, and are a potential source of trouble. Accordingly, a further object of this invention is for eliminating the aforementioned undesirable operating characteristics without increasing the number of trolley bars.

In carrying this invention into effect in one form thereoi', normally open contacts are arranged to complete, when closed, a shunt circuit around the armature of a hoist motor. An armature shunt on a series connected motor causes a slow motor speed because part of the current used in exciting the field is shunted from the armature, thus reducing to below normal the ratio of armature current to field current. The normally open contacts are arranged to be closed in response to energization of a winding connected in the motor circuit so as to be responsive to the counter voltage of the motor.

As soon as the motor has accelerated to a slow speed as it is lowering out of the limit stop as a series connected machine, the winding is energized sufficiently to cause closure of the contacts to effect completion of the armature shunt circuit and thereby to maintain the slow motor speed until the limit switch resets.

Other objects and advantages will become apparent from the following specification wherein reference is made to the drawing, in which Fig. 1 is a simplified wiring diagram 01 the control system;

Fig. 2 is a diagram indicating the position or the switches for the various master switch positions during a hoisting operation;

Fig. 3 is a diagram indicating the position of the switches for the various master switch positions during a lowering operation;

Fig. 4 is a simple diagrammatic sketch of a crane hoist drive and limit switch. operated thereby;

Fig. 5 is a simplified wiring diagram of a portion of the control system showing a modification of the invention; and

Fig. 6 is a simplified wiring diagram of a por tion of the control system showing another modification of the invention.

Referring to Fig. 1, a direct current motor Hi having an armature winding ii, a commutating field winding [2, and a series field winding I; is arranged to be connected for energization from direct current power supply conductors 20 and 2|. For the hoisting direction of operation switches l4 and I! are arranged to connect the armature winding ii, winding l2, the series field winding iii, a winding I! for an electromagnetically released brake, and a resistor 18 in series with each other across the conductors 2|! and 2|. With the switches l4 and I! closed the brake is released and the motor I0 is connected as a series motor and exerts a hoisting torque on a hook 4B suspended from a cable 45 wound on a drum 25 as shown in Fig. 4. The drum 25 is arranged to be driven by the motor l0 through gearing 48.

If the switch [4 is opened and a switch It is closed, a circuit is completed for lowering operation from the conductor 20 to the junction point 8 between the series and commutating field windings. The circuit divides at the point 8. One branch of the divided circuit includes the series field winding I3 and the other branch includes the commutating field winding 12, the armature winding li, a resistor l8, and a normally closed switch 22. The divided circuits join at the junction point 9 and the lowering circuit is completed to the conductor 2| through the brake the commutating field winding IS, the resistor l6, and the switch [1.

to provide a hoist control system including means With the switches l8 and i1 closed the motor I. is connected as a shunt motor across the conductors 20 and 2i and is adapted to drive the hoisting drum 25 in a direction tolower the hook 4G or to retard by dynamic braking action the lowering of an overhauling load attached to the hook 48.

Speed control during both lowering and hoisting is obtained by manipulation of the switches 22, 26, 21 and 28. The switches 26, 2'! and 28 are arranged to shunt respective series connected sections of the resistor [6. The switch 26 when closed short circuits the resistance section lie, the switch 21 when closed short circuits the resistance section I62), and the switch 28 when closed short circuits the resistance section l6c. Speed control in the lowering direction is ob tained also by manipulation of switches 29 and 30 which complete circuits from adjustable points on the resistor 19 to an adjustable point on the resistor l6 and to the terminal of the resistor 16 which is connected to the switch l1, respectively. The control system so far described is similar to that shown in my reissued Patent No. 19,5 47, reissued April 23, 1935, but this invention is applicable to other types of dynamic lowering hoist controllers as well.

When the control system is used for hoisting service the switches and resistors so far described are generally mounted on a relatively stationary part of the hoisting mechanism and the necessary electrical connections are completed through the trolley bars and trolley shoes to the motor III, brake winding 15, and a limit switch 4|] to be described. The switch 18 and the junction point 8 are connected through a trolley bar 3|, the switch 14 and the armature winding ii are connected through a trolley bar 32, the switch 22 and the junction point 9 are connected through a trolley bar 33, and the brake winding i5 and the resistor iii are connected through a trolley bar 34.

The hoist limit switch 40 which may be one of a number of well known types is shown diagrammatically in Fig. 4. The switch 40 comprises a weight 41 suspended by a cable 49 from a counter-balanced lever arm 4| which is arranged to actuate a contact arm 42 through a suitable snap action device, shown as a coil spring 43. Attached to the contact arm 42 are the bridging portions of a pair of normally-closed contacts 42a and 42b and a pair of normally-open contacts 420 and 42d. A projection 44 attached to the cable 45 is arranged to engage the weight 41 when the hoist motor Ill lifts the hook 4B beyond a predetermined point. Engagement of the projection 44 with the weight 41 releases the lever arm 4! and permits the spring 43 to move the contact arm 42 from the left-hand position shown to a right-hand position. This movement of the contact arm 42 causes opening of the contacts 42a and 42b and closing of the contacts 420 and 42d with a snap action.

Referring again to Fig. 1, opening of the contacts 42a of the limit switch 40 disconnects the armature winding H from the supply conductor 20 and opening of the contacts 42?) interrupts the direct connection between the commutating field winding l2 and the series field winding 13. Closure of the contacts 420 and 42d complete a dynamic braking circuit for the motor Ill from the left-hand terminal of the armature winding i i through the contacts 420, the series field winding Hi from left to right, the junction point 9, the contacts 4203, a resistor 50, the junction point 8 and the commutating field winding I2 to the right-hand terminal of the armature II. It is to be noted that the contacts of the limit switch 40 are all between the motor In and the trolley bars indicating that the limit switch 40 is mounted with the motor on the moving part of the hoist.

The switches I4, II, I8, and 26 through 30 are preferably normally-open contacts of electromagnetically operated contactors and the switch 22 is preferably a normally-closed contact of an electromagnetically operated contactor. The contactors may be operated from a multi-position, manually-operated master switch and the operating sequence of some of them controlled through suitable relays in a well known manner. The preferred sequences for hoisting and lowering operations are shown in Figs. 2 and 3, respectively. The dots indicate which of the switches are closed in the respective master switch positions.

As shown in Figs. 2 and 3 the switch 22 is closed when the master switch is in its off position. For lowering operation, movement of the master switch to the first position in the lowering direction causes closure of the switches I'I, I8, 21 and 28 as shown in Fig. 3. The motor III is now connected across the power conductors 20 and 2I as a shunt motor with the resistor section id in series with both the armature winding II and the field winding I3, and the resistor I9 is in series with the armature winding II through the switch 22. With this connection only a small lowering torque is developed if the load is not overhauling, and overhauling loads are prevented from lowering rapidly because of dynamic Ibraking action.

In the second lowering position the switch 22 opens and the switch 29 closes, causing an increase in motor torque or an increase in the dynamic braking resistance and a consequent increase in lowering speed. In the third lowering position the switch 28 opens to increase the resistance in the field circuit and also to increase the dynamic braking resistance. In the fourth lowering position the switch 21 opens, causing a further increase in the field circuit resistance and a further increase in the dynamic braking resistance. In the fifth lowering position the switch 29 opens and a switch 30 closes. All of the resistor I6 is now connected in the field circuit and in the dynamic braking circuit and but a small portion of the resistor I9 remains in the armature circuit. It is obvious that movement of the master switch from the off position through the lowering positions causes a gradual increase of motor speed for both overhauling and non-overhauling loads.

If at any time during lowering operations the master switch is returned to the off position, a

dynamic braking circuit is completed throughthe resistor I9 and the switch 22 causing retardation of the motor I and concurrently the brake winding I5 is deenergized which permits the fric tion, the switch 22 opens to interrupt the shunted motor connection causing an increase in hoisting speed. The switches 26, 21, and 28 are closed in sequence as the master switch is moved to the third, fourth, and fifth hoisting positions respectively, causing gradual acceleration of the motor III to its normal hoisting speed, theultimate speed depending upon the size of the load being lifted. Movement of the master switch from any of the hoisting positions to the off position sets up a dynamic braking circuit through the resistor I9 and the switch 22 to retard the motor by dynamic braking action. At the same time the brake winding I5 is deenergized and the electromagnetic brake consequently is released to assist in stopping the motor and to hold the load in its raised position.

If the load is hoisted too high. the projection 44 engages the weight 41 and permits the spring 43 to move the contact arm 42 to its right-hand position. Opening of the contacts 42a and 421) as a result of the operation of the contact arm 42 disconnects the motor I0 and the brake winding I5 from the source of power supply and closure of the contacts 420 and 42d completes the previously traced emergency dynamic braking circuit forthe 'motor I0 through the resistor 50. The motor I0 is brought to a stop by the combined action of the dynamic braking torque and the friction brake which was applied when the winding I5 was disconnected. The usual limit stop permits the crane hook to travel a short distance after the weight has been lifted, and the time for the dynamic braking and friction braking to become effective permits the hook to travel a short additional distance before it is stopped. This overtravel might b ten or twelve inches. To reset the limit switch in such case requires seven or eight inches of downward travel of the crane hook. If there is considerable gearing such as shown at 48 between the drum 25 and the motor III, this distance of downward travel will equal about 50 revolutions of the motor which is sufiicient to permit the motor III to accelerate to a rather high speed when connected as a series motor.

Movement of the master switch to th first lowering position with the limit switch 40 in the tripped position completes a circuit through the switch I8, the junction point 8, the commutating field winding I2, the armature winding II, the contacts 420, the field winding I3, from left to right, the junction point 9, the brake winding I5, the switches 28 and 21, the resistor section H541, and the switch H to the conductor 2|.

The motor III is thus connected as a series motor across the power conductors 20 and 2| through the resistor section Ilia. This connection results in a large downward torque which causes the motor III to accelerate rapidly so that it has reached considerable speed before the limit switch 40 trips. If a large overhauling load is attached to the hook 46, the speed is of course still greater. With this connection the motor III is shunted through the resistor 50 and the contacts 42d, but this motor shunt has little effect in reducing the motor speed due to the low resistance of the resistor section I6a.

When the limit switch 40 returns to its normal position with the master switch in the first or subsequent lowering positions, the motor I0 is reconnected as a shunt motor. As previously mentioned, the shunt motor connection for the first point lowering is a slow speed point. This causes a sudden slowing down of the motor III upon resetting of the limit switch "I as the hoist moves out of the limit. This slowing down is so severe in some cases as to cause damaging mechanical strains in the hoisting mechanism and also interferes with proper commutation of the motor [0. If the master switch is in other lowering positions the change in speed as the limit switch resets is also such as to cause serious damage.

To eliminate this operating defect in accordance with this invention, an electromagnetic switch 5| having an operating winding 5| w and normally-open contacts 5hr is utilized. The winding Blw is connected across the motor Ill from the junction point I through the winding H10 and the contacts "d to the junction point 0, and the contacts ila are arranged to complete a circuit around the armature winding 1 I and the commutating field winding I2 through a resistor 52. When the limit switch 40 is in its tripped position and the master switch is moved to a lowering position, the counter electromotive force of the motor I is impressed on the winding Slw. Due to the rapid acceleration of the motor Ill under these conditions, the counter voltage rises rapidly. The winding SM is arranged to cause closure of the contacts 5|a when it is subject to a voltage corresponding to a safe slow speed of the motor l0. Therefore, shortly after the switches l1 and III are closed to move the hoist out of the limit, the switch 5| closes its contacts Sla to complete the shunt circuit around the armature winding II. The rate of acceleration and ulti mate speed of the motor Ill under such conditions are greatly reduced, and the motor speed is at a very low value when the limit switch resets so that there is very little, if any, change in speed as the hoist mechanism comes out of the limit.

The winding Blw is apt to be energized and cause closure of the contacts 5la during emergency dynamic braking. This, however, does not greatly affect the dynamic braking torque. It is to be noted that the switch 5| and the resistor 52 are connected for mounting on the moving part of the hoist so that it is not necessary to in-- crease the number of trolley bars.

A certain '75 ton crane driven by a 230 volt, 100 horsepower motor, on which the control system of this inventionwas subsequently installed,

formerly attained a speed of 900 R. P. M. on r the first lowering point when moving out of the limit stop with an empty hook but when the limit switch reset, the speed was suddenly reduced to R. P. M. causing severe flashing on the commutator and dangerous strains on the hoisting mechanism. The resistance of the resistor of this installation is 0.74 ohm and the armature shunt resistor 52 having a resistance of 0.185 ohm was added in accordance with the present invention. Contactor 5| also was added and arranged to close the circuit through the resistor 52 when the counter voltage of the motor reached volts. The motor speed with the added equipment was 120 R. P. M. when lowering out of the limit stop with an empty hook as compared with the former speed of 900 R. P. M. In lowering a 30 ton load the lowering speed as a series motor with the present armature shunt was 160 R. P. M. which reduced to 62 R. P. M. when the limit switch reset. A crane which the purchaser refused to use because obviously the structure would be damaged and dangerous, was equipped with the present invention and thereafter used for heavy loads and found safe and efiicient.

The modification disclosed in Fig. 5 employs a limit switch similar to the limit switch l0 but which has an extra normally-open contact 42c arranged to close the armature shunt circuit through the resistor 52 concurrently with the closing of the contacts 420 and 42d when the limit switch trips. The same result is accomplished as is accomplished in the control system of Fig. 1, but the extra contact 42c on the limit switch requires a special construction.

Although operation of the contactor 5| by the voltage drop across the motor lll during emergency dynamic braking is of little consequence, such operation can be prevented by the insertion of a contact type rectifier 53 in series with the winding 5110. The voltage drop across the resistor 50 is in one direction during emergency dynamic braking and in the other direction when the motor in is connected to lower out of the limit stop Ml, and, therefore, with the rectifier 53 arranged at the correct polarity, the contactor 5| closes during lowering with the limit switch 10 tripped but does not close during emergency dynamic braking.

Although this invention has been described in connection with a dynamic braking hoist controller of the type shown in my reissue Patent No. 19,547, its operation in connection with other types of controllers is obvious and the invention is not to be limited to the exemplary structures and uses illustrated.

Iclaim:

1. A control system for an electric motor arranged to operate a member in two directions and having an armature winding and a field winding, which windings are adapted for shunt and series connection, selectively, with each other, for operation of the motor as a shunt and as a series machine, selectively, and adapted for connection to a source of power, said system comprising a dynamic braking circuit for said motor, a reversing controller for said motor, switch means responsive to movement of said member to a predetermined limit of travel in one direction for disconnecting the motor from the source of power and for completing said dynamic braking circuit for the motor, said reversing controller including means to connect the motor to a source of power as a series motor for moving said member in the opposite direction while said dynamic braking circuit remains completed, a slow speed means operable to modify said last named connection to the source of power while the last named connection is completed to change the excitation of the motor for moving said member at a slow speed, and electroresponsive means normally disconnected from the motor and arranged to be electrically connected to the motor by said switch means and when connected to the motor being responsive to an electrical condition of the motor for causing operation of said slow speed means only until said dynamic braking circuit is interrupted by said switchmeans as a result of the movement of said member in the opposite direction.

2. A control system in accordance wtih claim 1 characterized in that the slow speed means includes means for completing an armature shunt connection for said motor, whereby said change in the excitation of the motor is effected.

3. A control system in accordance with claim 1 characterized in that the electroresponsive means comprises a contactor responsive to the electrical condition of the motor and the slow speed means comprises an armature shunt connection for said motor controlled by said contactor.

4. A control system for an electric motor arranged to operate a member in two directions and having an armature windin and a field winding, which windings are adapted for shunt and series connection, selectively, with each other, for operation of the motor as a shunt and series machine, selectively, and adapted for connection to a source of power, said system comprising a dynamic braking circuit for said motor, a reversing controller for said motor, switch means responsive to movement of said member to a predetermined limit of travel in one direction for disconnecting the motor from the source of power and for completing saiddynamic braking circuit, said reversing controllerdrrcluding means to connect said motor as a seriemmotor to a source of power for moving said member in the opposite direction while said dynamic braking circuit remains completed, said switch means being operable consequent upon movement of said member in the opposite direction concurrently to interrupt said series motor connection, to connect said motor as a shunt motor to the source of power, and to interrupt said dynamic braking circuit, a slow speed means operable to provide a slow speed connection for said motor until said dynamic braking circuit is interrupted as a result of the movement of said member in the opposite direction, an electroresponsive means operable in response to an electrical condition of the motor for permitting operation of said slow speed means.

5. A control system in accordance with claim 4 characterized in that the slow speed connection includes an armature shunt circuit having a resistance such that the speed of said motor, when the armature shunt circuit is operative, is substantially the same as the speed oi. the motor after said dynamic braking circuit is interrupted.

6. A system of motor control for raising and lowering a load and employing a direct current motor having an armature winding and a field winding, which windings are adapted for shunt and series connection, selectively, with each other, for operation of the motor as a shunt and series machine, selectively, and adapted for connection to a source of power, a dynamic braking resistor, means for connecting the motor to said source of power as a shunt motor for lowering a load, means for connecting the motor to said source of power as a series motor for raising a load, switch means responsive to the elevated position of the load while a load is being raised for disconnecting the motor from said source and for connecting the motor in a dynamic braking circuit with said resistor, means for reconnecting the motor to said source as a series motor for lowering the load while said dynamic braking circuit remains intact, said switch means being operable after the load has been lowered a predetermined distance to interrupt said dynamic braking circuit and to reconnect the motor to said source of power as a shunt motor, an armature shunt circuit including an armature shunt resistor, fixed electrical connections between said resistor and said armature winding, and contact means in said connections operable for completing said armature shunt circuit while said motor is connected as a series motor and lowering the load.

7. A system in accordance with claim 6 characterized in that an electroresponsive means connected to the motor and responsive to the counter electromotive force of the motor effects operation of said contact means.

8. A system in accordance with claim 6 characterized in that the contact means is operated by operation of said switch means in response to the elevated position of the load during hoisting of said load.

9. In combination, an electric motor having a field winding and an armature winding adapted to be connected in series and in parallel with each other, selectively, limit switch means for disconnecting said motor from its source of supply at a predetermined limit of operation and for partially completing reverse power connections between said motor and said source, directional switching means for completing said partially completed connections, electroresponsive means normally disconnected from said motor, said limit switch means including means operable to connect said electroresponsive means to said motor upon disconnection of said motor from its source by said limit switch means, and an armature shunt circuit for said motor, said electroresponsive means being responsive to an electrical operating condition of said motor upon completion of said partially completed connections for connecting said armature shunt circuit to said motor.

10. In combination, an electric motor having an armature winding and a field winding and adapted to be connected to a source of power as a shunt and as a series motor, selectively, electroresponsive means disconnected from said motor while said motor is connected as a shunt motor and when connected to said motor while said motor is connected as a series motor operable in response to an electrical condition of said motor for completing an armature shunt circuit for said motor, limit switch means operable at a predetermined limit of operation of said motor for disconnecting said motor from said source and connecting said electroresponsive means to said motor, and directional switching means for reconnecting said motor to said source as a series motor with said electroresponsive means connected to said motor.

11. In a control system for an electricmotor arranged to operate a member in two directions and having an armature winding and a field winding, which windings are adapted for shunt and series connection, selectively, with each other, for operation of the motor as a shunt and as a series machine, selectively, and adapted for connection to a source of power, the combination with a dynamic braking circuit for said motor, a reversing controller for said motor, switch means responsive to movement of said member to a predetermined limit of travel in one direction for disconnecting the motor from the source of power and for completing said dynamic braking circuit, said reversing controller including means to connect the motor to the source of power for moving said member in the opposite direction while said dynamic braking circuit remains completed, and a slow speed means operable to modify said last named connection to the source of power while the last named connection is completed to change the excitation of the motor for moving said member at a slow speed, of electroresponsive means connected to the motor through said switch means and responsive to the countervoltage of the motor for causing operation of said slow speed means.

12. A control system for an electric motor having a winding adapted to be connected as a series field winding and comprising, in combination, a dynamic braking resistor, an electroresponsive means having an operating coil normally disconnected from said motor; limit switch mechanism operable at a predetermined limit of operation in one direction of rotation for disconnecting said motor from its source, for connecting said coil in parallel with said motor, and for completing a series dynamic braking circuit through said resistor, said field winding, and the armature of said motor; directional switching means for completing reverse power connections from a point in the circuit between the armature and said resistor to the source so that said motor is connected to the source with said dynamic braking resistor in parallel with said armature and series field winding; said electroresponsive means being responsive to the terminal voltage of said motor upon completion of said reverse power connections for completing an armature shunt circuit in parallel with the armature of said motor and being responsive to the operation of said limit switch at said predetermined limit in the reverse direction of rotation of said motor for interrupting said armature shunt circuit.

13. In combination, an electric motor having a field winding, limit switch means operable concurrently at a predetermined limit of motor operation to disconnect said motor from its source, to connect said motor in a dynamic braking circuit including a resistor, and to partially complete reverse power connections from said motor to said source, directional switching means for completing said reverse power connections, an armature shunt circuit for said motor, an electroresponsive means, and fixed connections completing a parallel circuit including said dynamic braking resistor and said electroresponsive means, said electroresponsive means being responsive to an electrical operating condition of said motor upon completion of said reverse power connections for connecting said armature shunt circuit to said motor.

14. A control system for an electric motor having a field winding and an armature winding, an armature shunt circuit having terminals adapted to be connected across said armature winding, an electromagnetic switch having an operating winding adapted to be connected across said mo tor and having contacts in said armature shunt circuit, and limit switch mechanism operable at a predetermined limit of motor operation for disconnecting said motor from its source and connecting said operating winding across said motor, said electromagnetic switch being responsive to a voltage of said motor while said operating winding is connected across said motor for closing said contacts in said armature shunt circult.

15. In combination, an electric motor having a field winding, an armature shunt circuit for said motor, an electromagnetic switch having an operating winding adapted to be connected to said motor and having contacts in said armature shunt circuit, which, when closed, complete said armature shunt circuit, and limit switch means for disconnecting said motor from its source and connecting said operating winding to said motor at a predetermined limit oif operation, said switch means being responsive to a voltage of said motor while said winding is connected to said motor for closing said contacts and completing said armature shunt circuit.

16. In a hoist control system and hoist motor combination, a hoist motor having a field winding adapted to be connected as a series and as a shunt winding. selectively, limit switch mechanism for disconnecting said motor from its source at a predetermined limit of operation and for preparing reverse power connections from said motor to said source, directional switching means for completing said reverse power connections, an armature shunt resistor having one terminal fixedly connected to said motor, and means responsive to an electrical operating condition of said motor upon completion of said reverse power connections for completing an armature shunt circuit for said motor including said armature shunt resistor.

17. In a motor and control system combination, a plurality of trolley bar mechanism; a series circuit including a pair of limit switch contacts, a motor armature winding, and a motor field winding connected between one pair of said trolley bar mechanisms; control means for said motor connected between said trolley bar mechanisms and a source of power for the motor; and an armature shunt circuit for said motor and a dynamic braking circuit for said motor each connected to said motor between said trolley bar mechanisms and said motor.

18. In a series type hoist motor and control system combination, a plurality of trolley bar mechanisms, control means for said motor connected between said trolley bar mechanisms and a source of power for the motor, a dynamic braking circuit for said motor arranged to be completed by a limit switch, and an armature shunt circuit for said motor arranged to be completed by an electroresponsive means, said dynamic braking circuit and said armature shunt circuit being fixedly connected to said motor inside of said trolley bar mechanisms.

19. A hoist motor and hoist control system combination comprising a first group of electrical devices with fixed electrical connections therebetween and including a plurality of switches and a plurality of resistors for controlling the operation of said hoist motor; a second group of electrical devices with fixed electrical connections therebetween and includin said hoist motor, a limit switch means, a dynamic braking circuit controlled by said limit switch means, an armature shunt circuit, and a switch having an operating means responsive to a voltage of said motor and operable to control a connection between said armature shunt circuit and said motor; and electrical connections between said groups of devices enabling relative movement of said groups.

20. A hoist motor and hoist control system combination comprising a plurality of trolley bar mechanisms; a first group of electrical devices with fixed electrical connectionsv therebetween and including a plurality of switches and a plurality of resistors for controlling the operation of said hoist motor; a second group of electrical devices with fixed electrical connections therebetween and including said hoist motor, a limit switch means, a dynamic braking circuit controlled by said limit switch means, an armature shunt circuit, and a switch means having an operating means responsive to a voltage of said motor for controlling a connection between said armature shunt circuit and said motor; and a plurality of electrical connections between said groups of devices including said trolley bar mechanisms, respectively, whereby relative movement of said groups is enabled while said electrical connections between said groups are maintained.

21. In combination, an electric motor having a field winding, a rectifier, electroresponsive means responsive to an electrical condition of said motor for completing an armature shunt circuit for said motor, and limit switch mechanism for disconnecting said motor from its source and connecting said electroresponsive means to said motor in series with said rectifier at a predetermined limit of operation.

22. A control system for hoists in which the hoist is driven by an electric motor having a field winding comprising in combination, an electromagnetic switching device for controlling the continuity of an armature shunt circuit for said motor and having an operating coil adapted to be connected to said motor so as to be responsive to an electrical condition of said motor, a rectitier, an overhoist limit switch mechanism for disconnecting said motor from its source at a predetermined limit or operation or said motor in one direction, for connecting said operating coil to said motor in series with said rectifier to provide for completion oi said armature shunt circuit in response to a predetermined electrical condition of said motor, and for deenergizing said coil at said limit or operation in the reverse direction of rotation of said motor, thereby to interrupt said armature shunt circuit.

23. In a system or motor control for raising and lowering a load and employing a direct current motor having an armature winding and a field winding, which windings are adapted for shunt and series connection, selectively, with each other, for operation 01' the motor as a shunt and series machine. selectively, and adapted for connection to a source of power, and a dynamic braking circuit for the motor, the combination with means for connecting the motor to the source of power as a shunt motor for lowering a load, means for connecting the motor .to said source of power as a series motor for raising the load, switch means responsive to the elevated position of the load while a load is being raised for disconnecting the motor from said source and for connecting said motor in a dynamic braking circuit, and means for reconnecting the motor to said source as a series motor for lowering the load while said dynamic braking circuit remains intact, said switch means being operable after the load has been lowered a predetermined distance to interrupt said dynamic braking circuit and to reconnect the motor to said source of power as a shunt motor, 01' an armature shunt connection for the motor and operable to slow down the motor, when the motor is operating as a series motor, to provide a motor speed approximately equal to the normal shunt motor lowering speed, and said switch means being operable for rendering said armature shunt connection efiective while said load is being lowered said predetermined distance, whereby the motor speed does not change appreciably when said switch means operates to reconnect the motor as a shunt motor.

24. A control system for a dynamo electric machine having an armature winding and a field winding and adapted to be connected as a series and as a shunt machine, selectively, comprising in combination, a dynamic braking resistor, an armature shunt resistor, limit switch means for disconnecting said machine from its source at a predetermined limit or operation in one direction of rotation and completing a series dynamic braking circuit through said dynamic braking resistor. the field winding, and the armature winding of said machine, directional switching means for completing reverse power connections to said source from a point in the dynamic braking circuit between said armature winding and said dynamic braking resistor so that said machine is connected to said source with said dynamic braking resistor in parallel with a series connection including the armature winding and the series field winding, said limit switch means including means operable upon completion of said series dynamic braking circuit for connecting said armature shunt resistor in parallel with the armature winding of said machine, and said limit switch means :being operable at said predetermined limit in the reverse direction of rotation oi. said motor for interrupting said armature shunt circuit and completing power connections irom said machine to said source with said field winding in a parallel circuit with said armature winding.

DAVID C. WRIGHT.

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