US2805377A - Crane hoist motor limit circuit - Google Patents

Description

Sept 3, 1957 D. PETTIT 2,805,377

CRANE} HoIsT' MOTOR LIMIT CIRCUIT Filed May 3, 1955 2 Sheets-Sheet 1 Z9 35 moan Bus sue I I I I l I 7.

INVENTOR. Dona L. PETTIT FIQI A-r-ronucv United States Patent Office CRANE HQIST MOTOR LIMIT CIRCUIT Born L. Pettit, Wauwatosa, Wis., assignor to hips are D Company, Detroit, Mich, a corporation of Micin an Application May 3, 1955, Serial No. 565,572

8 Claims. (Ci. 318--275) This invention concerns a novel control circuit for crane motors. In crane hoist applications there is generally provided some type of limiting device or arrangement which prevents the crane load from exceeding a predetermined limit of vertical movement. Such a limiting arrangement may take the form of a limit switch in the power circuit of the crane motor, which is actuated by the crane hook when it reaches a preferred height limit. However, the use of an ordinary limit switch introduces an inherent characteristic which permits the load to oscillate up and down, as the limit switch is tripped and reset. This action is especially evident with crane controllers which employ armature or motor shunts and is particularly annoying on cranes where head room is limited and the crane operator finds that his load automatically lowers below the hoist limit when attempting to obtain maximum height of the load. Also, it is evident that such oscillating action may move the load to a dangerous position or result in objectionable jarring of the load. Therefore, it is an object of this invention to provide a crane hoist limit circuit which incorporates a limiting means in circuit with the crane motor and a current responsive device operable in conjunction therewith to prevent oscillation of the motor during a hoisting operation.

In conventional crane installations it is necessary that a minimum number of power connections, such as trolley bus bars connecting the motor circuit and the control circuit (traveling with the crane) to the fixed power source, be provided because of the great expense and quantity of material required for this purpose. Some installations have solved the problem of oscillation at the'hoist limit by the use of added trolley bars which connect the limit switch in circuit with the control circuit to thereby effect non-oscillatable control. However, this costly expedient has met with little success and less expensive alternative methods have been sought. Therefore, it is a further object of this invention to provide a limit circuit as above, which can be applied to an existing hoist controller without the addition of extra trolley bus bars or mechanical alterations to the hoist by the use of a relatively simple and inexpensive relay operating in cooperation with a limit switch and a conventional type crane control circuit.

It is a still further object of the invention to provide a crane hoist limit circuit employing a lockout type controller which makes it necessary for the crane operator to move the master switch to the off or neutral position before further operating the hoist after actuation of a limiting means.

Other objects and features of the invention will be readily apparent to those skilled in the art from the specification and appended drawings illustrating certain preferred embodiments in which:

Figure 1 represents a diagram of the control circuit for a crane motor incorporating the present invention.

Figures 2 to 5 inclusive show the motor. loadv circuits under various operating conditions of the circuit.

2,805,377 Patented Sept. 3, 1957 Figure 6 represents a side plan view of the lockout relay.

Figure 7 represents a front plan view of the lockout relay.

Figure 8 is a diagrammatic representation of a crane hoist limit switch and brake.

Referring to Figure 1, 1t designates generally an electric motor suitable for operating a crane hoist. In this particular instance, a direct current motor having an armature 10, armature winding 12 and a field winding 14 is utilized.

A master switch or controller 20 is provided, having a number of carn operated contacts 22, 24, 26, 28, 32, 34, 36, 38 and 42, which are sequentially and alternately operated by means of a manual device such as a handle (not shown), to accomplish a multiplicity of switching functions. The master switch or controller 20, its corresponding controller contacts 22, 24, 26, 28, 30, 32, 34, 36, 38 and 42 and the contactors (11, 13, 15, 17, i9, 21, 23, 25 and 27 with their corresponding contacts) controlled thereby represent a convenient and conventional means of operating a crane hoist system.

A source of direct current for the circuit is obtained through power lines L1 and L2 from an electrical supply (not shown). The control circuit and its various switching devices are shown in the off or neutral position pending further operating by the crane operator, such as occurs between successive crane loads. In this standby position, the master switch or controller contacts 22 are normally closed, while controller contacts 24, 26, 28, 3t), 32, 34, 36, 38 and 42 are normally open. The master switch 20 has five motor speed selecting positions below its off position for lowerin crane loads and five positions above the off position for selecting the desired motor speed during hoisting. These positions are indicated by the dashed horizontal lines in Figure 1, and the crosses on these lines indicate closed contacts, while the absence thereof, at an intersection of the heavy connector lines and the dashed lines, indicates open contacts for the respective positions of the master switch. For example, in Speed 1 lowering position, controller contacts 24, 26 and 42 will be closed while controller contacts 22, 28, 30, 32, 34, 36 and 38 will be open. Note that the contactor coils 11, 13, 15, 17, 19, 21, 23, 25 and 27 are controlled by the controller contacts 26, 28, 32, 3t), 34, 36, 24, 42 and 38 respectively, although other contacts in series with some of the coils may effect control independently thereof, as will be subsequently described. The contactor coils, through their corresponding controller contacts, are connected to a common conductor 3 which connects with power input line L1 through either controller contacts 22 and conductor 4 or conductors 5, 6, contacts 43 and conductor 9. An undervoltage coil 40, which controls the energization of the contactor coils connected to conductor 41 by means of its contacts 44 is connected to L2 by a conductor 8, and initially through conductor 7, normally closed contacts 52, conductor 5, conductor 3, normally closed controller contacts 22 and conductor 4 to the line L1. As shown in Figure l, the coil 40 is energized because of the above circuit arrangement and the off position of master switch 20, and therefore its normally open contacts 43 and 44 are in their closed positions, preparatory for operation of the crane motor. Obviously, a switch would be required in the circuit to the coil 40 or in the lines L1 or L2 to completely deenergize the control system during prolonger off periods, but for purposes of this invention it is best described during an interim ofi? or standby position, such as between successive hook loads. The contacts 43 of coil 40, when closed, provide a holding circuit for the coil 40 through conductors 6 and 9 which allows continned energization of coil 40 after the master switch 20 is moved out of its off position whereby controller contacts 22 and the initial energizing circuit is opened. The undervoltage coil 40 provides asafety shut-down feature in the event of excessive reduction or complete failure of the line voltage in a manner well known in the art. It also provides a central control function for the controller circuit, i. e., generally the circuitry below conductor 41, in conjunction with lockout contacts 52 in a manner to be subsequently described.

The various controller contacts and the circuitry which they control will now be described.

The controller contacts 26 control the energization of a lowering control contactor coil 11 for operating contacts 45, 46, 47 and 48. Therefore, in any one of the five speed lowering positions of the master switch 20, the contacts 45, 47 and 48 will be closed and contacts 46 open. In all other positions of the master switch, these contacts will be in their opposite positions as shown. The contacts 45 are connected in circuit with the motor 10 to control the initial energization thereof during any of the lowering positions of the master switch 20.

The controller contacts 28 control the energization of a hoisting control contactor coil 13 for operating contacts 53, 54, 55 and 56. Therefore, in any of the five speed hoisting positions of the master switch 20, the con tacts 53, 54 and 55 will be closed and contacts 56 open. In all other positions of the master switch 24 these contacts will be in their opposite positions as shown. The contacts 53 are connected in circuit with the motor 10 to control the initial energization thereof during any of the hoist positions of the master switch 20. Note that the polarity of the motor armature 10 is reversed between lines L1 and L2 from the previous switch connections for lowering to obtainreversed operation of the motor, in a manner well known in the art.

The controller contacts 32 control the energization of a first dynamic braking contactor coil 15 for operating normally closed contacts 16 and normally open contacts 18. Therefore, in the 2, 3, 4 and speeding lowering positions and the 3, 4 and 5 speed hoisting positions, the contacts 16 will be open and contacts 18 will be closed. In all other positions of the master switch 2% these con tacts will be in their positions as shown.

The controller contacts 39, through either normally closed contacts 46 or normally open contacts '75, control the energization of a second dynamic braking contactor coil 17 for operating normally closed contacts 61, 62 and normally open contacts 63. Therefore, in the 4 and 5 speed lowering positions and the 2, 3, 4 and 5 speed hoisting positions, the contacts 63 will be closed and contacts 61 and 62 open, provided, of course, that either contacts 46 or 75 are in the closed position. In all other positions of the master switch the contacts 63 will be open and contacts 61 and 62 will be closed.

The dynamic braking coils 15 and 17 are utilized in a well known manner to furnish a safety feature which limits the speed of the crane motor in the event of voltage and brake failure.

The controller contacts 34 control the energization of a first acceleration contactor coil 19, for operating contacts 65 and 66. Therefore, in either the 4 and 5 speed hoisting positions of master switch 20, the contacts 65 and 66 will be closed and in all other positions of switch 20 they will be open. The contacts 65 are provided to short out the starting resistance 67 to increase motor speed in a manner well known in the art.

The controller contacts 36 control the energization of a second lowering control contactor coil 21 for operating contacts 74 and 75. Therefore, in the 3, 4 and 5 speed lowering positions of master switch 20, the contacts 74 and 75 will be closed, and in all other positions of the master switch 20 they will be open.

The controller'contacts 24 control the energization of a second acceleration contactor coil 23 foroperating con- 4 tacts 77, 78 and 79 provided that either contacts 66 or contacts 47 are closed. Also, the coil 23 may be energized through conductor 64, contacts 62, 48 and 47. The contacts 77 are provided to short out the second starting resistance 69 to obtain maximum speed of the motor 10 after starting.

The controller contacts 42 control the energization of a first main contactor coil 25, for operating contacts 81 and S2. The coil 25 may also be energized through con ductor 84, contacts 18 and either contacts 87 or contacts 82 and 71. The contacts 81 are provided to control the energization of the motor circuit conjointly with contacts 86 for connecting the motor circuit to line L2.

The controller contacts 38 control the energiztion of a second main contactor coil 27 through a parallel circuit. The parallel circuit has one branch with contacts 63, 79

I and 56, and the other branch with contacts 78 and 55.

The coil 27 controls the operation of normally open contacts 86 and normally closed contacts 87.

Many of the features briefly described above are more particularly pointed out and described in connection with the control system for hoist motors, Patent No. 2,590,453 issued to Dorn L. Pettit. Their reference in the subject invention is only to furnish an environmental and illustrative embodiment thereof.

Referring now to the upper portion of the circuit above conductor 9, which may be designated as the motor or power circuit in contradistinction to the master switch or controller circuit heretofore described, the contacts are operable to connect the motor circuit components to the line L1 when the master switch 20 is in any of its speed lowering positions, and contacts 53 are operable to connect the motor circuit components to the line L1 when the master switch 29 is in any of its speed hoisting positions. As shown, the motor circuit is deenergized because the master switch 20 is in its off position and both contacts 45 and 53 are open.

A limit switch 99 (Figs. 1 and 8) connected in the power circuit of motor 10 has normally open contacts 91 and 93 and normally closed contacts 92 and 94, which may be simultaneously operated by a limit determining means such as a hook block 95 on a hoist cable 96 to perform a switching function to be subsequently described. A high limit resistor 29 in series with the limit switch contacts 91 provides a shunting resistance for the motor fields 12 and 14 when the limit switch 90 is tripped to its opposite (from shown) position.

A brake coil 35 connected in series circuit with the motor field 14 is adapted when deenergized to operate a mechanical brake 35a (Fig. 8) to finally stop and hold a crane load in position with the master switch 28 in the off position and to release the brake when energized to allow controlled operation of the motor. The series brake is a conventional feature to provide safe operation in the event of voltage and current failure or to effect a complete stop in the off position. The motor 1%) with its field winding 14, the brake 35a, the limit switch 90, and the resistor 29 are arranged in a conventional manner to be mounted on a structure movable with respect to the power lines L1 and L2 and for this purpose only the conventional four trolley bus bars 95 are required.

A lockout relay 49 (mechanical details of which are shown on Figures 6 and 7) is provided having a shunt coil and a series coil with a linking arrangement 51 between their respective 'arrnatures, for a purpose to be subsequently described. The shunt coil 50 (Figure 1) connected in circuit with a current limiting resistance 57 is controlled by hoist contacts 54. The series coil 69 is connected in series circuit with limit switch contacts 94. motor 10 and armature winding 12. Normally closed contacts 52 of lockout relay 49 are connected in series circuit with the undervoltage coil 4% to control the energization thereof and consequently the entire master switch or control circuit' Obviously, this switch can be placed in other circuit positions to effect its lockout function, for example, in conductor 8 or line L2.

A limiting relay coil 70 and its associated relay contacts 71 are provided to limit the top speed (5) in the lowering direction, when heavy hook loads are encountered. The coil 70 is connected in series circuit with the armature when lowering contacts 45 are in the closed position. Thus, the coil 70 will be responsive to armature current, and correspondingly high armature current when heavy loads are present. The relay coil 70 is adapted to close contacts 71 with a normal current flow therethrough and open contacts 71 upon the appearance of an excessive current flow.

Starting resistances 67 and 69 are provided in series circuit with the armature 10, winding 12, limit switch contacts 92 and field winding 14 when the hoist contacts 53 are closed. The resistance 67 is shorted, during acceleration, by acceleration contacts 65 to eliminate part of the starting resistance, and the resistance 69 is shorted by acceleration contacts 77 to eliminate the remainder of the starting resistance to effect smooth acceleration of the motor 19. This arrangement constitutes a conventional method of motor starting and forms no part of the present invention. Also, during the initial lowering op eration, i. e., speed lowering positions 1 and 2, the resistances 67 and 69 are in series circuit with the parallel branch, of which winding 12, armature 10, contacts 94, lockout relay coil 60, limiting relay coil 70, dynamic braking resistors 58, 59 and their corresponding switch contacts 16 and 61 form one branch, and the field 14, now in shunt circuit with armature 10, forms the other branch.

Referring now to Figures 2 to 5 inclusive, in which are shown elementary motor load circuits during the off position (Figure 2), first speed hoist position (Figure 3), econd speed hoist position (Figure 4) and the hoist limit position after tripping of the limit switch 90 (Figure 5), respectively. In Figures 3 and 4 the motor 10 is connected across the lines L1 and L2, while in Figures 2 and 5 loop circuits disconnected from the lines L1 and L2 are shown.

In Figures 6 and 7 mechanical features of the lockout relay 49 are illustrated. As heretofore described, the series lockout relay coil 60 and the shunt lockout relay coil 50 are electrically connected in the circuit of Figure l. The upper contactor, with shunt coil 50, has an armature 68 which is shown in its open or presumably deenergized position. The lower contactor, with series coil 60, has an armature 76 which is shown in its closed or presumably energized position. A link 51 is mechanically coupled at one end to an extremity 96 of armature 76, and at its other end to an extremity 88 of armature 68, thereby linking the annatures to effect simultaneous but opposite armature movement. With coil 60 energized and holding armature 76 against its magnetic core, the link 51 aided by return spring 80 restrains the armature 68 from movement toward its energized position. The

I normally closed relay contacts 52, operable by the lockout relay 49, are physically actuated by the upper armature 68 to assume an open position when the armature 68 is moved counterclockwise to its energized position in juxtaposition to its corresponding magnetic core. This may be effected when two conditions are present, one, the relay coil 56} is energized and two, the relay coil 60 is deenergized. If both coils 6t) and 50 are energized or both deenergized, the armatures 68 and 76 remain in their positions as shown. Therefore, this arrangement permits a circuit lockout which is a combined function of two relay coils in such a manner that a particular condition and sequence of operation is necessary to effect a desirable switching function.

Operation During normal operation of the crane motor 10 in any of the following speed positions of the master switch 1 Lower, 2 Lower, 3 Lower, 4 Lower, 5 Lower, 3 Hoist, 4 Hoist and 5 Hoist, the function of the various components is substantially as described in the Patent No. 2,590,453 of Dorn L. Pettit. In those positions, as will be evident, from a study of Figure 1, operation of the crane motor will be effected from its off position through five speed lowering positions and three speed hoisting positions, providing dynamic braking as a result of the circuitry consisting of dynamic braking coils 15 and 17, their associated contacts 16, 18, and 61, 62, 63, respectively, and resistances 58 and 59; providing limited top speed in the lowering direction with heavy hook loads as a result of the circuitry consisting of limit relay coil 70 and its associated contacts 71, and providing smooth acceleration through the starting circuit heretofore described.

Operation in the speed hoisting positions 1 and 2 are particularly benefited by this invention because in those positions, oscillation is ordinarily encountered upon tripping of limit switch 90 and caused by the motor shunt components (Figures 3 and 4). Assuming that the operator has moved the master switch or controller 2-9 from its off position to speed 1 hoist position, the circuit of Figure 3 represents the load condition of the crane motor 1!} in this position and the following circuits are completed in Figure l: the undervoltage coil 40 (initially energized during the off position) has effectively placed the power source of lines L1 and L2 across conductors 3 and 41 respectively; controller contacts 42 close by action of the master switch 20, energizing the main control coil 25 thereby closing contacts 81 and 82, and controller contacts 28 close simultaneously with contacts 42, energizing the hoist control coil 13 thereby closing contacts 53, 54 and 55 and opening contacts 56. The closure of contacts 53 and contacts 81 completes a circuit from the line L1 to line L2 as follows: contacts 53, a parallel circuit consisting of one branch with series lockout coil 69, contacts 94, armature 10, winding 12, contacts 92 and series field 14, and the other branch with limiting relay coil '70, resistances 5S and 59 and contacts 16 and 61, series braking coil 35 (which releases the mechanical crane brake), start resistance 67, start resistance 69 and contacts 81. The closure of contacts 54 completes a shunt circuit to the motor 10 from line L1 to line L2 as follows: contacts 53, contacts 54, resistance 5'7, shunt lockout coil 50, resistance 73 and contacts 81. In normal hoisting the current drawn by the armature 1t} flowing through the series lockout coil 69 causes the lower armature 76 (Figures 6 and 7) to hold itself tightly sealed preventing the upper shunt coil 50 from moving its armature 63 and thereby contacts 52. The slow speed effected by the above circuitry moves the crane load in its hoisting direction until the limit switch 9t) is tripped mechanically by a limit device attached to the crane hook, for example. The limit switch 90 then closes contacts 9%) and 93 and opens contacts 92 and 94. With the opening of contacts 94, the series lockout coil is deenergized, permitting the armature 68 to move counterclockwise under the influence of its hitherto energized coil 59. This action effects operation of the contacts 52 to their open position, deenergizing the undervoltage coil 40 and thereby deenergizing the controller circuit. The operator must then return the master switch or controller 20 to the off position in order to reset it or reenergize the undervoltage coil 40. At this time it is expected that the operator will lower the load but should he attempt to hoist, the lockout coil 50 is reenergized and lockout coil 60 maintained deenergized will again open contacts 52 to deenergize the controller circuit.

Similarly, the movement of the master switch to speed 2 hoist position closes circuits as in speed 1 hoist position and in addition closes controller contacts 36 to energize the dynamic braking coil 17 through normally closed contacts 46. The energization of coil 17 opens contacts 61 to remove resistance 59 from the motor load circuit, as shown in Figure 4. This action effectively decreases the total current to the motor shunt circuit of coil and resistance 58 to increase the current through the motor circuit and thereby increase the motor speed in the hoisting direction; The limit switch 90 and the lockout relay 49 are then similarly Operated when the hoist limit is obtained to effect the operation as in speed 1 hoist position. The motor load circuit for either hoist speeds l or 2, after tripping of the limit switch 99, is shown in Figure 5. Here, a low resistance shunt (field 14) across the motor armature 110 causes the motor to come quickly to rest and thereby prevents motor oscillation. It will be observed that the shunt coil 59 is operative only during hoisting because contacts 54 are open during the lowering and 0E positions.

Therefore, while a certain preferred embodiment of this invention is disclosed, it is understood that the invention is not limited thereto, as many variations will be readily apparent to those skilled in the art and the invention is to be given its broadest possible interpretation within the terms of the following claims.

What is claimed is:

1. A control system comprising a pair of electric circuits to be interrupted one after the other, means for interrupting a first one of said circuits while a second one of said circuits is maintained, relay means having contacts and a pair of coils, means for energizing and deenergizing said coils independently of each other, said contacts being in a first position while both of said coils are do energized and remaining in said first position upon substantially concurrent energization of both of said coils and being operative to move to a second position upon deenergization of one of said coils while the other of said coils remains energized, said one of said coils being arranged to be deenergized upon interruption of said first .one of said circuits, whereby said contacts move to said second position, and means responsive to movement of said contacts to said second position to interrupt said second circuit.

2. A motor and control system combination comprising an electric motor, means for connecting said motor to a source of power, a shunting circuit for said motor connected to said source for causing operation of said motor at slow speed, a limit means operative at a limit of travel of said motor for disconnecting said motor from said source with said shunting circuit intact, relay means having contacts and a pair of coils, said contacts being in a first position while both of said coils are energized and both are deenergized and being operative to move to a second position upon deenergization of one of said coils while the other remains energized, means for energizing both of said coils while said motor is connected to said source and for deenergizing said one of said coils upon operation of said limit means at said limit of travel while maintaining said other one of said coils energized, and means operative upon movement of said contacts to said second position to disconnect said shunting circuit from said source.

3. A motor and control system combination comprising a direct current motor, means for connecting said motor to a source of power, a shunting circuit for said motor connected to said source for causing operation of said motor at slow speed, an electrically operated brake having an op rating coil connected in series with said motor and said shunting circuit between said source and said motor and shunting circuit, a limit means operated by said motor at a limit of travel of said motor for disconnecting said motor from said source with said shunting circuit intact, relay means having contacts and a pair of coils, said contacts being in a first position while both of said coils are energized and both are deenergized and being operative to move to a second position upon deenergization of one of said coils while the other of said coils remains energized, means for energizing both of said coils while said motor is connected to said source and for deenergizing said one of said coils upon operation of said limit means at said limit of travel with said shunting circuit intact and while maintaining said other of said coils energized, and means operative upon movement of said contacts to said second position to disconnect said shunting circuit from said source thereby to deenergize said brake operating coil. V

4. A motor andcontrol system combination comprising a motor, contactor means operative when energized to connect said motor to a source of power, means for energizing said contactor, a shunting circuit for said motor connected to said source through said contactor means for causing operation of said motor at slow speed, an electrically operated brake having an operating coil connected in series with said motor and said shunting circuit between said source and said motor and shunting circuit, a limit means operated by said motor at a limit of travel of said motor for disconnecting said motor from said source while said contactor means is closed and said shunting circuit remains intact, relay means having contacts and a pair of coils, said contacts being in a first position while both of said coils are energized and both are deenergized and being operative to move to a second position upon deenergization of one of said coils while tic other remains energized, one of said coils being in series with said motor to be energized and deenergized with said motor, means for maintaining the other of said coils energized while said contactor means is closed, whereby when said limit means operates at said limit of travel said relay contacts move to said second position, and means operative upon movement of said contacts to said second position to deenergize said contactor thereby to disconnect said shunting circuit from said source and to deenergize said brake operating coil.

5. In a control system for a direct current motor having an armature winding and a field winding, an armature circuit connected to one terminal of said armature, a connection' between one terminal of said field winding and the other terminal of said armature, a limit switch responsive to operation of said motor beyond a predetermined limit to interrupt said armature circuit and to reconnect said one terminal of said field winding to said one terminal of said armature for dynamic braking of said motor, a resistor connected in shunt relation with said armature during operation of said motor, a contactor in series with said resistor, a relay means having contacts and a pair of coils, said contacts being in a first position while both of said coils are energized and deenergized and being operative to move to a second position upon deenergization of one of said coils while the other of said coils remains energized, said one of said coils being connected in said armature circuit to be energized by armature current during operation of said motor and deenergized upon interruption of said armature circuit by said limit switch, means for energizing said other coil of said relay upon energization of said motor and for maintaining said other coil energized after said limit switch operates, and means responsive to movement of said contacts to said second position to open said contactor and thereby to interrupt said shunt circuit.

6. A motor and control system combination comprising an electric motor, an electromagnetic switch for controlling said motor and having an operating winding which when energized closes said switch, a control relay means having contacts and a pair of coils, said contacts being closed when both of said coils are energized and when both of said coils are deenergized and opened when one of said coils is. deenergized while the other of said coils is energized, circuit means for energizing said operating winding and including said contacts in series with said winding, an energizing circuit for said motor including one of said coils, means for opening said'motor energizing circuit independently of said switch, and means for maintaining said other coil energized after said motor energizing circuit is opened, whereby upon opening of said motor energizing circuit said contacts open to deenergize said winding. 7 i H 7. A motor and control system combination comprising an electric motor, a plurality of electromagnetic contactors for controlling said motor, an undervoltage relay for said contactors having a winding which when deenergized causes opening of said undervoltage relay and deenergization of all of said contactors, a control relay means having contacts and a pair of coils, said contacts eing in series with said winding, being closed when both of said coils are energized and when both of said coils are deenergized, and being open when one of said coils is deenergized with the other energized, an energizing circuit for said motor controlled by at least one of said contactors and including one of said coils, means for opening said motor energizing circuit independently of said contactors, means for maintaining said other coil of said relay energized after said motor energizing circuit is opened, whereby upon opening of said motor energizing circuit said contacts open to deenergize said undervoltage relay.

8. A motor and control system combination comprising an electric motor, a shunting circuit around said motor 20 for causing said motor to operate at a slow speed, an elec tromagnetic brake for said motor and having an operating '10 coil, electromagnetic contactor means operative when energized to connect said motor and said shunting circuit to a source of power through said brake coil to energize said motor for slow speed operation and to release said brake, limit means operated by said motor for disconnecting said motor from said source while said contactor means remains energized and while said shunting circuit remains intact thereby to maintain said brake coil energized, a relay having a coil connected in series with said motor so as to be energized and deenergized with said motor, a circuit for energizing said contactor means, contacts on said relay in said energizing circuit for said contactor means, and means associated with said relay for maintaining said relay contacts closed except when said limit means operates with said contactor means energized.

References Cited in the file of this patent UNITED STATES PATENTS 1,425,657 James Aug. 15, 1922 2,245,253 Cooke June 10, 1941 2,569,317 Jones Sept. 25, 1951 2,595,291 Ramsey et al. May 6, 1952

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