US1919989A - Motor control system - Google Patents

Description

Filed Jan. 9, 1930 Inventor: Carl .Schiebeler, Arno Weiss by Ma Their Attorney Patented July 25, 1933 UNITED srarss arser orr ce CARL SCI-IIEBELER, OF BERLIN-HALENSEE, AND Ali-N0 WEISS, OF BERLIN-OHARLOTTEN- BURG, GERMANY, ASSIGNORS TO GENERAL ELECTRIC COIVIPANY, A CORPORATION OF NEW YORK MOTOR CDNTROIi SYSTEM Application filed January 9, 1930, Serial No. 419,650, and in Germany Maya, 1929.

This invention relates to systems for controlling the operation of electric motors, more particularly to systems in which series wound motors are required to start heavy loads from rest; and has for an object the provision of a system for limiting the starting torque of the motor to a maxi mum permissible value and for causing the motor to deliver a series of impulses to the load in the event that the load does not start to move when the applied torque reaches its maximum permissible value.

The invention, although generally ap plicable to systems in which an electric motor is required to start heavy loads is particularly advantageous in its application to systems in which a series motor is employed to drive a pipe foundry crane. It is well known that unusually large pullin forces are required to Withdraw the core fiom the pipe after it has been cast and because of the large torque that a series wound motor is able to exert, when starting under load, motors of this type are usually employed in this connection.

Furthermore it frequently happens that despite the provision of intermediate layers of material designed to prevent their adhesion, the core and the casting bake together and the motor is thus unable to start the load immediately, in which event if a series motor is employed the torque will increase to a point at which the mechanical parts of the crane are endangered. In practice cases have occurred in which the torque developed by series motors employed in driving pipe foundry cranes has resulted in damage and even in destruction to the crane itself. 7 V

In the operation of a pipe foundry crane the operator returns thecontroller handle to the neutral position immediately he observes that the core has not started to move when in his judgment the motor torque has reached a value that may not be exceeded without endangering the mechanical parts of the crane and the windings of the motor. In order to withdraw the core from the cast pipe the operator then switches the controller on and off to cause the motor to deliver a series of impulses to the load until the core is loosened in the casting after which the core may easily be withdrawn therefrom. Our invention provides a solution for the above problem.

In carryingthis invention into effect in one form thereof, we provide means for lime iting the starting torque of a drive motor to a maximumpermissible value together with 7 means for automatically causing the motor to deliver a series of impulses to the load that does not start to move when the maximum permissible starting torque is applied.

For better and more complete understanding of'the invention, reference'should now be had to the following specification andto the accompanying drawing thesingle figure of which is a diagrammatical view of the embodiment of the invention.

Referring now to the drawing, an electric motor 10 is mechanically coupled to drive a suitable load such for example as a crane employed for the purpose of withdrawing cores from pipes after they have been cast. As shown the motor 10 is a series motor, that is to say it is provided with a field winding 11 which is connected. in series with the armature of the motor and thus conducts the entire armature current. The use of-a series wound motor is very advantageous in starting heavy loads since its torque increases with increasing armature current. The motor 10 is supplied with electrical energy from a suitable source of supply such for exampleas the generator 12 to which it is connected in Ward Leonard fashion'by means of'conductors 13 and 14:; thegenerator 12 itself being driven preferably ata substantially constant speed by any suitable driving means such ternatin'g current electric motor 27 which is supplied from a suitable source of electric power represented in the drawing by the three supply conductors 28. As shown in the drawing the generator 12 is provided with a series field winding 15, a shunt field winding 16 and a separately excited field winding 17 the separately excited field winding 17 being energized from any suitable direct current for example as the alsource such as that represented in the drawing by the two supply conductors 18.

The shunt field winding 16 and the separately excited field winding 17 are cumulative, that is to say, they are so wound and energized that the magnetic fields resulting from their energization are in the same direction and add to each other. The series field winding 15 on the other hand acts differentially with respect to the shunt field winding and the separately excited field winding 17, i. e., it is wound and excited in such a manner that the magnetic field resulting from its energization opposes the magnetic fields due to the field windings 16 and 17 Thus, the resultant or effective field of the generator 12 is the algebraic sum of the magnetic fields resulting from the field windings 16 and 17 diminished by the field produced by the field winding 15. The number of turns of the series field winding 15 is so chosen that the current which the generator 12 supplies to the motor 10 will be limited to a predetermined value which value will be low enough to prevent the current from doing damage to the windings of the motor and which will limit the torque applied by the motor to a value that will prevent the motor from doing damage to the mechanical parts of the crane in case the core does not start to move.

The strength of the separately excited field 17 of the generator is regulated by means of a regulating resistance 19 and a cooperating movable contact 20, the position of which on the resistor 19 isvaried as desired by means of any suitable controlling means'such for example as a manually operated controller. An additional control resistance 21 is connected in the circuit of the separately excited fieldwinding 17 which is arranged to be short circuited by the contacts of an auxiliary relay 22 when the re lay is energized. The ohmic value of the resistance 21 is so chosen that when the contacts 'ofthe relay 22 are open and it is in circuit with the field winding 17 the re sultant field of the generator, i. e., the algebraic sum of the magnetic fields produced by the field windings 15, 16 and 17 when field winding 15 is energized to the maximum permissible value is practically reduced to zero thereby causing the current supplied by generator 12 to motor 10 to decrease to a negligible value.

As shown, the auxiliary relay 22 is provided with an energizing winding which is supplied from the direct current source 18, and this relay serves when energized to bridge the contacts 23 to complete a short circuit about the control resistance 21 and when deenergized to open the contacts 23 and thereby remove the short circuit about the resistance 21,

The peration of t e o t el y .22 to insert and exclude .the resistance 21 from the circuit of the field winding 17 is controlled by means of a control relay 24, the energizing winding of which is connected in circuit with the conductor 14 in the supply circuit between the generator 12 and the motor 10. When the control relay 24 is deenergized its armature member bridges the contacts 25 in the circuit of the energizing coil of the control relay 22 to complete the energization of the latter whilst when the relay 24 is energized its armature is attracted to the open position to open circuit the contacts 25 and thereby deenergize the coil of auxiliary relay 22. The coil of the control relay 24 is so designed that it attracts its armature member to the open position when current of the maximum permissible value above described traverses its winding and retains its armature member in the attracted position until the current flowing in its winding is reduced practically to zero. A time delay device may be provided in conjunction with the control relay 24 to control the periodicity of its operation as desired. As thus. constructed the control relay 24 serves to control the auxiliary relay 22 to insert and exclude the control resistance 21 from the circuit of the separately excited field winding 17 in response to the current supplied by the generator 12 to the motor 10.

lVith the above understanding of the elements comprising the invention and their organization, the operation of the system itself will be readily understood from the de tailed description which follows:

When the movable contact 20 and the relay 22 are in the positions in which they are shown in the drawing, i. e., with the entire regulating resistance 19, and the control resistance 21 in the circuit with the sepa-. rately excited field winding 17 the generator is practically deenergized and supplies no current to the motor 10. The disconnect switch 26 is first closed to complete the energizing circuit for relay 22 which then closes and bridges the contacts 23 to short circuit the control resistance 21. Operation of the manually operated controller to vary the position of themovable contact 20 on the regulating resistance 19, i. e., to decrease the resistance causes. the current flowing in the field winding 17 to increase and thus causes the generator 12 to supply a currentto the motor 10 the value of which current increases in proportion to the increase of the current of the field winding17 as will be well understood. In addition the voltage generated by the generator 12 causes a cur rent to flow in the shunt field winding 16 which produces a field that adds to the field produced by the winding 17. However, as the current which the generator 12 supplies to the motor 10 increases in value the magnetic field produced by the series field winding 15 also increasesin value, and opposes orsubtracts from the ma netic field due to the windings 16 and 17. hen the current supplied to the motor 10 increases to the predetermined value above mentioned, i} e.,the value at which the current will do damage to the windings of the motor 10 or cause the motor 10 to do damage to the crane in the event that the core does not start to move, the subtractive efiect of the fields due to the series windings 15 is such that the current supplied by the generator 12' cannot 'increase further. Thus, it will be seen that the field windings '15, 16 and 17 are so designed and related to each other that the output current of the generator 12 is limited to a maximum permissible value.

WVhen the current supplied by the generator 12 to the motor 10 reaches this predetermined maximum permissible value the control relay 24 attracts its armature out of engagement with the contacts '25 thereby opening the energizing circuit of the auxiliar rela 22. Deener ization of auxiliar y y b relay 22 permits its armature member to drop out of engagement with the contacts;23 thereby removing the short circuit about the resistor 21 and inserting it in circuit with the separately excited field winding 17.

As a result of the insertion of resistance 21 in the circuit of the separately excited field winding 17 the magnetic field due to the field windings 16 and 17 is practically overcome by the opposing magnetic field due to the series field winding 15 and thus, the resultant energization of the generator 12 is practically reduced to zero. As a result of the deenergization of the field of the generator the current supplied to the motor 10 decreases to a practically negligible value thereby resulting in a deenergization of the control relay 24.

Due to the deenergization of the control relay its armature is permitted to descend under the attraction of gravity and again to bridge the contacts 25 to complete the energizing circuit of auxiliary relay 22. Auxiliary relay 22 in responding to the energization of its coil operates its armature to the upper or closed position to bridge the contacts 23 and again complete a short circuit about the control resistance 21 thereby excluding it from the circuit of the separately excited field winding 17, the magnetic field of which then builds up to its former value thus causing the resultant field of the generator 12 again to build up to a value sufiicient to supply current of maximum permissible value to the motor 10.

If the motor torque and current again reach their maximum permissible values before the core starts to move, the control relay 2 1 and the auxiliary relay 22 will again function in the manner described to decrease the current and torque of the motor to a minimum value and subsequently to rapidly increase them totheir maximum permissible values, thereby causing the motors to deliver a short sharp impulse to the load. It will be clear that if the load does not move that the above described operation of decreasing and subsequently increasing the torque and current of the motor will here peated' at regular intervals, thus in effect causing the motor 10 to deliver to the load a series of torque impulses of maximum value, that continues until the core is loosened. As soon, however, as the core becomes loosened in the casting andthe motor 10 starts to rotate, the current taken by the motor to produce the torque necessary to withdraw the core from the casting will actually decrease. Consequently, the current drawn by the motor 10 will not reach its maximum permissible value again"during that operation, and thus the control relay 24 will remain unattracted in the position shown, bridgingthe contacts 25' and maintaining the auxiliary relay 22 in its operated position short circuiting the control resistmice 21 for theremainder ofthe Withdrawal operation. i i

"From the foregoing description it will be clear "th'at'the operation of the system to limit the torque applied by the motor to the loadand-to cause the motor'to delivena series of impulses to the load in-the event that it does not start when the current and torque reach their maximum permissible values is entirely automatic; no action being required of the operator other than to move the controller handle to an operative position to initiate the operation.

0 Although in accordance with the provisions of the patent statutes we have described our invention as embodied in concrete apparatus organized in a particular manner, it is to be understood that the embodiment shown is merely illustrative and that the fications and alterations will readily suggest themselves to persons skilled in the art without departing from the true spirit of the invention or from the scope of the annexed claims.

What we claim as new and desire to se- 1. In a variable voltage control system, a generator provided with a field circuit, a motor supplied therefrom for driving a load body, a resistance connected in said field circuit, and means responsive to the current exchanged between said generator and said motor for intermittently short circuiting said resistance to produce variations in the current supplied to said motor between a maximum value and a substantially lower minimum value thereby to cause said motor cure by Letters Patent of the United States 11D invention is not limited thereto since modito deliver a Series of driving impulses to said load. a I

2. Ina variable voltage control system a generator provided with a separately excited field winding a-ndan oppositely Wound series field Winding for limiting the armature current of said generator to be aqpredetermined value, a motor supplied from said generator for driving a load, a normally short circuited resistance in said separately excited field circuit, means for increasing the currentsupplied to said motor to said pres determined value, and means responsive to said predetermined value of current to interrupt the short circuit about said resistor to decrease the current supplied to said motor and responsiveto a decreased value of said current'to reclose said short circuit to increasesaid current to said predetermined value. I

3. In a motor control system, a motor for citation of said separately excited Winding thereby to reduce the excitation of said generator substantially to zero and responsive to substantially ZGIO current for increasing the excitation of said separately. excited Winding so as to increase'said current to said maximum permissible value. I v

4. In a variable voltage control system, a supply generator and a motor supplied there from for driving a load, a separately excited field circuit for said generator and meansior varying the excitation of said circuit for varying the voltage and current supplied to said motor, an oppositely Wound series field circuit for said generator for limiting the current supplied to said motor to a maximum permissible value, a resistance in said separately excited field circuit, normally closed contacts for short circuiting said resistance, an electroresponsive device responsive to the maximum permissible value of current supplied to said motor for opening said contacts to interrupt the short circuit about said resistance to decrease the current supplied to said motor and responsive t'o'decreased current for closing said contacts to increase the current supplied to said motor to said maximum permissible value.

CARL SCHIEBELER. ARNO VV'EISS.

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