Patent GB-674932-A (GB)
Title: Improvements in or relating to alternating current electric motor equipments
IPC: H02P3/22
Filed: 1948-08-23
Abstract: 674,932. Control of A.C. motors. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S., July 21, 1949. [Aug. 23, 1948] No. 22189/48. Class 38 (iii). An alternating current motor equipment comprises a wound rotor induction motor, variable resistance arranged to be connected to one winding of the motor usually the rotor winding, a servomotor for varying the said resistance, and a differential electromagnetic device arranged to control the said servomotor, the said device being biased in effect, so that the servomotor is controlled to increase the said resistance, the said device having first and second electric windings arranged to be differentially energized to operate the device against the said bias, the first winding being arranged to be energized by a control current and the second winding by a restraining current which is arranged to be maintained substantially constant at least while the motor is operating in a given state of operation such as forward motoring or dynamic braking. A three-phase induction hoist motor 1 has its stator winding connectable to bus bars 3 through contacts 5 or 6 for motoring or reverse power braking operation. Dynamic braking is provided by connecting direct-current exciter 4 to the stator winding through contactor 7. The rotor resistance 8 is varied by means of an oil-operated servomotor having pump 11 and a valve 12 which is controlled by a magnet winding 15, excited from a two-phase generator 16 through an auto-transformer 17, and a restraining winding 32 which is arranged to be excited with constant alternating-current. The generator 16 is coupled to the motor 1, and, during motoring or reverse power braking operation, its three-phase rotor is supplied from the bus-bars 3 through a transformer 22. During such operation, the output of the generator 16, which is proportional to the slip speed of the motor 1, is applied to tappings in the transformer 17, the coil 15 being connected in series with a resistor 29 to the variable transformer tapping 30. The transformer 17 may consist of two parts wound on different cores, only one part being effective at a time according to the position of a control lever 31 which adjusts the tapping 30. The control lever is moved in alternate directions for forward or reverse operation, the taps 30 and 24 coinciding in the neutral position. During motoring, the valve 12 is adjusted until the force due to the winding 15 balances that due to the winding 32 minus the weight of the moving parts of the valve, the rotor resistance being varied, in the manner described in Specification 625,950, so that the torque of the induction motor is a maximum at all speeds. A synchronous motor 34 drives the exciter 4 and a pilot exciter 35 which supplies a filled winding 36 of the main exciter through an adjustable resistor 37. A rectifier 40, supplied through transformer 39, energizes the rotor winding of the generator 16, the output of which is proportional to the speed of the motor 1, during dynamic braking. The rotor resistance 8 is controlled to give maximum torque for a given position of the lever 31, the output of the generator 16 being applied between the centre tapping-point and the ends of the transformer 17. The top 44 of the resistor 37 is also controlled by the lever 31 which may be movable in a slotted gate and mechanically operate the contactors 5, 6, 7. When these contactors are open the windings 15, 32 are de-energized and the valve 12 falls under its own weight to adjust resistors 8, 29, to their maximum positions. An interlock device, such as described in Specification 674,931, may prevent closure of contactors 5, 6 until resistance 8 has reached its maximum value. The current in winding 32 is kept substantially constant by a network comprising saturable reactor 49 and auto transformer 50. Two resistors 52a, 52b are selectively connected to provide different values of restraining current for driving or power braking and for dynamic braking operation. The hoist may be held by a mechanical brake before starting, some electrical torque being applied before the brake is lifted. As the motor torque is increased, the torque on the brake reverses to operate a contact 54 (Fig. 2) acting through contacts 55, 56 on the main forward and reverse contactors. Contacts 61- 64, on contactors controlling the rotor resistance of the induction motor, control contacts 57-60 so as to co-ordinate the steps of a resistor 52, which is connected in series with the restraining winding 32, with those of the rotor resistance whilst the motor is at standstill. The amount of resistance connected in series with the coil 32 is thus dependent upon the motor load when the motor starts and the contact 54 changes over, the contacts 57-60 which have been operated being held in by coils 66. The restraining-current is therefore varied with the motor load to provide a desired rate of acceleration. Further adjustment of the resistor 52 is prevented when the motor has started by means of switch 67, operated by vane 68 in an oil-filled groove 69 around the motor shaft. Specification 674,930 also is referred to.