Patent GB-717253-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B66B1/12
Filed: 1952-01-17
Abstract: 717,253. Control of A.C. motors. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S. Jan. 9, 1953 [Jan. 17, 1952], No. 1379/52. Class 38(3) [Also in Group XXXV] A reversible A.C. motor winding system is provided with: (i) automatic speed control to a preset value by variation the rotor resistance; (ii) dynamic braking by D.C. energization of the stator for sudden speed reduction; (iii) plugging for reversing (iv) torque control to limit accelerator of the winder. General. As shown a wound rotor induction motor 10 has its rotor connected to a servo adjusted liquid resistor 13 and its stator energized through forward or reverse contactors 53, 55 during motoring or through braking contactor 83 from a D.C. supply during dynamic braking. The valve 26. which controls a hydraulic system to operate the movable contact 14 of the rotor resistance, is differentially actuated by coils 23, 32. The coil 23 is energized from a control generator 20, coupled to the winder motor and having its rotor energized from the A.C. supply during motoring and an auxiliary D.C. supply during braking. The coil 32 is energised by a speed control signal as described below. Auxiliary supplies are provided by a flywheel loaded motor generator set comprising a dynamic braking exciter 71 and pilot exciter, and an auxiliary A.C. and D.C. supply generator 48. Automatic speed control. A predetermined reference voltage from resistor 47 (selected by operation of the control lever) is applied to control a magnetic amplifier 60 connected in series with a magnetic amplifier 61 controlled by the voltage of a tachometer generator 33 driven by the winder motor. The voltage drops across the magnetic amplifiers are rectified and applied to the fields 70, 73 of a pilot exciter which energizes the dynamic braking exciter 72 and a further magnetic amplifier 80. The output of the amplifier 80 is applied through a resistor 90 to the other valve operating coil 32 to control the rotor resistor. The coil circuit is shunted by electrodes 16 immersed in the electrolyte of the rotor resistance to compensate for changes in its conductivity. Dynamic braking. Operation of the control lever to suddenly decrease speed causes operation of a polarized relay 81 to de-energize the forward or reverse contactors and energize the dynamic braking contactor 83 connecting the winder motor stator to the exciter 72. An additional current component is applied to the rotor-resistancecontrol-valve-coil 32 through resistance 91 from the auxiliary A.C. supply to modify the motor torque characteristic. Reversing. Contact 46 on the control lever cooperating with contacts 56, 57, which are connected to a "direction of rotation" polarized relay 77 energized from the tachometer-generator 33, causes operation of a plugging relay 74 to reverse the pilot exciter field 73 and release the polarized relay 81, whereupon the dynamic braking contactor drops out and the selected forward or reverse contactor is energized. Torque control. The Specification states that a strain gauge in the winder head-gear may be arranged to vary the resistance 90 in series with the rotor-resistance-control-coil 32; Modifications. (i) The pilot and main exciters for dynamic braking may be replaced by saturable reactors Fig. 2 (not shown); (ii) dynamic braking may be omitted and braking effected by plugging only Fig. 3 (not shown); (iii) The magnetic amplifiers may be arranged to saturate in order to restrict the rate of correction with large error signals. Specifications 625,950 and 715,340 are referred to.