GB656304A - Improvements in and relating to electrical control systems for winding and like motors - Google Patents

Abstract

656,304. Automatic speed control systems. METROPOLITAN-VICKERS ELECTRICAL CO., Ltd. Nov. 24, 1948, No. 30547. [Class 38 (iv)] [Also in Group XXXVII] In a control system for an A.C. winding or like motor having provision for dynamic braking, the stator 1 of the motor is excited during braking through a multiphase discharge rectifier 8, the grid voltage of which is controlled by the difference between the voltage of a tachometer generator 18 driven by the motor and that derived from a potentiometer 13 in accordance with the desired speed. In this manner the excitation of the motor 1 during braking conditions is adjusted so as to neutralize the discrepancy between the actual and desired speed of the motor. The potentiometer contact 12 is operated by a cam 10 driven by the winding motor, the cam being shaped in accordance with the desired speed control throughout the winding range. When the motor is driving the load, the stator 1 is excited from A.C. mains 2, 3, 4 over " forward " or " reverse " contactors Fl, F2, F3 or R1, R2, R3, the rotor 5 having in circuit an adjustable liquid rheostat 6, the electrodes of which are controlled by a motor 7 in accordance with the difference voltage between the tachometer generator 18 and the potentiometer 13 so as to regulate the speed. Under " driving " conditions the voltage of 18 is less than that derived from 13 and relay OCM is operated to close contacts M1, M2 and thus apply this difference voltage to the motor 7 through a transductor 16. Under " braking " conditions, i.e. when torque is applied to the winding motor from the load the generator voltage is greater than that derived from the potentiometer and a relay OCB is operated to (1) apply the difference voltage to the rectifier grids via a phase-shifter 22 at contacts B3, B4, (2) switch the rectifier 8 supplied by transformer 9 across the stator 1 at contacts B1, B2, and (3) connect a tachometer generator 23 and potentiometer 24 to the input of the transductor 16 at contacts B5, B6. The contact 25 of the potentiometer 24 is mechanically coupled to the movable electrodes of the rheostat 6, the arrangement being such that during braking, the rotor resistance is maintained proportional to the speed of the motor by the interaction of the generator 23 and potentiometer 24, and at the same time stator excitation is varied under control of generator 18 and potentiometer 13 to maintain the speed in accordance with that set by the cam 10. The potentiometer 13 may also be adapted for manual operation independently of the cam, the latter serving only to override the manual operation when the maximum desired speed is exceeded. In this case excessive input to the transductor 16 caused by rapid movement of the contact 12 may be neutralized by a current transformer 27 feeding an opposed signal into the transductor. In modifications, the potentiometer 24 may be replaced by a device for measuring the resistance of the liquid rheostat 6 as described in Specification 637,128, [Group XXXVII], and the relays OCM, OCB replaced by a biassed transductor responsive to reversal of the control voltage.