GB543788A - Improvements in the control of synchronous electric motors - Google Patents

Abstract

543,788. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO. Ltd. Sept. 2S. 1940, No. 14471. Convention date, Sept. 23, 1939. [Class 38(iii)] In a synchronous motor run up to near synchronism as an induction, motor with the field system 3 short-circuited through resistance 5, the resistance is removed and the field supplied with its D.C. excitation when the required minimum speed has been reached and at pre-determined favourable points in the slipcycle of the sub-synchronous operation in order to provide for the maximum pulling-in torque. The speed responsive means takes the form of an impedance relay 18 carrying an opposing torque voltage element which compensates for supply variations and is described in Specifications 512,361 and 530,122. At a speed corresponding to say 3 per cent. slip its contact arm moves relatively slowly between the contacts 28 and 30 under the action of the armature current pulsations. When the contacts 28 are opened continuously for a pre-determined time interval the operating winding 34 overcomes the holding winding 33 of a time-delay relay 32 (in which the time delay is determined by resistances and capacities across the two windings), and the relay 32 closes. This connects. the holding winding 14 of the. field switch 13 in series with the contacts 28 so that the circuits of the two holding windings are opened at the same points in the half-slip cycle when the motor is operating at the minimum speed at which it will effect the operation of the relay 32. The closing winding 15 therefore becomes effective, the field switch having an operating time about half that of the relay 32, to change over the field winding to excitation, with the resulting synchronization. The field switch, in closing, short-circuits resistances and condensers in the windings of the relay 18 thereby altering its characteristics to suit the reverse operation, i.e. the reversal of the field switch when the synchronous motor falls out of step. Specification. 530,123, [Group XXVII], also is referred to.