GB620207A - Improvements in or relating to alternating current electric motor equipments - Google Patents

Abstract

620,207. Control of A.C. motors. GENERAL ELECTRIC CO., Ltd., and COX, W. R. Jan. 8, 1947, No. 656. [Class 38(iii)] In order to prevent the overheating of a winding motor during dynamic braking at low speeds, means is provided so that the stator excitation (D.C. or low frequency A.C.) is automatically limited when the motor is running at low speeds and further means, dependent upon the motor speed, is provided to render the first means inoperative when the motor speed exceeds a predetermined value so that, in a preferred arrangement, the first means is incapable of again limiting the excitation should the motor speed fall below the predetermined value during the particular period of braking. As shown, the stator winding of a motor 1 is supplied during dynamic braking with D.C. from a generator 2 the field of which is limited initially by a resistor 14, At say 25 per cent full speed the tachometer generator 16 operates the relay 18 which in turn operates relay 21 to short-circuit the resistor 14, the braking torque then being that chosen by the setting of the control lever of the rheostat 15. The relay 18 becomes inoperative when the speed of the motor falls below say 15 per cent full speed, but the relay 21 remains energized through hold-in contacts 24 until a contact 22 is opened by the return of the control lever to the neutral position. The contacts 22 may alternatively form auxiliary contacts on the contactor 9. In another embodiment an additional relay provides an arrangement wherein maximum braking torque is applied on the failure of the relay 21. Specification 620,206 is referred to.