GB713483A - Improvements relating to electric motor control systems - Google Patents

Abstract

713,483. Control of A.C. motors. ENGLISH ELECTRIC CO., Ltd. Feb. 6, 1953 [Feb. 6, 1952], No. 3170/52. Class 38(3) [Also in Group XXXV] During dynamic braking operation, the primary winding 10 of a three phase induction motor is connected for direct-current excitation to the output stage of an amplifying exciter 11 which may be of the kind set forth, in Specification 579,570, [Group XXXV], the secondary winding 12 of the motor being connected to a rotor resistance controller 13. A control winding 11a of the exciter is supplied through rectifier 14 from the secondary winding 12 and is thus excited in accordance with the product of motor flux and speed. The winding 11b opposes the winding 11 a and is supplied from motor-driven tachometer generator 15 through rheostat 16 which is coupled to the controller 13. The rheostat 16 is arranged so that the full output of the tachometer generator is applied to winding 11 b when the controller 13 is in the higher torque range. For any given setting of the controller 13, assuming steady conditions, the voltage of the generator 15 will fall at a greater rate than that of the secondary winding 12 when the motor speed is reduced. The consequent unbalance of the excitation of the windings 11a, 11b, tends to reduce the stator excitation and so restore the ampere-turns of the two windings near to balance. This tends to maintain the flux constant irrespective of rotor current and gives a braking torque which, for a given speed, increases continuously with movement of the controller 13 to reduce the rotor resistance. The ganging of the rheostat 16 and controller 13 has the effect of extending the range of controlled braking torque. In an alternative arrangement, the amplifying exciter may have a single input winding. The Provisional Specification refers to the use of a magnetic amplifier.