GB666561A - Improvements relating to the control of electrically operated winding or like mechanisms - Google Patents

Abstract

666,561. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd., LIDDIARD, H. W. C., PETCH, T. H., and ROLFE, E. May 24, 1949. [May 24, 1948] No. 13991/48. Class 38 (iii). An induction motor driving winding or the like mechanism has a control system for providing a dynamic braking effect comprising means for connecting the primary of the motor to a source of direct-current, the voltage of which is made to vary in such a way that it causes the voltage generated in the secondary winding of the motor to be directly proportional to the frequency of .the current flowing in the aforesaid secondary winding, so that the torque is maintained substantially proportional to speed throughout the entire speed-range. The source of direct-current is an amplidyne generator A having a field winding F1 excited through rectifier B1 and saturable reactor SR, and a field winding F2, oppositely excited through rectifier B2 and resistor R2, both field windings being energized from the secondary circuit of the induction motor IM. A third field winding F3 of the amplidyne has constant excitation and is boosted by the winding F2. The element SR saturates at different frequencies of the applied voltage so that the excitation of the winding F1 depends upon the frequency of the rotor current of the induction motor. If the rotor voltage of the induction motor increases, the frequency remaining constant, the current rise in F1 is greater than in F2 thereby increasing the bucking effect and restoring the rotor voltage to its former value. A change 'of frequency alters the excitation of winding F1, the rotor voltage and that across winding F2 being forced to correspond so that the rotor voltage is varied in accordance with the rotor frequency. A stabilising transformer T may be connected to feed back into the circuit of the winding F3. The excitation of the field windings F1, F2, may be, in addition, responsive to the secondary current of the motor IM, a current transformer being connected in the secondary circuit and developing an appropriate voltage across a resistor in the circuit of the windings, Fig. 2 (not shown). The generator A may be replaced by a system of saturable reactors and rectifiers, Fig. 3 (not shown). Specifications 535,016 and 535,732 are referred to.