GB512361A - Improvements in and relating to electric motor control systems - Google Patents

Abstract

512,361. Control of A.C. motors. BRITISH THOMSON-HOUSTON CO., Ltd. July 16, 1938, No. 21157. Convention date, July 16, 1937. [Class 38 (iii)] The excitation of a synchronous motor 1 after it has run up to speed is effected under control of a control relay 15 energised in accordance with the frequency of the primary current pulsations when the motor is operating out of synchronism and the control relay is compensated for fluctuations in the supply voltage. Upon operation of a switch 7 a selfmaintaining main switch 5 is closed to connect the motor armature 2 to the supply. The relay 15 has a coil 18 energised by the armature current and a coil energised by the supply voltage. The torque exerted by these coils is opposed by that exerted by a compensating coil 21 energised by the supply voltage. Initially the armature current is such that the relay closes its contacts 22 and a relay 24 is energised. This relay 24 operates to energise a relay 25 which prepares at contacts 28 an energising circuit for a field switch 11. The armature current pulsations are such that the contacts 22 remain closed until a predetermined speed is reached whereupon the contacts 22 open periodically and when the motor speed reaches such a value that the contacts 22 remain open for a period sufficient for the relay 24 to open the field switch 11 is energised. This opens a closed circuit for the field and connects a direct current source 10. The coil 21 is effective to compensate for variations of supply voltage. The operation of the field switch de-energises the relay 25 which opens after a time lag. The field switch in closing short-circuits a resistance 33 in the circuit of the coil 21 so that the setting of the relay 15 is altered. If the motor falls out of synchronism the power factor becomes sufficiently lagging, and its armature current sufficiently great to cause the relay 15 to reclose its contacts 22 and re-energise the relay 24. This short-circuits the coil of the field switch 11 which moves to its open position so that the motor again runs as an induction motor until its speed again reaches a value at which the field is re-energised by the operation of the relays 15, 24, 25. Specification 464,866 is referred to.