GB1410141A - Synchronous motor control circuits - Google Patents

Abstract

1410141 Control of A.C. motors FIZIKOENEBGETICHESKY INSTITUT AKADEMII NAUK LATVIISKOI SSR 7 Feb 1973 6004/73 Heading H2J A control circuit for the induction starting of a synchronous motor comprises a rectifier 4 supplying the rotor field winding 2 through a transistor 1 controlled by a transistor 5 in turn controlled by a rectifier 16 and the difference between the outputs of rectifiers 6, 7, the rectifiers 4, 7, 16 being energized by a transformer 10 also supplying the stator winding (not shown) of the motor, the rectifier 6, together with a rectifier 15 being supplied by a transformer 11 whose primary is constituted by the field winding 2, and the rectifier 15 controlling the firing of a silicon-controlled rectifier 13 which controls the application to the transistor 5 of the difference between the outputs of rectifiers 6, 7. The control circuit is designed to prevent undesired energization of the field winding of the type of motor having damper cages producing an E.M.F./slip characteristic (B), Fig. 2 (not shown), having two values of slip corresponding to the same rotor-induced E.M.F., energization of the field winding being prevented over the range from the motor at rest to the first slip value. In a typical starting operation, the transistor 5 is initially controlled by the bridge 16 to make transistor 1 non-conductive, a diode and high value resistor combination across transistor 1 allowing a small current to pass to the winding 2, which is shunted by resistors 22, 23 during respective alternate half-cycles of the slip E.M.F. When the first value of slip has been reached, a Zener diode 14 breaks down to allow the rectifier 15 to fire the S.C.R. 13 and apply the output of divider 8 to the transistor 5, already receiving the output of rectifier 6 from a resistor 9. As synchronous speed is approached, the voltage U6 of rectifier 6 falls below that U7 of rectifier 7 and transistor 5 becomes progressively less conductive to progressively turnon transistor 1 and so increase the current to the winding 2, the transistors 1 and 5 rapidly becoming respectively ON and OFF to allow the motor to pull into step. The aforementioned control of the transistors 1, 5 also occurs if the motor falls out of step, thereby restoring the synchronous operation.