SPEED REDUCING CONTROL SYSTEM WITH VOLTAGE REGULATION FOR A POLYPHASE ELECTRIC MOTOR

摘要

SPEED REDUCING CONTROL SYSTEM WITH VOLTAGE REGULATION FOR A POLYPHASE ELECTRIC MOTOR A variable speed 3-phase a-c motor is connected via a plurality of controllable bidirectional switches to a 3-phase source of alternating voltage having a variable fundamental frequency (f) and an rums magnitude (v) that varies with f, and the switches are suitably arranged and controlled so that motor speed can be reduced by skipping selected cycles of the source voltage. When operating at a fraction of full speed, the magnitude-to-frequency ratio of motor voltage is maintained substantially the same as at full speed by varying the "firing angle" at which the switches start conducting current so as to minimize any deviation in the rms magnitude of the motor voltage from the product of v times said fraction.