Speed control circuit for cage-rotor motor - has power unit and control unit allowing drive frequencies lower than mains frequency

摘要

A power unit contains numerous phase thyristors and is connected to a 3-phase supply network and has a 3-phase output. A control unit is connected to the power unit and initiates the commands for controlling the appropriate thyristors in the power unit. The control unit contains a microprocessor that performs the arithmetic operations required by the wanted speed. The control unit also selects the required positive and negative halfwaves. The advantage lies in simplicity and cheapness and in being able to select frequencies lower than the mains frequency without lose of precision in setting the frequency.