CONTROLLER FOR STARTING AND STOPPING ELECTRIC MOTORS

摘要

A three-phase motor is operated by a controller having sets of thyristors which when triggered switch electricity from each phase of a supply to the motor. The controller measures the interval .PHI. between when the voltage and the current for one phase to the motor make zero crossings and measures the interval .gamma. that the thyristor are non-conductive during half-cycle of the voltage for the one phase. A control angle .THETA. is calculated according to the equation .THETA. = .PHI. + K.gamma., where K is a positive number. The control angle .THETA. is compared to a reference value to derive an error value. The times at which the thyristors are triggered is controlled in response to the error value. To start the motor, the reference value is gradually decreased to trigger the thyristors progressively earlier during each half cycle of the supply voltage. To stop the motor, the reference value is gradually increased to trigger the thyristors progressively later during each half cycle of the supply voltage.