Abstract: The motor speed control apparatus of the present invention includes a thyristor (14) connected in the current supply circuit of a motor (13) for phase controlling the motor to control its rotational speed. The determination of the trigger timing of the thyristor in the event that the actual trigger timing would occur at a point after the occurrence of a zero voltage point of the mains supply voltage, is made at a point in time advanced by an amount corresponding to the delay time between the time prior to a speed change and the time following the speed change, whereby the motor speed is rendered constant.

Abstract: A frequency converter having a silicon controlled rectifier comprises a protection circuit which includes a series resonant circuit connected in parallel with the silicon controlled rectifier to generate a reverse voltage thereacross in response to a short circuit established in response to a commutation failure of the silicon controlled rectifier. An inhibit circuit is provided to respond to the reverse voltage by inhibiting the current flow through the silicon controlled rectifier to allow the same to restore to its locking state.