APPARATUS FOR CONTROLLING AN ELECTRIC MOTOR

A motor controller includes a set of thyristors which couple the motor to a source of alternating electricity. A detector senses when the voltage of the source makes a zero crossing. A mechanism is provided to vary the frequency of a clock signal (GATE CLOCK) at a rate determined by a user selectable input which represents the rate at which the motor speed is to change during starting or stopping. The clock signal (GATE CLOCK) is applied to a counter (71) which produces an active trigger signal (FIRE) when a given number of clock signal cycles have been counted following a zero crossing of the source voltage. The active trigger signal causes a trigger pulse circuit (72) to apply a brief trigger pulse to gate electrodes (21, 22, 23) of the thyristors. The varying frequency of the clock signal (GATE CLOCK) alters the relative times at which the thyristors are triggered to vary the amount of voltage applied to the motor. Different frequency variation functions can be stored in a memory for the starting and stopping modes.