Abstract: The duty cycle of the alternating current waveform applied to an electric motor is varied by varying the firing point of an SCR circuit. This firing point is varied by logic circuitry which responds to the largest (or smallest) output of several integrators each of which provides an increasing or decreasing control voltage in response to different error signals. The error signals are related to different control criteria such as motor speed, motor current, etc. Thus the error signal indicating the greatest deviation from a set point controls the firing point. Since the output level of the integrators is slow to vary, a high degree of noise immunity is attained.