Abstract: A method of controlling the rotation speed of a motor to maintain it at a desired value, comprising a step of obtaining a deviation between the detected rotation speed and the desired value, and a step of changing the gain of a feedback loop, which serves to control the current flowing in the motor, to a value lower than a predetermined one during a period in which the deviation of the detected deviation in rotation speed is smaller than a preset maximum ripple value.

Abstract: A digitally controlled rectifying system used for driving a d.c. motor through antiparallel connected forward and reverse converters. The firing phase angle of the converters is shifted by 180.degree. at a time of gate switching operation for the converters so that the load current is formed to become zero, and after the gate switching operation the firing phase angle is shifted to a position at which the load current is substantially zero, whereby the time lag of control occurring at the switching operation of the converters can be minimized irrespective of the load condition.