To prevent the generation of starting noise due to the fluctuation of speed from by detecting the output pulse of a pulse generator, and by providing a means for turning off and on the connection between an auxiliary coil winding and a power supply when the frequency or the period of the output pulse is equal to or more and equal to or less than a setting value, respectively.

A pulse generator 10 outputs a pulse according to the speed of an electric motor M. A detection means uses three timers T1, T2, and T3 by a CR. The timer T1 starts clocking by the input of a trigger pulse, and the timer T2 is triggered by the inverse phase of the timer 1. The timer T3 is connected to a logic product circuit consisting of the timers T1 and T2. When the electric motor M is rotated, the output pulse is inputted to the timers T1 and T2 for starting clocking and the timer T3 increases speed with the clocking state being retained and an auxiliary coil winding being turned on. When the period of the output pulse becomes shorter than the clocking time of the timers T1 and T2, the timer T3 is not triggered and the auxiliary coil winding turns off a main coil winding for reducing the speed.