speed command voltage ls whichever has greater level the switch 44 is a normally closed switch which is opened when the control pedal 7 is moved down even if the movement is very small the rotating speed deviation calculating unit 33 has a first amplifier 45 which calculates the rotating speed deviation u03b4n between the rotating speed command voltage s and the rotating speed n of the motor 5 the negative maximum value of the rotating speed deviation u03b4n is limited by a zener diode 46 the deviation amplifying unit 34 has a second amplifier 47 which subtracts the current signal if from the rotating speed deviation u03b4n to limit substantially the maximum current supplied to the motor 5 the maximum current is regulated by a variable resistor 48 the pwm unit 35 has a first comparator 49 which compares a triangular pulse signal generated by a carrier pulse oscillator os1 50 and the output signal of the deviation amplifying unit 34 and provides a rectangular pulse signal of a pulse width corresponding to the rotating speed deviation u03b4n the frequency of a carrier pulse signal generated by the carrier pulse oscillator 50 is 2 khz the photocoupling unit 36 comprises as principal components the light emitting diode 51 i e one of the two components of the photocoupler ph and a transistor 52 connecting the light emitting diode 51 to the power source the light emitting diode 51 and the phototransistor 21 of the power transistor driving circuit 20 constitute the photocoupler ph while the transistor 52 of the photocoupling unit 36 is held on the light.