PURPOSE: To prevent a friction clutch on an electric motor from burning or wearing, and adverse effects of chips resulting from abrasion on bearings, etc., by cutting or reducing power supply to the motor when it rotates slower or is locked due to an excessive external load while monitoring the rpm of the output shaft.

CONSTITUTION: To start a motor (M), an ON-OFF signal generator 47 issues an ON signal, and on receiving the ON signal a timer 44 outputs an ON signal to a power control circuit 45, which then feeds electric power from a power source 46 to the motor (M). Thus, the motor (M) starts rotating. When an excessive load is applied to the motor (M), a built-in clutch mechanism will slip. When the motor rpm detected by a rotation detector 41 lowers below the rpm indicated by a reference speed indicator 42, a speed comparator 43 will issue a signal to timer 44, which will output an OFF signal to the power control circuit 45 after a pre-set time passes. On receiving the OFF signal, the power control circuit cuts the power supply from the power source 46 to the motor (M).