PURPOSE: To adjust the energizing current in response to drill loads by providing a voltage regulating circuit to adjust the voltage in response to the number of drill revolutions in the energizing circuit of an electromagnet absorbing a drill stand to a work under process.

CONSTITUTION: When a switch is turned on to rotate a motor 2, an output is generated from a revolution detecting circuit 16, and the output is fed into a comparator circuit 31 through a converter 24, a voltage follower 27, and an amplifier circuit 28, raising the negative input side voltage of the comparator circuit 31 higher than the positive input side voltage. Accordingly, the output from the comparator circuit 31 is made zero, a photocoupler 36 is made non- conductive, and a bidirectional thyristor 42 is made conductive at both sides of the AC current. Therefore, the voltage applied to a rectifier circuit 3 is doubled in appearance compared to that when the motor 2 is stopped, and the energizing current of the electromagnet 4 is made larger accordingly, i.e., when the motor is stopped a smaller energizing current flows through the electromagnet 4.