To prevent an element from being broken by the increase of a power supply voltage to a withstanding voltage of a connecting element or higher during the brake control or the deceleration control of a motor.
A voltage detection determination circuit 4 which detects and determines two voltages of an external torque command voltage and a first power supply 1 is used to detect that they are voltages for deceleration control from the brake control and high speed rotation to low speed rotation, and that the voltage of the first power supply 1 is raised over a specified threshold value. Based on the detected determination result, the device is controlled to turn on all power transistors which allow synchronous currents of the power transistors to be supplied so that the regenerated current does not flow to the first power supply 1. Alternatively, the device is controlled to turn on all the power transistors which allow source currents of the power transistors to be supplied, preventing the voltage of the first power supply 1 from being excessively raised to prevent the element from being broken.
NISHINO HIDEKI
YAMAMOTO YASUNAGA
