The control device detects the B-terminal voltage by a B-terminal voltage detection section. When an abnormal voltage detection section detects that a voltage value of the detected B-terminal voltage is equal to or higher than a predetermined voltage value, all switching elements of the negative pole-side arm in a power conversion section are brought into conduction by a negative pole-side arm short-circuiting section, whereas all switching elements of a positive pole-side arm are interrupted. In addition, a field current is limited to zero or to a limit value by a field current control section.