The throttle opening setting value provided from a throttle controller (108) may exceed the actual controllable range of the throttle opening. The value stored in an output voltage setter (20) is read out in response to its throttle opening setting value and transferred to a correction signal generator (21). The generator (21) supplies a correction signal determined by the output voltage setting to amodulator (23). PWM signal is transferred to an inverter (3) for determining the output voltage. When receiving the opening setting value indicative of overloading, the output voltage setter (20) releases a smaller value to decrease the output of the generator. <IMAGE>
