When a voltage abnormality error signal from a drive control section is at a low level, a control section determines a normal state. When the voltage abnormality error signal has arisen to a high level, the control section does not immediately determine an abnormal state and instead measures a time in which the voltage abnormality error signal is at the high level. If the measured time does not exceed a predetermined error timer value (limit value) T, the control section ignores this abnormal state as a temporary voltage abnormality. If the time in which the voltage abnormality error signal is at the high level exceeds the error timer value T, the control section determines a serviceman call error (SC) as the occurrence of voltage abnormality.