PURPOSE: To reduce noise and vibration when driving a delta connection three- phase motor by adding properly combined pulse voltages to a delta connection three-phase motor and bringing the phase current waveform of a motor coil winding closer to a fundamental sinusoidal waveform.

CONSTITUTION: Properly combined pulse voltages 10 are added to a delta connection three-phase motor 5 and a phase current waveform 9 of a motor coil winding is brought closer to a fundamental sinusoidal waveform. The properly combined pulse voltages 10 do not depend on the pulse width modulation system and are obtained by combining the a power supply voltage 2 (H), a grounding voltage 3 (L), and an open voltage by considering the operating characteristics of the delta connection three-phase motor 5. Therefore, a current closer to a sinusoidal waveform equivalent to the phase current waveform 9 flows in terms of a terminal current 4, thus causing three-phase current to flow in an improved balance and hence reducing noise and vibration when driving the delta connection three-phase motor 5.