To relieve the beat of vibration sounds accompanying the rotation of two or more motors at the vicinity of the same rotational speed, at a low price.

In a condition that each phase of a first polyphase motor 1 and a second motor of the same number of phases is connected to common output lines 11-13 for each phase, these motors are driven based on PWM (pulse width modulation) control in single mode by a single inverter circuit 5. Even if they are driven with the single inverter circuit 5, a slip occurs on the side of the second motor 3, so a difference occurs between the rotational speeds of both motors 1 and 3 even if the adjustment by a special control is not performed. Accordingly, the beat of the vibration sounds accompanying the rotation of two or more motors 1 and 3 in the vicinity of the same rotational speed can be relieved by driving them with one inverter circuit 5.

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