Study of the Further Reduction of Shaft Voltage of Brushless DC Motor With Insulated Rotor Driven by PWM Inverter

The authors previously succeeded in reducing the shaft voltage of a PWM driven motor with a rotor which had an outer core and an inner core (and the shaft), electrically insulated each other by a resin (hereafter, the insulated rotor). This paper proposes a new method for further reduction of the shaft voltage of a motor with an insulated rotor by adding a capacitor between brackets and N line of the dc link of the inverter. A common-mode equivalent circuit of the system with an ungrounded motor is examined, and the effect of further reduction by the new method is verified by calculation of the shaft voltage from the equivalent circuit and measurement of the shaft voltage of the motor.