Shaft Voltage Comparison between Grounded and Ungrounded Brushless DC Motors with Insulated Rotor Driven by PWM Inverter

The voltage source PWM inverter generally used to drive air-conditioning fans has posed a major problem, that being the high probability of electrical erosion of the bearing. Through inverter switching, a potential difference, called the shaft voltage, is generated between the inner and outer races, causing electrical breakdown of the bearing lubricant. This results in a current passing through the bearing that causes bearing erosion. We verified that shaft voltage is reduced by the use of an insulator inserted between the outer and inner cores of the rotor in an air-conditioning fan motor where the frame is not grounded. This paper describes the calculated and measured waveforms of shaft voltage for the insulated rotor brushless DC motor driven by a PWM inverter. We verified that the insulated rotor also has the effect of suppressing shaft voltage even when the motor frame is grounded.