Comparing Brushless DC Motors: A Method of Suppressing the Shaft Voltage Even in a Grounded Motor Frame

The voltage-source pulsewidth modulation (PWM) inverter generally used to drive air-conditioning fans has posed a major problem, i.e., 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 an electrical breakdown of the bearing lubricant. This results in a current, sometimes called the electric discharge machining current, passing through the bearing, which causes bearing erosion. The 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 article describes the calculated and measured waveforms of the shaft voltage for the insulated rotor brushless dc (BLdc) motor driven by a PWM inverter. The insulated rotor also has the effect of suppressing the shaft voltage even when the motor frame is grounded.