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The brushless DC motors are also becoming popular in air conditioning fan motors, and protection of bearing from electro-erosion is becoming a major challenge in the North Europe market in particular. The bearing electro-erosion is caused when the bearing lubricant is dielectrically broken down, making current run on the inner race and outer race of the bearing. As a countermeasure, we verified that shaft voltage was suppressed by the insulated rotor. Air conditioning fan motors are known to generate the necessary wind for cooling the heat exchanger and its motor rotating speed. It was also confirmed that the shaft voltage increased as the motor rotating speed increased. This paper describes the shaft voltage falls below the dielectric breakdown voltage of the bearing grease in the actual operating speed range and is also verified that insulated rotors are effective for suppressing electro-erosion with respect to changes in rotating speed.