A potential difference called the shaft voltage of brushless DC motors driven by PWM inverter is generated between the inner race and outer race of bearing due to inverter switching, and the voltage causes grease on the ball bearing to be dielectrically broken down, making current run between the inner race and outer race. As a result, electrical pitting is generated. In small ball bearings for air conditioning at the voltage endurance test of electrical pitting, the value of oil film parameter that is large enough to be abnormal has been reported in the past. In this paper, we proposes the brushless DC motor structure (We termed as the insulated rotor) for air conditioning which reduces the shaft voltage and an electric discharge phenomenon is not produced, and we verify the surface roughness of the inside of a bearing with paying attention to the influence of the oil film parameter in the proposal specification.