Suppressing Bearing Voltage in an Inverter-Fed Ungrounded Brushless DC Motor

This paper describes the bearing voltage reduction achieved by the brushless dc motor for air conditioning driven by the voltage source inverter. First, the simplified equivalent circuit of the brushless dc motor is derived based on the common-mode equivalent circuit where stray capacitance is taken into account. Then, the electrostatic capacity distributions between a winding and a stator core, a winding and a magnet, a stator core and a magnet, and magnet capacitance are measured, and the bearing voltage is calculated with the simplified equivalent circuit. It is confirmed with the experiment that the bearing voltage can be suppressed when the inside and outside of the rotor core are insulated with resin (hereafter, the insulated rotor) and that the electrostatic capacity of the rotor is made smaller. Finally, a bridge-type equivalent circuit is derived in an air conditioning fan motor where the frame is not grounded, an insulated rotor is actually designed, and the bearing voltage is measured.