Tatsuo Maetani

Approaches to suppressing shaft voltage in brushless DC motor driven by PWM inverter

This paper describes the shaft voltage reduction achieved by the brushless DC motor for air conditioning driven by the voltage source inverter. First, the simplified equivalent circuit of the concentrated winding 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 winding and stator core, winding and magnet, stator core and magnet, and magnet capacitance are measured, and the shaft voltage is calculated with the simplified equivalent circuit. Finally, the bridge type equivalent circuit is derived, and it is confirmed with calculation and experiment that the shaft voltage can be suppressed when the inside and outside of the rotor core are insulated with resin (hereinafter, the insulated rotor).

Influence of motor rotating speed on shaft voltage of brushless DC motor with insulated rotor driven by PWM inverters

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.

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.

Study of the Further Reduction of Shaft Voltage of Brushless DC Motor With Insulated Rotor Driven by PWM Inverter

Authors succeeded in reduction of the shaft voltage of a motor with an insulated rotor driven by PWM inverter previously. This paper proposes a new method of further reduction of the shaft voltage of the motor with the insulated rotor. A non-grounding common-mode equivalent circuit of the motor is examined, and the effect of the further reduction by a new method is verified by calculation of the shaft voltage from the equivalent circuit and measurement of the shaft voltage of the motor.

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.

Study of the further reduction of shaft voltage of brushless DC motor with insulated rotor driven by PWM inverter

The authors previously succeeded in reducing the shaft voltage of a PWM driven motor with a rotor which had an outer core and an inner core (and the shaft), electrically insulated each other by a resin (hereafter, the insulated rotor). This paper proposes a new method for further reduction of the shaft voltage of a motor with an insulated rotor by adding a capacitor between brackets and N line of the dc link of the inverter. A common-mode equivalent circuit of the system with an ungrounded motor is examined, and the effect of further reduction by the new method is verified by calculation of the shaft voltage from the equivalent circuit and measurement of the shaft voltage of the motor.