0093-9994 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TIA.2017.2717378, IEEE Transactions on Industry Applications Mitigation Method of the Shaft Voltage according to Parasitic Capacitance of the PMSM Jun-kyu Park 1 , Thusitha Wellawatta 2 , Sung-Jin Choi 2 , and Jin Hur 1 1 Dept. of Electrical Engineering, Incheon National University, Incheon, South Korea 2 School of Electrical Engineering, University of Ulsan, Ulsan, South Korea jinhur@inu.ac.kr Abstract— This study proposes the shaft voltage mitigation method according to change in parasitic capacitances of a permanent magnet synchronous motor. To consider the shaft voltage reduction in the initial motor design process without any filter, the parasitic capacitances affecting the shaft voltage are calculated using the motor geometry parameters. Then, the shaft voltage is analyzed according to change in parasitic capacitances using the equivalent circuit model and the torque characteristic is also analyzed to effectively mitigate the shaft voltage. As a result, the rotor-to-winding is determined as an appropriate parameter to mitigate the shaft voltage among the parasitic capacitances because it affects the shaft voltage and does not affect the output torque. Finally, the shaft voltage mitigation method according to variation of rotor-to-winding capacitance is verified by experiment. Keywords—Bearing current, common-mode voltage, equivalent circuit, parasitic capacitance, permanent magnet synchronous motor, shaft voltage. I. INTRODUCTION Permanent magnet synchronous motors (PMSMs) driven by a space vector pulse width modulation (SVPWM) inverter are widely used in lots of industrial fields due to their high power density and high torque. In the drive system, however, they have the major cause of motor bearing failure due to fast switching of the SVPWM [1]–[3]. Practically, all SVPWM inverters generate a common-mode voltage (CMV) relative to the ground, which creates a shaft voltage through parasitic capacitances of the motor [4]–[7]. The electric field of the stator winding allows many parasitic capacitance couplings in excrescent places. The high dv/dt in the CMV excites these capacitance links. The results of this connection generate a voltage in rotor shaft. This concept was mentioned by Chen and Busse [5]–[8]. Specifically, bearings are connected in two different paths. One is connected in parallel with the air–gap capacitor (capacitance between the stator and the rotor, defined as Csr), and another is connected in series with the parasitic capacitance between the winding and rotor (Cwr). Generally, bearing current by high dv/dt is not considered as risk factor because it is very small [8], [9]. However, if the effective bearing impedance becomes small due to electric discharge machining (EDM) effect (The electric breakdown field strength of the lubricant is at approximately 10–15 V/μm [8], [10]), bearing races and balls become short because EDM effect causes the dielectric breakdown of the lubricant film, as shown in Fig. 1. For this reason, the shaft voltage is discharged as a shaft current through the bearing. Owing to increase in the maintenance and downtime costs, attention on protecting the motor bearing has increased. Thus, lots of studies have been conducted on the shaft voltage reduction [11]–[17]. In [11], microfiber ring is applied as a basic solution. It grounded the shaft voltage directly, but it also needs to be replaced after degrading. In [12], a slot wedge based reduction method was proposed to mitigate shaft to ground voltage. In [13], an insulated hybrid bearing concept was proposed. The bearing is made by nonconductive materials for this technique but it limits the mechanical strength of the motor. In addition, slot- embedded partial electrostatic shield concept was proposed in [14] to reduce the capacitive coupling between the stator windings and the rotor. On the inverter side, lots of studies have conducted on the reduction of the CMV [15]–[17]. As a disadvantage of previous studies, those reduction methods require additional installations or nonconductive materials. In this study, a shaft voltage mitigation method is proposed by changing parasitic capacitances that can be considered in motor initial design process. Thus, parasitic capacitances coupled with winding, stator, and rotor are calculated using motor design parameters, and shaft voltage is analyzed using equivalent circuit model according to change in the effective parasitic capacitances. As a result, rotor-to-winding capacitance is determined as a variable parameter through the analysis of the correlation between the parasitic capacitances and output torque characteristics because rotor-to-winding capacitance has greatest effect on the shaft voltage and has no effect on the average torque. Finally, shaft voltage mitigation method according to change in winding-to-rotor capacitance is verified by experiment.