Power Electronics for Renewable Energy Systems, Transportation and Industrial Applications

Chapter 21Common-Mode Voltage and Bearing Currents in PWM Inverters: Causes, Effects and Prevention

Jaroslaw Guzinski

Faculty of Electrical and Control Engineering, Gdansk University of Technology, Gdansk, Poland

21.1 Introduction

Accelerated degradation of bearings of induction motors (IMs) operating with voltage inverters is an effect of parasitic current flow, defined as a bearing current. The first reports on bearing currents were published nearly 100 years ago [1]. The observed phenomena were reported only for high-power machines as a result of magnetic asymmetry [2]. Its value is negligible compared with the bearing currents that occur in machines with inverter-type supplies [3]. Bearing failure is now the most common failure of AC machines operating in adjustable speed drives (ASDs). Owing to the high number of ASDs, this type of failure requires special attention. The bearing current in modern ASDs is closely connected with the appearance of the common-mode (CM) voltage resulting from the operation of a voltage inverter with pulse-width modulation (PWM). For that operation, long-term current flow through motor bearings, with a density exceeding the ratings of rolling elements can completely destroy them. It is reported that a current density of J_b ≥ 0.1 A mm⁻² has no noticeable impact on the life of the bearing, but a current density of J_b ≥ 0.7 A mm⁻² can significantly shorten its lifetime [3].

CM voltage refers to the three-phase natural system and is defined as the ...