Electric Motors with Variable Speed Drives – Reducing the Effects of Switching Electronics

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Galina Mirzaeva
Associate Professor
The University Of Newcastle

Variable Speed Drives (VSD) offer numerous advantages and have been adopted in many industrial applications. However, they have also brought some new challenges. Many of these are associated with the so-called Common Mode Voltage.

If an induction motor is fed from an AC supply, the motor star point is usually not connected to earth, however, the potential at the star point is close to zero. If the same motor is fed from a VSD, then the motor star point sees a switched voltage of a high magnitude, or the Common Mode Voltage. Through parasitic capacitive coupling with the motor frame, this voltage creates currents that flow through the motor bearings (the so-called “bearing currents”), shortening their life.

Common Mode Voltage can also create severe electromagnetic interference with other devices, through a common supply. Additionally, high frequency switching creates challenges to protection, which is traditionally based on sensing and rms value calculation of currents and voltages at the supply frequency.

In this talk, the Common Mode Voltage and other aspects of Variable Speed Drive operation will be discussed, and the mitigation strategies will be explained.

Galina Mirzaeva received Bachelor and PhD degrees in Electrical Engineering from South Urals State University, Russia. In 2004-2010 she worked as a Research Academic with CRC Mining Australia. Since 2010 she is with the University of Newcastle, Australia, first as Senior Lecturer and since 2017 - as Associate Professor. Her research interests include Electric Drives, Machines and Power Electronics.