The doctoral dissertations of the former Helsinki University of Technology (TKK) and Aalto University Schools of Technology (CHEM, ELEC, ENG, SCI) published in electronic format are available in the electronic publications archive of Aalto University - Aaltodoc.
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Modeling and Reduction of Shaft Voltages in AC Motors Fed by Frequency Converters

Petri Mäki-Ontto

Dissertation for the degree of Doctor of Science in Technology to be presented with due permission of the Department of Electrical and Communications Engineering for public examination and debate in Auditorium S4 at Helsinki University of Technology (Espoo, Finland) on the 21st of April, 2006, at 12 noon.

Overview in PDF format (ISBN 951-22-8141-4)   [1611 KB]
Dissertation is also available in print (ISBN 951-22-8140-6)

Abstract

This thesis deals with the modeling and mitigation of shaft voltages and bearing currents in frequency converter-fed ac motors. Common-mode voltages produced by modern frequency converters can cause destructive currents in motor bearings. The distribution of the common-mode voltage between parasitic capacitances in the motor causes a capacitive shaft voltage, and high-frequency common-mode currents flowing in the stator core generate a shaft-encircling magnetic flux, which causes an induced shaft voltage. Simulation models for these phenomena are needed in the evaluation of new drive system designs that could cause shaft voltage problems. Methods for the mitigation of shaft voltages can also be investigated by such models. In the thesis, the circumferential magnetic flux that causes the induced shaft voltage is investigated experimentally and by numerical analysis. Analytical expressions are derived for the magnetic flux and the common-mode impedance of the stator core. A circuit model suitable for the shaft voltage analysis is developed for induction motors. Both the capacitive and induced shaft voltages are included in the model. A correct common-mode impedance distribution of the drive system in a wide frequency band is essential in the shaft voltage analysis. Therefore, the circuit models of all drive system components are developed, and a complete model of the electric drive is composed. The model is validated by comparing the simulations to experimental results. Some promising methods for bearing current mitigation in high-power low-voltage drives are investigated in the thesis using simulations, analytical expressions, and experiments. New design principles are proposed for mitigating the shaft voltages in induction motors.

This thesis consists of an overview and of the following 8 publications:

  1. P. Mäki-Ontto and J. Luomi, Circumferential flux as a source of bearing current of converter-fed AC machines, in Proceedings of the 2002 IEEE Nordic Workshop on Power and Industrial Electronics (NORPIE 2002), Stockholm, Sweden, 2002.
  2. P. Mäki-Ontto and J. Luomi, Common-mode flux calculation of ac machines, in Proceedings of the 15th International Conference on Electrical Machines (ICEM 2002), Bruges, Belgium, 2002.
  3. P. Mäki-Ontto and J. Luomi, Bearing current prevention of converter-fed AC machines with a conductive shielding in stator slots, in Conference Record of the 2003 IEEE International Electric Machines and Drives Conference (IEMDC 2003), Madison, WI, USA, 2003.
  4. P. Mäki-Ontto and J. Luomi, Design of converter-fed induction motors to reduce bearing currents, in Proceedings of the 16th International Conference on Electrical Machines (ICEM 2004), Cracow, Poland, 2004.
  5. P. Mäki-Ontto, H. Kinnunen, and J. Luomi, AC motor cable model suitable for bearing current and over-voltage analysis, in Proceedings of the 16th International Conference on Electrical Machines (ICEM 2004), Cracow, Poland, 2004.
  6. P. Mäki-Ontto and J. Luomi, Induction motor model for the analysis of capacitive and induced shaft voltages, in Conference Record of the 2005 IEEE International Conference on Electric Machines and Drives (IEMDC 2005), San Antonio, TX, USA, 2005.
  7. P. Mäki-Ontto, H. Kinnunen, and J. Luomi, Three-phase model for the circuit simulation of common-mode phenomena and shaft voltages in AC motor drive systems, in Conference Record of the 2005 IEEE International Conference on Electric Machines and Drives (IEMDC 2005), San Antonio, TX, USA, 2005.
  8. P. Mäki-Ontto, J. Luomi, and H. Kinnunen, Reduction of capacitive and induced shaft voltages in an induction motor drive using dual-bridge inverter approach, Electrical Engineering (Archiv für Elektrotechnik), in press.

Keywords: ac motor, frequency converter, common-mode, shaft voltage, bearing current

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© 2006 Helsinki University of Technology

Last update 2011-05-26