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.
Aalto

Electromechanical Interaction in Rotordynamics of Cage Induction Motors

Timo P. Holopainen

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 24th of August, 2004, at 12 noon.

Overview in PDF format (ISBN 951-38-6405-7)   [790 KB]
VTT Publications 543, ISSN 1455-0849

Dissertation is also available in print (ISBN 951-38-6404-9)
Copyright © 2004 VTT Technical Research Centre of Finland
VTT Publications 543, ISSN 1235-0621
VTT-PUBS-543
TKK-DISS-1880

Abstract

Electromagnetic fields in the air gap of an electric machine produce electromagnetic forces between the rotor and stator. The total force exerted on the rotor due to the eccentric rotor position is called the unbalanced magnetic pull. This eccentricity force is directed roughly over the shortest air gap. At low frequencies, the vibration amplitudes of flexural modes may be large enough to couple the electromagnetic system to the mechanical one. This electromechanical interaction changes the vibration behaviour of the system.

The main purpose of this dissertation is to reveal the main rotordynamic consequences induced by the electromechanical interaction in cage induction motors. Another goal is to achieve this by deriving a simple and representative electromechanical rotor model with physical variables and parameters.

In this study, a new parametric model was derived for the unbalanced magnetic pull induced by an arbitrary rotor motion in transient operation. The parameters of this model can be determined analytically from the basis of the machine characteristics or estimated numerically as in this study. To estimate the parameters, an efficient numerical method was developed from the analysis of impulse response. The numerical results showed that the simple electromagnetic force model, together with the estimated parameters, predicts the unbalanced magnetic pull fairly accurately.

An electromechanical rotor model was derived by combining the Jeffcott rotor model with the simple electromagnetic force model, including two additional variables for the harmonic currents of the rotor cage. Applying this model, the rotordynamic effects of electromechanical interaction were studied. Three induction motors were used in the numerical examples. The results obtained show that the electromechanical interaction may decrease the flexural frequencies of the rotor, induce additional damping or cause rotordynamic instability. These interaction effects are most significant in motors operating at, or near, the first flexural critical speed. Excluding the potential rotordynamic instability, the numerical results indicate that the electromechanical interaction effectively reduces the unbalance response close to the first flexural critical speed.

This thesis consists of an overview and of the following 6 publications:

  1. Holopainen, T. P., Tenhunen, A. & Arkkio, A. 2002. Electromagnetic Circulatory Forces and Rotordynamic Instability in Electric Machines. In: Hahn, E. J. & Randall, R. B. (eds.). Proceedings of the 6th International Conference on Rotor Dynamics. Sydney, Australia, 30.9-4.10, 2002. Vol. 1. Sydney: University of New South Wales Printing Services. Pp. 446-463. ISBN 0-7344-1963-1. © 2002 by authors.
  2. Holopainen, T. P., Tenhunen, A. & Arkkio, A. 2002. Electromechanical Interaction in Rotor Vibrations of Electric Machines. In: Mang, H. A., Rammerstorfer, F. G. & Eberhardsteiner, J. (eds.). Proceedings of the 5th World Congress on Computational Mechanics. Vienna, Austria, 7-12 July, 2002. Vienna: Vienna University of Technology. 10 p. © 2002 by authors.
  3. Holopainen, T. P., Tenhunen, A., Lantto, E. & Arkkio, A. 2004. Numerical identification of electromagnetic force parameters for linearized rotordynamic model of cage induction motors. Journal of Vibration and Acoustics, Vol. 126, Issue 3, pp. 384-390. ISSN 1528-8927.
  4. Holopainen, T. P., Tenhunen, A., Lantto, E. & Arkkio, A. Unbalanced magnetic pull induced by arbitrary eccentric motion of cage rotor in transient operation. Part 1: Analytical model. Electrical Engineering. In press. ISSN 0948-7921 (Paper) 1432-0487 (Online).
  5. Holopainen, T. P., Tenhunen, A., Lantto, E. & Arkkio, A. Unbalanced magnetic pull induced by arbitrary eccentric motion of cage rotor in transient operation. Part 2: Verification and numerical parameter estimation. Electrical Engineering. In press. ISSN 0948-7921 (Paper) 1432-0487 (Online).
  6. Holopainen, T. P., Tenhunen, A. & Arkkio, A. Electromechanical interaction in rotordynamics of cage induction motors. Journal of Sound and Vibration. In press. ISSN 0022-460X.

Keywords: unbalanced magnetic pull, electromechanical interaction, rotors, electric motors, vibration characteristics

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© 2004 Helsinki University of Technology

Last update 2011-05-26