Magnetic End Leakage Flux in a Spoke Type Rotor Permanent Magnet Synchronous Generator

Authors: Petter Eklund, Jonathan Sjölund, Sandra Eriksson, Mats Leijon

Abstract:

The spoke type rotor can be used to obtain magnetic flux concentration in permanent magnet machines. This allows the air gap magnetic flux density to exceed the remanent flux density of the permanent magnets but gives problems with leakage fluxes in the magnetic circuit. The end leakage flux of one spoke type permanent magnet rotor design is studied through measurements and finite element simulations. The measurements are performed in the end regions of a 12 kW prototype generator for a vertical axis wind turbine. The simulations are made using three dimensional finite elements to calculate the magnetic field distribution in the end regions of the machine. Also two dimensional finite element simulations are performed and the impact of the two dimensional approximation is studied. It is found that the magnetic leakage flux in the end regions of the machine is equal to about 20% of the flux in the permanent magnets. The overestimation of the performance by the two dimensional approximation is quantified and a curve-fitted expression for its behavior is suggested.

Keywords: End effects, end leakage flux, permanent magnet machine, spoke type rotor.

Digital Object Identifier (DOI): doi.org/10.5281/zenodo.1340130

References:

[1] S. Eriksson and H. Bernhoff, “Rotor design for PM generators reflecting the unstable neodymium price,” in XXth International Conference on Electrical Machines, Sept 2012, pp. 1419–1423.
[2] W. Kakihara, M. Takemoto, and S. Ogasawara, “Rotor structure in 50 kW spoke-type interior permanent magnet synchronous motor with ferrite permanent magnets for automotive applications,” in IEEE Energy Conversion Congress and Exposition, Sept 2013, pp. 606–613.
[3] X. Ge, Z. Q. Zhu, J. B. Li, and J. T. Chen, “A spoke-type IPM machine with novel alternate airspace barriers and reduction of unipolar leakage flux by step-staggered rotor,” in IEEE International Electric Machines Drives Conference (IEMDC), May 2015, pp. 53–59.
[4] P. Eklund, S. Sjökvist, S. Eriksson, and M. Leijon, “A complete design of a rare earth metal-free permanent magnet generator,” Machines, vol. 2, no. 2, p. 120, 2014.
[5] S. Eriksson, A. Solum, M. Leijon, and H. Bernhoff, “Simulations and experiments on a 12 kW direct driven PM synchronous generator for wind power,” Renewable Energy, vol. 33, no. 4, pp. 674–681, 2008.
[6] P. Eklund and S. Eriksson, “Air gap magnetic flux density variations due to manufacturing tolerances in a permanent magnet synchronous generator,” in XXII International Conference on Electrical Machines (ICEM), Sept 2016, pp. 93–99.