Analysis of the Far Field of Permanent-Magnet Motors and Effects of Geometric Asymmetries and Unbalance in Magnet Design

Abstract

In the design of permanent-magnet synchronous machines for naval applications, exterior magnetic fields are of interest. These decay at a rate depending on the number of poles, with magnetic fields due to a higher number of poles decaying more rapidly. We have developed multipole expansion methods to study the effects of geometric asymmetries and unbalanced pole strength on the components of the far field. We have found that, if there is an imbalance in the set of poles, the lower order decay of the unbalanced poles dominates in the far field, and the advantage of using a higher number of poles is diminished. Multipole expansion in combination with the charge simulation method offers a quick and easy method of determining effects of imbalance on the far field of motors at the design stage. We present the results for a variety of imbalances and pole numbers, and discuss the unbalanced terms due to the demagnetization and space imbalance. We also explain the behavior of the far-field decay with the aid of analytical expressions.