Radial Forces in Brushless Doubly-Fed Machines

This paper investigates the radial forces in brushless doubly-fed machines (BDFMs). The calculation of radial forces in traditional electric machines is reviewed first as the foundation for radial force calculation in BDFMs. The radial force patterns are presented in detail and found to be related to the pole pair combination and supply frequencies. The influence of saturation effect is also investigated. It is shown that the unbalanced magnetic pull (UMP) in BDFMs is mainly contributed by airgap flux density harmonics whose pole pairs are differed by 1. These harmonics can be the dominating ones that contribute to the average torque or those useless parasitic ones. To completely cancel the UMP in BDFMs, the pole pair combination should meet the constraint that the equivalent pole number of rotor is even. Finite element results are provided to validate the effectiveness of the presented analysis.