Unbalanced force calculation in switched-reluctance machines

Abstract

This paper presents a detailed analytical model for computing the radial magnetic forces that arise in switched reluctance machines (SRM's). The model is general and includes iron saturation, displacements of the rotor from its center location, and arbitrary angular rotor positions. The force between an individual stator pole and its corresponding rotor pole is calculated. The model is used to calculate the unbalanced magnetic forces on the SRM rotor, due to the rotor being displaced from its center location, by calculating the difference in the radial magnetic forces on opposite stator poles. The calculation of the unbalanced magnetic rotor forces requires an especially accurate model for the radial magnetic forces since the unbalanced forces are the difference between the two large radial forces on opposite sides of the rotor. The side pull created by the unbalanced forces will stress the bearings of the motor. The detailed analytical model presented here will simplify the bearing system design and will be especially useful if less stiff magnetic bearings are being employed. Finite-element analysis is used to validate the detailed analytical model. This same model for calculating the radial magnetic forces can be used as the input to a calculation of stator yoke ovulation due to radial magnetic forces and of the resulting acoustical noise production.