Minimum airgap-permeance data for the doubly-slotted pole structures common in variable-reluctance-motors

Minimum airgap-permeance data for double-salient pole structures, such as those common in variable-reluctance motors (VRMs), are presented. The data are derived from a finite-element analysis of a family of doubly salient pole structures. The ratio of the rotor interpolar arc to the stator pole arc and the ratio of the rotor pole undercut to rotor interpolar arc length are identified as the critical parameters governing minimum airgap permeance. The data are normalized to the active axial length of the motor, and are independent of the active radius of the motor. The permeance data are used to compute minimum inductance for three VRMs which have been experimentally characterized. Comparison of the computed and measured values indicates that end turn effects comprise a significant portion of the measured minimum inductance for motors with a short axial length. A second comparison is made between the data presented and the conventional flux tube analysis, indicating that the finite-element data are about as accurate as those obtained with flux tubes but are easier to implement.<>