MINIMUM AIRGAP-PERMEANCE DATA FOR THE DOUBLY-SALIENT POLE STRUCTURES COMMON IN VARIABLE-RELUCTANCE MACHINES

This paper presents minimum airgap-permeance data for doubly salient p ole structures, such as those common in variable-reluctance motors (VR Ms). The data are derived from a finite element analysis of a family o f doubly salient pole structures. The ratio of the rotor interpolar ar e to the stator pole are and the ratio of rotor pole undercut to rotor interpolar are length are identified as the critical parameters gover ning minimum airgap permeance. The data are normalized to the active a xial length of the motor, and are independent of the active radius of the motor. The numerical permeance data are compared to experimental d ata. The experimental data provide information on third dimension effe ct, and also indicate that localized pole tip saturation occurs at ver y low values of exciting current when the effective airgap is small. T he numerical permeance data are used to compute minimum inductance for three VRMs which have been experimentally characterized. Comparison o f the computed and measured values are consistent with the other exper imental results, indicating that third dimension effects comprise a si gnificant portion of the measured minimum inductance for VRMs with a s mall axial-length to active-radius ratio. A second comparison is made between the data presented here and the conventional flux tube analysi s, and indicates that the finite element data are more accurate than t hose obtained with flux tubes and are easier to implement.