Relationship Between Slot-Pole Combination and Performance of Partitioned-Stator Field-Excitation Brushless Machines by Air-Gap Field Modulation Principle

In this article, the relationship between slot-pole combination and the electromagnetic performance of a partitioned-stator field-excitation (PS-FE) brushless machine is analyzed by the air-gap field modulation principle (AFMP). First, the air-gap field modulation process of two PS-FE machines is investigated with the inner stator (IS) pole number N-i/rotor pole number N-r/outer stator (OS) slot number No of 8/18/42 and 12/10/12 respectively, and verified by finite element analysis (FEA) and experiments. Then, a general combination (Rule-I), a deprecated combination (Rule-II), and a compensatory combination (Rule-III) are defined to guide the slot-pole selection. It is found that if Nr is an integer multiple of N-i/2 (i.e., the IS pole pairs number p(i)), the air-gap flux-densities will contain only the integer multiple harmonics of p(i), resulting in an asymmetric flux-linkage and a back-electromotive force (back EMF) in the armature windings. Although symmetric flux-linkage and back EMF can be performed by Rule-III, a higher torque ripple and a higher dc-winding-induced flux-linkage exist in the PS-FE machines. Hence, Rule-IV and the saturated IS effect are discussed to pursue a lower dc-winding-induced flux-linkage, thereby a lower induced voltage under both open-circuit and on-load. Finally, rules are listed and the important findings are summarized.