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 electromagnetic performance of a partitioned-stator field-excitation (PS-FE) brushless machine is analyzed by air-gap field modulation principle (AFMP). Firstly, the air-gap field modulation process of two PS-FE machines is investigated with the inner stator pole number N_i /rotor pole number N_r /outer stator slot number N_o 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 N_r is an integer multiple of N_i /2 (i.e. the inner stator 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 inner stator 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.