The equivalent winding — A solution to a unified approach for the magnetic force inside round airgaps of electrical machines

The electromagnetic complex phenomena inside electrical machines make the determination of specific mathematical models rather laborious. Often, the electrical machine analysis is made based on simplified assumptions according to which iron losses are neglected and iron permeability is considered infinite. Thereby, the whole magnetic energy, on which is based the entire electromechanical conversion, is considered located only in the air gap, between the two armatures. The paper presents the advantages of using the air gap magneto motive force (MMF) as an intermediate quantity between ampere-turns produced by a field coil and the space variation of the magnetic field produced by the field current. The MMF in the air-gap along a field line does not depend on the air-gap length and superposition of magnetic waves is possible. The space-time variation of the magnetic force in the air-gap depends on the field coil and the way conductors are distributed in the slots around the armature. In order to achieve a unified and consistent approach regarding distribution of the magnetic force inside electrical machine air-gaps, the authors are introducing the concept of the equivalent winding placed on an equivalent surface attached to the main field armature. The constructive elements, the characteristics of the equivalent windings and the features of the equivalent surface are presented. Examples are provided to underline the advantages of the equivalent winding concept.