INDUCTION MACHINE WINDINGS AND THEIR MMFs
As shown in Chapter 2, the slots of the stator and rotor cores of induction machines are filled with electric conductors, insulated (in the stator) from cores, and connected in a certain way. This ensemble constitutes the windings. The primary (or the stator) slots contain a polyphase (threephase or two-phase) a.c. winding. The rotor may have either a 3(2) phase winding or a squirrel cage. Here we will discuss the polyphase windings. Designing a.c. windings means, in fact, assigning coils in the slots to various phases, establishing the direction of currents in coil sides and coil connections per phase and between phases, and finally calculating the number of turns for various coils and the conductor sizing. We start with single pole number three-phase windings as they are most commonly used in induction motors. Then pole changing windings are treated in some detail. Such windings are used in wind generators or in doubly-fed variable speed configurations. Two-phase windings are given then special attention. Finally, squirrel cage winding mmfs are analysed. Keeping in mind that a.c. windings are a complex subject having books dedicated to it [1, 2], we will treat here first its basics. Then we introduce new topics such as “pole amplitude modulation”, “polyphase symmetrization” , “intersperse windings” , “simulated annealing”  and “the three equation principle”  for pole changing. These are new ways to produce a.c. windings for special applications (for pole changing or mmf chosen harmonics elimination). Finally, fractional multilayer three-phase windings with reduced harmonics content are treated in some detail [8,9]. The present chapter is structured to cover both the theory and case studies of a.c. winding design, classifications, and magnetomotive force (mmf) harmonic analysis.