There are several basic concepts that must be established before the analysis of electric
machines can begin. The principle of electromechanical energy conversion is
perhaps the cornerstone of machine analysis. This theory allows us to establish an
expression of electromagnetic torque in terms of machine variables, generally the
currents and the displacement of the mechanical system. Other principles that
must be established are the derivation of equivalent circuit representations of
magnetically coupled circuits, the concept of a sinusoidally distributed winding,
the concept of a rotating air-gap magnetomotive force (MMF), and the derivation
of winding inductances. The above-mentioned basic principles are presented
in this chapter, concluding with the voltage equations of a 3-phase synchronous
machine and a 3-phase induction machine. It is shown that the equations, which
describe the behavior of alternating-current (ac) machines, contain time-varying
coefficients due to the fact that some of the machine inductances are functions of
the rotor displacement. This establishes an awareness of the complexity of these voltage
equations and sets the stage for the change of variables (Chapter 3), which
reduces the complexity of the voltage equations by eliminating the time-dependent
inductances.
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