**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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