The SRM is a doubly-salient, singly-excited machine with independent windings of the stator. Its stator structure is same as PM motor, but the rotor is simpler having no permanent magnet on it. Stator windings on diametrically opposite poles are connected in series or parallel to form one phase of the motor.

Several combinations of stator and rotor poles are possible, such as 6/4 (6 stator poles and 4 rotor poles), 8/4, 10/6, 12/6 etc.
4/2, 2/2 configurations are also possible, but with these it is almost impossible to develop a starting torque when the stator and rotor poles are exactly aligned.

The configurations with higher number of stator/rotor pole combinations have less torque ripple. The design objectives are to minimize the core losses, to have a good starting capability and to eliminate mutual coupling.

Energy partitioning during one complete working stroke. (a) Linear case. (b) Typical practical case. W = energy converted into mechanical work. R = energy returned to the dc supply.The nonlinear saturating characteristics of real magnetic steel has a marked influence on the energy conversion process in an SRM.

Only for very low values of saturation, the characteristics approximate the ideal linear case. The flux-current characteristics in the unaligned position is approximately linear, because the magnetic path is dominated by large airgap and flux densities are small.

In the aligned position the airgap reluctance is small and flux density is high, which causes high saturation at higher currents.
The SRM is always driven into deep saturation to maximize the energy transfer in each stroke.

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