Why the shape of rotor-poles of salient pole synchronous machines are designed as sinusoidal in shape?
The salient poles produce a rather non-sinusoidal back emf in the stator coils (which, from Fourier theory, can be thought of as a sinusoid with harmonic components). The net of these harmonics in a multi-phase machine is non zero, which results in residual torque ripple that is undesirable. Shaping the rotor poles helps to reduce these harmonics and allow for smoother operation, albeit with some loss in efficiency.