Q.Give the principle of AC generator.

Principle. An AC generator (alternator) converts mechanical energy, used to rotate a coil or a field magnet, into electrical energy, working on the same electromagnetic-induction principle throughout this unit: relative motion between a conductor and a field changes the linked flux, inducing an emf (Faraday's law) whose direction follows Fleming's right hand rule.

Deriving the sinusoidal emf. For a coil of N turns, area A, rotating with angular velocity $\omega$ in a field B, starting (t = 0) with the coil's plane perpendicular to B (normal along B, maximum flux linkage $\Phi_m=NBA$), at angle $\theta=\omega t$ the flux linkage is $N\Phi_B=NBA\cos\omega t$, and Faraday's law gives the induced emf

-- a purely sinusoidal (alternating) emf, completing one full cycle per one full mechanical rotation of the coil (or field magnet). This is the fundamental principle behind every AC generator. …