In an AC generator, a coil with N turns, all of the same area A and total resistance R , rotates with frequency ω in a magnetic field B . The maximum

Flux = B(NA) cos (wt)

e.m.f. = rate of change of flux = - NBAω sin(ωt)

magnitude of emf = NBAω sin(ωt)

now maximum occurs when sin(ωt)=1

so maximum value of e.m.f. =

and it occurs when area vector is perpendicular to the magnetic field vector.