For the generation of elect power we need a prime mover which supplies mechanical power input to the alternator, can be steam turbines,or hydro turbines .When poles of the rotor moves under the armature conductors which are placed on the stator ,field flux cut the armature conductor ,therefore voltage is generated and is of sinusoidal in nature…due to polarity change of rotor poles(i,e) N-S-N-S.