ac generator

Hi,

The AC Generator needs to produce the AC Voltage in the Main Stator of the Generator.

Secondly the Rotor of AC Generator is rotating with the help of prime mover.

If you take a look at Faraday's Law of Electromagnetic Induction - to induce the voltage in the Main Stator coil, you need to have changing magnetic field.

If you supply Rotor an AC Voltage of (say 50 Hz) & the 4 pole rotor is rotating at 1500 RPM ( I.e Synchronous Speed), the Changing magnetic field will not be faced by the Main Stator Coil, Hence no emf induction.

Yes, In case of cyclone generators the Rotor is fed with the AC voltage with controlled frequency to get the Constant Voltage & Constant frequency at Fluctuating Rotor Speed.

For EM induction you need a relative movement between conductor and the magnetic field.

a) Rotating coil and stationary field (normal alternator- you need DC for stationary field)

b) Rotating field and stationary coil (moving pole generators)

c) Stationary coil and time varying field (Transformer)

d) Rotating coil and rotating field, but at different speeds (Induction motors)

If the field and coil movement synchronise, you won't have EM Induction - and as #1 said, you have to keep on changing the terminals of AC field as the rotor rotates to maintain the relative movement. It is much more easier not to do it and keep it nice DC.