Why the generators are always excited using dc voltage and not ac?

Answer 1

Mainly there are three points,

1. If you give AC to the field coil, there is no need of rotating it, as AC itself produces alternating flux.Thus generator will give Transformer action which actually transfer the power not actually it generates or produce as per the basic law of mutual Induction.

2.In case if you rotate the field giving AC Supply by a prime mover , actually constant poles(as in physical magnet) will not be produced.For every cycle of rotation,there will be continuous change in the poles in respect to three windings of Armature.So as it results irregular waveforms of voltages,we cant tap the pure sinusoidal 3 phase supply from armature.Most probably it will get distorts & might be chances of burning of Stator(armature)coil in Alternator.

3.So to overcome come from all above disadvantages, DC excitation will be given so that constant field strength will be maintained as in case if we employ permanent magnets which produces constant flux

Answer 2

AC generators convert rotational energy into AC by moving a static magnetic field through the vicinity of the stator windings. You create this static magnetic field with DC. If you tried to use AC, the magnetic field would be varying, and this is more corectly known as a transformer. Since you want to boost the energy using rotational energy, you want the static field, hence the use of DC of excitation.

Its not any different than the alternator in your car. It takes DC from the regulator producing a rotating field, and induces three phase AC in the stator. The difference is that the car alternator then rectifies the AC to make DC to recharge the battery and run the car's electrical system, where in a power plant you take the AC and step it up to the electrical transmission system.

Answer 3

all voltage is ac until it comes to a transformer when needed.ac voltage travels farther.