can u solve(dc excitation)

It is not necessary to use DC excitation to generate AC power if you have a permanent magnet generator. The permanent magnet, however, is slowly degraded over time and becomes less effective with extended use. So, for long term applications, a DC field is used to simulate a permanent magnet.

Cutting the flux lines of a static magnetic field induces current in the object (i.e. coil) which is how AC power is created. DC creates a static field, AC is dynamic. Frequency of DC = 0, of AC = rate of rotation in the static field.

Does this help?

hi friend:

on generator we use dc excitation and we turn the rotor with engine ..etc, to get an AC power on the stater but in transformer we use something like AC excitation without moving parts.

the mechanical energy is converted to electrical on generator but its the same on transformers, just stepped up or down!

the rotor of gen- is turning at one direction ''engine direction'' so u need its magnetic field to be at one direction same as its rotation to generate power on the stater. if you use AC excitation some times the direction of the flux will be opposite to the rotor direction and will be no power generated.

thanks

Yes field is exited by AC,Because that is generated by mutual induction of same filed, and that make it of exactly of same frequency and exactly synchronised,

This means excitation frequency is exactly equal to output frequency. and it is exactly synchronised.

If you try to exite field with some external AC Source it is not going to produce output of right frequency and right wave sahpe.unless exitation source is not of exact frequency and not sunchronised with output voltage,