Initial Exitation

Rajesha, you need to provide more information .

I know your english may be a challenge but you need to include the following information so that more folks in this forum can help you:

what size of generator is it?

Is the excitation power shunt fed or Permanent magnet generator ( PMG) fed?

If I am allowed to volunteer an answer, here goes:

Small generators in the less than MW range do have enough residual magnetism to get voltage build up without having to flash the field.

Large generators do not have enough residual magnetism so some means have to be used to get the exciation going.

Generators which are shunt fed( meaning that the generator output power is used to derive the DC for field excitation) then at the appropriate generator speed during startup , DC from the machine battery supply is supplied for a brief moment called, "field flashing" , to start the excitation and voltage buildup going after which the generator output would be high enough to build up excitation and keep it going.There is usually a voltage relay in the control system which senses the buildup from field flash and drops off the battery DC.

If the machine does not have shunt fed excitation the it may be equipped with a Permanent Magnet Generator (PMG). Most PMGs I have seen provide 420 HZ at 110- or 220 volts single or 3-phase depending on the amount of excitation power required.Very early in the startup , you will have enough power to start excitation. But at about the same speed where you would "field flash " ( usually around 90-95% speed) you would close the field breaker to get the PMG power into the automatic voltage regulator and start excitation. Exciting too early can result in a over flux condition and damage the generator rotor.A Voltz/HZ relay usually protects you from this occurrence.

Generators which are black start are invariably PMG type exciation so that you can use some of the power generated during startup to feed cooling fans or other equipment , depending on the startup time.

Send more information for a more refined answer.