Relatively simple question here.

I am curious as to why there is a stator voltage controller in the excitation equipment for a synchronous machine.

I understand that the controller's output affects the firing of the pulses for the thyristor bridge therefore change the excitation voltage.

Is it because stator current (and therefore flux) needs to be equal to rotor current (and therefore flux) to maintain the machine's synchronicity?

 

M,

Synchronous generator terminal (stator) voltage is a function of the speed of the generator rotor (field) and the strength of the magnetic field (rotor). Since, when the synchronous generator is operating at rated frequency, the speed will be constant the only way to change terminal voltage will be to change field strength--which, as you rightly noted, is a function of excitation.

So, field strength (excitation) is modulated to maintain a generator terminal (stator) voltage setpoint. This is usually called AC mode, or Automatic Mode, of an exciter or excitation system.

If the excitation (field strength) is too low then, yes, the generator rotor can "fall" out of synchronism, called "slipping a pole." This is very destructive for the generator, and usually or the coupling between the generator and the prime mover driving the generator. There is usually a protective relay or function to prevent under-excitation that might result in slipping a pole.

But, under normal circumstances, when the generator is being operated at or near rated voltage and frequency there is no chance of losing synchronism.

Hope this helps!

 

Yes, that was great!

Thanks!