Consider a three-phase synchronous motor. If the field current is varied for constant load power then we get the famous V curves of Ia versus If. When the back-emf of the motor is larger than the phase supply voltage the motor is said to be OVER-EXCITED. How can you get such a thing? Isn't it then a generator? Of course this is when it is used as a synchronous capacitor.
K.
It can't be a generator until you're putting mechanical power _into_ it and getting electrical power out.
The over/under excitation just comes about because the 'correct' excitation is taken to be when the reactive VARs are zero, so less than the correct excitation must be under excitation, and more must be over, yes?
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Ok thanks - of course that makes sense.
K.
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