One of the most common errors we see when rewinding synchronous alternators is specific to the exciter rotor. It’s common for a winder to assume the “poles” match the main rotor when in fact that is rarely the case. The exciter rotor MUST be wound for the same number of poles as the stationary exciter fields.
The output frequency of the exciter can be determined by: Exciter poles / main rotor poles x line frequency
- Main rotor has 4 poles
Exciter fields have 10 poles 10/4 x 60 (50) = 150 Hz (125 Hz)
- Main rotor has 6 poles
Exciter fields have 10 poles 10/ 6 x 60 (50) = 100 Hz (83 Hz)
There are two reasons for an exciter to be wound with more poles than the main stator and rotor of an alternator. First, the higher output frequency results in a cleaner DC waveform when rectified. Second, a higher frequency allows a greater power rating in a smaller core.