Excitation Method : By Brushless Excitation
In this method, main shaft of prime mover drives Pilot excitor, main excitor and the main alternator.
Diode rectifier wheel is also mounted on the same shaft.
Pilot exciter is a permanent magnet alternator with permanent magnet (PM) poles on the rotor and 3- phase armature winding on the stator. 3-phase power from the pilot exciter is fed to thyristor controlled on the floor. After rectification, the controlled d.c. output is supplied to stationary field winding of main exciter.
The 3-phase power, developed in the rotor of main excitor is fed through hollow shaft to the rotating silicon diode rectifier mounted on the same shaft.
The d.c power from diode rectifier bridge is delivered along the main hollow shaft, to the main alternator field winding without brushes and slip rings.
A signal, picked from alternator terminals through CT and PT controls the firing angle (a) of the thyristor bridge. This enables to control the field current of main exciter which given the alternator output voltage.
This schemes does not require any sliding contacts and brushes.
Therefore called brushless excitation.