The word Excitation means the production of flux by passing current in the field winding.The arrangement or the system used for the excitation of the synchronous machine is known as Excitation System. To excite the field winding of the rotor of the synchronous machine, direct current is required. Direct current is supplied to the rotor field of the small machine by a DC generator called Exciter. A small DC generator called Pilot Generator, supplies the current to the Exciter.
The Exciter and the Pilot Exciter both are mounted on the main shaft of the Synchronous generator or motor. The DC output of the main Exciter is given to the field winding of the synchronous machine through brushes and slip rings. The pilot exciter is excluded in smaller machines.
For medium size machines, AC Exciters are used in place of DC Exciter. AC Exciters are three phase AC generators. The output of an AC Exciter is rectified and supplied through the brushes, and the slip rings to the rotor winding of the synchronous machine.
For large synchronous generators having few hundred megawatt ratings, the Excitation System requirement becomes very large. The problem of conveying such a large amount of power through the high-speed sliding contacts becomes formidable.
Presently, the large synchronous machines are using Brushless Excitation System. A Brushless Exciter is a small direct coupled AC generator with its field circuit on the stator and the armature circuit on the rotor. The three phase output of the AC exciter generator is rectified by solid state rectifiers. The rectified output is connected directly to the field winding, thus eliminating the use of brushes and slip rings.
A Brushless excitation system requires less maintenance due to the absence of brushes and slip rings. The power loss is also reduced. The DC required for the field of the exciter itself is sometimes provided by a small pilot exciter. A pilot exciter is a small AC generator with a permanent magnet mounted on the rotor shaft and the three phase winding on the stator. It provides the field current of the exciter. The exciter supplies the field current of the main machine. Thus, the use of a pilot exciter makes the excitation of the main generator completely independent of external supplies.