Excitation system for the three-phase synchronous machine.

Excitation system for the three-phase synchronous machine.

Excitation means the production of flux by passing current in the field winding.Direct current is required to excite the field winding on the rotor of the synchronous machines. for small machines, dc is supplied to the rotor field by a d.c. genrator called exciter.the eciter may be supplied current by a smaller d.c. generator called pilot exciter.the main and th pilot exciters are mounted on the main shaft of the synchronous machine(generator or motor). the main and Pilot exciter are mounted on the main shaft of the synchronous machine (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. in smaller machines, the plot exciter may be omitted,but this arrangment is not very sensitive are quick acting when change of field current are required by synchronous machine.

For medium size machine AC exciter are used in place of DC exciters.A.C. Exciters are three phase AC generator. The output of an AC exciter is rectified and supplied through bruses and slip rings to the rotor winding of main synchronous machine.

Large synchronous generator with rating of order of few MW requirements become very large. The problem of conveying such amount of power through high speed sliding contacts becomes formidable. At present large synchronous generators and synchronous Motors are using brushless excitation system. Brushless exciter is a a small direct coupled AC generator with its field circuit on the stator and armature circuit on the Rotor. The output of DC exciter generator rectified by solid state rectifiers. The rectifier output is connected directly to the field winding eliminating the use of brushes and slip rings.

A brushless citation system require less maintenance due to absence of brushes and slip rings the power loss is also reduced.

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 permanent magnet mountain on root shaft  and three phase winding on the stator. Permanent magnet of the pilot exciter produced the field current of the exciter. The exciter supplies field current of the main machine. thy the use of a pilot excited by the excitation of the main generator completely independent of external supplies.