Excitation in an electrical machine means production of magnetic flux by passing current in the field winding. In order to excite the field winding on the rotor of the synchronous machine the direct current is required.
For small size synchronous machines (motor or generator), the direct current is supplied to the rotor field winding by a DC generator, known as exciter. The exciter is mounted on the shaft of the synchronous machine. The DC output of the exciter is fed to the field winding of the synchronous machine through brushes and slip rings.
For medium size synchronous machines, AC exciters are used in place of DC exciters. The AC exciters are 3-phase AC generators. The output of the AC exciter is rectified and given to the rotor field winding of the synchronous machine through brushes and slip rings.
For large size synchronous machines, the excitation requirements become very large. The problem of conveying such large amount of power through high-speed sliding contacts becomes formidable. Therefore, in large-size synchronous machines, brushless excitation systems are used.
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 is rectified with the help of solid-state rectifier.
The rectified output is connected directly to field winding of the synchronous machine which eliminates the use of brushes and slip rings. Due to the absence of brushes and slip rings, the brushless excitation system requires less maintenance. Also, it uses solid state rectifier, thus the power loss is also reduced.