Generators require direct current to energize its magnetic field. The DC field current is obtained from a separate source called an exciter. Either rotating or static-type exciters are used for AC power generation systems. There are two types of rotating exciters: brush and brushless. The primary difference between brush and brushless exciters is the method used to transfer the DC exciting current to the generator fields. Static excitation for the generators fields is provided in several forms including field-flash voltage from storage batteries and voltage from a system of solid-state components. DC generators are either separately excited or self-excited.
Excitation systems in current use include direct-connected or gear-connected shaft-driven DC generators, belt-driven or separate prime mover or motor-driven DC generators, and DC supplied through static rectifiers.
The brush-type exciter can be mounted on the same shaft as the AC generator armature or can be housed separately from, but adjacent to, the generator. When it is housed separately, the exciter is rotated by the AC generator through a drive belt.
The distinguishing feature of the brush-type generator is that stationary brushes are used to transfer the DC exciting current to the rotating generator field. Current transfer is made via rotating slip rings (collector rings) that are in contact with brushes.
Each collector ring is a hardened-steel forging that is mounted on the exciter shaft. Two collector rings are used on each exciter, each ring is fully insulated from the shaft and each other. The inner ring is usually wired for negative polarity, the outer ring for positive polarity.
A rotating-rectifier exciter is one example of brushless field excitation. In rotating-rectifier exciters, the brushes and slip rings are replaced by a rotating, solid-state rectifier assembly. The exciter armature, generator rotating assembly, and rectifier assembly are mounted on a common shaft. The rectifier assembly rotates with, but is insulated from, the generator shaft as well as from each winding.