PROCESS POWER ELECTRIC.

Generator repuire direct current to energize it's magnetic field.The DC field current is obtained from a sparate source called an exciter.Either rotating or static-type exciters are used for AC power generator 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 field static excitation for the generator fields is provided in several froms including field-flash voltage from storage batteries and voltage from a system of solid-state components DC generator are either separately excited of 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 the 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 outher ring for positive polarity.
A rotating-rectifier exciter in 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 mounthed on a common shaft.The rectifier assembly rotates with,but is insulated from,the generator shaft as well as from each winding.
Static exciters contain no moving parts.A portion of the AC from each phase of generator output is feedback to the field windings,as DC excitations.Through a system of transformers,rectifiers,and reactors.An external source DC is necessary for initial excitation of the field windings.On engine driven generators,the initial excitation may be obtained from the storage batteries used to start the engine or from control voltage at the switchgear.