Slip rings and brushes have certain disadvantages: increased friction and wear
(therefore, needed maintenance), brush voltage drop can introduce significant
power losses. Still this approach is used in most small synchronous machines.
On large generators and motors, brushless exciters are used.
A brushless exciter is a small AC generator whose field circuits are
mounted on the stator and armature circuits are mounted on the rotor
shaft. The exciter generator’s 3-phase output is rectified to DC by a 3-
phase rectifier (mounted on the shaft) and fed into the main DC field
circuit. It is possible to adjust the field current on the main machine by
controlling the small DC field current of the exciter generator (located on
the stator).
Since no mechanical contact occurs between the rotor and the stator, exciters of
this type require much less maintenance.
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Slip rings are metal rings completely encircling the shaft of a machine but insulated
from it. One end of a DC rotor winding is connected to each of the two slip rings on
the machine’s shaft. Graphite-like carbon brushes connected to DC terminals ride on
each slip ring supplying DC voltage to field windings regardless the position or speed
of the rotor.
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