Prerunning Checks. The circumferential position of
the brushes on the commutator is important for commutation and also to provide
the voltage characteristics set at the factory. Brushes should be on the
factory test setting.
The toes of the brushes should be aligned and should have no
skew. The spacing between adjacent arms of brushes should be identical within
0.032 in.
The brushes should move freely in their holders and should
have a pressure against the commutator of 2 to 3 lb/in2 on the basis of brush
cross section. The faces of the brushes should accurately match the curvature
of the commutator surface.
The polarity of the main fields may be checked by tracing
the wiring around the frame or by lightly exciting the fields and using a
compass around the frame behind the poles.
The oiling system for the bearings should be checked and the
oil rings tested for freedom. The entire machine, particularly its air gaps,
should be inspected for foreign material.
Running Checks. Note any unusual noise as the unit is
brought up to speed. Bearing temperatures should level out at acceptable values
within a few hours.
The voltage should be slowly raised at no load and
commutation observed. If satisfactory, the voltage should be raised to 110% of
rated and then reduced.
The generator may then be loaded gradually while commutation
is observed, until rated current is reached. If commutation remains
satisfactory until stable temperatures are achieved, the generator is ready for
work.
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