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 ELECTRICAL
Motors and Generators-Continued
Permanent records should be kept of measured insulation values
on all integral hp motors
To ascertain trends, graphs showing the long-time relationship of
insulation resistance to time should be prepared on all large or
critical equipment
INSULATION-RESISTANCE VALUES
Insulation Resistance
Insulation Resistance
Preferred
Minimum
Machine Rating
(Megohms)
(Megohms)
(volts)
0.20
0.11
110
0.50
0.22
220
0.75
0.42
440
SYMPTOM OBSERVED/Probable Cause
NOISY BEARING (Popping or churning):
Overlubrication
Excessive moisture in lubricant
EXCESSIVE ARCING AT BRUSHES:
Incorrect brush position
Improper type, size or span of brushes
Incorrect brush pressure or contact
Loose brush rigging
Dirty or rough commutator or slip ring
High or low commutators bars
Short-circuited commutator bars
Overload or excessive vibration
NOISY BRUSHES (SINGING):
Excessive brush pressure
Brushes too hard
Holders improperly adjusted
High and low commutator bars
Loose commutator bars
High mica between commutator bars
7-36
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