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Why is shaft current insulation required on large motors &
when should it be checked i.e. before or after assembly?
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Answer Posted / sunil kumar sinha
Shaft of large motors need to be insulated at one end and
earthed at other end.Due to some unbalance in electrical
system (Which can not be avoided)very small amount of
voltage induces along the rotor shaft. One end of shaft is
normally earthed through carbon brush or through earthed
bearing, hence one end of rotor shaft is anchored at earth
potential and other end will float and experience small
voltage w.r.t earth as voltage is induced along the shaft.
Now the shaft is supported on bearings. One
end of bearing is either earthed if no earthing brush is
provided and the other end support bearing is insulated.The
insulation resistance of shaft, by lifting earthed beating
end of the shaft is to be measured to ensure that the
insulated bearing is healthy. This is to be measured during
assembly of motor because the healthy mess of insulated
bearing can be checked at this stage only.
Further when motor is in operation shaft voltage is to be
measured. Some voltage must appear. If there is no voltage
it indicates that the insulated bearing has also earthed.
this will make path for shaft current to flow through the
bearings. this will result in pitting of babit of bearings
and finally its failure, increase in shaft vibration.
Hence for healthiness of support bearings of motor current
insulation is must and need to be checked periodically for
healthyness of motor rotor bearings.
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