Answer / 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.

Answer / avd

In large motors both bearings needs to be insulated to
avoid flow of current through bearings.This flow of current
is caused in rotor due to high flux in the large motors. If
current flows through bearings there will be pitting in the
bearings and will cause premature bearing failure. To give
path to current, genaraly carbon brushes are used at motor
shaft which are grounded at other end.Hence flow of current
through bearings avoided.
After assembling of motor on bearing padastal both
bearings insulation resistance shall be checked by megger.

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