Why is shaft current insulation required on large motors &
when should it be checked i.e. before or after assembly?

Answer Posted / 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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