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Current Analysis:
Although
squirrel-cage induction motors are very hardy and require little
maintenance, it is possible for rotor bars or end rings to crack
or break, particularly during starting when excessive heating
and vibration may occur. This is because the starting current
can be as high as eight to ten times normal full load current.
A
cracked rotor bar will overheat resulting in mechanical failure.
Arcing normally occurs across the broken bar and this usually
damages the rotor laminations. The adjacent bars are then subjected
to greater thermal and mechanical stresses during subsequent
starting and the motor condition deteriorates with time and
the number of starts experienced.
Failures
are greater when frequent starts are required. We use online
spectrum analysis of the stator current signal as an additional
tool to vibration to detect broken rotor bars in squirrel-cage
induction motors.
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