Maintenance Technology June 2017 - 41
Insulation problems are to blame for a high percentage
of motor failures and associated unplanned costs.
ACCORDING TO SKF (skf.com, Gothenburg, Sweden, and
Lansdale, PA), 40% of failures in electric motors are caused
by bearing problems. Another 40%, however (a percentage
that's even higher in motors operating above 4 kV), are
caused by insulation problems associated with coil windings
or loose connections. Unfortunately, predictive-maintenance
techniques to detect insulation weakness typically aren't
employed as much as those used to keep tabs on bearing
health, i.e., vibration analysis and infrared thermography.
(Ground-wall-insulation "megger" testing is common in
plants, but, as SKF explained, it's not a complete test.)
The problems caused by insulation weakness, including
catastrophic motor failures and, in some cases, fires, can be
just as serious as those caused by worn bearings or overheating. For this reason, it is important for personnel to have
a way of assessing insulation integrity and be able to take
There are two types of insulation in an electric motor.
Groundwall insulation is found between the motor stator
and the electrical windings. The insulation strength of new
groundwall insulation is very high, often 40 times operating
voltage. Winding insulation is the thin insulation on the
wires used in the motor windings. The insulation strength of
new winding insulation is about 15 times operating voltage.
Most motor insulation failures start as winding insulation failures since that insulation strength is vastly weaker.
When a winding insulation failure occurs, the motor can
fail quickly, often becoming so hot as to also damage the
groundwall insulation, causing it to fail.
Automated motor-insulation testing, using a device such as
the SKF Baker AWA-IV, has been shown to make insulation
testing easy and remove operator error and inconsistency.
Static insulation testing is done with the motor disconnected from the power supply, and typically performed from
the motor control cabinet (MCC). Testing from the MCC
also allows detection of electrical faults outside the motor
itself, such as in junction boxes or feeder cables. Motors
also can be electrically tested in situ through dynamic
monitoring, which can reveal problems in the wider powermachine-load system. Typical insulation testing includes:
■ Coil resistance tests
■ Meg-ohm test
■ Polarization Index (PI) test
■ DC step-voltage test
■ Hipot test
■ Surge test.
continued on p. 42
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