A motor’s heat signature will
tell you a lot about its quality
and condition. If a motor is
overheating, the windings will
rapidly deteriorate. In fact, every
increase of 10 °C on a motor’s
windings above its design operating
temperature cuts the life
of its windings’ insulation by 50
percent, even if the overheating
is only temporary.
If a temperature reading in
the middle of a motor housing
comes up abnormally high, take
a thermal image of the motor and
find out more precisely where
the high temperature is coming
from, i.e. windings, bearings or
coupling. (If a coupling is running
warm it is an indicator of misalignment.)
There are three primary causes
for abnormal thermal patterns;
typically most are the result of
a high-resistance contact surface,
either a connection or a
switch contact. These will usually
appear warmest at the spot
of high-resistance, cooling off the
further away from the spot. This
thermal image shows a classic
pattern in the center phase
connection on the line-side of a
breaker; note how the conductor
cools off at the top of the image.
Load imbalances, whether
normal or out of specification,
appear equally warm throughout
the phase or part of the circuit
that is undersized/overloaded.
Harmonic imbalances create
a similar pattern. If the entire
conductor is warm, it could be
undersized or overloaded; check
the rating and the actual load to
determine which.
Failed components typically
look cooler than similar, normally
functioning ones. The most
common example is probably a
blown fuse. In a motor circuit
this can result in a single phase
condition and, possibly, costly
damage to the motor.