meter (DMM) with VFD measurement
capability; combination full-function
DMM with the specialized insulation tests
described above, plus a low-pass filter.
“The electrical tests on the motor itself
are specialized and not normally found on
a digital multimeter,” Silva said. “An insu-
lation tester that performs basic DMM
functions, plus the other motor tests
described above, can be extremely helpful
to electricians and technicians responsible
for servicing a large number of motors.”
Fluke offers a full line of testers for
electric motors, including DMMs and
Hioki USA Sales Engineer Bob
D’Amico said the company markets
power analyzers, insulation testers and
DMMs for testing electric motors.
“Basic motor tests include shorting
between the windings, capacitor testing,
test for winding resistance, insulation
tests, and test of power consumption,”
he said. “Whether multifunction testers
or individual testers are used depends on
the price users want to pay. All-in-ones
can be very expensive while individual
models can be less than half the cost.”
Megger Senior Applications Engineer
Jeffrey R. Jowett said the first step in
testing a motor is to distinguish between
electrical failure and bearing or other
“If electrical failure is the cause,
nameplate information is important;
parameters indicated on the nameplate
should be tested for compliance,” Jowett
said. “Perform an insulation test to iso-
late deteriorated windings and shorts
to ground. A clamp-on ammeter can be
used to look for excessive or insufficient
current draw and for load imbalance.
Similarly, a voltmeter can check low- and
“By far, the most important electrical
test is the insulation resistance test. Insulation resistance is a handy and easily
tested indicator of overall motor condition. Dirt, moisture, corrosives, electrical
and mechanical stresses all cause insulation to deteriorate over time until eventual
breakdown results. This deterioration can
be readily assessed and tracked to head off
final breakdown and loss of the motor.
“Multimeters should be used to
check voltage and current draw and
load imbalance, power quality meters
and data loggers to check for harmon-
ics, which can overheat the motor. A
low-resistance ohmmeter or dedicated
winding-resistance tester checks for
deterioration of windings.
“For other functions, multifunction
testers are being steadily improved in
their ability to provide quality tests and
features. But there is still the risk of
quantity over quality. Currently, single-function testers, multimeters aside, are
the tried-and-true option.
“A surge tester will pinpoint turn-to-turn shorts, where the insulation
around the winding has broken down
and shorted across to the adjacent turn.
These are cumulative failures that, at
some point, begin to affect performance.
But they can often be corrected on the
spot by application of insulating varnish.
“A power quality meter can be left in
place for extended periods of time and
will capture harmonics, which can also be
a source of heat and failure,” Jowett said.
Megger's offerings include insulation
testers, which are most important. Others
include hipots, multimeters and clamp-on ammeters, motor and phase-rotation
testers, winding resistance testers, low
resistance ohmmeters, power quality
testers and data loggers.
GRI FFI N, a construction and tools writer
from Oklahoma City, can be reached at
Hioki PW6001 power analyzer
Megger insulation tester