Product Overview

The wire itself is rarely, if ever, destroyed. It is always
the motor-winding insulation that determines if the motor
is good, bad, or on the verge of becoming bad. The winding
insulation prevents the current, which is going through the
wire, from traveling into the motor body. The expression,
“THAT MOTOR IS GROUNDED” simply means that the insula-
tion of the winding has deteriorated or has become damaged
from heat and can no longer resist the current from traveling
into the motor body. You can TEST the resistance of the insu-
lation, or you can MEASURE the resistance of the insulation.
THERE ARE TWO WAYS OF
TESTING THE INSULATION:
1. A high voltage test from wire to ground (motor body).
2. A high voltage breakdown test from wire to ground. A
high voltage test is usually performed at the factory where
the product is manufactured. The Underwriters Labora-
tory standard is to test an electrical product from wire
to ground with a voltage equal to 1,000 plus twice the
operating voltage, as an example: If a motor was operating
at 115VAC, you would conduct the test at 230VAC from wire
to ground and measure if there was any current going into
the body. A high voltage breakdown test is performed to
determine at what voltage point the insulation will break
down and allow current to pass into the motor body. Since
you are actually destroying the insulation, a breakdown test
is usually performed in a laboratory where different insula-
tions are being tested for their resistance to high voltage,
or a more technical term, dielectric strength. The two
above methods are used to TEST the insulation resistance.
MEASURING THE
INSULATION RESISTANCE:
Since the measurement of electrical resistance is the OHM,
we use an Ohmmeter. When measuring the insulation re-
sistance of a motor winding, we are involved in resistances
in the range of 50 to 100 million ohms. As an example, the
starting winding of a small motor may measure 115 ohms,
and the resistance of the insulation of a larger motor may
measure 90,000,000 ohms. Most ohmmeters are incorporat-
ed in the popular multi-meters where one meter can serve
as an amp-meter, voltmeter and ohmmeter. However, when
this meter is used to read Ohms, the circuit does not permit
measurements more than a few million ohms or megOhms.
The SUPCO M500 megOhmmeter is specically designed to
detect Ohms with a scale that reads up to 1,000 megOhms
(1,000,000,000 Ohms). To accomplish this with some degree
of accuracy, you have to apply at least 500 volts. Some
instruments use 1,000 volts. Although you can “feel” the
500 volts, it is not lethal because the instrument current is
self-limiting, allowing only micro-amps to go through. This
limiting circuit is also the main reason why the use of a
megOhmmeter can never damage the
insulation of a winding.
100-200 MegOhms
Readings
Most electrical engineers and motor designers agree
that a measurement of 150 or more megOhms across
an electrical terminal to its ground would be consid-
ered excellent insulation, and a reading of 100 to 150
megOhms is very good.
60-100 MegOhms
Readings
Measurements of 60 to 100 megOhms would show a
decline in the insulation resistance, either in an area
or a specic spot. This is a most important measure-
ment. It tells you that if you do not take corrective
steps, the insulation will completely breakdown.
With open type motors it is a matter of good house-
keeping by cleaning the dust and any grime from the
windings, using a recommended solvent.
20-60 MegOhms
Readings
A 0 to 60 megOhms reading in a hermetic compres-
sor is an indication that you can have any one of
a few problems; a winding that was overheated,
contaminated oil, or moisture circulating in the
system. Sampling the oil for a burning odor will tell
you immediately that the winding was overheated.
If the oil is clean and odor free, it can still have
some contamination that is causing the low megOhm
reading. A 20 megOhm reading shows severe contam-
ination and failure of the system is likely. The best
overall protective procedure is to dump and replace
all the oil and change and install a new liquid line
drier. If the oil had an odor, a thorough check of the
condensing medium, air fans, cooling tower and all
motors for worn bearings should be made. Now that
you have discovered a low megOhm reading and have
taken the necessary preventative measures, the
follow-up is very important. Take another reading in
a few weeks, and if the reading remains the same
or improves, take another measurement in a few
months. If the reading shows more deterioration,
you can forewarn the owner that there is going to
be a major breakdown in their system. One of the
best maintenance programs for a new system is to
log the megOhmmeter readings when the unit is rst
installed and compare the readings every six months.