Datasheet Content
HARD STARTING AND
OVERLOAD PROBLEMS:
SPECIFICATIONS
Range 20 to 1000 MegOhms
M500
MEGOHMMETER
Applied Voltage 500 Volts
Answering a call where the problem is a hard-starting Power Source (2) “C” Size Batteries
condition is more involved than merely replacing a Test Leads (2) 9” Long (Store within case)
defective relay, start capacitor, or installing a hard
start. The unknown factor is how long did the unit
cycle on the overload before the consumer realized
the food compartment was getting warm and called
OPERATION
the service dispatcher. In other words, was the wind- 1. Install two new alkaline “C” size batteries. Polarity is
ing overheated to a point that could have damaged indicated on bottom of battery compartment.
the insulation? This unknown factor of overheated 2. Remove ALL wires that are connected to the terminals
windings is also present on calls that relate to dirt- that are to be tested. Attach one test lead from the meg-
clogged condensers, defective condenser fan bearings Ohmmeter to ANY WINDING TERMINAL and the other test
or motors and brownouts. Whenever a sustained motor lead to the METAL CASE of the motor. Make sure the point of
overload condition existed before you arrived on the contact is clean to the BARE METAL (no paint).
job site, a megOhmmeter test tells both you and the 3. Depress “POWER” button, wait approximately 10 seconds
consumer if there was any damage to the winding then proceed to read the insulation value shown on the cor-
insulation. Putting this information on the consumer’s responding illuminated LED lamp.
receipt and recording it in the office records is sound 4. Illuminated “BELOW 20” lamp indicates low insulation or a
business practice for any future problems and assures short circuit in the unit being tested.
the customer that the service engineer “KNOWS HIS 5. If the “ON” lamp is illuminated but no light appears on the
BUSINESS”. megOhm scale, then the insulation value is MORE THAN 1000
MEGOHMS.
APPLICATION 6. If the “ON” light is not illuminated when the “POWER”
button is depressed, then the batteries are installed improp-
Hermetic Refrigeration Motors erly or the batteries are weak and should be replaced.
7. Remember, all tests are made from the motor
Hermetic motors require special test parameters since
TERMINAL to the METAL CASE.
the winding operates in an oil and refrigerant atmo-
sphere. Under normal conditions the oil and refriger-
ant have a high electrical resistance. However, when
Why All HVAC/R
they are contaminated with moisture, the electrical
resistance changes and a low megOhm reading of
the winding terminal to ground may actually be the
resistance of the contaminants in the oil and not in
the winding insulation. New hermetic motors have a
Service Technicians
resistance value of more than 100 megOhms. Readings
of 50 megOhms or less would indicate that either the Need This Instrument
winding insulation is deteriorating or the oil/refrig-
erant is contaminated. Installing an oversized drier
may clean up the system or another possible solution
could be replacing all the compressor oil. In either You have often heard the expression, “THE MOTOR
case, if the megOhm tests show even the slightest IS BURNT OUT” or “THE MOTOR IS HALF COOKED”
improvement in resistance values, the remedy may or “IT WAS OVERHEATED”. Actually, the reference is
have checked a declining condition. A good “rule of F583 being made to the condition of the motor winding
thumb” is to smell a small sample of the oil. If at any 09092015 insulation...
time in the history of the unit the motor was severely
overloaded, it will leave a tell-tale trace in the oil
that can never be removed other than by discharging SEALED UNIT PARTS CO., INC.
all refrigerant and replacing oil. The odor of burnt PO Box 21, 2230 Landmark Place
insulation is easily recognized and a low resistance Allenwood, NJ 08720 USA
value of 20 megOhms or less will verify that the motor www.supco.com
insulation may be severely damaged. www.supco.com
The wire itself is rarely, if ever, destroyed. It is always 100-200 MegOhms 20-60 MegOhms
the motor-winding insulation that determines if the motor Readings Readings
is good, bad, or on the verge of becoming bad. The winding
insulation prevents the current, which is going through the Most electrical engineers and motor designers agree A 0 to 60 megOhms reading in a hermetic compres-
wire, from traveling into the motor body. The expression, that a measurement of 150 or more megOhms across sor is an indication that you can have any one of
“THAT MOTOR IS GROUNDED” simply means that the insula- an electrical terminal to its ground would be consid- a few problems; a winding that was overheated,
tion of the winding has deteriorated or has become damaged ered excellent insulation, and a reading of 100 to 150 contaminated oil, or moisture circulating in the
from heat and can no longer resist the current from traveling megOhms is very good. system. Sampling the oil for a burning odor will tell
into the motor body. You can TEST the resistance of the insu- you immediately that the winding was overheated.
lation, or you can MEASURE the resistance of the insulation. If the oil is clean and odor free, it can still have
some contamination that is causing the low megOhm
THERE ARE TWO WAYS OF reading. A 20 megOhm reading shows severe contam-
ination and failure of the system is likely. The best
TESTING THE INSULATION: overall protective procedure is to dump and replace
all the oil and change and install a new liquid line
1. A high voltage test from wire to ground (motor body).
2. A high voltage breakdown test from wire to ground. A drier. If the oil had an odor, a thorough check of the
high voltage test is usually performed at the factory where condensing medium, air fans, cooling tower and all
the product is manufactured. The Underwriters Labora- motors for worn bearings should be made. Now that
tory standard is to test an electrical product from wire you have discovered a low megOhm reading and have
to ground with a voltage equal to 1,000 plus twice the taken the necessary preventative measures, the
operating voltage, as an example: If a motor was operating follow-up is very important. Take another reading in
at 115VAC, you would conduct the test at 230VAC from wire a few weeks, and if the reading remains the same
to ground and measure if there was any current going into or improves, take another measurement in a few
the body. A high voltage breakdown test is performed to months. If the reading shows more deterioration,
determine at what voltage point the insulation will break you can forewarn the owner that there is going to
down and allow current to pass into the motor body. Since be a major breakdown in their system. One of the
you are actually destroying the insulation, a breakdown test 60-100 MegOhms best maintenance programs for a new system is to
log the megOhmmeter readings when the unit is first
is usually performed in a laboratory where different insula-
tions are being tested for their resistance to high voltage,
Readings installed and compare the readings every six months.
or a more technical term, dielectric strength. The two Measurements of 60 to 100 megOhms would show a
above methods are used to TEST the insulation resistance. decline in the insulation resistance, either in an area
or a specific spot. This is a most important measure-
MEASURING THE ment. It tells you that if you do not take corrective
steps, the insulation will completely breakdown.
INSULATION RESISTANCE: With open type motors it is a matter of good house-
keeping by cleaning the dust and any grime from the
Since the measurement of electrical resistance is the OHM,
windings, using a recommended solvent.
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…
