Product Overview

M500
MEGOHMMETER
Why All HVAC/R
Service Technicians
Need This Instrument
You have often heard the expression, “THE MOTOR
IS BURNT OUT” or “THE MOTOR IS HALF COOKED”
or “IT WAS OVERHEATED”. Actually, the reference is
being made to the condition of the motor winding
insulation...
www.supco.com
F583
09092015
SPECIFICATIONS
Range
Applied Voltage
Power Source
Test Leads
20 to 1000 MegOhms
500 Volts
(2) “C” Size Batteries
(2) 9” Long (Store within case)
OPERATION
1. Install two new alkaline “C” size batteries. Polarity is
indicated on bottom of battery compartment.
2. Remove ALL wires that are connected to the terminals
that are to be tested. Attach one test lead from the meg-
Ohmmeter to ANY WINDING TERMINAL and the other test
lead to the METAL CASE of the motor. Make sure the point of
contact is clean to the BARE METAL (no paint).
3. Depress “POWER” button, wait approximately 10 seconds
then proceed to read the insulation value shown on the cor-
responding illuminated LED lamp.
4. Illuminated “BELOW 20” lamp indicates low insulation or a
short circuit in the unit being tested.
5. If the “ON” lamp is illuminated but no light appears on the
megOhm scale, then the insulation value is MORE THAN 1000
MEGOHMS.
6. If the “ON” light is not illuminated when the “POWER”
button is depressed, then the batteries are installed improp-
erly or the batteries are weak and should be replaced.
7. Remember, all tests are made from the motor
TERMINAL to the METAL CASE.
SEALED UNIT PARTS CO., INC.
PO Box 21, 2230 Landmark Place
Allenwood, NJ 08720 USA
www.supco.com
Answering a call where the problem is a hard-starting
condition is more involved than merely replacing a
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
the service dispatcher. In other words, was the wind-
ing overheated to a point that could have damaged
the insulation? This unknown factor of overheated
windings is also present on calls that relate to dirt-
clogged condensers, defective condenser fan bearings
or motors and brownouts. Whenever a sustained motor
overload condition existed before you arrived on the
job site, a megOhmmeter test tells both you and the
consumer if there was any damage to the winding
insulation. Putting this information on the consumer’s
receipt and recording it in the ofce records is sound
business practice for any future problems and assures
the customer that the service engineer “KNOWS HIS
BUSINESS”.
HARD STARTING AND
OVERLOAD PROBLEMS:
Hermetic motors require special test parameters since
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
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
resistance value of more than 100 megOhms. Readings
of 50 megOhms or less would indicate that either the
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
case, if the megOhm tests show even the slightest
improvement in resistance values, the remedy may
have checked a declining condition. A good “rule of
thumb” is to smell a small sample of the oil. If at any
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
all refrigerant and replacing oil. The odor of burnt
insulation is easily recognized and a low resistance
value of 20 megOhms or less will verify that the motor
insulation may be severely damaged.
APPLICATION
Hermetic Refrigeration Motors