inexpensive alternative to taut band (swing needle) megohmeters, the
M500 provides fast and accurate measurements to verify the condition of
- Zone scale to indicate insulation condition
- Measure insulation values up to 1000 megohms @ 500VAC
- Hand held
- Test leads store in case
- Operates on 2 C batteries (not included)
- Impact resistant
- Soft carrying case included
||32°F to 120°F (0°C to 50°C)
||20 to 1000 megohms
:DMEG3 Part Description
|Relay Control circuits
|Hermetic Refrigeration Compressors|
M500 MEGOHMMETERWHY ALL 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.
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 insulation of the winding has
deteriorated or has become damaged from heat and can no longer resist the current from going into the motor
body. You can TEST the resistance of the insulation, 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 Laboratory 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 breakdown 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 insulations
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 resistance 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 11 5 ohms, and the resistance of the insulation of a good motor may measure 90,000,000 ohms. Most ohmmeters are incorporated
in the popular multimeters where one meter can serve as an ampmeter, 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 specifically
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 microamps 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.
100-150 Megohm 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 considered excellent insulation, and a reading of 100 to 150 megohms is very good.
60-100 Megohm Readings
Measurements of 60 to 100 megohms would show a decline in the insulation resistance, either in an area or a specific spot. This is a most
important measurement. 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 housekeeping by cleaning the dust and any grime from the windings, using a recommended solvent.
20-60 Megohm Readings
A 40 to 60 megohms reading in a hermetic compressor 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 contamination and failure of the system is likely. The best overall protective procedure is to dump and replace
the entire oil, 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 his system. One of the best
maintenance programs for a new system is to log the megohmmeter readings when the unit is first installed and compare the readings every
HARD STARTING AND OVERLOAD PROBLEMS:
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 starting "Pow-r-pak". 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 winding 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 office records is sound business practice for any future problems and
assures the customer that the service engineer "KNOWS HIS BUSINESS".