Helpful megohmmeter

Question: I have recently experiences two incidents where I believe the use of a megohmmeter may have been helpful.

I was dispatched for two service calls on a couple of residential split systems in the 3-ton range, 230 volt single-phase. Both compressors were drawing locked rotor amperage. I checked and determined that the run capacitors were okay, so I installed an SPP-6 start capacitor (no expansion valve). Both compressors came on, and after about ten minutes of run time, the amperages on both compressors were under the rated R.L.A.

Within a month, I was sent back to both calls and found that the compressors were shorted to ground, which resulted in compressor replacements.

My question is, had I used a megohmmeter to measure the insulation on the compressor motor windings during the first calls, could I have detected a potential problem? Could you also explain the advantages of a megohmmeter for proper use in the refrigeration field?

Joel Johnson, Pinellas Park, FL

Answer: A megohmmeter or “megger” is an insulation tester, which checks the value of the insulation resistance to the frame or ground of electrical apparatus. The scale is calibrated in millions of ohms resistance or megohms. The megger is a small dc generator, and by turning the crank (handle), a voltage is generated and a very small current flows across the insulation. This current and applied voltage are measured by the ohmmeter. The scale shows ohms and megohms resistance. For motors and generators with ratings of 2400 volts or less, a 500 volt megohmmeter should be used.

This insulation resistance test is useful in determining when to remove a motor from service for a reconditioning or rewinding. To be effective, this test must be conducted at regular intervals and the readings recorded for comparison with future readings. This is known as “trending.” High humidity can cause resistance readings to drop. Also, readings should be made at room temperature and the temperature recorded. Readings must be corrected for temperature variation. A graph of megohm readings at regular time periods should be made, and if resistance drops for two or three successive tests, the motor should be serviced. These comments apply to open, totally enclosed fan-cooled, and other non-hermetic motors and generators.

In hermetic compressors, the amount of moisture which can affect the megger reading may be such that it will have no detrimental effect on the operation of the compressor motor. Therefore, it is difficult to evaluate such a megger reading unless the amount of moisture is known. Liquid refrigerant which is in contact with the windings of a hermetic motor may give low megger test readings.

Always contact the compressor manufacturer for recommendations on evaluation of the hermetic motor condition.

-Richard Beard, PE, Consultant