Many factors affect megohm readings

Question: My coworkers and I have been taking megohm readings on some of out 20-and 25-ton Copeland compressors. As we know, this test is for checking the integrity of the insulation of the motor windings. Why is that when we change the oil we get a better reading?

Also, the reading would be considerably different if it wasn't running for a while. Shouldn't the integrity of the windings be consistent no matter what happens? If the insulation of the windings is damaged, why do we get a better reading after changing oil? Does the oil coat the windings, giving them a better insulation reading?

Answer: There are many factors that affect megohm readings, including contamination of the oil and refrigerant. Because the oil in the crankcase is a scavenger for everything that gets past the filter driers, is the quickest way to clean a system. Contaminants may be moisture if the driers are full.

They also may be carbon, oxides or metal particles if the compressor is failing or is replacing one that has failed. If the compressor has not operated for a period of time, liquid refrigerant may have migrated to the oil and this lowers its resistance. If you have a low megohm reading, operate the compressor for a period of time, shut it off and take another reading. Also, be sure that the terminals and connections are clean and snug. Dirt and moisture in the terminal box can be the problem.

Megohm readings should not be considered a “go” or “no-go” indication of the compressor: It should be used as a trend tool in preventive maintenance programs. Generally, the compressor should not be condemned if the megohm reading is .5 megohm or higher. If the readings are between .5 and 20 megohm, the filter driers at the very least should be changed. If the oil also is changed at that time, it may not be necessary to change the filter driers again.

­ -Glenn Hannegan Jr., consultant, retired from Copeland Corp.