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1 2010 Emerson Climate Technologies
Printed in the U.S.A.
AE4-1294
Application Engineering
B U L L E T I N
For years servicemen have used megohmeters toevaluate compressor
motor windings. However, mostmegohmeter manufacturers publish
guidelines thatapply to open motors. For this reason, Emerson
ClimateTechnologies has investigated the use of megohmeterson
hermetic and semi-hermetic compressors.
When using megohmeters to evaluate the motorinsulation of
compressors, it is important to understandthat they should not be
used as one would a volt-ohmmeter. A single megohmeter reading
gives little insightinto the condition of a motors insulation.
Megohmeters are best used as a part of a regularmaintenance
program to monitor trends (over severalmonths). For example, one
might record a megohm valueand compare it to a previous reading. If
subsequentreadings show a trend of lower and lower values,
thencorrective action (such as system clean up) should betaken.
Emerson does not incorporate the megohmeter intoany of its
quality checks. All Copeland compressorsmust pass U.L. required
tests using hi-potential currentleakage testers (hi-pot). Studies
performed by Emersonhave found that compressors with megohmeter
readingsas low as 0.5 megohms still pass the hi-pot.
There are many factors that affect megohm readingsincluding
contaminated refrigerant, oil level, refrigerantin oil and current
leakage through electrical fusites orterminal plates.
Any external electrical components connected to thecompressor
terminals also affect megohm readings.Wires, contactors and relays
all leak current and willdecrease compressor megohmeter readings if
notdisconnected.
As mentioned earlier a single megohm reading cannotbe used to
condemn a compressor since many otherfactors are involved. However,
limits can be placed onmegohm values that dictate action be taken.
Emersonhas found that these limits are related to the rated
voltageof the compressor. Megohm values equal to or greaterthan
1000 ohms per volt are probably acceptable. Forexample, a 460 volt
compressor might show a megohmreading of 460,000 ohms or 0.46
megohm. Compressorswith rated voltages of 208 to 230 volts would
then beoperable at megohm values of 0.208 to 0.230 megohms;for
simplicity, Emerson has set the limit at 0.5 megohmsbefore a
compressor is condemned. Figure 1 showsthe required procedure for
checking compressors witha megohmeter.
New compressors that have never been installed will notneed any
system clean-up procedures so long as themegohm reading is above
0.5. A baseline reading mustbe established for comparison purposes
and since thisis its first reading this will be its baseline
value.
Megohm Values of Copeland Compressors
AE4-1294 August 1992Reformatted November 2010
Application Engineering
B U L L E T I N
