Before condeming any semi hermetic due to a suspected short circuit I remove the terminal plate and disconnect the wires to test the windings with a megger.
The service valves must be front seated and the refrigerant recovered from the crankcase prior to removing the terminal plate.
Compressor Testing Guide Please use this guide if you suspect a faulty compressor To verify basic compressor function in a field environment the following will demonstrate whether further investigation will be necessary or not. 1 Earth Test Using a 1000v Megger check each field winding to earth. 1st check the earth connection is sound by an earth to earth test & repeat this following the motor test.
If reading < than 2 MÙ Stator probably burnt out, but do not rule out a contaminated / damp terminal plate.
Reading 2 MÙ - 20 MÙ Probably not burnt out, terminal plate contaminated or damp. Separate terminal plate from main casting & repeat check to casting. If fault clears then terminal plate is faulty.
Reading > 20 MÙ No earth Fault
Precautions Do not effect Megger checks under deep vacuum. Do not Megger overloads.
2 Field Balance Using an Ohmmeter set on Ohms verify balance 3ø motors only of motor field coils. If balance is out by more than 10 % suspect motor is defective. Separate terminal plate & repeat measurements, if readings do not improve rewind is probably required.
Application Engineering Bulletin
AE-1294 August 1, 1992
MEGOHM VALUES OF COPELAND COMPRESSORS
......There are many factors that affect megohm
readings including contaminated refrigerant, oil
level, refrigerant in oil and current leakage through
electrical fusites or terminal plates.
Any external electrical components connected to
the compressor terminals also affect megohm readings.
Wires, contactors and relays all leak current
and will decrease compressor megohmeter readings
if not disconnected.
As mentioned earlier a single megohm reading
cannot be used to condemn a compressor since
many other factors are involved. However, limits can
be placed on megohm values that dictate action be
taken. Copeland has found that these limits are
related to the rated voltage of the compressor. Megohm
values equal to or greater than 1000 ohms per
volt are probably acceptable. For example, a 460 volt
compressor might show a megohm reading of 460,000
ohms or 0.46 megohm. Compressors with rated
voltages of 208 to 230 volts would then be operable
at megohm values of 0.208 to 0.230 megohms; for
simplicity, Copeland has set the limit at 0.5 megohms
before a compressor is condemned.
Application Engineering Bulletin
AE-1294 August 1, 1992
MEGOHM VALUES OF COPELAND COMPRESSORS
......There are many factors that affect megohm
readings including contaminated refrigerant, oil
level, refrigerant in oil and current leakage through
electrical fusites or terminal plates.
Any external electrical components connected to
the compressor terminals also affect megohm readings.
Wires, contactors and relays all leak current
and will decrease compressor megohmeter readings
if not disconnected.
As mentioned earlier a single megohm reading
cannot be used to condemn a compressor since
many other factors are involved. However, limits can
be placed on megohm values that dictate action be
taken. Copeland has found that these limits are
related to the rated voltage of the compressor. Megohm
values equal to or greater than 1000 ohms per
volt are probably acceptable. For example, a 460 volt
compressor might show a megohm reading of 460,000
ohms or 0.46 megohm. Compressors with rated
voltages of 208 to 230 volts would then be operable
at megohm values of 0.208 to 0.230 megohms; for
simplicity, Copeland has set the limit at 0.5 megohms
before a compressor is condemned.
Thanks! I bought a megger a few months ago. Hadn't had a chance to figure out how to use it. This helps me get a feel for it. This thread is long out of play, so don't know if it will be seen, but what Megger setting recommended?
Thanks! I bought a megger a few months ago. Hadn't had a chance to figure out how to use it. This helps me get a feel for it. This thread is long out of play, so don't know if it will be seen, but what Megger setting recommended?
Download "a stich in time". Its a really long read and some of it is not related to what we do but it gives you more insight into megging.
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Originally Posted by heavymetaldad
