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xchcui
20-08-2017, 07:01 AM
Hello.

I test an air conditioner's compressor 3.5HP with megger(500V) and i got a result,0.5MΩ.
Good results should be above 50MΩ-100MΩ,while the result that i got is very low.
The compressor is working very good and the current consumption is normal.So,i wonder if i did something wrong which gave me an error result.
Before i took the measurment,i disconnected the three wires from the compressor pins(C/R/S).
One probe i connected to one of the pins and since there wasn't any bare metal on the compressor body to connect the second probe,
i connected the second probe to the metal sqaure box which is located above the compressor(the metal box that contains the capacitors,the contactor etc.).
The compressor's body and the metal box are both grounded as the whole body of the outer unit,so i assumed that it will be the same.
But as i mention above i got a very low result 0.5MΩ and i would like to know:
Could it be that i got very low result since the megger,actually,measured something else or was influenced from something else due to a wrong measurment procedure?Or maybe i had to disconnect some more component before the testing?

Thanks.

Greek_engineer
20-08-2017, 10:13 AM
which is the voltage applied to the compressor?

xchcui
20-08-2017, 10:20 AM
which is the voltage applied to the compressor?

The compressor input voltage is 220VAC.
The megger test made with 500VDC.

narkom
20-08-2017, 12:01 PM
Is there a crankcase heater on the compressor?

FaultCode
20-08-2017, 12:09 PM
Use the copper pipe as the earth connection.

chemi-cool
20-08-2017, 02:30 PM
You must connect both wires to the compressor, one to the pin and the other to one of the copper pipes entering, as close as possible to the compressor body or even better to the ground terminal.

Why did you check megger it in the first place?

monkey spanners
20-08-2017, 02:42 PM
I don't see a problem with where to placed the test wires, if it had made a bad connection the resistance reading would be higher and not lower (the only thing that would do is make a faulty compressor look ok and not the other way round)
Was the compressor warm? If it has not been running any liquid refrigerant in the compressor will give a lower reading on a megger test. Another issue could be damness around the terminals. Or the motor could be well on the way to failing.

xchcui
20-08-2017, 04:53 PM
Is there a crankcase heater on the compressor?
Not that i can see.


...Why did you check megger it in the first place?
I change a run capacitor,so on the way i did some test to the compressor.


I don't see a problem with where to placed the test wires, if it had made a bad connection the resistance reading would be higher and not lower (the only thing that would do is make a faulty compressor look ok and not the other way round)...
Yes,this is what i thought.I am aware to the situation that other ground point,besides the compressor body,may give me higher resistance,but my concern was about influences of other components on the measurmemt.
Such as:the compressor body,the condenser motor fan,the evaporator motor fan and actually every metal object of the air conditioner is grounded,i thought that,maybe,this shared ground gave me somehow a wrong reading,
so it doesn't,right?and i don't need to seperate the compressor from the ground in order to get a correct result,only the three wires(C/R/S),am i right?

...Was the compressor warm? If it has not been running any liquid refrigerant in the compressor will give a lower reading on a megger test. Another issue could be damness around the terminals. Or the motor could be well on the way to failing.
Yes,i turn it on for 15 minutes and i cleaned the terminals(though they were not wet).

malik55
21-08-2017, 01:14 AM
When testing a compressor with a Megger you must have to isolate compressor terminal wires completely which you did it as mentioned in your mail, Terminals should be clean and there should be no moisture or rust present, also a clean earth point on compressor body or on suction/discharge pipes required for to connect the Megger wires as suggested by Mr. Chemi-cool.
If your motor has .5M reading, It is working nicely and has no problem then it is ok. As per formula the M-Ohms a single phase motor for 230volt may work less then .5M but it is a normal practice that reading less then .5 is dangerous and we have to be cautioned.
Operation voltage x 1 K-Ohm = M-Ohm
230x1000=0.230000M-Ohms or 0.25 M-Ohms(for single Phase motors).
460x1000=0.460000 M-Ohms or 0.5 M-Ohms (for three phase motors) industry practice as minimum.
There are other factors which also effects resistance such as moisture in the system.
for more info please visit the link>
https://opi.emersonclimate.com/CPID/GRAPHICS/Types/AEB/ae1294.pdf

xchcui
21-08-2017, 07:29 AM
Okay,thanks,i understand the resistance parameters issue and that my result 0.5M is considered okay.but i see that no one refer to my question about the fact that the compressor body is grounded with other components.I mean that when i takes the measurment the compressor is not,actually,disconnected completely.It is connected to ground and sharing the same ground,also,with the condenser motor fan,the evaporator motor fan and actually every metal object of the air conditioner.Can that situation affect the reading result?
I believe it won't,as I know that the current path should flow from the megger to the winding,to the body of the compressor and back to the megger,but i would like to verify this.

hyperion
21-08-2017, 12:22 PM
The mechanical earth connection between the various components should not make any difference as the megger circuit will take the least line of resistance and will not be affected by other components on the unit, provided that all of the electrical cables have been disconnected from the compressor terminals.

xchcui
21-08-2017, 05:14 PM
Besides the three wire(C/S/R),i didn't see any other wires that are connected to the compressor,so i guess that everything is fine.
Thanks alot for all the responses.
There were very helpful to solve my issue.