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reggie
29-01-2003, 06:10 AM
I had a compressor motor today that tripped an MCB. I took a reading and was 18Megs to earth across the windings.
After resetting the MCB the motor ran pulling the correct amps and valves all ok. Obviously its on the way out and requires changing but what is the lowest reading to earth detected on a motor before replacement is required and what can you get away with??

herefishy
29-01-2003, 06:27 PM
My thoughts....

Why risk a burnout?

I have experienced small hermetics that would cool fine, proper amp draw, but not until testing to ground in the MegOhm range understand why the dern thing would warm up and trip thermal overload.

I would certainly explain to customer the apparent derioration of the motor, and absolutely recommend immediate remedy, illustrating the cost of replacement and further complications of a "burnout" scenario.

I would think that any tolerence for motor winding insulation deterioration would be specified by the manufacturer, and that is from whom I would seek advisement for your specific question.

Is there any tolerence for such a condition, at all?

Andy
29-01-2003, 08:56 PM
Hi,:)
the normal test figure used is 2 meg or above for electrical installations up to the old 15th edition, the newer IEE 16 edition states 0.5 meg. I still use 2 meg as it sits better with me.
Hope this helps, Regards. Andy :)

zolar1
30-01-2003, 05:06 AM
There are an enormous number of reasons a compressor could trip an MCB. Power outages, flood-backs or flooded starts, excessive head or suction pressures beyond the compressors operating envelope, short cycling, low oil, poor off-cycle equalisation etc. etc. etc. etc........ and many are inter-related... one the cause of the other.

You forgot ALIENS! LOL

Perhaps a 'brownout' due to insufficient incoming electrical service could also be the culprit?

angryk
31-01-2003, 02:08 AM
I heard from a copeland representative that they don't approve of megaohming a compressor because it is damaging to the windings. Anybody else hear of anything similar?

Dave Goodings
31-01-2003, 07:25 PM
John
What I think the copeland rep is talking about, and I thought Mark might have mentioned it!!! is that you should always check the pressures on a system before megger testing if it is a hermetic or semi because if the system is under a vacuum you can breakdown the insulation damaging the windings/insulation hope this helps

CuGe
14-08-2005, 06:15 AM
If you ever megger a copeland recip, even a new one they always seem to have low readings(bit suspect to say the least!!) thats why the Copeland rep probebly told you that. But as Marc said, change the oil and driers and see how how you go.

Robearbam
16-08-2005, 06:25 PM
Found this in my notes, please forgive any redundancy.

The use of a megohmmeter to test motor windings to ground will often prove a convenient means of determining moisture level in the refrigerant. Readings to ground of 50 megohms and above indicate that the refrigerant in the system is free of moisture. A reading of 20 to 50 megohms indicates that a replacement of the filter drier is necessary. A reading below 20 megohms indicates that the refrigerant has a very high moisture content.Refrigerant should be recovered and reclaimed.

bernard
18-08-2005, 08:07 PM
Hi

I,m scratching my head trying to understand why changing a drier would give you a better megger reading.Would it be moisture turning the oil acidic breaking down the windings??Sorry if this is a silly question but I,ve got to know


Thanks

Bernard

chemi-cool
18-08-2005, 08:24 PM
Hi Bernard.

It cant be true.

The wires inside the motor are covered with lacquer, therefor there is no connection to the body.
There are other ways to check the moisture in the system or the oil condition.

Chemi:)

bernard
18-08-2005, 08:45 PM
Hello

I agree,but I can,t understand what the rest of the posts are on about or I,m I missing something:confused:


Regards Bernard

malik55
18-08-2005, 10:55 PM
Yes it is the moisture contamination which produces acids, These acids chemically attacks the motor winding insulation and as insulation becomes weeker and weeker, it results in minor leakage currents that causes overheating and further weeks insulation, at last heavy leakage currents starts and arcing and burn out occurs.
The question how much is the safe readings, as mentiontiond in other posts IEEE STANDARD is 1MEG OHM+1MEG OHM per KILOVOLT of machine operating voltage,
for a 380 volt motor it will be 1.38MEG OHMS
As a rule of thumb you can take it as 2MEG OHMS.
But for a refrigeration compressor motor a 100MEG OHMS >inf is Exellent,
50/100MEG OHMS some moisture is presest replace driers,
20/50MEG OHMS sever moisture is present replace oil and driers,
Below 20MEG OHMS totally contaminated if oil and drier replacement does not improve motor must be open for to check its condition.
you also have to check the manufacturer literature for their equipment safe limits.

Robearbam
19-08-2005, 11:25 AM
Hello

I agree,but I can,t understand what the rest of the posts are on about or I,m I missing something:confused:


Regards Bernard

In electricity we learn that everything conducts, even the best insulator. It's just the matter of how much resistance an insulator can offer before it breaks down and conducts.

chemi-cool
19-08-2005, 09:41 PM
Hi Malik.

It is best, to check the compressor insulation when starting the system, any changesmight require replacing the windings.

Chemi:)

malik55
20-08-2005, 02:51 PM
Yes Chemi, you are right I agree with you.
It is also better to test insulation on yearly base and keep the record for future refrence.

adacus1
15-10-2005, 10:29 PM
The normal Megger meter operates at 500v and this would be considered a megohm test. Units should normally only be tested at this value and should not suffer any damage from repeated tests, power wiring is usually rated at 600v for industrial applications.

Hi pot uses much higher voltages 1000v + and are generally carried out by manufacturers using specialised time controlled equipment, that is the voltage is applied for a very limited time only. At these voltage levels repeated testing can cause electrical damage.

I wont start any equipment until I have meggered it out no matter who or how competent the last person to touch it was particularly new equipment being brought into service or portable transport equipment being restarted. My life is on the back end of this machine and have seen just about every wiring fault possible including direct connection to ground of power wiring. A megger check particularly on transport equipment takes only 2 minutes maximum start to finish, would you risk your life to avoid a 2 min test?

hybridjunction
28-01-2006, 12:19 PM
Found this in my notes, please forgive any redundancy.

The use of a megohmmeter to test motor windings to ground will often prove a convenient means of determining moisture level in the refrigerant. Readings to ground of 50 megohms and above indicate that the refrigerant in the system is free of moisture. A reading of 20 to 50 megohms indicates that a replacement of the filter drier is necessary. A reading below 20 megohms indicates that the refrigerant has a very high moisture content.Refrigerant should be recovered and reclaimed.

I have read that somewhere myself. :D

walden
28-01-2006, 06:22 PM
The Copeland engineer could have been referring to the thermistor unit, inside the compressor motor terminal box, when he said that it was not recommended to Megger compressor windings. I cannot see that the windings themselves would be adversely affected.