Semihermetic compressor winding resistance measurement

[racktech2008]

Hi to everybody. I´m a newcomer both in this forum and in rack refrigeration. Mostly work on single compressor systems before. Hope to be able to learn and share experiences with all of you.
My question is: what method is the best for measuring semihermetic compressor winding resistances, often in the range of tenths of an ohm (for instance, 0.30 ohms)? In this range, regular vohms probably will become unreliable due to the insternal resistance of the device and test leads. I know that there are some milliohmeters used in special labs worth a fortune. But for the regular tech, are there any cheaper options availabe? I`m trying to get some insight on the actual winding condition in several compressors located in this rack.

[Brian_UK]

HI and welcome to the forum.

A post last month by Nike123 gave some useful information, have a look...

+
Nice Fluke workbook for measuring refrigeration compressors.
http://www.mediafire.com/?11izldhsd3d

+

[nike123]

I think he is more concerned about winding resistance than winding insulation. If someone know good and reasonable priced miliohmmeter, is his point here.;)

Something like this:
http://tinyurl.com/ysx3tq

[Brian_UK]

Very true, I stand corrected and will slink off stage right.... .... ...

Mind you, I'm not quite sure why anyone would need to measure resistances to that level on a compressor motor.

[nike123]

Large compressors have low resistance, and manufacturers gave us data of the resistance of the windings. If you try to measure that resistance with ordinary multimeter, you could have big error.
Measuring resistance of winding could tel us if we have partial short-circuit in winding.

You could have lot of small short-circuits in windings without damaging insulation resistance between winding and ground, and insulation measurement could gave you pass for that winding even if it is, in realty, damaged and draw excess current.
Therefore , it is important to measure that resistance with great accuracy's. Also, you could compare resistance of all windings, as diagnostic tool for evaluation of compressor condition.

[racktech2008]

Thank you very much Brian and Nike for your kind answers.
Nike accurately described the nature of my questioning: I would like to know the resistance of the windings, not really the megaohm value of the insulation. I was told that winding resistance, taken at the proper temperature, is much more useful than megaohm values when trying to determine the current status of the stator. But for this measurement, the only available and reliable meters are miliohmmeters, right?

[Electrocoolman]

I use a Fluke DVM that has a low ohms range that allows one to zero out the test lead resistance.

The good old AVO 8 also had a low ohms range.

[nike123]

Thank you very much Brian and Nike for your kind answers.
Nike accurately described the nature of my questioning: I would like to know the resistance of the windings, not really the megaohm value of the insulation. I was told that winding resistance, taken at the proper temperature, is much more useful than megaohm values when trying to determine the current status of the stator.

Both of measurements are equally useful, and one without other could not gave us full picture about state of winding.



But for this measurement, the only available and reliable meters are miliohmmeters, right?

Quite right.

[nike123]

I use a Fluke DVM that has a low ohms range that allows one to zero out the test lead resistance.

The good old AVO 8 also had a low ohms range.

Yes, but that is like measuring vacuum with slightly more accurate gauge then ordinary suction gauge, in comparison with electronic vacuum meter.

[Brian_UK]

Thanks Nike, we learn a little more each day.

I have to say that large compressor and me don't normally get together very often and with my ignorance it's probably just as well ;)

[Electrocoolman]

Any low ohm measurement will not distinguish a shorted turn though.

[nike123]

I have to say that large compressor and me don't normally get together very often and with my ignorance it's probably just as well ;)

Don't dramatize now, Brian. You are big boy!:D:D

[nike123]

Any low ohm measurement will not distinguish a shorted turn though.

One turn, certainly not, but could reveal much more then simple DMM.

Here is article about complete measuring of windings:
http://www.alltestpro.com/pdf/Static_Winding_Analysis.pdf

[Grizzly]

Hi to everybody. I´m a newcomer both in this forum and in rack refrigeration. Mostly work on single compressor systems before. Hope to be able to learn and share experiences with all of you.
My question is: what method is the best for measuring semihermetic compressor winding resistances, often in the range of tenths of an ohm (for instance, 0.30 ohms)? In this range, regular vohms probably will become unreliable due to the insternal resistance of the device and test leads. I know that there are some milliohmeters used in special labs worth a fortune. But for the regular tech, are there any cheaper options availabe? I`m trying to get some insight on the actual winding condition in several compressors located in this rack.


Far be for me to disagree with any of the Knowledgeable answers already given all valid.
BUT RACKTECH,

Why are you wanting to read the winding resistance values.
To such a high degree of accuracy?

Unless you are not trying to accomplish this "in the field"

please could you let me know why you want to read to such a fine tolerance?
Many thanks Grizzly

[jjthefridge]

Hi,

can't go far wrong with a fluke digital multimetre (I prefer auto ranging) a meggar and a clip on ameter, if your balancing the windings, worth checking the insulation of the winding, then check the running current of the compressors, when you've got those reading, in time just check the running current if there's over the different over the phases then there's a problem. Wouldn't bother going into miliohns, no point. should be able to zero multi meters with leads, failing that measure the resistance of the leads, subtract that from the windings reistance.

:rolleyes:

[racktech2008]

Thank you again for your interest in this topic.

I was told that although megger tests are an useful tool when determining insulation condition, the readings must be taken over a period of time in order to determine a trend. For instance, in the case of a contaminated system that is slowly eating away the winding varnish, the trend should show a decaying insulation value.
On the other hand, direct resistance readings on the windings, when compared with the given values usually included in the manufacturer´s data, offer a quicker view of the current status of the windings: if they differ a lot, something must definitely be wrong with the windings. The problem is that the manufacturer´s values are usually in the tenths of one ohm.
This is the reason why I'm really insterested in finding a way to accurately detect variations in the resistance. I know that Fluke and Simpson and other test equipment manufacturers supply miliommeters, but the cheapest are worth about US$ 600. I guess that I'm going to have to go out and buy the most accurate digital VOM I can afford in order to be able test these windings.

[Springbok]

Hey racktech2008,you're spot on about the insulation tester.Having the megger,insulation tester,ohm etc all in piece of equipment can be handy,but then you mentioned the price tag of 600 smackers.:eek:

The fluke 1587 IRT instrument is perfect for the job,but not the cheapest.Investing in something like that is always a winner.Good luck...:D

[petewa]

bitzer compressors have a thermistor usually in the electrical box but can be in the switchboard, and should read 500ohms across the probes when the compressor is warm. cheers

[Grizzly]

Racktech.
Thanks for replying.
You original post seemed to be asking for laboratory accurate meters,

But as jj and springbok point out you don't need to be that accurate in the field.

It is worth pointing out that the resistance to earth and within the windings.
ARE greatly affected by the atmospheric conditions of where the motor is!

A damp motor that has been standing for a long time
can fail an Insulation test.
Whereas warm it up for a while and it will pass!

Please follow the Link below as the booklet I posted previously has lots of helpful hints.

Especially to do with the Electrical side of fridge!

http://www.refrigeration-engineer.com/forums/showthread.php?t=10419

Cheers Grizzly