Checking compressor is down to earth or shot

[strongman]

Hey guys , my name is dave and i jsut started my apprenticeship in refrigeration and wanting to learn quickly. Ive got a question , on how to check 3 phase and single phase motors with a multimeter.. How do you do it on a multimeter and then use the results to determine what the compressor is doing?

i know you set it on ohms and go each terminal to earth and between the phases, but what amount of ohms is good bad etc ??? sorry as i dont understand this

[nike123]

First, you need to have data about compressor winding resistance for certain temperature. That you could find from compressor manufacturer catalogs or from equipment service manuals. Second, you need to have precise ohmmeter for measuring that resistance.
With multimeter you could measure that windings are not discontinued and that are not grossly short circuited between individual windings and between windings and housing.
Check this thread:
http://www.refrigeration-engineer.co...ad.php?t=10419

And this article:
http://www.alltestpro.com/pdf/Static...g_Analysis.pdf

For proper insulation measurement you need mega ohm tester.
This is description about insulation measurements:
http://www.mediafire.com/download.php?11izldhsd3d

[strongman]

i still dont get how to determine it is shot ... say i got on site compressor not running has burnt out, if i were to ohms the winding or phases of the unit with the figures given , how do i determine physical with a multimeter For eg i ohms an 3 phases motor and i get 4ohms 10ohms 4ohms Does this mean the motor is shot as its pulling higher ohms, also how do you check on a single phases run start common windings???. Can you just continiuty test each phase/whinging to an earth , is that how you see if its down to earth ?

[Chunk]

Bonjour

If you are just using a multimeter to test if a compressor is down to earth,you just need to meter between the terminals and an earth point,if it is completely damaged you will get resistance,this works with single and 3 phase.

To test roughly if the windings are faulty on a single phase unit,meter between common and start,common and run,run and start and the two smallest resistances should add up to the largest.

To test 3 phase you have windings u1 v1 w1 and u2 v2 w2 depending on the configuration,part wind,u1 and u2 will show resistance when measued together same with v1 v2 and w1 w2.Star delta (if i remember correctly)u1 and v1 have resistance,v1 and w2,then w1 and u2 likewise.If you dont get resistance between the windings then something is wrong.

To do a proper test you will need a proper ohm tester as multimeters will not show everything,but they give you a rough idea.

I hope that made sense.

[Chunk]

Heres another very usefull link Samarijt Sen posted

http://openbookproject.net/electricCircuits/

Bonsoir

[Thermatech]

Measuring resistance of each winding terminal to earth on 3 ph compressor motor with a multi meter only gives you an aproximate indication.
The reading should be completely open circuit.

To properly test the motor windings you must use a Mega meter. This slams 500v or 1000v through the windings. This is much more accurate as it effectively tests the windings under load.
Typically you are looking for a reading higher than 1 meg ohm. On alot of air conditioning compressors up to say 10hp a good motor will show 20 to 40 meg ohm.
You might just get some resistance reading on a standard multi meter at 1 meg ohm reading from a Mega meter test.

As an example ,,,, you might go to site to invetigate why a compressor has tripped the power supply mcb.
Multi meter shows all 3 windings open circuit so no ground fault.
Mega meter shows 1 meg ohm. So you suspect possible ground fault.
You reset power supply & run compressor. After a few seconds it trips the power supply again.
So you do another mega test & this time find less than 1 meg ohm. So now you are sure the motor windings have ground fault.

Some times when motor windings start to fail to ground it will only happen under load conditions & the motor windings have heated up to expose the winding damage.

If you find a compressor like this try doing a mega test with the compressor cold & then heat the compressor shell up & then mega test again.
You might find that a cold test does not conclusively show a definate ground fault but the hot test shows a clear ground fault.

[strongman]

Thanks guys