3 Phase Motor - Insulation Resistance Testing

[netblindpaul]

1kv is what I was taught, never done me any harm. Although I don't usually work with anything small.

My thaught is if you test 500v line neutral, double the working voltage it makes sence to IR 1kv phase to phase when the normal working voltage is 415.

Rob,
I was a staff engineer with a motor manufacturer, so I was party to design drawings, calculations & test specifications & this was our spec.
So, I did not make this up.

BTW I "DID" the electrical engineer course, & ended up with BSc(Hons) & MEng on top of my apprenticeship & HNC, FWIW!
Amongst other qualifications.

 

[Bobster]

Rob,
I was a staff engineer with a motor manufacturer, so I was party to design drawings, calculations & test specifications & this was our spec.
So, I did not make this up.

By no means am I saying your wrong, infact after looking over quite a few online documents I'm in the wrong. It's just how I was taught.

 

[Darkwood]

Neither are wrong in set circumstances the use of 1000v A.C. is permitted but its stressed this is a routine mainenance check for insulation degregation and shouldn't be applied to motors with low standard IR tests or against manufacturers guide.
The 500v DC used is below the 625v peak on the A.C. half cycle so dosn't always give a true reflection of condition.

But as a standard IR test to check the winding on a suspected fault 500v DC is addequate.

 

[Bobster]

I'll normally test at 500v first and if the results are good, test at 1kv for filling in the test sheet.

 

[netblindpaul]

IF you are "looking" for a possible fault then a 1kV test could be justified.
However, it is not considered acceptable for routine maintenance checks as it CAN damage the equipment under test.

 

[Knobhead]

Like RoB2 I use 500V followed by 1000V and have done so for the past 40 years. And to be honest with you I see no reason to change my methods.

 

[Knobhead]

IF you are "looking" for a possible fault then a 1kV test could be justified.
However, it is not considered acceptable for routine maintenance checks as it CAN damage the equipment under test.

I believe the thread started with a fault situation.

 

[Engineer54]

With such small motors that the OP is talking about, 500V IR tests are more than ample. When you start talking about the bigger motors, say from around 5 to 7.5 HP upwards, i can see where Tony is coming from.

Darkwood made good sense in his observation about these motors being suitable for the environment they are being used in. He is probably right too, that these failing motors are not constructed to meet the conditions they are being subjected too. Dampness and humidity has nothing to do with a motors IP rating, that's down to the winding construction. On the HK MTRC system, any motors (normally sump pumps and fans etc,) located in the very humid tunnel sections would have encapsulated windings, among other physical protective measures....

 

[ryanm]

Thanks for the replies. Just to clarify, I am not an electician, I am an apprentice, working with people who don't seem to know too much either. Which is why I wanted to ask for some advice more than anything, as I won't be going back to college for a while.

Any test I do is offline, and no power is ever present (as I'm not competent enough yet).

The motors are IP56, but the cleaning people get very enthusiastic, and manage to get water in everywhere it seems!!

So it seems as a rule of thumb, you would IR test at double the rated voltage?

Rob2. You asked why IR test the windings? Mainly as I have been told to, and a recent motor which failed was good to earth between phases, but when tested between 2 windings, it failed. Not the same terminals of the same windings by the way So should the windings to windings test be done with an ohm meter, and not a IR Tester?



Also, WHY do you test a motor (small) on 500v? Is there a reason, or is it just because it is double the winding voltage?




Thanks for all the replies.
Ryan

 

[Engineer54]

You asked why IR test the windings? Mainly as I have been told to, and a recent motor which failed was good to earth between phases, but when tested between 2 windings, it failed. Not the same terminals of the same windings by the way So should the windings to windings test be done with an ohm meter, and not a IR Tester?

No, winding to winding tests are IR tests, your checking for insulation breakdown and/or weakness in the insulation between the two windings. Individual winding tests (end to end) are conducted with a low ohm meter. Your typical MFT or multimeter, are not really suitable for accurate results, you really need a bit of current flowing such as you would get with a Ductor Tester set at the right range, or a 4 point low ohm meter...

 

[ryanm]

Oh I see. That is what I was testing for, insulation breakdown on the motor windings as the motor had failed.

 

[Engineer54]

The motors are IP56, but the cleaning people get very enthusiastic, and manage to get water in everywhere it seems!!

At IP 56, the cleaners must be jet washing them individually. If that's the case, the company either needs to have words with these cleaners, or purchase very expensive IP 67 replacements!! lol!!

 

[Engineer54]

Oh I see. That is what I was testing for, insulation breakdown on the motor windings as the motor had failed.

Then you have done what you have been asked to do then, ...confirmed an insulation breakdown between two windings.
Not much else you can do....

 

[Bobster]

Sorry dont think I made myself clear, I thaught you had been IR testing the windings end to end as well as phase to phase.

As E54 has said just a low ohm meter will not show up all faults testing windings end to end, but if the results per winding are different it usually indicates something unterward.

I've said on post#19 how I would IR a motor although others will disagree on my methods. Testing at 500v I'd done as it's above the operating voltage of the motor, testing at lower than the rated voltage isn't putting enough pressure into the circuit to show a possible fault.

And as for why it's 500v, it's an industry standard.

 

[Silly Sausage]

At IP 56, the cleaners must be jet washing them individually. If that's the case, the company either needs to have words with these cleaners, or purchase very expensive IP 67 replacements!! lol!!

I've used jet washers in the past that would instantly strip the paint off motors, forget IP67, water gets everywhere