3 Phase Motor - Insulation Resistance Testing

[ryanm]

Hi all,

Could someone please help with a query I have?

At a factory I work in, we get a lot of damaged motors (due to it being a very wet environment), they are only quite small normally, varying around the 0.25-0.37kW size.

Normally when I test them, I remove everything so I am left with the 6 terminals and earth. I test the normal way phases to earth, and phase to phase, and have always been told to test with the Megga on 500v.

Why is this though? I have never learn't any theory on IR testing, just been shown how to and away we go (obviously no power is present), but a guy at work asked my why I wasn't using the 250v or 1000v setting, and to be honest I don't know why....he wouldn't tell me his thoughts on it either.

So, is there anything I can read up on, or could any one here explain different situations when the different settings apply please?

I suppose if you are testing something in a domestic home, you would test on 250v as most items will normally run off 230v? So a 400v motor would be tested on the 500v setting?
If that is correct, why is it? Just to not damage the insulation?



Thanks a lot

 

[Bobster]

You should be testing at double the rated voltage, unless sensitive components are in the circuit.

Yes phase to earth should be tested but at 1000v, why are you IR testing the windings? Use a low ohm meter and make sure there balanced.

Can I ask if you an electrician as this is a pretty basic test.

 

[stef]

Ihe IEE regulations recommend on motors a test voltage of 500 V for insulation resistance test, minimum acceptable result should be 1 MOhm. This is to test whether the insulation can withstand the voltage, in case of a missing earth the motor housing could become live. This is whynyou measure all three windings to earth.
More sensitive kit, as you already know, should be tested at 250 V.

Domestic appliance can be tested at 250 V, however installations should be at 500V

This depends on the application.

 

[telectrix]

IR test at double the rated voltage. i.e. 500V for L to N and L to E. 1000v for L1-L2-L3. continuity test each winding on low ohms. this however will not usually show shorted turns. for that an inductance meter sometimes helps.

 

[netblindpaul]

You should never test motors at 1kV unless the maker says its OK!
You can cause pin point shorts in the windings.
The correct test voltage is 500V d.c. between phases & each phase to earth.
You MUST also ensure that any internal components are correctly connected prior to IR testing, this may be open, or short circuit, or earthed depending on the motor design.

 

[telectrix]

i stand corrected. :banghead:

 

[stef]

You should never test motors at 1kV unless the maker says its OK!
You can cause pin point shorts in the windings.
The correct test voltage is 500V d.c. between phases & each phase to earth.
You MUST also ensure that any internal components are correctly connected prior to IR testing, this may be open, or short circuit, or earthed depending on the motor design.

Yes, this is actually true, esp for small motors with a small wire gauge winding. The enamell ( or whatever it is) insulation is too thin to withstand 1000 V

 

[Bobster]

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.

 

[Darkwood]

If your environment is wet then the motors themselves should be IP rated for the situe as too the control systems etc, any motors that dont meet this should be replaced for suitably rated versions or others measures taken if possible. The fact you say that alot of the motors are damaged would suggest hey are not suitable and if you are responsible for the maintenance of them then you also are responsible that they suit the environment they are in.

You can IR test till the cows come home but the minute you test and report on a motor you become responsible for the suitability of the motor for the job in hand, the environment it is in and the safety measures implemented or its safe use.

Be weary not to dig yourself a hole here, in your shoes i would be inclined to explain upfront if the motors are incorect for the job and save all the testing... if moisture is ingressing into the motor and/or its controls then the risk of electric shock becomes real.

 

[Darkwood]

i stand corrected. :banghead:

Its one of your rare moments Tel but nice to see im not the only one who wants to shoot myself in the head occasionally.

 

[Deleted member 26818]

Yes, this is actually true, esp for small motors with a small wire gauge winding. The enamell ( or whatever it is) insulation is too thin to withstand 1000 V

Shellac.

 

[telectrix]

it's what i was taught at college, way back when the regs were written in latin. then again, any motor then that was under 50HP was classed as a toy.

 

[Bobster]

it's what i was taught at college, way back when the regs were written in latin. then again, any motor then that was under 50HP was classed as a toy.

Join the club

 

[Darkwood]

I was taught the same but since leaving college alot of what was taught was soon to be realised was basic background but what they didnt say was it dosn't apply in all cases .... you need electrical engineer courses for that as they dont bull.... as much as more lifes are at stake.
We would have had to have done an 8yr course to get the full spec ..... thats why i still laugh at some of these instant sparkies (although not all i must add)

 

[MarkieSparkie]

Shellac.

I don't think Tel needs Shellacing he was just expressing a honest opinion! :jester:

 

[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

 

[Engineer54]

I meant to say IP 68, lol!!

Doesn't matter if you strip the paint off them, you shouldn't be able to get water past that degree of protection, into electrical parts that will be detrimental to it's running.... Not at the price they normally charge for such motors anyway....lol!!!