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.