Insulation testing results

[Tweaker]

Hi,

Very new to this forum so excuse any faux pas's.
Was wondering if anyone has come across this situation: Was doing some insulation testing as part of our planned maintenance and came across a motor that gave an odd result.

Capstan Motor 440V/60Hz, 24kW, Star/Delta configuration.

Removed all wiring and tested between windings 1U,1V,1W and earth and all 3 showed approx 1550 MOhm so pretty normal but between the phases all showed 0 Ohm. Almost like they were all shorted together.

Motor seems to be working normally.
Any explanations?

 

[sparkdog]

Well,they would read approx 0 between phases on a 3 phase motor,wouldn't they?

 

[Bobster]

Testing between the phase's, where exactly did you connect your test probes?

 

[telectrix]

you're probably reading the winding resistances. that will give zero on megohms. try a continuity test. should read a few ohms.

 

[Bobster]

P.s looking at that drawing, I personally don't like the positioning of the overload in that circuit. It is allowed to be like that. However if for whatever reason it fails to transition into delta running it is afforded no O/L protection, coupled with the fact it will be running with 1/3torque it's highly probable the motor will burn out.

 

[Geordie Spark]

Hi,

Very new to this forum so excuse any faux pas's.
Was wondering if anyone has come across this situation: Was doing some insulation testing as part of our planned maintenance and came across a motor that gave an odd result.

Capstan Motor 440V/60Hz, 24kW, Star/Delta configuration.

Removed all wiring and tested between windings 1U,1V,1W and earth and all 3 showed approx 1550 MOhm so pretty normal but between the phases all showed 0 Ohm. Almost like they were all shorted together.

Motor seems to be working normally.
Any explanations?

If it ain't broke ............ :sailor:

... and ....:welcome:

 

[Tweaker]

Well,they would read approx 0 between phases on a 3 phase motor,wouldn't they?

Maybe you misunderstood me, I mean across the phases as in between 1U and 1V for example not from 1U to 2U.

 

[Tweaker]

Done the usual test, checked balance from one end of the winding to the other for all phases and it reads a few ohms for each but as mentioned meggering across the phases almost reads like the insulation has broken down between all the windings but motor runs so surely can't be the case.

insulation test from each phase to ground reads approx 1550 M Ohms so all other factors are normal.

 

[Marvo]

Looking at your control circuit has that motor got an internal condensate heater/defrost heater or something similar? Are you sure you've disconnected all the wiring before testing. Maybe remove the winding wires U1, V1, W1, and U2, V2, W2 and run the IR tests again with them in free air.

 

[Bobster]

So your testing U1 to V1 and getting 0Mohm? Can you get a low ohm reading between? The motor might be goosed. They will carry on running to well below 1Mohm that will probably be the lower range on your tester fine.

If this is the reading it doesn't comply with bs7671 and will need replacing/rewinding/drying out.

 

[Bobster]

Marvo, heaters will be completely separate from windings. For the size of the motor I don't see where they would be fitted. I've only come across heaters in slip ring chambers.

 

[Marvo]

Yep the heaters is a long shot and they shouldn't be in the same cct as the windings but it's always worth a retest with the winding wires off the terminal block. The OP is in the United Arab Emirates so he's probably working to codes other than the BS7671. One other thing I just thought of is we've had some pretty weird readings in the past from motors that are freewheeling whilst under test. Again a long-shot but it'sstrange he's only seeing a complete breakdown between windings and nothing whatsoever to earth.

 

[Bobster]

I realise that Marvo, a heater circuit would not show a s/c with all windings.

Having the motor spinning would show a fault with most modern meters and wouldn't let the test begin. Long shot as you've said.

my money's being on the motor has a fault in the windings, low enough to go under scale on his meter but high enough not to cause a fault.

 

[silva.foxx]

Hi,

Very new to this forum so excuse any faux pas's.
Was wondering if anyone has come across this situation: Was doing some insulation testing as part of our planned maintenance and came across a motor that gave an odd result.

Capstan Motor 440V/60Hz, 24kW, Star/Delta configuration.

Removed all wiring and tested between windings 1U,1V,1W and earth and all 3 showed approx 1550 MOhm so pretty normal but between the phases all showed 0 Ohm. Almost like they were all shorted together.

Motor seems to be working normally.
Any explanations?

StarStar/Delta... two speed motor. Tapped windings. 1UVW internally wound as delta with a tapping on each as 2UVW. So 1U to 1V should ohmically be, say twice that of 1U to 2V. So as you have a connection between all points on the motor terminals you will have negligible insulation resistance.

Hope this helps.

 

[Tweaker]

StarStar/Delta... two speed motor. Tapped windings. 1UVW internally wound as delta with a tapping on each as 2UVW. So 1U to 1V should ohmically be, say twice that of 1U to 2V. So as you have a connection between all points on the motor terminals you will have negligible insulation resistance.

Hope this helps.

Thanks for clearing up the confusion. Appreciated.

 

[Tweaker]

So how would you IR test this motor? Or just check the IR phase to GND only?
Thanks again.

 

[Knobhead]

Sounds like a Dahlander motor. There’s four ways to connect them. PM you’re mail address.