What would cause earth short in 3PH motor?

[-Matt]

Have had bit of a nightmare with a job of installing a replacement 15KW extraction system motor in a workshop.

The previous motor was just DOL so only one 3 core cable between the workshop and the motor which is outside, the otherside of building.

The new extraction system came with star-delta starter.

The customer needed this done ASAP and being very busy i agreed to wire the motor in delta and connect to main contactor in star-delta starter (knowing there would be a 10sec lag). Already 50A type D mcb which seemed sufficiant to me?

With the intention of replacing the cable with a 7 core when have a bit more time.


However when starting, it would trip the MCB, which had me slightly puzzled, as whilst the start up current will be high a 50A D should have covered this.

So spoke with the manufacturers technical department, who basically said, why is it in Delta it should be wired in star-delta!
And thats about all he could tell me.


So spent yesterday afternoon fitting a whole new cable, all wired up correctly in star-delta .
Press the start button, Thump.

At this point im smashing my head on a wall.

I had only tested the wiring up to the motor to this point, so did a IR test on the motor, to my discovery there was a short on the L1 coil to earth. Ohm meter showing 0.07 ohms on both sides of the coil to earth.
L2 and L3 coil was good with no short.

So really kicking myself for not testing this brand new “tested” motor to start with!!!

So now need to speak to manufacturer again, however im worried they may claim i damaged the motor by not wiring to their reccomended specification.

Im no expert with motors etc, however i cant see how i could have caused an earth short by starting up in delta?

So what would/could cause an earth to coil short in a motor?

 

[suffolkspark]

Just dont tell them youd already called up !

If its a brand new motor then it can surely only be a manufacturing defect unless anything has been nicked or caught in the term box

 

[Pete999]

Have had bit of a nightmare with a job of installing a replacement 15KW extraction system motor in a workshop.

The previous motor was just DOL so only one 3 core cable between the workshop and the motor which is outside, the otherside of building.

The new extraction system came with star-delta starter.

The customer needed this done ASAP and being very busy i agreed to wire the motor in delta and connect to main contactor in star-delta starter (knowing there would be a 10sec lag). Already 50A type D mcb which seemed sufficiant to me?

With the intention of replacing the cable with a 7 core when have a bit more time.


However when starting, it would trip the MCB, which had me slightly puzzled, as whilst the start up current will be high a 50A D should have covered this.

So spoke with the manufacturers technical department, who basically said, why is it in Delta it should be wired in star-delta!
And thats about all he could tell me.


So spent yesterday afternoon fitting a whole new cable, all wired up correctly in star-delta .
Press the start button, Thump.

At this point im smashing my head on a wall.

I had only tested the wiring up to the motor to this point, so did a IR test on the motor, to my discovery there was a short on the L1 coil to earth. Ohm meter showing 0.07 ohms on both sides of the coil to earth.
L2 and L3 coil was good with no short.

So really kicking myself for not testing this brand new “tested” motor to start with!!!

So now need to speak to manufacturer again, however im worried they may claim i damaged the motor by not wiring to their reccomended specification.

Im no expert with motors etc, however i cant see how i could have caused an earth short by starting up in delta?

So what would/could cause an earth to coil short in a motor?

How was the Motor linked to Delta configuration at the motor terminal block?

 

[-Matt]

L1- U1/W2
L2- V1/U2
L3- W1/V2

Which i believe to be correct, and used this before. But even if i got a coil mixed up would it cause an earth to coil short?

 

[Pete999]

Have had bit of a nightmare with a job of installing a replacement 15KW extraction system motor in a workshop.

The previous motor was just DOL so only one 3 core cable between the workshop and the motor which is outside, the otherside of building.

The new extraction system came with star-delta starter.

The customer needed this done ASAP and being very busy i agreed to wire the motor in delta and connect to main contactor in star-delta starter (knowing there would be a 10sec lag). Already 50A type D mcb which seemed sufficiant to me?

With the intention of replacing the cable with a 7 core when have a bit more time.


However when starting, it would trip the MCB, which had me slightly puzzled, as whilst the start up current will be high a 50A D should have covered this.

So spoke with the manufacturers technical department, who basically said, why is it in Delta it should be wired in star-delta!
And thats about all he could tell me.


So spent yesterday afternoon fitting a whole new cable, all wired up correctly in star-delta .
Press the start button, Thump.

At this point im smashing my head on a wall.

I had only tested the wiring up to the motor to this point, so did a IR test on the motor, to my discovery there was a short on the L1 coil to earth. Ohm meter showing 0.07 ohms on both sides of the coil to earth.
L2 and L3 coil was good with no short.

So really kicking myself for not testing this brand new “tested” motor to start with!!!

So now need to speak to manufacturer again, however im worried they may claim i damaged the motor by not wiring to their reccomended specification.

Im no expert with motors etc, however i cant see how i could have caused an earth short by starting up in delta?

So what would/could cause an earth to coil short in a motor?

Sorry for all these questions, just trying to get my head around the problem, you said you did an IR test

Have had bit of a nightmare with a job of installing a replacement 15KW extraction system motor in a workshop.

The previous motor was just DOL so only one 3 core cable between the workshop and the motor which is outside, the otherside of building.

The new extraction system came with star-delta starter.

The customer needed this done ASAP and being very busy i agreed to wire the motor in delta and connect to main contactor in star-delta starter (knowing there would be a 10sec lag). Already 50A type D mcb which seemed sufficiant to me?

With the intention of replacing the cable with a 7 core when have a bit more time.


However when starting, it would trip the MCB, which had me slightly puzzled, as whilst the start up current will be high a 50A D should have covered this.

So spoke with the manufacturers technical department, who basically said, why is it in Delta it should be wired in star-delta!
And thats about all he could tell me.


So spent yesterday afternoon fitting a whole new cable, all wired up correctly in star-delta .
Press the start button, Thump.

At this point im smashing my head on a wall.

I had only tested the wiring up to the motor to this point, so did a IR test on the motor, to my discovery there was a short on the L1 coil to earth. Ohm meter showing 0.07 ohms on both sides of the coil to earth.
L2 and L3 coil was good with no short.

So really kicking myself for not testing this brand new “tested” motor to start with!!!

So now need to speak to manufacturer again, however im worried they may claim i damaged the motor by not wiring to their reccomended specification.

Im no expert with motors etc, however i cant see how i could have caused an earth short by starting up in delta?

So what would/could cause an earth to coil short in a motor?

 

[Pete999]

What sort of Meter did you use to conduct the IR test?

Sorry for all these questions, just trying to get my head around the problem, you said you did an IR test

Something up with my device, I'll start again, you mentioned IR testing and a reading of 0.07 Ohms, along with Ohm Meter what sort of tester were you using for the IR testing?

 

[-Matt]

Sorry, i probably wasnt very clear.

I put clamp meter on earth and started, it showed 40A before MCB tripped.

I then noticed continuity on my voltage meter between L1(U1) and earth, so then disconnected the motor.


I then used the IR tester to double check the cable between the started and motor, which was clear.

Then used multimeter on ohm setting to check the 6 winding ends on the motor:

U1 + W2 both had 0.07 ohm to earth.

The rest were infinity.

Hope that explains.

 

[Pete999]

Sorry, i probably wasnt very clear.

I put clamp meter on earth and started, it showed 40A before MCB tripped.

I then noticed continuity on my voltage meter between L1(U1) and earth, so then disconnected the motor.


I then used the IR tester to double check the cable between the started and motor, which was clear.

Then used multimeter on ohm setting to check the 6 winding ends on the motor:

U1 + W2 both had 0.07 ohm to earth.

The rest were infinity.

Hope that explains.

No not really what were the IR readings on all three motor windings?

 

[Lucien Nunes]

OK yes it does sound like one winding is S/C to frame. I don't think you damaged it. Star-delta starting reduces the peak current demanded from the supply, and the mechanical shock on the driven load, but most motors of that size are OK with DOL starting.

If you had wired one coil reversed in phase relative to the other two, it would have taken a heavy overload - the MMFs inside oppose each other and it behaves like a transformer with a shorted secondary. Left connected, this would result in rapid overheating, but it would have likely got too hot to touch, or given off smoke, before winding damage resulted. Which it didn't, not least because the MCB tripped.

The likely cause was a manufacturing fault, or something simple that has been disturbed during fitting, such as one of the winding tails being crushed against the entry hole into the terminal box.

 

[Marvo]

I'd loosen the back motor housing and visually check the windings, you can probably do this with the motor still in situ. Looking at the colour of them it will be immediately obvious if you've damaged one of them by overheating. If they're all bright shiny copper coloured then return it for warranty, if one is a darker colour compared to the others then I doubt they'll warranty it so maybe a rewind.