480v Motor wired for Delta(run) ground fault

E

Eddie Smith

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Location
USA
Occupation
Maintenance
Hello All.
Thanks for having me here.

I have a Toshiba 7.5hp 460vAC 3PH motor that is wired for Delta run and is showing a ground fault at the 525 VFD.
Isolated the Motor output from the FVD and simulated a run = no issue.

Megger'd the wiring from VFD to Pecker head = good.

Q: I am getting an ohm reading, but not a continuity buzzer from the fluke measuring at T1/12 to ground, T2/10 to ground, T3/11 to ground. It is consistent between all three phase wires - 3.5kohm or so.

Between each other, I'm getting around 3ohms.

I'm yet to break open the tails and test.

Why am I seeing a resistance value as described above?

Thanks all!
Eddie
 
There's no point in measuring from each lead to ground. Once you verify nothing's open, you only need to measure from any one lead to ground.

Can you separate the different sections of coils, so that you have multiple coils, then measure from case to each coil? If you're seeing open from some coils, but 3.5 kohm from one of the coils,then you know it's got insulation problems.

You're getting a ground fault, because 3,500 ohm from winding to case = messed up insulation. What reading do you get with a normal multimeter?

See https://docs.johnsoncontrols.com/chillers/api/khub/documents/wcgBZTjhXiXUwwH~P7dFMA/content
 
Eddie Smith said:
Hello All.
Thanks for having me here.

I have a Toshiba 7.5hp 460vAC 3PH motor that is wired for Delta run and is showing a ground fault at the 525 VFD.
Isolated the Motor output from the FVD and simulated a run = no issue.

Megger'd the wiring from VFD to Pecker head = good.

Q: I am getting an ohm reading, but not a continuity buzzer from the fluke measuring at T1/12 to ground, T2/10 to ground, T3/11 to ground. It is consistent between all three phase wires - 3.5kohm or so.

Between each other, I'm getting around 3ohms.

I'm yet to break open the tails and test.

Why am I seeing a resistance value as described above?

Thanks all!
Eddie
Click to expand...
Your motor is grounded but not bad enough to be a dead short. Your meter only has a nine volt battery, the motor gets to see 480V when it's energized.
 
Hi all.
My apologies for the delay in writing. Just so many challenges here all happening at the same time!

So, to update - here's what I found.
Indeed, there is a ground fault, as detected by the 525 and my fluke and Megger. It is nominal, although as mentioned above by Tom; substantial enough for the VFD to detect.
I removed the 7.5hp unit ( Y754SDSR42A-P) and found remnants of conductive Carbon and Graphite had found their way through the P/H and into the windings through a poor installation. There was no CGB used by whomever installed.

I appreciate you all chiming in with your expertise and suggestions. I am guilty of over-thinking this one! although in 30 years, I have never seen scenario with such a nominal bleed across to ground like this.

Thank you again all - best wishes for your weekend to those that are at home; and 'Work Safe' to those like me who are again at the coal face!

Eddie



​

 
VFD is sending out a voltage with rather high spikes instead of a true sinusoidal wave form. They are good at detecting ground faults, these voltage spikes are probably higher than your test volts was with the megger. They will detect the fault when first accelerating the motor, when the effective run volts is now where near what it is at full speed operation.

If the motor was not designed for use on VFD and/or is a long run from the drive to the motor you probably should consider putting a line reactor on the drive output then feed to the motor it will lessen the intensity of those voltage spikes and the effect they will have on motor insulation. This sort of a bigger issue with 480 volts than with 208-240 volts as often the motor windings have same insulation. In fact below about 50 HP often the motors are dual voltage and can run on either supply, but the spikes are much higher with a 480 volt supply and more damaging than when running on 208-240 volts.
 
Hello Sir.
You raise an interesting point.
The power at the facility is notoriously unstable. I've seen 509Vac to ground with a >11% delta between phases.
The newer panels do indeed run line reactors and it's my plan to upgrade that panel to be unified.
One day about a month ago I lost two 753's and a 525... the smoke was released!

Good analytics fella.
Eddie
 
Eddie Smith said:
Hello Sir.
You raise an interesting point.
The power at the facility is notoriously unstable. I've seen 509Vac to ground with a >11% delta between phases.
The newer panels do indeed run line reactors and it's my plan to upgrade that panel to be unified.
One day about a month ago I lost two 753's and a 525... the smoke was released!

Good analytics fella.
Eddie
Click to expand...
Are you on an ungrounded Delta ?
Corner ground?
Something's not right. VFDs typically don't like either.
 
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