Ground fault on VFD

I was always taught to use 500v test on 480 DOL motors and 1000V test on VFD (480v) driven motors.

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Best advice. The reason for the 1000V Megger on 480V motors driven by VFDs is because when powered by a VFD, there is a potential for voltage spikes to be created that exceed 1000V. If tested at 500V, it's entirely possible for there to be leakage of those spikes that a 500V Megger would not have triggered. Even at 1000V there is a risk of missing some of the worst, but if you are having spikes that exceed the motor insulation rating, you have other problems to solve anyway.

Old darlington transistors, i.e. what was used at the time of the old A-B bulletin 160, were very robust in terms of surviving things like Megger testing. But you can't get those any more, the new ones are smarter, cheaper, faster etc., but are less forgiving.

Mr OkiePC,

Sounds like we've done battle on the same fronts!
10 years combined amongst: pork plant, turkey plant, cheese plant, and kosher plants.
With a foundry and glass tempering plant thrown in for good measure.

The ground in certain position sounds a little wierd I know, but motors can get really hot, and when they get really hot, stators and rotors can warp just a few thousands.

.001 inch is all you need to get 20Mohms or so impedance. Turn that rotor till the warps have only .00001 inch... and Voila(french- 'see this')... now you have your ground.

I have also seen several motors which run fine until they reach operating temperature, then get a short or ground fault... As the heat builds and things expand,the winding gets close enough to the stator laminations and the drive faults.

Waiting for the motor to cool down, the megger shows it is fine. Testing it while still hot shows the failure, though!

Rejoe
What drive and motor?

RobertB said:

Looks like you got a lot of advise here. One thing no one has mentioned yet is.... TURNING the motor WHILE watching for a ground. I've encountered this problem and when my PITA (Pain In The A**) motor spun to a stop for Meg testing, "what do you know.... no ground again."
Turned out that only at a certain rotation did the ground appear. With it happening in only 5 degrees of a certain position, I guess the chances of it stopping on the ground was 1 in 72.
I have a few more suggestions, but give this a try 1st if no one else gotcha ya going.

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What has rotation of a motor got to do with the ground fault???

JRW said:

Rejoe
What drive and motor?

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Drive - SINAMICS
motor - servo motor , FT series , siemens

ok..Replaced the drive earlier & now the cable, still my PITA motor continues to frustrate me..What next ?

Did you read the other replies posted about rotation and ground fault???

What I would try next is hooking the VFD up to a brand new motor, (1 hp, 2hp) smaller the better. (easier on the back) Then see if the VFD will run it. If it runs now, then it surely was your motor.
If it doesn't run, look at the VFD manual and see if there are ADJUSTABLE parameters for declaration of the ground fault that someone might have played with and made it TOOO sensitive.
Hope this helps.

Robert
There is for vector mode (p1901)
But I don't see it for servo mode
That's why I asked about model and motor...

JRW said:

Robert
There is for vector mode (p1901)
But I don't see it for servo mode
That's why I asked about model and motor...

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Sinamics is a general mane for Siemens' high end drive lines. The Sinamics V90 is a servo drive, the S120 / S150 can be Servo or Induction.

But it's still a good point. If it is a Sinamics V20 or a Sinamics G-anything, it's not set up to operate a servo motor.

1 more detail can change everything!

Problem solved

Ok. guys after several hours of searching & analyzing the problem has been resolved.

The servo motor was at fault.Although the megger values didnt indicate anything, it was found that the insulation one of the phases was partially damaged causing windings to come in contact with ground.

Anways thank you guys for the support & advice.