VFD Ground Fault

[Tim Ganz]

I know many VFD's will fault on a ground fault but that same motor will run fine accross the line starting. Does the VFD do something like a megger test before it starts? How does it detectn an insulation problem while running?

 

[Bit_Bucket_07]

I suspect that they detect ground faults in the same manner that a GFCI does, by continuously measuring for an imbalance between outgoing and incoming current between phases.

 

[ASF]

I suspect that it would vary between drives, but most likely it comes down to a measurement of current flow, in much the same way that an RCD works (albeit with a few more smarts). If current sent to the drive does not equal current returned, it must be going to ground.

I'm sure in reality it's a bit more complicated than that, but that would have to be the basic premise behind it I would imagine

(edit: seems that Bit_Bucket has faster fingers than me today )

 

[jraef]

Yes, it is residual current detection, what the rest of the world calls RCD. GFCI is a very specific term for a type of low level tripping Ground Fault protection circuits in bathrooms and kitchens that would be way too sensitive for something like a VFD or anything else running an industrial motor, but our equivalent to an RCD is GFP, equipment ground fault protection, also sometimes called "arcing ground fault protection" (but again, not to be confused with AFCI)

So as said, the VFD looks at the current from each phase and sums them up, expecting that everything that goes out comes back and nets out to be equal. Anything that doesn't come back, meaning the net sum has a residual value, must therefore be going to ground.

The reason that an across-the-line starter doesn't trip is likely because the leakage to ground is at such a high resistance that the amount of current flow is not seen by anything in a simplistic cheap starter. But a good solid state OL relay would see it, or a GFP device. But people often don't spend the few extra dollars those cost, so the leakage is allowed to continue. Sometimes that can go on for years, but usually it just slowly gets worse until the motor smokes. The VFD is just better at protecting the motor than a simple X-line starter is.

 

[alan_505]

Most of the AB drives that I remember looking at have a CT(Halleffect) around the 3 output wire so if there is a current measured it assumes that there is some current going to earth as the sum of all the currents in the 3 wires should equal zero, the drives also must do other measurements as well.

Alan

 

[seppoalanen]

Return Current is harmonic current from motor and/or Leakage
current from cable capacitance.
If cable is too length or its is not symmetric, part of harmonic returns outside the motor cable.
I don't know your case, but in some ABB's drive it is possible switch of "Motor Earth fault property".

 

[GeoffC]

Seppoalanen is correct in a vsd it is all about harmonics. You need to make sure the installation is as per manufacturer guidelines
Screened motor cable with the screen earthed at both the motor and vsd ends using earthing glands or an earth strap to completely contact all the screen. A length of wire connected to the screen is not a solution and may even be worse than no connection

 

[DickDV]

Sine wave power to a motor only stresses the insulation to the peak value of the sine wave. PWM power, due to ringing and reflected waves on motor leads of any length, often stress motor insulation at double the sinewave peak voltage. The VFD will fault on leakage current to ground at these high voltages as well as voltages in the "normal" range. I've seen many cases where the VFD reports the motor grounded and, on 480V service, a 1000V megger will test the motor as not grounded.

 

[seppoalanen]

Here 2 Symmetric! motor cable types.