ECN Electrical Forum
Discussion Forums for Electricians, Inspectors and Related Professionals

My latest trouble: A large portable centrifuge on a wheeled cart is tripping gfi's
after running a short time & particularly on spin-up. This is a centrifuge manufactured in the last 6 years in the USA, so the quality is good and it's not a relic from some earlier era. No problems with leaks or anything else related to water infiltration. FLC for the motor is 7.4A, and it's being run off a 20A circuit. Before the receptacles where it was used were changed to GFI's,
the thing worked fine on regular "dumb" receptacles. Only since the change to GFI has the trouble started, and only, it seems, with this piece of equipment. So I take it apart, figuring the manufacturer wired it wrong.. Let me describe the setup.. The centrifuge had a 2hp variable speed motor powered through a DIN rail VFD, installed at the factory and sized for this motor. The motor is configured for 240v 1ph operation, as is the VFD. To get 240v, the manufacturer is running a 2KVA autotransformer backwards to "up" 120V to 240V which then goes on to the VFD and then to the centrifuge motor.

I'm thinking the nuisance tripping is caused by the following things and want to bounce it off some other sound minds before I squeeze the trigger and buy a transformer that will boost 120V to 240V "the right way"...

So here's the scenario..

The ground wire off the power cord goes to the case, the ground in the VFD and the motor and frame of the centrifuge, as it should. No problem there.

The autotransformer (Acme T-2-53062) is fed on the low (120v) side
with the (120v) hot on X1, the neutral on X2, nothing on X3. On the high
side, H1 and H2 go to the VFD. Yes, they are running it backwards.. the manufacturer's decision, not mine.

Now I am reasoning that the cause for the nuisance tripping is that when this autotransformer is run in
reverse, counter to the what the (transformer) manufacturer intended, the 120v hot is sending current into the autotransformer windings, and that current has 2 paths to the neutral wire in the 120v cord feeding the autotransformer.. one via the neutral center tap on the low side (X2), and also via the other end of the winding (H2) opposite H1. I suspect that is sending some uneven current down the neutral wire, or even a slightly out of phase "delayed second bump" that's causing the GFI to act weird. Sound reasonable? I'm doubting myself as I sit here looking at the price of autotransformers.

I think the transformer wiring is okay. Have you meggered the transformer windings to ground to check for a fault?

Do you have access to a scope and a current probe?

Does your clamp on show any current on the EGC? (although they are not usually that reliable at GFCI trip currents)

If you are doing this I would start on a dumb receptacle so nuisance tripping is not shutting you down before you see the fault.

I have done this to prove refrigeration equipment had a real fault to people who say "they all do that".

Have a clamp meter and oscilloscope, but no megger.

I was going to hook up the scope and watch what happens. Although
the megger would tell a story quickly, It'll have to wait till I can borrow
one. Until then..

If you put a current probe on the EGC, I bet you will see the problem. Just be sure you are looking at the only path to ground. It should be easy on a wheeled cart.
Then the picture will tell the story.

The GFCI Trip issue is definitely due to Current "Leaking" someplace in the Circuitry.

Since the Voltage is increased via an Autotransformer - as opposed to an Isolated Transformer, the Circuitry leaving the Transformer will also be "Monitored" by the GFCI Device - as will the Circuitry at the input side of the Transformer.

If the Transformer was an Isolated type (separate non-interconnected Primary and Secondary Windings), the GFCI Device will only "Monitor" the Circuitry which it is connected to - i.e.: the "Input" to the Transformer.

Since the OP's scenario uses an Autotransformer, all Currents flowing through the connected Equipment's Circuitry will flow through the GFCI Device (through the Device's CT).

Judging from the trip events:
after running a short time & particularly on spin-up
the trip issue may be One of the following:

A: VFD is shunting TV Surges via MOVs,
B: MOVs at the VFD are "Leaking" at an increasing rate during usage,
or,
C: Current is "Leaking" from One or more Windings - either at the Motor, at the Transformer, or both.

All Three of the above possible causes have a common link: Heat

I would bet the trip issue is related to an L-G connected Component at the VFD - most likely an MOV or other type of TVSS / SPD.

The GFCI Device is tripping due to an imbalance of Current flowing between the Ungrounded Conductor ("Hot"), and the Grounded Conductor ("Neutral"), as sensed via the Device's CT (Current Transformer).
Somewhere in the Circuitry, there is an L-G (or even an N-G) Leakage of Current.

The "Reverse Connection" of the Autotransformer will not cause an L-G Current Leak, and is not an Operating issue.
There should not be any L-G Current flowing "Directly" from the Autotransformer, unless a Winding Tap is directly Bonded to the Equipment Grounding Conductor.

If the trip issue turns out to be normal operation of the VFD's MOVs, the Autotransformer may need to be replaced with a Standard Isolated Transformer.

-- Scott (EE)

Yes, heat is an issure.. the whole thing is used in a hot environment in an already hot climate.. Southern Cal.. you know.

My gut instinct jumped to isolation transformer, but I need to back this up with some reason and logic before spending
the money on an ultimate solution. L-G leakage is a possibility, but I'll have to chase it down. There was nothing wrong
with the setup that was visually apparent. The things in pretty good shape. My mind went to N-G problems first. None of the
taps in the autotransformer is directly grounded. That would be crazy in a cord-connected device. I already see some cords
around the place with the ground pins snipped off (for whatever reason..). I'll look into the VFD as the culprit.. that didn't
occur to me, the focus of my thinking was always on the autotransformer. Guess I needed to take a few steps back from the
problem, which was why I posted in the first place.

I would think the problem can be traced to the particular VFD. I recently had a small VFD on a saw conveyor, newly installed that would trip a high voltage fault due to our ungrounded 440 delta system. The solution was to isolate the drive the drive chassis from ground. We had measured 35 volts to ground from the drive case. This type of leakage current could easily cause the GFCI to trip.

Some, if not most, manufacturers of small inexpensive VFDs purposely connect filter circuits L-G, thereby creating 'current leaks'.

As others pointed out, the solution is a isolation transformer.

And that's what it appears to be.. 4-9 volts on the ground, depending on speed setting. Thanks for the logic check, all who
resopnded..