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.
