Autotransformer grounding

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DW98

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when a machine with a 480/400 autotransformer is turned on or off, lights fed from same panelboard as machine go off and back on. Machine is wired with a three-wire + ground SO portable cord drop. Electrician attempting to troubleshoot problem, connected a ground from the ground terminal to building steel. problem went away. Ultimately, equipment ground run with branch circuit is landed on the panelboard ground bus and the panelboard feeder equipment ground is landed on the switchboard ground bus supplying it and so on back to the grounding electrode system bus bar from which a grounding electrode conductor is connected to steel.

Any thoughts on why problem goes away when bonded locally to building steel. Since this local bond is connected to the equipment ground, potential should be zero. Am I missing something related to code or ground loops that prohibits this?
 
the autotransformer is a wye with a neutra l. I'm verifying if the neutral is used and if it is the point that is grounded. If the neutral point is not grounded, I'm wondering if a neutral with the branch circuit will help.
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petersonra said:
hmmmmmm. Is that even possible with an autotransformer?
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It is possible to make a three phase autotransformer which goes from wye with neutral to wye with neutral. Has to be non SDS. It can be either buck or boost. It cannot be fed from a three wire delta.

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DW98 said:
when a machine with a 480/400 autotransformer is turned on or off, lights fed from same panelboard as machine go off and back on. Machine is wired with a three-wire + ground SO portable cord drop. Electrician attempting to troubleshoot problem, connected a ground from the ground terminal to building steel. problem went away. ...

Any thoughts on why problem goes away when bonded locally to building steel. ...
Click to expand...
DW98 said:
the autotransformer is a wye with a neutral. I'm verifying if the neutral is used and if it is the point that is grounded. If the neutral point is not grounded, I'm wondering if a neutral with the branch circuit will help.
Click to expand...
It sounds like the equipment may have some internal L-N loading that's unbalancing the wye of the autotransformer and therefore causing the voltage at its common point to shift around (since you did not feed it with a neutral). This might result in excess voltage across one or two legs of the wye autotransformer, causing that part of the autotransformer core to saturate. This might produce a spike in the current drawn and cause lights on the same panelboard to blink.
Also, the fact that grounding the machine to building steel seemed to fix the problem suggest that a neutral terminal in the machine is improperly bonded to equipment ground. If so, then the bond should be disconnected and a neutral conductor from the panel should be provided instead.
 
On a 3 wire delta system you need a zig zag transformer to recreate the missing neutral. Several business loss insurance carriers require conversion anyway to high resistance grounding.

Auto transformers by nature need a neutral. No getting around it.
 
I got more information from electrician troubleshooting this transformer. Equipment that transformer feeds does use the neutral. Connecting the neutral from the branch panelboard did not help. What did make the problem go away was disconnecting the equipment ground run with the branch circuit. It's a 4-wire SO cord connection. Obviously, we can't leave the equipment ground unconnected, but some spike must be getting on the ground when the machine is turned on/off. I'm not sure what to do here. transformer is three wire y autotransformer.
 

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DW98 said:
... Equipment that transformer feeds does use the neutral. Connecting the neutral from the branch panelboard did not help.
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Where did you connect the neutral to? From your picture it appears that the wire labeled 4 is the neutral conductor feeding the equipment.
 
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