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Discussion Starter · #1 ·
This looks like an auto transformer I think (never hooked one up before). It's going to be feeding a 3 phase 3D printer (no neutral). 3w 3phase 208v coming in, 3w 3phase 230v coming out. I was going to ground the #1 on the diagram but it feels like I'd be grounding the secondary and primary with this configuration. Anyone connected something like this up before?
 

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I googled this for the manufactures website, but I was unable to find anything on this transformer

It really isn't a true buck boost as the primary or part of the primary is not common with the secondary.

When connected what do you measure for voltage on the secondary?
 

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Donuts > Fried Eggs
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As drawn, it is an autotransformer: Your primary and secondary connections share the same winding. Easy way to confirm that is check for continuity before it's hooked up. Primary and secondary should have no continuity in SDS.

Not sure what the heck is going on with 1. I agree, grounding that is verboden because you'll be putting current on your equipment grounding system. But it almost looks like a zig-zag diagram? Does this thing need a neutral?
 

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Discussion Starter · #7 · (Edited)
I'm assuming its a neutral, I haven't checked anything yet. There are coils between 1 and 2.

I'm thinking it ok to ground it because it's not like I'm ground a neutral again, there is no neutral brought to it. I'm just letting the secondary 230v know 'hey this is ground'.
 

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Donuts > Fried Eggs
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I'm assuming its a neutral, I haven't checked anything yet. There are coils between 1 and 2.

I'm thinking it ok to ground it because it's not like I'm ground a neutral again, there is no neutral brought to it. I'm just letting the secondary 230v know 'hey this is ground'.
Negative. Think of that center point as your autotransformer neutral, the same way a neutral is derived in a normal wye transformer. Any current imbalance between the windings on that autotransformer will create a voltage difference between phases and that center point.

Because the transformer winding is electrically continuous with your feeder circuit, if that center point is grounded you'll have current flow on your EGC back to the feeder-circuit transformer.
 

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It is a 3 phase auto transformer and no you do not ground it. Your Equipment grounding (bonding) wire is all you need. A fault on the load will transmit through the winding as the primary and secondary are electrically connected.

You are using a grounded 3 phase supply at 208 volts? the neutral is grounded before the transformer and after the transformer just like any 3 phase load fed right off the 208 volt supply you do not take a neutral if the load does not require it. A ground on the winding appears as a fault to the supply breaker. Unlike a distribution transformer where the only connection from the primary windings to the secondary is magnetic. An auto transformer is electrically as well as magnetically connected to the secondary.
 

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Discussion Starter · #12 ·
Then I don't need indicator lights either. That's a good point that a fault on the secondary would travel right back to the feeders.
 
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