240 Delta to 208Y No XO

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Trickyflea

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I have a 240 delta to 208Y transformer for an elevator motor. I am getting quite unbalance secondary voltages
A to ground 105; B to ground 190; C to ground 105
But pretty close on the phase to phase readings
A to B = 210; A to C = 212; B to C = 209
Is this normal?
 
Trickyflea said:
I have a 240 delta to 208Y transformer for an elevator motor. I am getting quite unbalance secondary voltages
A to ground 105; B to ground 190; C to ground 105
But pretty close on the phase to phase readings
A to B = 210; A to C = 212; B to C = 209
Is this normal?
Click to expand...
Sounds like the secondary is ungrounded. You need to ground the XO per 250.30.
 
It looks like you may have an autotransformer, with 208V or 230V taps, are you sure the secondary is a wye connection and not a delta?
What does the wiring on the nameplate diagram show?
 
You have to look closely at the label. That is _not_ a delta:wye transformer, it is an autotransformer. The grounding needs to be provided by the source feeding the transformer because this transformer is _not_ a 'separately derived system'.

If the supply feeding this transformer is a 240V high leg delta, then I'd expect the output of the transformer to be unbalanced with respect to ground.
 
winnie said:
If the supply feeding this transformer is a 240V high leg delta, then I'd expect the output of the transformer to be unbalanced with respect to ground.
Click to expand...
As it was. I noticed the voltages were proportionate to a high-leg delta.
 
You appear to have the required grounding, I would remove the white wires since they have no purpose and call it good. The voltages you measured to ground will be lower since you are autotransforming down by 32 volts.
 
The label on the center coil indicates the output is a wye.... there should be an XO someplace...
as Larry suggests, contcat the manufacturer
 
augie47 said:
The label on the center coil indicates the output is a wye.... there should be an XO someplace...
as Larry suggests, contcat the manufacturer
Click to expand...
As its an autotransformer, grounding the Center of the wye (XO) would be a catastophic problem, since its not a separately derived system. You would be effectively connecting the XO to the midpoint of the center-tapped delta transformer. This would parallel the 120v winding with the autotransformer 240v winding.
 
I see the "auto" on the label now....
Is it not starnge that the label also shows the output as "Wye" ?
 
augie47 said:
I see the "auto" on the label now....
Is it not starnge that the label also shows the output as "Wye" ?
Click to expand...
jim dungar said:
No. It is likely a Wye connected autotransformer: 240V Wye to 208V Wye.
Click to expand...
A picture is worth 1000 words. Here is a diagram of a typical wye connected autotransformer. But I agree with Augie, I dont like referring to the "output" or "input" of an autotransformer. It should just be called a wye connected autotransformer and nothing else.
 

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electrofelon said:
A picture is worth 1000 words. Here is a diagram of a typical wye connected autotransformer. But I agree with Augie, I dont like referring to the "output" or "input" of an autotransformer. It should just be called a wye connected autotransformer and nothing else.
Click to expand...
It isn't clear to me that this is a wye connected transformer. Despite the wye in the label, it might actually be delta connected.

If the L-N voltages are stable, and the connected load working properly, and the 240V supply high leg delta, I would leave everything alone.

If something isn't working, then the next steps would be to find out what the 240V supply grounding is, and get a circuit diagram for the transformer

Jon
 
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