High secondary voltage

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macguyver2.0

macguyver2.0

Assumptions are the mother of all **ck ups.
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Ok so I have a 480 3 phase primary that I am stepping down to a 240 single phase. I ran 2 legs of the 480 (which is reading 488 phase to phase and 280 phase to ground) to the primary side of the transformer (H1 & H4). I have H2 & H3 jumped together, an equipment ground running through the system, and my center tap X2 & X3 grounded and bonded on the transformer. On the secondary side of the transformer (X1 & X4) I am getting 272 volts. This is well above the 10% range that I am wanting. Am I missing something here or do i have a defective transformer?
 
Your secondary line voltage should be 1/2 of the primary voltage so 488/2=244 volts. What's the secondary voltage to ground on each leg?
 
Does your transformer have taps ? If so, likely you can remidy the problem by moving the tap conductor.
 
macguyver2.0 said:
Ok so I have a 480 3 phase primary that I am stepping down to a 240 single phase. I ran 2 legs of the 480 (which is reading 488 phase to phase and 280 phase to ground) to the primary side of the transformer (H1 & H4). I have H2 & H3 jumped together, an equipment ground running through the system, and my center tap X2 & X3 grounded and bonded on the transformer. On the secondary side of the transformer (X1 & X4) I am getting 272 volts. This is well above the 10% range that I am wanting. Am I missing something here or do i have a defective transformer?
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Single phase transformers are not very complicated. With 488V between H1, H4, is very difficult for both H1 and H4 to be 288V to ground reference. Unless the 480 is from something screwie - 480 ungrounded delta? Measure H1 to H2,H3, and H4 to H2,H3. You should be seeing 244V

As for the 272V secondary:
Is the xfm a small control transformer? If so those are wound to give a high secondary voltage when unloaded

Here is a link to an AB paper: https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1497-wp001_-en-p.pdf

Still, with 488V on the primary I would not expect more than 260V on the secondary (unloaded). Put a resistive load on the xfm. Lightbulbs work well.. Or if it is in the 1kva-3kva range - a heater. See if the voltage cones down to expected - 122V

If it is not a control transformer - Augie's comment on the taps is good
 
iceworm said:
Single phase transformers are not very complicated. With 488V between H1, H4, is very difficult for both H1 and H4 to be 288V to ground reference. Unless the 480 is from something screwie - 480 ungrounded delta? Measure H1 to H2,H3, and H4 to H2,H3. You should be seeing 244V

As for the 272V secondary:
Is the xfm a small control transformer? If so those are wound to give a high secondary voltage when unloaded

Here is a link to an AB paper: https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1497-wp001_-en-p.pdf

Still, with 488V on the primary I would not expect more than 260V on the secondary (unloaded). Put a resistive load on the xfm. Lightbulbs work well.. Or if it is in the 1kva-3kva range - a heater. See if the voltage cones down to expected - 122V

If it is not a control transformer - Augie's comment on the taps is good
Click to expand...

Yes it is a small control transformer.
 
iceworm said:
Single phase transformers are not very complicated. With 488V between H1, H4, is very difficult for both H1 and H4 to be 288V to ground reference. Unless the 480 is from something screwie - 480 ungrounded delta? Measure H1 to H2,H3, and H4 to H2,H3. You should be seeing 244V

As for the 272V secondary:
Is the xfm a small control transformer? If so those are wound to give a high secondary voltage when unloaded

Here is a link to an AB paper: https://literature.rockwellautomation.com/idc/groups/literature/documents/wp/1497-wp001_-en-p.pdf

Still, with 488V on the primary I would not expect more than 260V on the secondary (unloaded). Put a resistive load on the xfm. Lightbulbs work well.. Or if it is in the 1kva-3kva range - a heater. See if the voltage cones down to expected - 122V

If it is not a control transformer - Augie's comment on the taps is good
Click to expand...
If the source is 480/277 wye, he is pretty close to what he should have - which he said was 488 and 280 (not 288). 280 x 1.732 is just under 485

I am with others in that one should check any primary taps and make sure they are set where needed for the application. If there are no taps then ratio should be much closer to 2:1 than it is.
 
kwired said:
If the source is 480/277 wye, he is pretty close to what he should have - which he said was 488 and 280 (not 288). 280 x 1.732 is just under 485 ....
Click to expand...
That's rue - primary H1 - G and H4-G measurements are okay.
macguyver2.0 said:
No it doesn't have taps. If it did I would have tried that.
Click to expand...
Yeah, I would have thought you would have noticed them.

The AB paper says they wind control transformers for 1.05X nominal secondary voltage, unloaded, Secondary voltage drops to 1.0X at full load. The one you have could be different.

Still suggest to load it up to 100%. Check the voltage. With 488V primary, I expect 244V secondary. Again, yours could be different.
 
Double check to make sure that H2 and H3 are not grounded. ptonsparky mentioned in post #2 that nothing else should be connected there.
Measure the voltage between H1 and H2-H3, and also H4 and H2-H3. They should be about half of your 488V.
H2-H3 to ground should be about 141V.
 
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