Using a single phase of 3 phase transformer
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Elect117
Senior Member
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- California
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- Engineer E.E. P.E.
Yes but the KVA is reduced 1/3 and some manufacturers have guidance on this when it is a dry type. Some specify a 5% kVA for the center tap for the unbalance or something like that.
junkhound
Senior Member
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- Renton, WA
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- EE, power electronics specialty
Should run cooler also as core losses reduced, 1/2 of core operates at 1/2 flux level but is 2X volume of that 1/2 and flux core loss is a power function (typically 2),
Elect117
Senior Member
- Location
- California
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- Engineer E.E. P.E.
huh?
Also, having re-read this a few hours later I should elaborate on what I was thinking of doing:
We currently have a 480V to 240V delta/delta transformer that has center taps on the secondary. I was planning on powering all three phases of the primary, then powering a 120V/240V load center for automation purposes off the secondary. X1 and X2 would be my hots, and the center tap would be bonded to ground and be my neutral. This would be the only load on the secondary.
We currently have a 480V to 240V delta/delta transformer that has center taps on the secondary. I was planning on powering all three phases of the primary, then powering a 120V/240V load center for automation purposes off the secondary. X1 and X2 would be my hots, and the center tap would be bonded to ground and be my neutral. This would be the only load on the secondary.
Elect117
Senior Member
- Location
- California
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- Engineer E.E. P.E.
I understood. That is what I responded to.
A three phase transformer is 3 single phases. The split phase is 120/240 on a single phase. So, for example, the 3 phase KVA is 30kva, then your single phase kva is 10kva and you might only be able to have an unbalance of 5% of that based on the manufacturer's guidance.
junkhound
Senior Member
- Location
- Renton, WA
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- EE, power electronics specialty
I misread your op, thinking you were only powering one of the 480 phases...
wwhitney
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- Berkeley, CA
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- Retired
Seems to me that the 5% limit on the center tap would not apply for the condition that all of the secondary loading is single phase C-N-A.
The reason for the 5% limit on the center tap is the effect of circulating current in the delta secondary: for any given load currents and coil impedances, KVL around the delta loop requires that the voltages sum to 0, which determines the circulating current necessary to make that happen.
Consider the best case that the primary voltages are all balanced (if not, there will be a no-load circulating current in the secondary) and that the coil impedances are balanced; i.e if the impedance C-N is Z, then N-A is also impedance Z, and A-B and B-C are impedance 2Z. Z can also incorporate the effect any of any (balanced) primary impedance.
Then for the case of a secondary load current +I that is only supplied C-N, no other loads, the circulating current will be -I/6. So the net currents will be -I/6 in A-B, B-C, and N-A, and 5I/6 in C-N (all in phase). The voltage change around the loop is - 2Z * I/6 - 2Z * I/6 +Z * 5I/6 - Z * I/6 = 0.
So for a single phase only loading the circulating current actually decreases the maximum current in the coils, and if all these balance assumptions hold, the available kVA would actually be more than 1/3 of the transformer kVA.
But suppose we have the unbalanced C-N-A loading along with full loading A-B and B-C. E.g. a resistance R applied A-B, another resistance R applied B-C, and a resistance R/2 applied N-A, where R is chosen so the load current I equals the transformer rated current. This is the loading case of a fully balanced 3 phase load (which would have no circulating current) minus the first case above.
Now the circulating current will be I/6, the negative of the circulating current in the first case. This won't overload the A-B or B-C coils, as their currents are 120 or 240 degrees out of phase with I, so the magnitude of the vector sum is not increased. But this circulating current of I/6 is in phase with the load current I in the N-A coil, and the total current in the N-A coil will be 7I/6, an overload.
This phenomenon is the reason, as I understand it, for the 5% limit on the center tap of 240V/120V delta secondary, and as we can see it only arises with 3 phase loading.
Cheers, Wayne
Last edited:
wwhitney
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- Berkeley, CA
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- Retired
Seems to me it would be better to only connect H1 and H2 and not H3. If you do connect all three, then if there is any voltage imbalance on the primary, you will get a circulating current that could reduce the available kVA.
Also, if the transformer has identical coil impedances, I believe that used in this way the single phase rating is 1/2 the 3 phase rating for 240V only single phase loading, or 1/5 the 3 phase rating for completely unbalanced 120V loading, rather than 1/3 and 1/6, respectively. The coil path X1-X3-X2 is parallel to the coil X3-X0-X1, and the parallel path would have twice the impedance of X3-X0-X1. So the load current would divide between the two coil paths, reducing the X3-X0-X1 coil current.
Cheers, Wayne
petersonra
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- Location
- Northern illinois
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- engineer
I think you can do it, but it might be simpler to just run 480 single phase to the control panel and add a small single phase transformer there.
If I was going to go to the trouble to add a single phase panelboard near the control panel I would run 240/120 there. I know that extra wire is really expensive but it might come in handy down the road.
If I was going to go to the trouble to add a single phase panelboard near the control panel I would run 240/120 there. I know that extra wire is really expensive but it might come in handy down the road.