Gerald Flingos
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Can you power a single phase panel from a 3 phase transformer where the 3rd leg of the transformer has no load?
3 phase Transformer
- Thread starter Gerald Flingos
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Yes, but the capacity is reduced.
Yes, for example a 3ph 30kva transformer when used to supply a single phase load by using only 2 of the transformer's secondary lines will only provide 1/3 of the transformer"s rated kvan or 10kva.
Please correct me and point me where I got it wrong here. I tried using your figures but I got a different answer:
Capacity = 30 kVA, 120V, 3-phase, FLA = 144.34 amps.
max. load = ? (assuming PF = 1)
Max. line amps = 30,000/(1.732 x120) = 144.34 amps
Load kVA (one phase loaded) = 144.34 x 120 = 17,321 VA (57% of total capacity?)
Tony S
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57.7% = 1/√3
http://www.mgmtransformer.com/faqs/...rmer-be-loaded-as-a-single-phase-transformer/
Can a three phase transformer be loaded as a single phase transformer?
Yes, but the load can not exceed the rating per phase and the load must be balanced. (KVA/3 per phase)
For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.
Can a three phase transformer be loaded as a single phase transformer?
Yes, but the load can not exceed the rating per phase and the load must be balanced. (KVA/3 per phase)
For example: A 75 kVA 3 phase transformer can be loaded up to 25 kVA on each secondary. If you need a 30 kVA load, 10 kVA of load should be supplied from each secondary.
Exactly! But which is which?
The website posted above (transformer manufacturer) or our calculations?
Tony S
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- Resting under the Major Oak UK
I would say the manufacturers are referring to Ph→N while the forum is working on Ph→Ph
Power
Ph→N=1/3
Ph→Ph=1/√3
Both are correct, depending how you look at the problem.
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If wye, the full potential of two windings can be realized (20kVA or 2/3). Note this is winding kVA calculated with actual line amperes and winding voltage. Load kVA may be different. For example, you can power up to 83.3 amperes of loads with each of the two lines. If this were a 208Y/120V system, that be 83.3A x 120V x 2 = 20kVA of LN loads, but only 83.3 x 208 = 17.3kVA of LL loads. Combinations thereof will fall somewhere in between.
If delta, the full potential of the direct winding can be utilized, but only half of the other two. The two indirect windings, being in series, have twice the impedance. You have two current paths, direct at 1Z and the other at 2Z. As such, when the direct winding is fully loaded, the other two windings will be at half the current, half the power. 10+5+5= 20kVA. Note this calculation assumes all three primary conductors are energized.
So, since the other 2phases are 120deg appart you are essentially changing 3ph to supply the single phase then?
Yes.
As a demonstration, remove the direct winding from the bank. The load is now connected across the open end of an open delta.
Sorry but not enough info given to answer the q.......
Specify if trafo is Y or Delta. Specify the voltages:
Assume either 480/277 or 208/120 on 3 phase trafo; what is single phase panel voltage and is it split phase?????
Actually, enough information is provided. Connecting to any two points on a 3Ø supply, delta or wye, yields a single phase voltage. Any such connection in common US 3Ø configurations can be used as a single phase source (the high leg to neutral of a delta secondary is a special case which I will be glad to discuss only if necessary), just as any split voltage 3Ø source can supply a split phase 1Ø panel. There are some slight caveats on this latter part but the statement still holds true under scrutiny. Utilizing a 208Y/120V 3Ø 4W system to supply 120/208V 1Ø 3W systems is commonplace in the US for apartment complexes.
No argument regarding supplying 1ph from any (2) 2 L-L or a L-N of a 3ph3w delta, 3ph4w delta, or 3ph4w wye transformer source which is common place. Understanding that any of these transformers is capable of supplying 1ph when their primaries are energized with 3ph power when any of these is suppling just a single 1ph load the KVA available for that load is !limited to 1/3 of the rated 3ph kva of the transformer with the exception of a single (1)ph L-N load of a 3ph4w 240/120 delta supplying 1ph120v load.
Not, just making s statement. That a 3ph transformer when supplied my all three phases, when only a single 1ph load is bring a supplied that the load is limited to 1/3 of the kva rating of the transformer.
The only time that is true is if the load is connected L-N to a wye secondary.
Then if the the 1ph load is connected to L1-N and that is the only 1ph load then you could exceed the 1/3 the kva rating. As such is the L1 to X0 winding rated for that?
No. One winding can handle 1/3 the total kVA rating of the transformer. When a load is connected L1 to X0, only that one winding delivers power.
Connect a load across any two lines (L1-L2, L2-L3, L1-L3) and a wye secondary transformer is using two windings to deliver power... potentially more than 1/3 the transformer rated kVA.
Spot on! 57% as my calcs tell me!
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