I have a Square D 75T3H 75 KVA dry-type transformer connected 480V Delta to 120/208 wye. I need to supply some single-phase load at 208 Volts by connecting to only two of the secondary 208 volt terminals. I will not be connecting to the third secondary terminal. There is no 3 phase load connected to the transformer. How much single phase 208 volt load can I safely feed without severely overheating and unbalancing the transformer? My first inclination is to say 50 KVA, but I'm just not sure nor do I know how to calculate the answer. Please help. Thanks.
Single Phase Loading of 3 phase transformers
- Thread starter hughbrew
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LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
I agree with Roger, Hugh. You should be distributing the single-phase 208v loads among the three phases. The difference would be the panel; the breakers would be the same 2-pole units.
Technically speaking, the loads would be distributed as A-B, C-A, B-C, A-B, etc. as you go down one side of the panel.
Technically speaking, the loads would be distributed as A-B, C-A, B-C, A-B, etc. as you go down one side of the panel.
plate
Senior Member
- Location
- South East PA
The full kVA would be available, in this case 75kVA. (If I am reading your message properly.)
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
To a rough approximation, you could get 58% of the full three phase kVA delivered to a single phase line to line load.
Think about it: each terminal of this transformer is rated to deliver kVA/ V / sqrt(3) amps. So you can get 208A out of terminal A and 208A out of terminal C. If you connect a load from A to C, then you have 208V at 208A, or 43kVA available to deliver to the load.
This is, of course, an approximation, since under full loading legs A and C would not only be self heated, but also heated by leg B.
-Jon
Think about it: each terminal of this transformer is rated to deliver kVA/ V / sqrt(3) amps. So you can get 208A out of terminal A and 208A out of terminal C. If you connect a load from A to C, then you have 208V at 208A, or 43kVA available to deliver to the load.
This is, of course, an approximation, since under full loading legs A and C would not only be self heated, but also heated by leg B.
-Jon
plate
Senior Member
- Location
- South East PA
Yes. I can see that now. The power delivered to the line to line side of the transformer would be one third the total times sqrt3 (75kVA*0.333*1.732)or as you said approx 58%. Taking power fromt the line to neutral would give you approx one thirs (75kVA*1/3).
Thanks to all of you
Thanks to all of you
After thinking about it and opening some old books, I think the 58% of 75 KVA is the correct answer. The situation is a little unusual which is why I wasn't using the third secondary terminal or trying to balance the load by using all three phases. The transformer is an existing one, connected to a 480 volt 3-phase source with no load on it at the present time. All I need is to feed a 240 volt single phase load for a few days for a street carnival. I plan to use a buck-boost transformer to boost the 208 single phase to 240 volt. One of my other concerns was what would happen to the Primary side of the transformer i.e. would I be causing heating by causing circulating current in the Delta primary due to the imbalance? Thank you all so much for your interest and your help. I'm open to any other suggestions.
Thanks to all of you
After thinking about it and opening some old books, I think the 58% of 75 KVA is the correct answer. The situation is a little unusual which is why I wasn't using the third secondary terminal or trying to balance the load by using all three phases. The transformer is an existing one, connected to a 480 volt 3-phase source with no load on it at the present time. All I need is to feed a 240 volt single phase load for a few days for a street carnival. I plan to use a buck-boost transformer to boost the 208 single phase to 240 volt. One of my other concerns was what would happen to the Primary side of the transformer i.e. would I be causing heating by causing circulating current in the Delta primary due to the imbalance? Thank you all so much for your interest and your help. I'm open to any other suggestions.
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