delta-wye transformer load

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brentp

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What problems might arise by only loading up 2 of the 3 phases of a delta wye transformer? (112.5kva, 480v pri, 208/120v sec, feeding a 400a 1ph panel)

Thanks in advance
Brent
 
charlie b

charlie b

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One is that you can only load it to about 57% of its capacity (not the "two thirds" that you might have expected). That means you can only supply about 178 amps to the 400 amp panel.

But more importantly, you would be serving a single phase panel at 208/120 volts, and not at 240/120. So if you have loads that require 240 volt single phase, the source voltage will be wrong.
 
brian john

brian john

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If as noted you actuall need 240/120 VAC not 208/120, then if you change the Taps to approach 240 (about 227 VAC depending on primary voltage) you will have a high phase to neutral voltage, of about 131 VAC.

Right transformer for the right load would be best.
 
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brentp

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My original post had alot to be desired information wise. What I really am asking for help with is the formula/calculation showing why the transformer can only be loaded to 57% as Charlie B stated. My electrical theory needs some work.
 
Smart $

Smart $

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brentp said:
My original post had alot to be desired information wise. What I really am asking for help with is the formula/calculation showing why the transformer can only be loaded to 57% as Charlie B stated. My electrical theory needs some work.
Click to expand...
I'm going to go out on a limb and say I disagree with charlie b. I believe he is confusing the issue with an open delta arrangement. The arrangement you are inquiring about is loading 2 of 3 wye Lines. IMO you will have full usage of 67% for Line to Neutral loads, 33% for Line to Line loads, and somewhere in between for a combination of such loads.
 
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jim dungar

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Because each winding in a wye connected transformer is fully rated you can get 112.5kVA/(208V*1.73) = 312A from each secondary line.


Time for fun with numbers:
If you have (2) 120V loads connected line to neutral you will be using 624A out of the possible full load 936A which agrees with the 67% mentioned by Smart $.

But, if your load is (1) 208V load, you will be using 208*312 = 65kVA out of the possible 112.5kVA available which is the 57.7% mentioned by Charlie B.
 
Smart $

Smart $

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jim dungar said:
Because each winding in a wye connected transformer is fully rated you can get 112.5kVA/(208V*1.73) = 312A from each secondary line.


Time for fun with numbers:
If you have (2) 120V loads connected line to neutral you will be using 624A out of the possible full load 936A which agrees with the 67% mentioned by Smart $.

But, if your load is (1) 208V load, you will be using 208*312 = 65kVA out of the possible 112.5kVA available which is the 57.7% mentioned by Charlie B.
Click to expand...
After considering this further, I have a question: Isn't Line to Line current and voltage 30? out of phase with the winding voltage? As such, I don't see how the full 312A is available for Line-to-Line loads.

The Line to Neutral voltage on the winding at peak Line to Line voltage is Erms-winding ? cos 30? ≈ 103.9Vrms. Thus isn't available current Irmsmax-winding x cos 30? = 312.5 ? √3/2 ≈ 270.6A?

Therefore [(120 ? √3) or 208V] ? 312.5A ? √3/2 ≈ 56,250 VA or 50% (not the 33% I posted earlier, or the 57% posted by cb and you).
 
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mikehughes8

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Location
NC
loading 2 phases of 3 phase transformer

loading 2 phases of 3 phase transformer

Well, my biggest concern with this configuration is the phase imbalance thats created by loading two phases of a three phase transformer. Regardless your 208 loads will have approx 312amps available as mentioned and the 120V loads will have 937 amps available.
 
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