high leg delta

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badgers

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75KVA dry type.
240V 3 phase high leg at 208 to ground.
A and C phase 120 to ground

Why is the 120V KVA limited to 5% of the 3 phase rating?
why dont I have 25KVA on that winding at 120V. 3.75KVA at 120V seems to be nothing.

I would think each winding would have about 104A current and if I center tap one of the windings I am not seeing the source of 5% limit on KVA.
any help is appreciated.
thank you
 
badgers said:
75KVA dry type.
240V 3 phase high leg at 208 to ground.
A and C phase 120 to ground

Why is the 120V KVA limited to 5% of the 3 phase rating?
why dont I have 25KVA on that winding at 120V. 3.75KVA at 120V seems to be nothing.

I would think each winding would have about 104A current and if I center tap one of the windings I am not seeing the source of 5% limit on KVA.
any help is appreciated.
thank you
Click to expand...

Is this a single unit 3 phase transformer or 3 single phase transformers operating as a 3 phase bank?
 
badgers said:
Why is the 120V KVA limited to 5% of the 3 phase rating?
why dont I have 25KVA on that winding at 120V. 3.75KVA at 120V seems to be nothing.

I would think each winding would have about 104A current and if I center tap one of the windings I am not seeing the source of 5% limit on KVA.
any help is appreciated.
thank you
Click to expand...
1. Where does it say that (5%) in the manufacturer's specs?
2. One partial explanation is that the winding wire size on the center tapped winding will not allow you to tap off the full 25kVA from just one half of the winding. But that would correspond to a 12.5% limit, not 5% limit.
3. Beyond that, it might be related to the magnetic field balance on the common core, something that I know little about. In that case it would go along with a restriction on drawing unbalanced 240V VA from any of the three windings. Is that mentioned?
 
A single core transformer has historically been limited to 5% single phase loading. While the math is straight forward,it is still fairly advanced.
The quick answer is it has to do with unbalances and circulating currents (in particular single L-N, 120V, loading). This loading problem does not exist with multiple core installations.

However, in the past 5-10 years, some manufacturers have dropped this 5% loading requirement, so they must be doing something different to handle the heating.
 
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