Open Delta

[sdilucca]

Anyone have a clue why open deltas only put out 58% of the power a regular delta would?

 

[bphgravity]

Re: Open Delta

1.732 / 3 = 58%

 

[ramsy]

Re: Open Delta

Got this old book, Shultz. "Transformers and Motors." 1989 Newnes, Boston p41. that says, when a 3-bank is converted from a closed delta to open delta (V) system, the load cannot be greater than 58% of the kVA capacity of the 3? system.

While each winding of a closed delta provides 1/1.73 58% max current, open delta secondaries windings must each supply 100% of the current, since the winding and the line are now in series with each other and are equal.

 

[rattus]

Re: Open Delta

J. C.,

It is all about phase shift.

Consider a fully loaded full delta with each transformer delivering a phase current, Ip.

Now if one transformer is removed, its phase current must be supplied by the other two, and this phase current will be out of phase with the others.

The phase currents of these two jump to 1.732Ip. Therefore, the transformers must be derated by a factor of 1/1.732 = 0.577

This factor is determined by vectorial combination of the currents.

Rattus

 

[sdilucca]

Re: Open Delta

thanks. I actually found a good explanation in the book "Principles of Electric Machines and Power Electronics" pg.75-78. Pretty much backs what everyone has said. It actually helps in seeing Ratuss's point about the phase shift being the 58% culprit.

 

[sdilucca]

Re: Open Delta

BTW... ramsy, thanks for telling me about that book. It turns out that one of my coworkers has it. It's very practical. I just ordered one. thanks again.