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Open Delta

Open Delta

Open Delta

(OP)
Why does an open delta provide 58% power instead of 2/3 the power of a regular delta?

RE: Open Delta

Think about what the difference is between the open delta xfmr, and it's line currents compared to what the "delta currents" would be.  Then compare that to a regular delta.  A standard delta's line currents differ by root 3 of the "delta currents".  Make sense?

Mike

RE: Open Delta

(OP)
What I know about open delta is:

  I-phase  =  I-line

  V-phase  =  V-line

For regular delta:

  1.73*I-phase  =  I-line
  
  V-phase = V-line

Is this correct?

RE: Open Delta

In a Closed Delta the line current is shared by two windings.
When used in Open Delta, the transformers must be derated 86.6% to avoid overloading. After you have derated the transformers 86.6% you multiply by 2/3.

Look at it another way;
One 33 Kva transformer = 33 KVA
Two 33 1/3 Kva transformers, derated for open delta service.
66 2/3 Kva x 86.6% = 57.7 Kva
Three 33 1/3 Kva transformers = 100 Kva.

RE: Open Delta

In an open-delta (V) connection, the line current is limited to the phase current of Iline/√3 to avoid transformer overload. The maximum power available is:

a) Two transformer (Open Delta):  PV=√3Elinex Iline/√3  = Elinex Iline.

b) Three transformer (Closed Delta):  PΔ=√3Elinex Iline .
                                                                                         PV/ PΔ= 1/√3= 57.7%

RE: Open Delta

(OP)
thanks everyone... I also found a great reference in the book "Principles of Electric Machines and Power Electronics" pg.75-78.  It's all about those sneaky phase shifts.  thumbsup2

RE: Open Delta

You should give Cuky a star for the answer.  I suggest you draw out the circuit and work it out long-hand.  If you assume a voltage of 1<0, 1<-120 and 1<120 and impedances of 1<0, you can easily determine what the power is on an open delta system.  Then use the simple 3 phase power equation for a closed delta system.  You'll then find the ratio Cuky gave you.

Mike

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