unbalanced load calc for 3 phase delta system

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Re: unbalanced load calc for 3 phase delta system

I have got a small doubt in this regards, In a delta wired 120/240 system , whether all phases would be center-tapped (or) any one of three phase would be center tapped ?.

As far my knowledge, any one of three phase would be center-tapped to get 120V supply.Whether Phase A (or) B(or) C.So this case , the number of single phase load will be connected / loaded with the same phase.

See here :

If phase A is center-tapped, It would draw the three phase load of 80Kva as well as single phase load of (34+41) Kva.So Phase A will be loaded with 80+(41-34) = 87 Kw

Other phases will take care of only 80 Kva load.

could someone clarify me ?

Jeyakumar
 
Re: unbalanced load calc for 3 phase delta system

Kal-

What are you referring to as a phase? In a delta system it is easier for me to think of phase A-B, A-C, and B-C. In a wye system I think of phase A, B, and C. Does that help you?

Phase A-B is transmitting 80 kVA, phase B-C is transmitting 80 kVA, and phase A-C is transmitting 121 kVA. The neutral of the center tapped A-C phase will transmit (41 kVA -34 kVA) = 7 kVA.

Therefore the answer to the original question (Rattus' second post):

Ia = 835 A
Ib = 890 A
Ic = 577 A

I left the relative angles out.

[ February 11, 2006, 09:49 AM: Message edited by: Zifkwong ]
 
Re: unbalanced load calc for 3 phase delta system

Jeyakumar,

You would center tap only one transformer because the CT is also the grounded neutral for the 120/240V loads.

In this case, winding A-C is center tapped and it provides 80KVA + 34KVA + 41KVA = 155KVA. You add these powers, not subtract.

zifkwong got the correct answers first; then I went back and corrected my error. However, I think Ib should be 577A if we center tap winding AC. Zif's numbers are correct though. Go back and review these posts for the correct numbers.

[ February 11, 2006, 09:52 AM: Message edited by: rattus ]
 
Re: unbalanced load calc for 3 phase delta system

Originally posted by RUWired:
rattus, i finally see where our differences are.When someone says to me they have a "connected" load of 80 kva a-b-c,i think they are saying its 80 between a and b, 80 between b and c, 80 between c and a.Your saying each line to the coil is 80kva making a-b 240kva and so on. now making it a "connected" load of 240 kva.(and then doing the math from there).
Rick
Click to expand...
The original post stated 80KVA per phase. That says it all. Now you need to convince yourself that,

Ib = 333Ax2cos(30) = 333Ax1.732 = 577A

The reason that 30 degrees appears instead of 120 degrees is that we must take the difference of the two phase currents rather than the sum--Kirchoff's Law.

Calculation of Ia and Ic is a bit more difficult.

Edited to correct DA mistake.

[ February 11, 2006, 12:33 PM: Message edited by: rattus ]
 
Re: unbalanced load calc for 3 phase delta system

When someone says to me they have a "connected" load of 80 kva a-b-c,i think they are saying its 80 between a and b, 80 between b and c, 80 between c and a.
Click to expand...
I could interpret that to mean 80 kva total or as you say, 80 kva between phases. I would have to ask for a clarification of the statement.
Your saying each line to the coil is 80kva making a-b 240kva and so on. now making it a "connected" load of 240 kva.(and then doing the math from there).
Click to expand...
To me the phrase "line to coil" is confusing. Each secondary coil has 80 kva of load making the load connected line to line (ab,ac,bc).
 
Re: unbalanced load calc for 3 phase delta system

I saw a lot of calc's but i did not see any one say anything about this being a high leg. If you center tap B and C on a Delta on 3 phase 3 wire system to get 120/240, your voltage on A to ground is 277V?
 
Re: unbalanced load calc for 3 phase delta system

If you center tap B and C on a Delta on 3 phase 3 wire system to get 120/240, your voltage on A to ground is 277V?
Click to expand...
No, it is 208 volts.
Don
 
Re: unbalanced load calc for 3 phase delta system

I have to say once again,The best thing about our trade is seldom does a day go by with out learning something new, seeing something in a different light, or understanding different concepts.
Rick
 
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