Single Phase Transformers on Three Phase Applications

[TimWilborne]

I'm trying to clarify the following. If I have 3 single phase 250KVA transformers used for three phase (one transformer on each leg) isn't my 3 phase KVA around 375KVA? Can someone verify this and explain why? I know it has something to do with the offset sine waves but can't remember the technical details of why and how to figure the exact KVA

[BobLfoot]

Might want to read this thru. I cannot recall the exact calculation but square root of 3 and 1.732 willbe involved in power calculations. I know that due to phase angles and coordination of efforts the power of three phases is not each phase times 3 except maybe for balanced loads.

[TimWilborne]

Boy you got any lighter reading Bob? That document might sink in later after my morning coffee but it isn't right now. The square root of 3 = 1.732 and I'm thinking your are right there. I just can't seem to find the formula in my reference books. For some reason I'm thinking it is 250/1.732*3 = 433KVA 3 phase or is it 250*1.732 = 433KVA 3 phase Both give roughly the same answer but I'm trying to remember why and what the proper calculation is

[TERdON]

Uhm, both formulas are equivalent. sqrt(3)/3 = sqrt(3). Duh. And yeah, it's correct, at least in every "normal" case.

[TimWilborne]

Yes now that I look at it again your are correct. This getting up at 4:00 in the morning is killing me Thanks for the clarification. So since we are looking at 3 250KVA single phase transformers that would be the equivalent of a 433KVA 3 phase transformer Edited May 19, 2006 by TWControls

[ECSI]

TWControls, just out of curiosity what are you trying to do here? Are you converting single phase to three phase using 3 transformers (I don't think this can be done), or are you trying to step up or step down a three phase supply voltage to a different voltage using 3 single phase transformers? I believe the total KVA may also depend on whether its wye or delta connected.

[TimWilborne]

There are currently 3 single phase 12,470 to 208/120 VAC transformers hooked to a 12,470 3 phase system. One transformer to each leg. We are looking at upgrading it to a larger 3 phase transformer I called a transformer company and it looks like I was incorrect though. If you have a 750 KVA 3 phase transformer it is actually 3 250 KVA single phase transformers. Where my confusion came in is the calculation for KVA on a 3 phase system is Amps*Volts*1.732/1000. I thought that the 3 single phase transformer configuration required some type of calculation to determine its 3 phase capacity but it does not. 3 250 KVA single phase transformers with one hooked to each of the incoming 3 phase power is the same as a 750 KVA three phase transformer Does that sound right to everyone?

[larry818]

I just think of the three phase calculations as having a sqrt(3) "advantage", just add the sqrt(3) where it favors three phase. I think the transformers would yield the sum of their values, tho, 'cos watts is watts, and they still need to dissipate their waste heat. While you get an amperage advantage using three phase, the watts are still higher.

[panic mode]

I didn't try it but in three phase circuit where angle between phases is same (120deg) power is P=U*I*sqrt(3)*cos(a)=U*I*1.73*cos(a) Single phase power calculates as P=U*I*cos(a) So If you have three single phase transformers wired one on each phase to emulate single 3-phase transformer, equivalent power should be 433kVA. They don't just add up to 750kVA because of phase angles. To get 750kVA, phase current would have to be bigger on each leg. Use that and calculate power for the single phase transformer. It should be more than 250kVA is rated meaning overload condition. Btw. sqrt(3)/3<>sqrt(3) 3/sqrt(3)=sqrt(3)

[TimWilborne]

That is what I thought to but I just contacted Acme transformer for a second opinion and they are saying the same thing. 250KVA single phase transformers, one to each leg is equal to one 750KVA 3 phase transformer. Either I'm not explaining it right to them or I just learned something knew

[Money4Nothing]

ACME (haha) is correct. The VA of 3 phases is the sum of the VA on each individual phase. KVA per phase = V(phase)I(phase)sqrt(3). Total KVA = 3x(KVA_per_phase). The square root of 3 factor comes from the difference between the line and phase currents or voltages. If you vectorally sum them around a whole 360 degrees with three phases as you calculate power, they cancel out. A wye or delta load connection does not matter either. Total KVA = 3 x Per Phase KVA. My comany has many times in the past used banks of single phase transformers instead of a 3-phase transformer, and they used the same calculation. And it worked, too $ Edit: I'd be happy to go more in depth with the math if you really want me to. Edited May 19, 2006 by Money4Nothing

[TimWilborne]

Yes thanks, first what is the advantage of using a bank of single phase transformers instead of a 3 phase transformer?

[Money4Nothing]

If you have a coil or winding burn out, you only have to replace one single phase transformer instead of a 3 phase transformer. Once we even used a bank of 4 single phase transformers, so if we lost one, we had immediate redundancy available for a single phase. But in general, for space-savings and ease of maintenance, its best to use a 3-phase. Its also slightly less costly. $