Transformer KVA
- Thread starter WCR
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jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Transformer KVA
Each power transformer provides 1/3 of the three phase load, and 1/3 of the single phase load. The lighting transformer provides 1/3 of the three phase load, and 2/3 of the single phase load.
Jim T
Each power transformer provides 1/3 of the three phase load, and 1/3 of the single phase load. The lighting transformer provides 1/3 of the three phase load, and 2/3 of the single phase load.
Jim T
hardworkingstiff
Senior Member
- Location
- Wilmington, NC
Re: Transformer KVA
Kind of demanding, no?
- Location
- Lockport, IL
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- Semi-Retired Electrical Engineer
Re: Transformer KVA
Is this something that your supervisor asked you to figure out?
Don't let the razzing from the other Forum members get to you. Welcome to the Forum. It's a great place to learn things about the trade that you might not be able to learn in the field.
That said, I am not sure I understand the question. This is not a situation where you apply a simple formula, like Ohm's Law (E = IR). You have to start by understanding how transformers work, how they are connected, and how you determine the power output.
Is this something that your supervisor asked you to figure out?
Don't let the razzing from the other Forum members get to you. Welcome to the Forum. It's a great place to learn things about the trade that you might not be able to learn in the field.
That said, I am not sure I understand the question. This is not a situation where you apply a simple formula, like Ohm's Law (E = IR). You have to start by understanding how transformers work, how they are connected, and how you determine the power output.
Re: Transformer KVA
"Each power transformer provides 1/3 of the three phase load, and 1/3 of the single phase load. The lighting transformer provides 1/3 of the three phase load, and 2/3 of the single phase load." Jim T
jtester,
Is that 1/6 of the single phase load for each "power" transformer? So the single phase load is 2/3 from the "lighting" (center tapped) transformer and 1/3 from the other two non tapped transformers?
"Each power transformer provides 1/3 of the three phase load, and 1/3 of the single phase load. The lighting transformer provides 1/3 of the three phase load, and 2/3 of the single phase load." Jim T
jtester,
Is that 1/6 of the single phase load for each "power" transformer? So the single phase load is 2/3 from the "lighting" (center tapped) transformer and 1/3 from the other two non tapped transformers?
Re: Transformer KVA
This is not as simple as it looks:
The 3-phase loading is a no brainer--25KVA/3 for each, Ta, Tb, & Tc.
Now let Ta be directly connected to the 1-phase load, then, the load current, Isp, divides 2:1 between Ta and the other two. Then the apparent power computations are,
Pa = 240 x Isp*2/3 for the first
Pb = Pc = 240 x Isp*1/3 each for Tb & Tc.
Pb + Pc = 240 x Isp*2/3 for both
Ta provides half the apparent power,
Tb & Tc provide one quarter each.
You must consider each individual transformer when you have such an unbalanced load.
This is not as simple as it looks:
The 3-phase loading is a no brainer--25KVA/3 for each, Ta, Tb, & Tc.
Now let Ta be directly connected to the 1-phase load, then, the load current, Isp, divides 2:1 between Ta and the other two. Then the apparent power computations are,
Pa = 240 x Isp*2/3 for the first
Pb = Pc = 240 x Isp*1/3 each for Tb & Tc.
Pb + Pc = 240 x Isp*2/3 for both
Ta provides half the apparent power,
Tb & Tc provide one quarter each.
You must consider each individual transformer when you have such an unbalanced load.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Transformer KVA
Rattus and ccjersey
The reason 1/3 of the lighting load comes from each power pot is that between the two lighting legs, say phase A and B, there are two paths, the first is across the lighting transformer which has impedance z, and the second is around the two power transformers in series which have total impedance 2Z. The different impedances will cause the current to split, 2/3 thru the lighting transformer and 1/3 thru the two power transformers. Since the two transformers are in series they each contribute 1/3 to the total. That is the reason for 4/3 lighting load on a closed bank.
I know it doesn't seem reasonable first, but it is true. Draw out the circuits and you will see what I mean.
Jim T
Rattus and ccjersey
The reason 1/3 of the lighting load comes from each power pot is that between the two lighting legs, say phase A and B, there are two paths, the first is across the lighting transformer which has impedance z, and the second is around the two power transformers in series which have total impedance 2Z. The different impedances will cause the current to split, 2/3 thru the lighting transformer and 1/3 thru the two power transformers. Since the two transformers are in series they each contribute 1/3 to the total. That is the reason for 4/3 lighting load on a closed bank.
I know it doesn't seem reasonable first, but it is true. Draw out the circuits and you will see what I mean.
Jim T
Re: Transformer KVA
Jim T.,
Indeed the load current does split 2:1 for the reasons you present, and looking at the load only, one xfrmr provides 2/3 of the load, and the other two provide the remaining 1/3.
Now, looking at the xfrmr loading which is the proper thing to do, the xfrms are loaded 2/3, 1/3, and 1/3. Is this what you are telling us?
Now adding in the 3-phase loads with different phase shifts complicates the matter. I would want to compute the currents in this one.
Jim T.,
Indeed the load current does split 2:1 for the reasons you present, and looking at the load only, one xfrmr provides 2/3 of the load, and the other two provide the remaining 1/3.
Now, looking at the xfrmr loading which is the proper thing to do, the xfrms are loaded 2/3, 1/3, and 1/3. Is this what you are telling us?
Now adding in the 3-phase loads with different phase shifts complicates the matter. I would want to compute the currents in this one.
jtester
Senior Member
- Location
- Las Cruces N.M.
Re: Transformer KVA
rattus
You have it right.
The original question was what size transformers are required. I provided the proper equation to determine transformer size. I agree that the electricity absorbed by the load is 2/3, 1/6, and 1/6. However since the question was about transformer sizing, the sizes are based on 2/3, 1/3, 1/3.
Jim T
rattus
You have it right.
The original question was what size transformers are required. I provided the proper equation to determine transformer size. I agree that the electricity absorbed by the load is 2/3, 1/6, and 1/6. However since the question was about transformer sizing, the sizes are based on 2/3, 1/3, 1/3.
Jim T
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