# Two transformers 'trick' for 3 phase

• posted

Question: If you need a certain amount of power (kva) 3 phase, are 2 transformers each with half the total kVA rating cheaper than 3 each with

1/3rd the total?

I was thinking of the Scott-T transformer configuration, which generates

90 degree 2 phase from 3 phase with 2 transformers. It also can produce 3 phase from 90 degree 2 phase when used "backwards". So if you have two Scott-T configurations back-to-back you have 3 phase to 3 phase with 4 transformers. Not very useful. But since one transformer simply feeds a second, they can be combined. So now you can have 3 phase to 3 phase with only 2 transformers. But they are different: Transformer 1 has primary and secondary A-CT-C and Transformer 2 is wired B-CT(1) (CT(1) means to centertap of 1) The two transformers have different voltage/current levels but have the same turns ratio.

Is there any reason why this configuration isn't used for commercial power systems? The need for different transformers outweighs the fact you need only 2 instead of 3? I don't know what the minimum kVA of a pole pig is but I would think this combo might be useful for serving low power

3 phase loads.

I know about open-delta configurations that also use 2 transformers, but if I understand correctly they have a substantial reactive current so they can provide less than their rated kVA even to a 1.0 powerfactor load.

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• posted

The capacity of open-delta-secondary transfomers is 86.6% of the sum of kVA ratings for the pair. Haven't heard of 'substantial reactive current' as the reason for load limitation.

A significant limitation of open-delta sets can be poor voltage balance at the secondary terminals with exactly balanced primary voltage. Per NEMA (MG1, C84.1), 5% voltage imbalance requires about a 25% horsepower derating of the served motor(s) for otherwise-equivalent motor life..

--s falke

• posted

| Question: If you need a certain amount of power (kva) 3 phase, are 2 | transformers each with half the total kVA rating cheaper than 3 each with | 1/3rd the total? | | I was thinking of the Scott-T transformer configuration, which generates | 90 degree 2 phase from 3 phase with 2 transformers. It also can produce 3 | phase from 90 degree 2 phase when used "backwards". So if you have two | Scott-T configurations back-to-back you have 3 phase to 3 phase with 4 | transformers. Not very useful. But since one transformer simply feeds a | second, they can be combined. So now you can have 3 phase to 3 phase with | only 2 transformers. But they are different: Transformer 1 has primary | and secondary A-CT-C and Transformer 2 is wired B-CT(1) (CT(1) means to | centertap of 1) The two transformers have different voltage/current | levels but have the same turns ratio.

Yes, you can do a Scott-T to transform 3 phase. If the primary is connected to WYE, then one can be wired between phases A and C (for example getting

208 or 480 volts) and the other wired between phase B and neutral (for example getting 120 or 277 volts). Now you have 2 transformers at 90 degrees. The secondaries would be wired as a T, as well, but you can do it either way as long as the voltages are right. If one has 240/120 out and the other has 208 out, then the 208 is the high leg that attaches to the center of the 240/120. Likewise for 480/240 and 416. If you have a load that wants 208 volts (like from 208Y/120), then you'll have to get some unusual voltages to start with (208/104 for one and 180 for the teaser leg).

If you get power that is 240 delta center tapped, it is similarly easy.

If you get power that is delta without any taps, you still have to tap at the center of one primary winding and attach the teaser primary to it, but nothing else. That derates the main winding.

I have not done the calculations and do not know any of the rating values needed for Scott-T. I won't use Scott-T if I can avoid it.

| Is there any reason why this configuration isn't used for commercial power | systems? The need for different transformers outweighs the fact you need | only 2 instead of 3? I don't know what the minimum kVA of a pole pig | is but I would think this combo might be useful for serving low power | 3 phase loads.

I have run across a maker of dry-type transformers that makes Scott-T versions only. I would think if it isn't being used, they would go out of business.

• posted

In general, a 3 phase transformer uses less iron than 3 single phase transformers to accomplish the same result.

However, having said that, most small 3 phase dry transformers are indeed wired as you suggest; this is called the T connection. Many, if not most, dry LV transformers below 9 kVA or so use the T connection.

I have seen one or two instances of individual large transformers arranged as a T, but nothing modern. There is really no benefit to it, as the two single phase cores will require more iron than a single 3 phase core.

As to the open delta connection, assuming a balanced secondary load of 100% power factor, one of the transformers will see an 86.6% lagging PF and one will see 86.6% leading. If we assume the case of 2 - 1kVA transformers, we see that the maximum power available will be 2 kVA X .866 = 1.732 kVA. If we add the third transformer, however, the power factor in all units is now

100%, for a capacity of 3 kVA, as against 1.732 kVA for two transformers. Adding 50 % more transformer capacity gives 73.2% more bank capacity.

That is the story on the "reactive power" in the open delta connection. Obviously, for anything other than unity PF, the voltage drops on the two transformers will be different, causing an unbalance which would not exist with the full delta configuration.

• posted

On Tue, 25 May 2004 20:07:11 -0400 BFoelsch wrote: | In general, a 3 phase transformer uses less iron than 3 single phase | transformers to accomplish the same result. | | However, having said that, most small 3 phase dry transformers are indeed | wired as you suggest; this is called the T connection. Many, if not most, | dry LV transformers below 9 kVA or so use the T connection. | | I have seen one or two instances of individual large transformers arranged | as a T, but nothing modern. There is really no benefit to it, as the two | single phase cores will require more iron than a single 3 phase core.

What if the phase loads are expected to to be substantially out of balance, such as 80% of the load being single phase lighting and appliances and the two big three phase motors need 240, not 208? Why not put in a big single phase 240/120 output and a small 208 output at 90 degrees?

| As to the open delta connection, assuming a balanced secondary load of 100% | power factor, one of the transformers will see an 86.6% lagging PF and one | will see 86.6% leading. If we assume the case of 2 - 1kVA transformers, we | see that the maximum power available will be 2 kVA X .866 = 1.732 kVA. If we | add the third transformer, however, the power factor in all units is now | 100%, for a capacity of 3 kVA, as against 1.732 kVA for two transformers. | Adding 50 % more transformer capacity gives 73.2% more bank capacity. | | That is the story on the "reactive power" in the open delta connection. | Obviously, for anything other than unity PF, the voltage drops on the two | transformers will be different, causing an unbalance which would not exist | with the full delta configuration.

In cases of getting single phase from three phase, I've seen these wiring configurations:

• * * / \ -or- / \ / \
• *---* * *---*---* * | | | | | |
• * / \ / \
*---*---* | | |

Wouldn't the first one be putting a 50% power factor on two phases? But it seems the 2nd one wouldn't really be any better, other than sharing the load. But all load on 2 of the phases is always going to be at 60 degrees (one leading, one lagging).

Also, in WYE configuration, if a single phase load is put on phases A-B in:

-A \ N--C- / \

-B

wouldn't there be an 86.6% power factor on A and B as passed back through a delta primary to the supply? Would a delta-delta with a zig-zag neutral be a way around this or would the imbalance distort the neutral?

• posted

You could, but it would offer no benefit over the open delta connection, and the open delta uses a standard transformer.

transformers.

I haven't! These look like questions out of an engineering text, but I can't imagine why anybody would actually hook anything up that way!! No matter how you slice it, a single phase load is a single phase load.

Think about it; if I take any of those contraptions and put single phase into the secondary, will I get three phase out of the primary? (Hint: NO!)Why not? Now, understanding that transformers are reversible, do these hookups REALLY convert 3 phase to single phase? NO. They just use 3 phases worth of hardware to get a single phase output. Might as well have used a single phase transformer to begin with.

All three configurations are ridiculous.

The secondaries will see the 86.6 PF, but in the delta primary you will see one winding at 100% PF and two windings with no load.

• posted

|> | As to the open delta connection, assuming a balanced secondary load of | 100% |> | power factor, one of the transformers will see an 86.6% lagging PF and | one |> | will see 86.6% leading. If we assume the case of 2 - 1kVA transformers, | we |> | see that the maximum power available will be 2 kVA X .866 = 1.732 kVA. | If we |> | add the third transformer, however, the power factor in all units is now |> | 100%, for a capacity of 3 kVA, as against 1.732 kVA for two | transformers. |> | Adding 50 % more transformer capacity gives 73.2% more bank capacity. |> | |> | That is the story on the "reactive power" in the open delta connection. |> | Obviously, for anything other than unity PF, the voltage drops on the | two |> | transformers will be different, causing an unbalance which would not | exist |> | with the full delta configuration. |>

|> In cases of getting single phase from three phase, I've seen these wiring |> configurations: |>

|> * * * |> / \ -or- / \ / \ |> * *---* * *---*---* * |> | | | | | | |>

|> * * |> / \ / \ |> *---*---* |> | | | |>

| | I haven't! These look like questions out of an engineering text, but I can't | imagine why anybody would actually hook anything up that way!! No matter how | you slice it, a single phase load is a single phase load.

Some of these are taken from actual generator specifications. See the single phase columns on the right side of the tables on page 6 of the folloring:

formatting link

| Think about it; if I take any of those contraptions and put single phase | into the secondary, will I get three phase out of the primary? (Hint: | NO!)Why not? Now, understanding that transformers are reversible, do these | hookups REALLY convert 3 phase to single phase? NO. They just use 3 phases | worth of hardware to get a single phase output. Might as well have used a | single phase transformer to begin with.

There really is single phase output from three phase input. The catch is that the current phase remains single. Half the load is on one phase and the other half the load is split on the other 2 phases at what appears to me to be a 0.5 power factor, one leading, one lagging.

|> Wouldn't the first one be putting a 50% power factor on two phases? But | it |> seems the 2nd one wouldn't really be any better, other than sharing the | load. |> But all load on 2 of the phases is always going to be at 60 degrees (one |> leading, one lagging). | | | All three configurations are ridiculous.

But are used in real practice.

|> Also, in WYE configuration, if a single phase load is put on phases A-B | in: |>

|> -A |> \ |> N--C- |> / \ |> -B |>

|> wouldn't there be an 86.6% power factor on A and B as passed back through |> a delta primary to the supply? Would a delta-delta with a zig-zag neutral |> be a way around this or would the imbalance distort the neutral? | | The secondaries will see the 86.6 PF, but in the delta primary you will see | one winding at 100% PF and two windings with no load.

If the single phase connection is between A and B (getting 208, 400, or 480 volts depending on whether you are using 208Y/120, 400Y/231, or 480Y/277), then there would be current on the A-N winding, and on the B-N winding.

Now look at the delta primary for that:

*-----* \ / \ / *

The horizontal winding across the top is on the same core as C-N, so I will label it as "Cp". The left side (backslashes) is on the same core as A-N, so I will label it as "Ap". The right side (slashes) is on the same core as B-N, so I will label it as "Bp".

Since there is current on A-N, and B-N on the secondary, that current is thus present on the "Ap" and "Bp" windings. The phase angle of the current is vertical in this diagram (because the line from A to B in the secondary vector diagram is vertical). So the currents on "Ap" and "Bp" are vertical. That would mean one is leading and one is lagging, by 30 degrees. So I say the power factor on the delta primary is 0.866, and current is divided between "Ap" and "Bp" (none on "Cp").

Now, where it gets interesting is if that delta primary is attached to a wye secondary of another delta-wye transformer upstream. Since two phases of the delta have current (load), that means on the upstrea wye, two legs have one each of those loads, and one leg has both. Because a delta is connected to as wye, there is a 30 degree difference between the winding angles. Now you will end up with most of the current on one phase with a power factor of 1, but some (I'll have to go draw the vectors and figure out the amounts) on the other two phases with 60 degree (0.5 pf) angles, one leading and one lagging.

That's what I say will be happening.

• posted

The first arrangement is sometimes called a "LeBlanc" connection, and as already stated, worthless for the described task.

--s falke

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