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#1
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Three phase from 2 transformers?
In a current thread, gfretw told a poster that one
way to indentify 3 phase service is if it's connected to two or three transformers. That brought back a question that has stumped me for a very long time. I know that it's possible to get 3 phase service with just two transformers. Apparently that is used for cost savings on lighter 3 phase loads. But what I don't understand is how you can get true 3 phase with only TWO transformers. Coming down the road you have 3 high voltage lines. Each of those has an AC waveform and each is 120deg off in phase from the other. If you have three step-down transformers, then you have one transformer connected to each high voltage line and obviously you get 3 waveforms out that have the same 120deg phase relation as those on the high voltage side. But how do you get 3 waveforms at 120deg shift relative to each other with only two transformers connected to only two of the 3 high voltage lines? |
#2
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Three phase from 2 transformers?
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#3
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Three phase from 2 transformers?
On 1/14/2013 9:31 AM, wrote:
In a current thread, gfretw told a poster that one way to indentify 3 phase service is if it's connected to two or three transformers. That brought back a question that has stumped me for a very long time. I know that it's possible to get 3 phase service with just two transformers. Apparently that is used for cost savings on lighter 3 phase loads. But what I don't understand is how you can get true 3 phase with only TWO transformers. Coming down the road you have 3 high voltage lines. Each of those has an AC waveform and each is 120deg off in phase from the other. If you have three step-down transformers, then you have one transformer connected to each high voltage line and obviously you get 3 waveforms out that have the same 120deg phase relation as those on the high voltage side. But how do you get 3 waveforms at 120deg shift relative to each other with only two transformers connected to only two of the 3 high voltage lines? Would be a lot easier to explain with a pencil and piece of paper. The usual delta connection has a 120/240V transformer, and then adds 3-phase to that. A diagram is about halfway down http://en.wikipedia.org/wiki/Three-phase_electric_power where the transformers are connected in a triangle. L1-N-L2 is a single phase 120/240V Edison circuit. Two transformers add L3 - L1-L2-L3 is 3 phase 240V. The voltage from N-L3 is 208V and is called the "high" leg (or several other names). This is "high-leg delta". But it still works if the L1-L3 transformer is not used. It is then called open delta and only uses 2 transformers. This used to be moderately common and is still around. It is probably not used for anything new. It is pretty easy to identify because of 2 utility transformers, only one of which has a secondary neutral connection. ======================================= For a wye connection, a Scott (or T) transformer connection can be used. See the diagram about halfway down labeled "Scott T transformer 3ph to 3 ph" http://en.wikipedia.org/wiki/Scott-T_transformer X1-X2-X3 are 3-phase with X0 being the neutral. This gives a wye 3-phase. A rather common use would be 480V 3-phase connected to H1-H2-H3 and 277/120V coming out X1-X2-X3-X0. This is used on some relatively small "dry" transformers in buildings with a 480V service. The advantage is you have 3 phase with only 2 transformers. The disadvantage is you screw up the power factor on both transformers and have to derate them to 87% (if I remember right). (And if I remember right, the delta transformers above also have to be derated for the 3-phase power.) I don't think utilities ever use this connection. The transformers actually run at real 2-phase at 90 degrees. You can use a Scott connection to actually use the 2-phase. |
#4
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Three phase from 2 transformers?
On Jan 14, 11:22*am, bud-- wrote:
On 1/14/2013 9:31 AM, wrote: In a current thread, gfretw told a poster that one way to indentify 3 phase service is if it's connected to two or three transformers. * That brought back a question that has stumped me for a very long time. I know that it's possible to get 3 phase service with just two transformers. *Apparently that is used for cost savings on lighter 3 phase loads. *But what I don't understand is how you can get true 3 phase with only TWO transformers. *Coming down the road you have 3 high voltage lines. *Each of those has an AC waveform and each is 120deg off in phase from the other. *If you have three step-down transformers, then you have one transformer connected to each high voltage line and obviously you get 3 waveforms out that have the same 120deg phase relation as those on the high voltage side. But how do you get 3 waveforms at 120deg shift relative to each other with only two transformers connected to only two of the 3 high voltage lines? Would be a lot easier to explain with a pencil and piece of paper. The usual delta connection has a 120/240V transformer, and then adds 3-phase to that. A diagram is about halfway downhttp://en.wikipedia.org/wiki/Three-phase_electric_power where the transformers are connected in a triangle. L1-N-L2 is a single phase 120/240V Edison circuit. Two transformers add L3 - L1-L2-L3 is 3 phase 240V. The voltage from N-L3 is 208V and is called the "high" leg (or several other names). This is "high-leg delta". But it still works if the L1-L3 transformer is not used. It is then called open delta and only uses 2 transformers. This used to be moderately common and is still around. It is probably not used for anything new. It is pretty easy to identify because of 2 utility transformers, only one of which has a secondary neutral connection. ======================================= For a wye connection, a Scott (or T) transformer connection can be used. See the diagram about halfway down labeled "Scott T transformer 3ph to 3 ph"http://en.wikipedia.org/wiki/Scott-T_transformer X1-X2-X3 are 3-phase with X0 being the neutral. This gives a wye 3-phase. A rather common use would be 480V 3-phase connected to H1-H2-H3 and 277/120V coming out X1-X2-X3-X0. This is *used on some relatively small "dry" transformers in buildings with a 480V service. The advantage is you have 3 phase with only 2 transformers. The disadvantage is you screw up the power factor on both transformers and have to derate them to 87% (if I remember right). (And if I remember right, the delta transformers above also have to be derated for the 3-phase power.) I don't think utilities ever use this connection. The transformers actually run at real 2-phase at 90 degrees. You can use a Scott connection to actually use the 2-phase.- Hide quoted text - - Show quoted text - But the question is, when using only two transformers tied into two of the 3 phases, what generates the missing third phase that is 120deg away from the other two? Does a special transformer give the phase shift? A typical transformer that you would use in a 3 phase installation using 3 transformers, tied to all 3 high voltage phases gives outputs that are not phase shifted, or at least by only a little, right? |
#5
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Three phase from 2 transformers?
On 1/14/2013 11:22 AM, bud-- wrote:
On 1/14/2013 9:31 AM, wrote: In a current thread, gfretw told a poster that one way to indentify 3 phase service is if it's connected to two or three transformers. That brought back a question that has stumped me for a very long time. I know that it's possible to get 3 phase service with just two transformers. Apparently that is used for cost savings on lighter 3 phase loads. But what I don't understand is how you can get true 3 phase with only TWO transformers. Coming down the road you have 3 high voltage lines. Each of those has an AC waveform and each is 120deg off in phase from the other. If you have three step-down transformers, then you have one transformer connected to each high voltage line and obviously you get 3 waveforms out that have the same 120deg phase relation as those on the high voltage side. But how do you get 3 waveforms at 120deg shift relative to each other with only two transformers connected to only two of the 3 high voltage lines? Would be a lot easier to explain with a pencil and piece of paper. The usual delta connection has a 120/240V transformer, and then adds 3-phase to that. A diagram is about halfway down http://en.wikipedia.org/wiki/Three-phase_electric_power where the transformers are connected in a triangle. L1-N-L2 is a single phase 120/240V Edison circuit. Two transformers add L3 - L1-L2-L3 is 3 phase 240V. The voltage from N-L3 is 208V and is called the "high" leg (or several other names). This is "high-leg delta". But it still works if the L1-L3 transformer is not used. It is then called open delta and only uses 2 transformers. This used to be moderately common and is still around. It is probably not used for anything new. It is pretty easy to identify because of 2 utility transformers, only one of which has a secondary neutral connection. Open delta is our fairly large utilities standard implementation for light commercial loads that need 3 phase. ======================================= For a wye connection, a Scott (or T) transformer connection can be used. See the diagram about halfway down labeled "Scott T transformer 3ph to 3 ph" http://en.wikipedia.org/wiki/Scott-T_transformer X1-X2-X3 are 3-phase with X0 being the neutral. This gives a wye 3-phase. A rather common use would be 480V 3-phase connected to H1-H2-H3 and 277/120V coming out X1-X2-X3-X0. This is used on some relatively small "dry" transformers in buildings with a 480V service. The advantage is you have 3 phase with only 2 transformers. The disadvantage is you screw up the power factor on both transformers and have to derate them to 87% (if I remember right). (And if I remember right, the delta transformers above also have to be derated for the 3-phase power.) I don't think utilities ever use this connection. The transformers actually run at real 2-phase at 90 degrees. You can use a Scott connection to actually use the 2-phase. |
#6
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Three phase from 2 transformers?
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#7
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Three phase from 2 transformers?
On Jan 15, 12:03*pm, wrote:
On Tue, 15 Jan 2013 06:11:52 -0800 (PST), " wrote: But the question is, when using only two transformers tied into two of the 3 phases, what generates the missing third phase that is 120deg away from the other two? *Does a special transformer give the phase shift? *A typical transformer that you would use in a 3 phase installation using 3 transformers, tied to all 3 high voltage phases gives outputs that are not phase shifted, or at least by only a little, right? With the open delta, usually the only time you have phase problems is if you connect 240v line to line loads across the "missing" transformer. Then you will see *voltage differences phase to phase. But I'm not talking about phase problems. I'm talking about what creates the missing third phase period. Coming down the road we have 3 high voltage phases. One is at 0deg, one is at 120deg, one is at 240deg. If I hook up 3 seperate transformers to get 3 phase, which I see a lot, then coming out of those 3 transformers are lower voltage waveforms that are at 0deg, 120deg, 240deg, neglecting any unintentional shift. So, you have 3 phase just like on the high voltage side, just stepped down. It makes sense. But now if they only use 2 transformers, I can see where you directly get any two of those 3 waveforms. Let's say I hook one of the two transformers to the 0 deg high voltage line. I hood the other to the 120deg high voltage line. I can see how out of that I get 0deg and 120deg for the 3 phase service, just like I did before. The question is, what generates the 240deg phase? Phase shift from one of the transformers? |
#8
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Three phase from 2 transformers?
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#9
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Three phase from 2 transformers?
Look up open delta transformer, you will get the answer you seek.
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