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?

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?

Hi,
I can see ut, it depends how the windings are wound and
how they are connected. I can visualize Delta 3-phase.

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.

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?

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.

On 1/15/2013 8:11 AM, wrote:
On Jan 14, 11:22 am, 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?

Crappy ASCII art (requires fixed width font):

B
\
\
\
\
\
A ___________C
N

This is a 240V open-delta secondary connection diagram (all diagrams are
show pseudo-phasor relationships). A center-tapped 240V transformer is
shown between A and C. A 240V transformer is shown between B and C.

Transformer connections on the primary are different voltage but connect
to the 3 phase supply the same way (with no neutral). *The primary
connections use all 3 phases*. The primary connections to the 3-phase
supply put transformer AC and transformer BC at a 120 degree phase
relationship. As a result you have 3-phase at secondary A-B-C just like
you have 3-phase where the transformer primaries are connected.

==============================================
Any modern 3-phase is likely to be 208/120V wye, or 480/277V wye for
larger buildings and industrial.

A
|
|
|
|
N
/ \
/ \
/ \
/ \
B C


There isn't a simple trick like open delta for wye (because you need to
create the neutral), but the Scott connection is about as simple.


B
|
|
|
|N
|
A____________C
Q


On the primary side one transformer connects between phases A and C. The
other transformer connects from phase B to the centertap (Q) of
transformer AC. As a result the transformers are electrically at 90
degrees.

The secondary connections are similar and convert the 2-phase from the
transformers back to 3-phase. The critical addition is a tap at N to
supply the neutral.

For a 208/120V secondary you have AB=BC=AC=208V. AN=BN=CN=120V (BQ=180V)


============================
For both open-delta and Scott the transformer primaries connect to all 3
phases, which determines the phase relationships between the 2
transformers, and the phase relationship of the transformers creates the
3-phase at the secondary connections.

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?

On 1/14/2013 9:31 AM, wrote:
....

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?

Look up "open delta" ...

--

Look up open delta transformer, you will get the answer you seek.