3 phase and 2 phase in the same system

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pv_n00b

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winnie said:
The existing 'high leg' delta already gives the 90 degree phase difference (A to B is 120 degrees from B to C which is 120 degrees from C to A, a normal 3 phase delta. But the _midpoint_ to C is 90 degrees out of phase with A to B). Since you already have the phase difference, you only need to adjust voltage.

-Jon
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If that were the case then why would there ever be a need for the Scott T transformer? You could just use a regular transformer to do the voltage change. I don't think it is that easy.
 
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winnie

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pv_n00b said:
If that were the case then why would there ever be a need for the Scott T transformer? You could just use a regular transformer to do the voltage change. I don't think it is that easy.
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I think it might actually be that easy. Take a look at the schematic of a Scott-T transformer. You have 2 coils, one of which derives a center point between two phases, the other which reduces the voltage between that center point and the third phase. If you already have the center point existing in your system, you only need the second coil.
 
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Hv&Lv

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winnie said:
I think it might actually be that easy. Take a look at the schematic of a Scott-T transformer. You have 2 coils, one of which derives a center point between two phases, the other which reduces the voltage between that center point and the third phase. If you already have the center point existing in your system, you only need the second coil.
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I disagree. A normal transformer configuration cannot be Scott T connected as there aren’t enough high side lead taps on them to tap the 86% tap or the midpoint high side tap. These coils have to be built for this purpose.
 
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winnie

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Hv&Lv said:
I disagree. A normal transformer configuration cannot be Scott T connected as there aren’t enough high side lead taps on them to tap the 86% tap or the midpoint high side tap. These coils have to be built for this purpose.
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My apologies for not being clear.

A Scott T transformer is used to convert a 3 phase 3 wire system to two phase.

A Scott T transformer consists of two sets of coils. They are often shown schematically in an inverted 'T' shape because that matches up with a graphical representation of the phases and voltages present.

The 'base' set of coils consists of a primary connected between two of the input 3 phases. This primary has a center tap. The secondary of this set of coils gives you one of the two output phases.

The 'vertical' set of coils consists of a primary connected between the third input phase and the center tap of the 'base' primary. It is this phase to midpoint connection that derives the 90 degree phase difference needed for the second output.

In addition to this phase difference, a voltage adjustment is required, so the primary is often described as using an 86% tap. Another way of describing this is that the voltage ratio of the 'base' set of coils is 0.5 + 0.5 : 1 (the 0.5 + 0.5 to describe the center tap) and the 'vertical' set is 0.86:1.

Now take a look at the phase angles and voltages present in a 'high leg delta' three phase supply. You _already_ have the 'midpoint' and the 'midpoint' to 'high leg' phase angle is _already_ the 90 degree offset required for your 2 phase system. What is wrong is the voltage. So if you start with a 'high leg delta' system, you already have several of the bits of the Scott T system already present in your 3 phase source. You still need the 86% tap, but you don't need to duplicate the center tap between 2 of your 3 phases.

-Jon
 
LarryFine

LarryFine

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winnie said:
Now take a look at the phase angles and voltages present in a 'high leg delta' three phase supply. You _already_ have the 'midpoint' and the 'midpoint' to 'high leg' phase angle is _already_ the 90 degree offset required for your 2 phase system. What is wrong is the voltage. So if you start with a 'high leg delta' system, you already have several of the bits of the Scott T system already present in your 3 phase source. You still need the 86% tap, but you don't need to duplicate the center tap between 2 of your 3 phases.
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In other words, if I'm reading you correctly, a high-leg delta and an inverted T (especially when super-imposed) share the same four points.
 
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Hv&Lv

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winnie said:
My apologies for not being clear.

A Scott T transformer is used to convert a 3 phase 3 wire system to two phase.

A Scott T transformer consists of two sets of coils. They are often shown schematically in an inverted 'T' shape because that matches up with a graphical representation of the phases and voltages present.

The 'base' set of coils consists of a primary connected between two of the input 3 phases. This primary has a center tap. The secondary of this set of coils gives you one of the two output phases.

The 'vertical' set of coils consists of a primary connected between the third input phase and the center tap of the 'base' primary. It is this phase to midpoint connection that derives the 90 degree phase difference needed for the second output.

In addition to this phase difference, a voltage adjustment is required, so the primary is often described as using an 86% tap. Another way of describing this is that the voltage ratio of the 'base' set of coils is 0.5 + 0.5 : 1 (the 0.5 + 0.5 to describe the center tap) and the 'vertical' set is 0.86:1.

Now take a look at the phase angles and voltages present in a 'high leg delta' three phase supply. You _already_ have the 'midpoint' and the 'midpoint' to 'high leg' phase angle is _already_ the 90 degree offset required for your 2 phase system. What is wrong is the voltage. So if you start with a 'high leg delta' system, you already have several of the bits of the Scott T system already present in your 3 phase source. You still need the 86% tap, but you don't need to duplicate the center tap between 2 of your 3 phases.

-Jon
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I guess I see where your going, but the open delta won’t be as balanced as the Scott t, if by chance you could rearrange the connections to give two currents offset by 90 degrees. The voltages in an open delta may be off by 90, but the currents are 120 degrees apart.
 
mbrooke

mbrooke

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Hv&Lv said:
I guess I see where your going, but the open delta won’t be as balanced as the Scott t, if by chance you could rearrange the connections to give two currents offset by 90 degrees. The voltages in an open delta may be off by 90, but the currents are 120 degrees apart.
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How does a T balance?
 
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winnie

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Location
Springfield, MA, USA
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Electric motor research
Hv&Lv said:
I guess I see where your going, but the open delta won’t be as balanced as the Scott t, if by chance you could rearrange the connections to give two currents offset by 90 degrees. The voltages in an open delta may be off by 90, but the currents are 120 degrees apart.
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I think that I see what you are saying. The voltage between the points in question (high leg to neutral) is 90 degrees (relative to the 'base') but the current must flow in the coils. However the circuit suggested in the OP is not an open delta, but a _closed_ high leg delta.

There would still be the power factor of current flowing in the coils out of phase with the voltage developed in the coils, but I think the closed delta situation is significantly more balanced.

-Jon
 
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