3 phase to single phase supply

[GoldDigger]

SO, I have always wanted to ask. If put a current oscilloscope in the primary, what would it look like? :?

The primary associated with A will have current Ipa as determined by the secondary current and the turn ratio.
The primaries associated with B and C will have that same secondary current but a different turn ratio and therefore more current than Ipa.

 

[ggunn]

The primary associated with A will have current Ipa as determined by the secondary current and the turn ratio.
The primaries associated with B and C will have that same secondary current but a different turn ratio and therefore more current than Ipa.

I believe he said that it was critical to keep the load on the primaries balanced.

 

[mbrooke]

The primary associated with A will have current Ipa as determined by the secondary current and the turn ratio.
The primaries associated with B and C will have that same secondary current but a different turn ratio and therefore more current than Ipa.

But it will be the same through out the sign wave at all points? Ie, each half wave will not be more than the other?

 

[don_resqcapt19]

With the double delta or the zig zag, a single phase 240 volt load will load the three primary phases equally.

 

[Ingenieur]

It is not a simple as three primaries and one secondary.
Basically you interwire three individual secondary windings in a complicated way. It is similar to the complicated interconnection of the Scott T circuit between two phase and three phase.

I'm not talking about his method
a common connection is
prim open delta 2 windings 3 phase lines
sec 2 windings in series summing dot polarity
but the prim phases are not balanced

 

[winnie]

If you use a 'double delta' secondary to derive single phase from three phase, you will place equal current loading on all of the transformer windings.

However the phase angle of all the current flow will be the _single phase_ current flow.

The net result is that the three phase current that this transformer draws from the supply will still be unbalanced.

You cannot turn a single phase load into a _balanced_ three phase load without some form of energy storage. Consider a resistive single phase load: power peaks and falls to zero twice a cycle. Now consider a resistive three phase load: power delivered to the load is _constant_, with each phase in turn carrying more or less of this constant load.

There is not enough information to even guess at a 'best' solution. What is more important: long term efficiency or short term cost? What is the duty cycle of this load? Exactly how well balanced must the 3 phase loading be? What sort of scrap hardware do you already have in your 'lab'?

-Jon

 

[Jraef]

You can use a Scott T transformer configuration to give you a 2 phase output from a 3 phase source. If the 1000V AC is feeding a rectifier, it would not matter.

https://electricalnotes.wordpress.com/2012/05/06/scott-t-connection-of-transformer/

D'oh, GoldDigger already posted it... :slaphead:

 

[Tony S]

A Scott T or LeBlanc transformer and use just L1 and L2

 

[Phil Corso]

Gentlepeople...

Fiddling with various xfmr connections is a total waste of time!

There is no known practical methodology available to service a single-phase load (AC or DC) from a 3-phase source, unless very unbalanced line-currents, excessive losses, and poor V-regulation, is acceptable!

For the DC load, presented by the OP, I suggest a 3-phase diode-bridge!
Phil Corso

 

[Ingenieur]

Gentlepeople...

Fiddling with various xfmr connections is a total waste of time!

There is no known practical methodology available to service a single-phase load (AC or DC) from a 3-phase source, unless very unbalanced line-currents, excessive losses, and poor V-regulation, is acceptable!

For the DC load, presented by the OP, I suggest a 3-phase diode-bridge!
Phil Corso

I'd like to know what he needs 600A at 1000 VAC single phase for! lol
must be a test rig or something

 

[Phil Corso]

Injunear...

Ask him


Phil

 

[winnie]

I agree, the 1000V 600A _single phase_ requirement is a bit odd.

I have to wonder if this is the intermediate for something that might otherwise be better served by 3 phases directly. For example, if the 1000A 600A is being rectified, or being used for electric heating or to run a motor....

-Jon

 

[Ingenieur]

I agree, the 1000V 600A _single phase_ requirement is a bit odd.

I have to wonder if this is the intermediate for something that might otherwise be better served by 3 phases directly. For example, if the 1000A 600A is being rectified, or being used for electric heating or to run a motor....

-Jon

agree
without knowing function it's hard to determine the best solution
duty cycle: continuous, intermittent, periodic pulsed
what is the load type: motor, inductive, capacitive, resistive, etc.
constant load or variable output, etc

 

[don_resqcapt19]

If you use a 'double delta' secondary to derive single phase from three phase, you will place equal current loading on all of the transformer windings.

However the phase angle of all the current flow will be the _single phase_ current flow.

The net result is that the three phase current that this transformer draws from the supply will still be unbalanced.

You cannot turn a single phase load into a _balanced_ three phase load without some form of energy storage. Consider a resistive single phase load: power peaks and falls to zero twice a cycle. Now consider a resistive three phase load: power delivered to the load is _constant_, with each phase in turn carrying more or less of this constant load.

There is not enough information to even guess at a 'best' solution. What is more important: long term efficiency or short term cost? What is the duty cycle of this load? Exactly how well balanced must the 3 phase loading be? What sort of scrap hardware do you already have in your 'lab'?

-Jon

The very reason that generator manufacturer's use 12 lead generators, is to provide balanced current on all of the windings in the generator where the generator is connected to supply single phase loads. Are you saying that does not work that way?

 

[Phil Corso]

Don...

While I agree that a 12-lead engine-driven generator can provide single-phase, 3-wire (e.g., 120/240v) I seriously doubt that it would an economic choice for a the OP's 600 kVA Load!

Furthermore, I don't believe the benefits associated with the 12-lead Engine-driven Generator are readily accessible in a commercial Xfmr!

Phil

 

[GoldDigger]

I believe he said that it was critical to keep the load on the primaries balanced.

In this case the power load on the three is balanced, but the VA is not. B and C have low power factor while A has a power factor of 1. (Ignoring magnetizing current).

 

[winnie]

The very reason that generator manufacturer's use 12 lead generators, is to provide balanced current on all of the windings in the generator where the generator is connected to supply single phase loads. Are you saying that does not work that way?

I agree that the 'double delta' connection will cause current to flow in all three phases, achieving the goal you describe above.

What I am saying is that the phase angle of the current flow in the three phase windings will not be a balanced three phase load.

Imagine a delta to double delta transformer where the single phase loading is between A and C. All of the coils will have current flow, but the supply current going into primary terminal B will be very small. Some large fraction of the single phase current will be on the A to C coil, and some will 'circulate' from A to B to C.

-Jon

 

[don_resqcapt19]

Don...

While I agree that a 12-lead engine-driven generator can provide single-phase, 3-wire (e.g., 120/240v) I seriously doubt that it would an economic choice for a the OP's 600 kVA Load!

Furthermore, I don't believe the benefits associated with the 12-lead Engine-driven Generator are readily accessible in a commercial Xfmr!

Phil

It would require that you build a 3 phase bank using 3 transformers that have all 4 leads brought out on the secondary...not a common transformer, but they exist.

I am not sure that there is an economic choice to provide single phase at 600 kVA and still keep the 3 phase load balanced.

 

[don_resqcapt19]

I agree that the 'double delta' connection will cause current to flow in all three phases, achieving the goal you describe above.

What I am saying is that the phase angle of the current flow in the three phase windings will not be a balanced three phase load.

Imagine a delta to double delta transformer where the single phase loading is between A and C. All of the coils will have current flow, but the supply current going into primary terminal B will be very small. Some large fraction of the single phase current will be on the A to C coil, and some will 'circulate' from A to B to C.

-Jon

Maybe the double delta doesn't work, but if it doesn't work why have the generator makers gotten away from the zigsag in favor of the double delta?

The zigzag will put equal load on all of the windings on both the the primary and secondary when you are supplying a 240 volt single phase load. One of the three coils directly supplies 120 volts or half of the 240, they other 2 connected so that 120 + 120 at 120° =120 and that supplies the other have of the 240.

 

[winnie]

Maybe the double delta doesn't work, but if it doesn't work why have the generator makers gotten away from the zigsag in favor of the double delta?

The zigzag will put equal load on all of the windings on both the the primary and secondary when you are supplying a 240 volt single phase load. One of the three coils directly supplies 120 volts or half of the 240, they other 2 connected so that 120 + 120 at 120° =120 and that supplies the other have of the 240.

Again, I agree that both the zig-zag and the double delta _use_ all of the three phase coils in the generator, and that such a configuration would similarly use all of the coils of a transformer. I also agree with your phase angle math describing how the terminal voltages are produced.

My point is that the current flow though the 'angled' coils will be in phase with the _single phase_ output voltage. The same current has to pass through all coils in series.

The power factor for the 'inline' coil is the expected single phase power factor. The power factor for one of the angled coils leads this by 60 degrees, the power factor for the other lags. All of the coils see the same full current, and all are utilized. Using the zig-zag or double delta arrangement clearly improves the alternator power handing capacity for single phase loads.

When the source of power is a spinning field moved by an engine, the engine sees a pulsing mechanical load (smoothed by inertia).

When the source of power is a set of primary coils, you see the same phase angle and power factor effects. The net result is that current flows in all of the coils. But what changes is the supply current going into the primary terminals.

In the transformer setup, you have 12 different currents: the 3 phase supply currents, the currents in the three primary coils, the currents in the three secondary coils, and the 3 load currents.

With single phase loading one of the load currents is 0.

With the zig-zag or double delta connection, all of the primary and secondary coils are carrying current. It is to utilize all of the coils that the generator guys use the double delta arrangement.

I claim that in the double delta transformer setup, one of the _primary_ supply currents will be close to zero.

-Jon