3 Phase Secondary From Single Phase Primary

[Seven-Delta-FortyOne]

I’ve read the threads that touch on this, but my question is a little different.

Ive heard and read that one way to get 3 phase power where it would otherwise be unavailable, besides using either a RPC or digital phase converter, is by feeding the primary side of an open delta with the two available hots. Then you’ll get 3 phase on the secondary.

How does this work? What creates the third phase, and what would make the secondary voltage 120 degrees apart, when the primary is 180?

Please note: I am not nearly where most members are at, as far as electrical knowledge. So go easy on me.

 

[xptpcrewx]

I’ve read the threads that touch on this, but my question is a little different.

Ive heard and read that one way to get 3 phase power where it would otherwise be unavailable, besides using either a RPC or digital phase converter, is by feeding the primary side of an open delta with the two available hots. Then you’ll get 3 phase on the secondary.

How does this work? What creates the third phase, and what would make the secondary voltage 120 degrees apart, when the primary is 180?

Please note: I am not nearly where most members are at, as far as electrical knowledge. So go easy on me.

Not possible unless the two hots you are referring to are displaced by 60 degrees.

 

[Seven-Delta-FortyOne]

Ah..

So you’re saying the poco could bring just 2 of the 3 phases to feed the primary?


That would make a lot more sense.

Then my only question would be, what makes the third phase? Just inductance on the secondary side?

 

[Besoeker3]

I’ve read the threads that touch on this, but my question is a little different.

Ive heard and read that one way to get 3 phase power where it would otherwise be unavailable, besides using either a RPC or digital phase converter, is by feeding the primary side of an open delta with the two available hots. Then you’ll get 3 phase on the secondary.

How does this work? What creates the third phase, and what would make the secondary voltage 120 degrees apart, when the primary is 180?

Please note: I am not nearly where most members are at, as far as electrical knowledge. So go easy on me.

You could convert it to DC then use a three-phase inverter.

 

[ActionDave]

The power company can do it with their two hots and neutral, but it still takes three wires.

 

[xptpcrewx]

Ah..

So you’re saying the poco could bring just 2 of the 3 phases to feed the primary?

As ActionDave mentioned, you still need 3-wires... 2 phases and a neutral.

Then my only question would be, what makes the third phase? Just inductance on the secondary side?

If it was an open-wye primary (two phases and neutral) connected with single-phase units, you could configure the secondary as open-delta to output three-phase. The third phase is simply created by the phase displacement (difference) of the other two phases. You don't need a third transformer to utilize three-phase power, but you would be operating the bank at reduced capacity when compared to the full-wye, closed-delta configuration.

 

[Seven-Delta-FortyOne]

The power company can do it with their two hots and neutral, but it still takes three wires.

How would the POCO get a neutral, except by center-tapping the secondary side of a transformer?

Then the phase angle on the hots would be 180, correct?

 

[Besoeker3]

How would the POCO get a neutral, except by center-tapping the secondary side of a transformer?

Then the phase angle on the hots would be 180, correct?

That's what I would call it. I think others here may disagree but I'm just a Brit................

 

[jim dungar]

Ah..

So you’re saying the poco could bring just 2 of the 3 phases to feed the primary?

Actually the utility needs to bring 2 hot legs and 1 neutral which come from a three phase wye source.

 

[Besoeker3]

Actually the utility needs to bring 2 hot legs and 1 neutral which come from a three phase wye source.

So you can't get it from a single phase primary?

 

[ActionDave]

How would the POCO get a neutral, except by center-tapping the secondary side of a transformer?

The power company always has a grounded / neutral conductor on the primary side.

The secondary could be corner grounded and not have a neutral.

Then the phase angle on the hots would be 180, correct.

Only relative to the neutral, which is only center tapped on one transformer winding. Phase to phase is still 120 degrees.

 

[Besoeker3]

The power company always has a grounded / neutral conductor on the primary side.

The secondary could be corner grounded and not have a neutral.


Only relative to the neutral, which is only center tapped on one transformer winding. Phase to phase is still 120 degrees.

I still don't get this. For me single phase is just single phase. If you introduce 120 deg that is another, different phase.

 

[ActionDave]

I still don't get this. For me single phase is just single phase. If you introduce 120 deg that is another, different phase.

You have made this clear in several other threads. Kindly leave this one alone.

 

[Seven-Delta-FortyOne]

The power company always has a grounded / neutral conductor on the primary side.

So you’re talking about the POCO supplying this after a transformer, not directly from the distribution lines.

 

[ActionDave]

So you’re talking about the POCO supplying this after a transformer, not directly from the distribution lines.

You lost me here.

 

[Seven-Delta-FortyOne]

In my area, and every place I’ve worked or seen, POCO a doesn’t have a neutral.

3 phase lines on a pole, period.

Then a transformer gets hung in the pole, or in some cases goes under ground to a transformer.

If the transformer is single phase, 2 of the distribution lines get tapped for the transformer primary, and off the secondary is two hots 180 degrees apart and a center tapped neutral.

If it’s a 3 phase transformer, all three go to the primary side. Still no neutral till the secondary.

So I’m not understanding where the POCO is getting this neutral from, to bring to the primary side of a xfrm.

 

[jim dungar]

So you can't get it from a single phase primary?

Of course you can't, that is basic physics.

 

[jim dungar]

In my area, and every place I’ve worked or seen, POCO a doesn’t have a neutral.

3 phase lines on a pole, period.

Then a transformer gets hung in the pole, or in some cases goes under ground to a transformer.

If the transformer is single phase, 2 of the distribution lines get tapped for the transformer primary, and off the secondary is two hots 180 degrees apart and a center tapped neutral.

If it’s a 3 phase transformer, all three go to the primary side. Still no neutral till the secondary.

So I’m not understanding where the POCO is getting this neutral from, to bring to the primary side of a xfrm.

Are you sure that are not using the grounding wire often found at the top of the structure. Sometimes this is called a static wire.

 

[Hv&Lv]

See if this drawing helps.
Delta primary on distribution like your talking about, two transformers, three phases secondary with a neutral after the transformer.

figured the drawing would be easier than me trying to explain it..

 

[LarryFine]

So you’re saying the poco could bring just 2 of the 3 phases to feed the primary?

As long as there's a neutral, yes.

Then my only question would be, what makes the third phase? Just inductance on the secondary side?

The primary isn't important to get this:

Picture a full delta. Then remove one secondary. The voltage across the two open lines is still approximately what it was.