high leg load

[wwhitney]

You are saying that you have a 208v delta system with a high (wild) leg?

No, we are talking about a 240V delta with a center-tapped winding to provide a single phase neutral, and that the high leg (the one not connected to the center tapped winding) is then 208V to that neutral.

While such systems generally are not designed for 208V loads from the high leg to the neutral, obviously that voltage is available, and a system could be designed for such loads. So this thread is about what primary currents you'd get from such a load, for both the delta and wye primary configurations.

Cheers, Wayne

 

[Do You Like Ham]

@herding_cats I'm not sure where you got '208v delta system'... the question is about connecting a load high-leg to neutral (208v) on a 240/120 3ph 4w delta system. Attempting to understand how one would determine the primary currents (L1, L2, L3) in the event the primary is delta or wye.

"there is no usable function for any load hot to neutral off of a delta transformer from the wild leg."
https://forums.mikeholt.com/threads/loads-on-high-leg-of-open-delta.60272/post-996134 refers to a 'handyman special'
The other scenario I've heard from personal anecdote is similar.. "local electrician" connected loads in this way.
I don't have direct experience of seeing or using this myself.. the question is more academic... how would it be computed.

 

[herding_cats]

Ok I see.

 

[wwhitney]

the question is about connecting a load high-leg to neutral (208v) on a 240/120 3ph 4w delta system. Attempting to understand how one would determine the primary currents (L1, L2, L3) in the event the primary is delta or wye.

So did you have a chance to look at my answers to see if you believe them? I'm still looking for confirmation that I didn't make any errors.

Cheers, Wayne

 

[jim dungar]

I am still looking at the issues with saying there is 0A flowing in the C primary side. If current is flowing on the secondary, then due to it being a transformer you are going to have a magnetic field in the core which would also need to be addressed. The original magnetic field would have been created, in one direction, by the primary voltage.

 

[wwhitney]

I am still looking at the issues with saying there is 0A flowing in the C primary side. If current is flowing on the secondary, then due to it being a transformer you are going to have a magnetic field in the core which would also need to be addressed. The original magnetic field would have been created, in one direction, by the primary voltage.

So, for an idealized transformer, the primary coil current is just the referred current from the secondary loads. And this is what the computation in post #8 means: for a 208V load supplied L2 - N, with the secondary coils A+C1 and B+C2 identical, the load current divides evenly and the referred current in the primary C coil from C1 cancels exactly the referred current from C2. Introduce a difference between the two paths A/C1 and B/C2, and the current won't divide evenly, and it won't fully cancel.

In a more realistic transformer model, there are other factors that will affect the current in the primary coil C. I'm only passingly familiar with the models, but there will certainly be the magnetization current. So you wouldn't measure 0 current in primary coil C; but the difference in current in primary coil C should be close to zero if you turn the L2-N load on vs off.

Cheers, Wayne

 

[jim dungar]

The current in a transformer secondary coil is related to the magnetic field in the transformer core created by the primary coil current. The magnetic field cannot 'flow' in two directions at once if there is only one magnetic circuit. This is a constraint you need to include in your analysis.

Just because you can analyze the center tapped secondary coil as if it were two independent circuits, its physical construction must not be ignored. A center tapped transformer output is physically two voltages in series addition not two voltages 180° out of phase with each other.

 

[ggunn]

A center tapped transformer output is physically two voltages in series addition not two voltages 180° out of phase with each other.

Incoming!

 

[wwhitney]

The current in a transformer secondary coil is related to the magnetic field in the transformer core created by the primary coil current. The magnetic field cannot 'flow' in two directions at once if there is only one magnetic circuit. This is a constraint you need to include in your analysis.

OK, my understanding of the magnetic circuit, and how that affects the electrical circuit, is definitely very weak. If you have a pointer to where I could learn about that it would be super appreciated.

Right now my analysis is just based on the idea that if the turns ratio is T, then the voltage ratio will be T (and in phase), and the current ratio will be 1/T (and in phase). So if there's something about the magnetic circuit that I'm missing, and that changes the ultimate result, one of those rules will need to be modified.

But let's amend the diagram in post #8 so that (a) the transformer is built up from 3 separate single phase transformers (separate cores) and (b) the C transformer is further split into two separate 480V : 120V transformers C1 and C2. The analysis in post #8 should then apply, as I have removed all magnetic circuit / common core constraints.

Is that correct? So primaries C1 and C2 would see would see opposite sign currents. If they are jointly supplied by a 2 wire feeder (not shared with transformers B and C), the current in that feeder would be zero. There would just be a circulating current around the primary C1 / C2 loop, which is possible in this impedance free idealization.

So then if go back to a single C transformer with split secondary coil, what changes? I have trouble seeing how any imbalance could be introduced, as the circuit has mirror symmetry (at least for the case of a resistive load on L2 - N).

Thanks,
Wayne

 

[jim dungar]

Incoming!

Hopefully not.

 

[kwired]

Hi! I'm trying to understand how one would determine the primary line currents in a 480 delta to 240/120 split phase 3ph 4w delta system. E.g., for example if there is an 80A (208v) load connected on high-leg to neutral on the secondary, how does one determine the currents on the primary line conductors?

it is 80 amps on the high leg and 80 amps on the neutral. What your concern really is all about is what is current flow through the delta windings? That current will depend first on whether it is open or closed delta, then comes what is there for other load on same winding sections.

Perfectly balanced conditions and half the neutral current flows toward A phase and other half towards C phase in a closed delta.

Open delta, the neutral current flows entirely toward the closed corner of the delta, there is no path to the opposing corner via the open side of the delta. There is no "balancing " so any current already flowing in other circuits is additive to this current in any particular transformer coil section.

 

[kwired]

it is 80 amps on the high leg and 80 amps on the neutral. What your concern really is all about is what is current flow through the delta windings? That current will depend first on whether it is open or closed delta, then comes what is there for other load on same winding sections.

Perfectly balanced conditions and half the neutral current flows toward A phase and other half towards C phase in a closed delta.

Open delta, the neutral current flows entirely toward the closed corner of the delta, there is no path to the opposing corner via the open side of the delta. There is no "balancing " so any current already flowing in other circuits is additive to this current in any particular transformer coil section.

this reply was how current flows on the secondary side.

Primary side it should depend if wye or delta as well as if open or closed delta for the secondary.

 

[LarryFine]

Perfectly balanced conditions and half the neutral current flows toward A phase and other half towards C phase in a closed delta.

Wouldn't that cause the current to want to flow in opposite directions in each half of the B winding?

 

[wwhitney]

Wouldn't that cause the current to want to flow in opposite directions in each half of the B winding?

Yes, but why is that a problem?

The question that Jim and I have been going back and forth over is then whether the primary B coil sees 0 load current, since the referred current from the two halves of the secondary would cancel in the primary.

Cheers, Wayne

 

[jim dungar]

Yes, but why is that a problem?

The question that Jim and I have been going back and forth over is then whether the primary B coil sees 0 load current, since the referred current from the two halves of the secondary would cancel in the primary.

Cheers, Wayne

And my point is the direction of the current is dictated by the magnetic field in the core, which cannot move in two different directions at the same time.

 

[LarryFine]

And my point is the direction of the current is dictated by the magnetic field in the core, which cannot move in two different directions at the same time.

That's what I'm thinking. If the high leg is carrying current, the neutral would have to, also.

The net current might be mathematically zero, but it seems some heat would be generated.

 

[jim dungar]

...it seems some heat would be generated.

Which is why it is rare to see a closed delta that allows for any significant neutral loading.

 

[wwhitney]

And my point is the direction of the current is dictated by the magnetic field in the core

I'm not following this statement.

If we ignore parasitic secondary and primary impedances, but include in our transformer model the primary inductance that creates the magnetic flux in the iron core, my understanding is that the magnetic flux is constant, independent of the load currents. The primary magnetizing current creates the magnetic field in the iron core; that's end of the story when the secondary is open circuit. If we add a secondary load, current flowing in the secondary creates a magnetic flux, which will be precisely canceled by the magnetic flux from the referred current in the primary.

So what's wrong with saying that since the secondary current is equal and opposite in the two half of the split secondary coil, the magnetic fluxes from those two halves will cancel each other, and the referred current in the primary is zero?

Thanks,
Wayne

 

[LarryFine]

So what's wrong with saying that since the secondary current is equal and opposite in the two half of the split secondary coil, the magnetic fluxes from those two halves will cancel each other, and the referred current in the primary is zero?

How can a secondary load be supplied if there is no primary current?

 

[wwhitney]

How can a secondary load be supplied if there is no primary current?

What I understand the math in post #8 to show is that the primary B coil (what today we are calling the coil corresponding to the center-tapped secondary, in post #8 we were calling this C coil) is not providing any of the energy to the load. It's all coming from the A and C coils (again, today's nomenclature). The B secondary coil's only job is to act as a voltage divider to give us a midpoint voltage reference.

Cheers, Wayne