Same Phase, Different Voltage

[thomasmwilson89]

OK, I recently had a issue that has since been resolved, but inevitably myself and coworkers end up chatting about things we aren't sure about (theories, what-ifs, etc) So this time I was curious about a two-parter.

Part 1- I've searched pretty hard but couldn't find any diagrams about it, which was kind of surprising. Let just say you had a 480V to 240V transformer (not sure if the transformer type would matter). Lets say you use A phase and B phase from 480V and it yields A phase a B phase on the 240V. Are the 480V and the 240V phases in sync (480V A phase and 240V A phase for example)? Like I said, I've looked for sinewave graphs comparing a primary voltage to secondary voltage but can't seem to find anything. In theory I would assume that the sinewaves would be the same, but I was curious to have some input.

Part 2- Provided that answer to the question above is that the phases are in fact in sync, what would happen if you touched the 480V A phase and 240V A phase together? Maybe I'm overlooking basic theory here, but since they're on the same wavelength (assuming same Hz), my brain is telling me that maybe it will be OK. But on the flipside, my brain also says that something about mixing voltages especially through a transformer seems horribly catastrophic. This is strictly one of those, I'm just curious type of questions, obviously I have no intention of trying it out. Any input?

 

[gar]

220513-0622 EDT

thomasmwilson:

If you have two sine waves of identical frequency, and the phase relationship between the two source sine waves is invariant, then no matter what is the constant phase shift between the two sine waves the resultant sum or difference of the two signals is still a sine wave of the same frequency, but the phase relationship may be different than either source sine wave. Take clear note that I am talking about the sum of the sine waves. This may mean you want to work with a transformer with no electrical conductivity between primary and secondary until you provide that connection, and that you do not parallel the primary and secondary. Only use a series connection. See ---

https://www.dsprelated.com/showarticle/635.php

on sine wave summing at Table 2 for --- A sin ( wt + a ) + B sin ( wt )

.

 

[bwat]

Assuming they have a common reference point and are in phase, you'll measure 240V between A-pri and A-sec... so connecting them together would be just the same as connecting 240V "hot" to a "neutral". Voltage is relative to something else. Voltage is the "potential difference".

 

[winnie]

Part 1- I've searched pretty hard but couldn't find any diagrams about it, which was kind of surprising. Let just say you had a 480V to 240V transformer (not sure if the transformer type would matter). Lets say you use A phase and B phase from 480V and it yields A phase a B phase on the 240V. Are the 480V and the 240V phases in sync (480V A phase and 240V A phase for example)? Like I said, I've looked for sinewave graphs comparing a primary voltage to secondary voltage but can't seem to find anything. In theory I would assume that the sinewaves would be the same, but I was curious to have some input.

Transformer type does matter.

If you are talking a common single phase transformer, the secondary voltage will be approximately in phase with the primary voltage. There are small phase shifts because of the leakage inductance of the transformer and load current, but for a good approximation voltages are in phase.

On the other hand the extremely common delta:wye three phase transformers introduce an over-all phase shift, because the line-line coils of the delta primary are in phase with the corresponding line-neutral coils of the secondary.

Part 2- Provided that answer to the question above is that the phases are in fact in sync, what would happen if you touched the 480V A phase and 240V A phase together? Maybe I'm overlooking basic theory here, but since they're on the same wavelength (assuming same Hz), my brain is telling me that maybe it will be OK. But on the flipside, my brain also says that something about mixing voltages especially through a transformer seems horribly catastrophic. This is strictly one of those, I'm just curious type of questions, obviously I have no intention of trying it out. Any input?

Even if the two are 'in phase', they are still at different voltages. If there is a common reference to close the circuit (say both 480V and 240V systems are grounded), then touching a 480V A phase to 240V A would result in a short circuit and high current flow.

The easiest transformer to see this on is an autotransformer where you have 480V and 240V taps on the same coil. Short two taps together if you want to convert your transformer to a smoke generator.

-Jon

 

[SceneryDriver]

Transformer type does matter.

If you are talking a common single phase transformer, the secondary voltage will be approximately in phase with the primary voltage. There are small phase shifts because of the leakage inductance of the transformer and load current, but for a good approximation voltages are in phase.

On the other hand the extremely common delta:wye three phase transformers introduce an over-all phase shift, because the line-line coils of the delta primary are in phase with the corresponding line-neutral coils of the secondary.



Even if the two are 'in phase', they are still at different voltages. If there is a common reference to close the circuit (say both 480V and 240V systems are grounded), then touching a 480V A phase to 240V A would result in a short circuit and high current flow.

The easiest transformer to see this on is an autotransformer where you have 480V and 240V taps on the same coil. Short two taps together if you want to convert your transformer to a smoke generator.

-Jon

"Short two taps together if you want to convert your transformer to a smoke generator. "

Every machine is a smoke machine if it's operated wrongly enough and you're brave enough.


SceneryDriver

 

[rambojoe]

There is no smoke.. it is a bomb and quite disturbing to witness.
It wasnt pri/sec and it wasnt my gear... it was a sharp gear edge that cut through (2) 500mcm..

 

[gar]

220513-1217 EDT

thomasmwilliams:

Going back to your original posts. It appears that your question is about a single phase isolation transformer with a 480 V primary, and a 240 V secondary. Being this is an isolation transformer that means there is no significant conductivity between primary and secondary. Thus, we have two coils of wire that are not conductively electrically connected together, but are magnetically very closely connected. Thus, a 4 terminal network.

When talking about the characteristics of this network we are not concerned with the origin of the source of voltage to the primary, it is just a voltage generator. It could be 1,000,000 V above some neutral. Doesn't matter. Neutral has nothing to do with the question.

If I apply 480 V from somewhere to the primary, then I will see 240 V across the secondary. If no connections are made to the secondary, then the secondary is only referenced to something else by leakage currents that are quite small.

This now defines what I believe is your circuit of the fundamental question.

Now if I connect one primary terminal to one secondary terminal, then there is no short circuit.

The various voltage I can obtain are:

Phasing 1 --- 480 V, 240 V, and 720 V.

Phasing 2 --- 480 V, 240 V, and 240 V.

Simple.

.

 

[ggunn]

Even if the two are 'in phase', they are still at different voltages. If there is a common reference to close the circuit (say both 480V and 240V systems are grounded), then touching a 480V A phase to 240V A would result in a short circuit and high current flow.

I. e., what we euphemistically refer to as a "thermal event".

 

[thomasmwilson89]

Now if I connect one primary terminal to one secondary terminal, then there is no short circuit.

The various voltage I can obtain are:

Phasing 1 --- 480 V, 240 V, and 720 V.

Phasing 2 --- 480 V, 240 V, and 240 V.

So... does this mean that you disagree with the idea that touching these two wires together would go boom?

Even if the two are 'in phase', they are still at different voltages. If there is a common reference to close the circuit (say both 480V and 240V systems are grounded), then touching a 480V A phase to 240V A would result in a short circuit and high current flow.

The easiest transformer to see this on is an autotransformer where you have 480V and 240V taps on the same coil. Short two taps together if you want to convert your transformer to a smoke generator.

I guess I was mostly thinking of an Isolation Transformer for my question. I'm still a bit confused on how this would create a short circuit? Maybe I'm WAY oversimplifying it in my head, but the reason this questions even came up was because I was thinking if something is perfectly in phase, then electrons are flowing the same direction at the same time, sooo... no collision of electrons... maybe no boom lol?

Maybe this is different because both are coming in at 12kV, but we have a Cogeneration Plant that powers pretty much half of our plant. When starting Cogen it syncs up the phases with our power supplier and then closes the breaker to connect each power source. I know it's different because the source isn't coming from the same place and the voltages are the same, but kinda similar, no?

 

[Hv&Lv]

OK, I recently had a issue that has since been resolved, but inevitably myself and coworkers end up chatting about things we aren't sure about (theories, what-ifs, etc) So this time I was curious about a two-parter.

Part 1- I've searched pretty hard but couldn't find any diagrams about it, which was kind of surprising. Let just say you had a 480V to 240V transformer (not sure if the transformer type would matter). Lets say you use A phase and B phase from 480V and it yields A phase a B phase on the 240V. Are the 480V and the 240V phases in sync (480V A phase and 240V A phase for example)? Like I said, I've looked for sinewave graphs comparing a primary voltage to secondary voltage but can't seem to find anything. In theory I would assume that the sinewaves would be the same, but I was curious to have some input.

Part 2- Provided that answer to the question above is that the phases are in fact in sync, what would happen if you touched the 480V A phase and 240V A phase together? Maybe I'm overlooking basic theory here, but since they're on the same wavelength (assuming same Hz), my brain is telling me that maybe it will be OK. But on the flipside, my brain also says that something about mixing voltages especially through a transformer seems horribly catastrophic. This is strictly one of those, I'm just curious type of questions, obviously I have no intention of trying it out. Any input?

Use this picture as your example.
Say the green is your primary at 480, the blue and red are split phase 240/120.

measure the peaks. Let’s keep it simple and forget 1.414 for now..
peak of red/blue to black line will be 120.
Peak of blue to red will be 240. (120+120)
peak of green to black line will be 480
peak of green to blue, that are in phase, will be 360. (480 peak to black line - 120 peak of blue to black line)

 

[thomasmwilson89]

This is exactly the diagram I was looking for but couldn't find it anywhere confirming it was how phases work in a transformer. But yes, assuming this is accurate for a transformer, it is exactly how I pictured it, and exactly what I meant by the primary leg and secondary leg being in phase with each other. I don't doubt that voltage would be strange, but does that create an explosion or any sort of "thermal event", afterall, electrons will be moving in the same directions at the same time (Hz), some of them just don't get up to the same speed as the others (Volts).

 

[winnie]

Gar and I are both correct.

Gar describes what happens if one terminal of the 240V secondary gets connected to the 480V primary.

I described what would happen if both systems 'have a common reference', meaning if both systems were grounded to the same point.

If no terminals of the secondary are connected to the primary, then you have an unreferenced system, and primary to secondary voltage is undefined. (In the real world it would be defined by leakage and capacitive coupling, but the voltage measured with a meter could vary widely.)

If only a single terminal of the secondary is connected to the primary, then you don't have a closed loop for current to flow, and thus you don't have a short circuit. This single point of connection creates the solid reference between primary and secondary. As Gar notes different connections create different voltage arrangements.

With two terminals of the secondary connected to the primary you have a closed loop where current could flow. How much current depends on the net voltage around the circuit and any impedance in the circuit. If you just have transformer coils of different voltage, and wires connecting everything, then you have a short.

Jon

 

[Strathead]

Transformer type does matter.

If you are talking a common single phase transformer, the secondary voltage will be approximately in phase with the primary voltage. There are small phase shifts because of the leakage inductance of the transformer and load current, but for a good approximation voltages are in phase.

link

Unless I am misunderstanding then no they are 180 degrees apart.

 

[gar]

220514-1040 EDT

I suspect that most electricians have very little training in transformer theory, and that is probably OK if they do not work in an industrial environment.

But here are some basic concepts to consider ---

1. Assume we are talking about a transformer with very tight coupling between different windings on the same core, that these windings as built are all separate two terminal coils ( meaning there is no resistive coupling between between the coils even though there is very tight magnetic coupling ).

2. A change in voltage on any one winding will produce an identical change in voltage on any other winding only modified by the turns ratio between the windings.

3. Whether the phase difference between the windings is 0 or 180 degrees is simply dependent on which of the two possible ways a two terminal coil can be connected.

With these basic concepts in mind you can combine any two or more windings in SERIES and produce no short circuit.

An exception to the no PARALLEL connection is ---
Coils can be paralleled if their no load output voltages are the same, and that their phasing is such that the coils all have the same phase.

From a practical point of view you won't find a typical transformer with precisely identical separate windings, but close to one another is adequate. The criteria being that the unwanted circulating current between the two windings is not excessive. There are a very large number of machine tool transformers wound with two 240 V primary coils, and two 120 V secondaries. The primaries can be used in series for 480 V input or in parallel for 240 V input. Independently the secondaries can be connected in series for 240 V output or in parallel for 120 V output.

Also note you can use any transformer for voltages less than the transformer voltage rating. Doing this just means you can not transfer full rated power, because the real transformer rating is based on current, and not power.

.