How does the neutral wire prevent MWBCs from operating at 240v?

[SG-1]

:lol::lol::lol: I guess you're right. But to repeat, being a parallel circuit does not explain why the voltage from black to white and red to white are the same, because they're not.

I fear I'll have to take back my posts above, but it will just confuse people instead of helping.

Do we agree that black to white & red to white are the same magnitude ?

Do we agree that they differ only in that when black to white is at its positive peak, red to white is at its negative peak ?

 

[jaggedben]

Do we agree that black to white & red to white are the same magnitude ?

Do we agree that they differ only in that when black to white is at its positive peak, red to white is at its negative peak ?

NO. Their voltage similarity depends entirely on the precision of the manufacturing of the source components, which will never be theoretically perfect. Moreover, imbalanced loads will cause voltage drop and divergent voltage readings on various parts of the system.

Go measure voltage on an actual split-phase system sometime. The chances that the voltages will be within .1 volts of each other are relatively small.

On an MWBC, you only see a true identical voltage between nodes due to parallelism if you measure between the two ends of the neutral. This requires flipping your thinking and considering what you normally think of as the 'loads' to be part of the sources. You have to think of it as two sources parallel to one load instead of two loads parallel to one source. See gar's post.

 

[SG-1]

MESH analysis of an unbalanced MWBC:

I have added some notes & expanded the equations to include units for clarity.

Notice that the 1 amp of current on the neutral conductor is not returning to the transformer, but is leaving the transformer (lower coil) at the polarity mark.

It then adds to the 1 amp of current from the 28 ohm load.

Then 2 amps goes down through the lower load & back to the transformer.

One amp is the total current in the upper transformer coil.

Two amps is the total current in the lower transformer coil. Notice it splits, so one amp continues up through the top coil & the other one amp goes out through the neutral point.

 

[MyCleveland]

SG-1

is there not a polarity problem...your sketch, earlier sketches, and many I have seen on google show something similar as yours with two windings stacked at (120/phase ang 0). What they don't show is the center tap grounded. In your sketch with the center terminal labeled as "+" and "N", is this not a dead short ?

Should not the terminal top to bottom be " + - +", where the "-" is grounded ?
Each winding would be 120/0 deg...top and 120/180 deg on bottom ?

My theory classes left me long ago and I cannot generate the math to show the phase ang separation.

Can anyone step up and resolve this please...
Your primary winding will be served via some 3P system at X volts(phase angle ~)....derived the three voltages and phase angles on the LV side indicating polarity.

 

[jaggedben]

SG-1

is there not a polarity problem...your sketch, earlier sketches, and many I have seen on google show something similar as yours with two windings stacked at (120/phase ang 0). What they don't show is the center tap grounded. In your sketch with the center terminal labeled as "+" and "N", is this not a dead short ?

Should not the terminal top to bottom be " + - +", where the "-" is grounded ?
Each winding would be 120/0 deg...top and 120/180 deg on bottom ?

Go back and look at posts 51 and 55 in this thread. Phase angle 0 is the same as phase angle 180 with the polarity flipped. You can show it either way but if you're not designating phase angles then it has to be stacked like in SG-1s sketches.

There's no short because there's no low resistance path directly from the + to the - of the same source. Grounding is irrelevant. The 'primary' source is irrelevant; it would usually be primary winding but it could be two batteries, or two synced generators.

 

[SG-1]

SG-1

is there not a polarity problem...your sketch, earlier sketches, and many I have seen on google show something similar as yours with two windings stacked at (120/phase ang 0). What they don't show is the center tap grounded. In your sketch with the center terminal labeled as "+" and "N", is this not a dead short ?

I am using the transformer polarity marks as my reference for + & -. Then I am freezing time & looking at the positive half cycle of the sine wave.

For a quick review of transformer polarity: As current enters H1 (x at the top of the primary winding) current exits X1 (x at the top of both transformer secondary coils) as if they were physically connected together.

When H1 is positive, then both X1 terminals are positive.

I am trying to avoid the use of phase angles to keep things as simple as possible. The sketch represents a common single phase service or a simplified MWBC. This keeps the math on the real number line. It still gets too complicated somehow.

Thank you Jaggedben for responding & answering MyCleveland questions.

 

[MyCleveland]

SG-1
The center tap is grounded...not floating. If grounded then we are back to 120/0 --- 120/180.
Please help me with the math on how the lower section of winding is at 180 deg, or point me to a PDF on the subject.

 

[kwired]

SG-1
The center tap is grounded...not floating. If grounded then we are back to 120/0 --- 120/180.
Please help me with the math on how the lower section of winding is at 180 deg, or point me to a PDF on the subject.

grounding won't change how current flows in the two loads in that drawing. In fact you could ground one of the outer ends instead of the mid point and it still won't change anything, it is still one voltage from end to end and half that voltage to the center tap the two loads are still connected across each half.

 

[MyCleveland]

grounding won't change how current flows

Really...ground the center and see what happens with the math.

I am just asking someone to show me the phase angle difference math...0 / 180.

 

[ActionDave]

Really...ground the center and see what happens with the math.

I am just asking someone to show me the phase angle difference math...0 / 180.

I don't do math but grounding doesn't change the current flow through the neutral.

 

[kwired]

Really...ground the center and see what happens with the math.

I am just asking someone to show me the phase angle difference math...0 / 180.

Yes really, all grounding does is place whichever point is grounded at ground potential. Connect additional components into the circuit to ground and then it can make some difference, but in the simple circuit that was drawn for this discussion it doesn't change current flow at all.

 

[MyCleveland]

I don't do math but grounding doesn't change the current flow through the neutral.

Looking at sketch in post 83, grounding the center tap does not change the current flow direction in lower section ?
Polarity has to change to -,+, or the lower 28V source positive terminal is shorted...no ?

 

[ActionDave]

Looking at sketch in post 83, grounding the center tap does not change the current flow direction in lower section ?
Polarity has to change to -,+, or the lower 28V source positive terminal is shorted...no ?

No. Ground is just a reference.

 

[kwired]

Looking at sketch in post 83, grounding the center tap does not change the current flow direction in lower section ?
Polarity has to change to -,+, or the lower 28V source positive terminal is shorted...no ?

shorted to what? the lower 28 volt source only has the lower load connected across it, it's resistance is what is limiting current.

Since we are primarily talking about AC + and - can add confusion - they only represent an instant in time when talking about an AC circuit here. But the concept of how this circuit operates is mostly the same whether AC or DC. Place two batteries in series with loads connected across each battery and the drawing is about the same thing, you will have a + terminal of one battery connected to the - terminal of the other battery, but no short circuit - you would have to run a low resistance (conductor) to opposite end of an individual battery to have a short circuit.

 

[roger]

Really...ground the center and see what happens with the math.

I am just asking someone to show me the phase angle difference math...0 / 180.

There is no 0/180

It would be the same grounded or not grounded, the tap in the center of a winding is all that matters.

or

Roger

 

[mbrooke]

There is no 0/180

It would be the same grounded or not grounded, the tap in the center of a winding is all that matters.

or

Roger

I know I am the nitpicker of the forum- :angel:- however, that graphic is technically not right. The cancellation does not take place in the neutral wire itself, rather the electrons never make the left turn back to the trafo. In essence cancellation is taking place at the "T" point. I know most folks know that, but I just want to clarify for future readers who may be confused.

 

[roger]

I know I am the nitpicker of the forum- :angel:- however, that graphic is technically not right. The cancellation does not take place in the neutral wire itself, rather the electrons never make the left turn back to the trafo. In essence cancellation is taking place at the "T" point. I know most folks know that, but I just want to clarify for future readers who may be confused.

And I agree, it is just providing a visualization for effect.

Roger

 

[SG-1]

Really...ground the center and see what happens with the math.

I am just asking someone to show me the phase angle difference math...0 / 180.

The phase angle math is beyond me.

Think about an ungrounded delta, does corner grounding any one phase create a short circuit ?

 

[jaggedben]

The phase angle math is beyond me.

Think about an ungrounded delta, does corner grounding any one phase create a short circuit ?

Grounding only one node of a circuit does not create a short circuit on any kind of circuit, anywhere, whatsoever. Grounding has nothing to do with phase angles, either.

 

[SG-1]

Why does all the current coming from both phases end up traveling down the neutral and not from phase to phase? Thank you.

As we can now see the current travels both paths, phase to phase & phase to neutral.

Some more sketches to show the current paths a little more clearly.

I did the MESH analysis again using the actual current paths to see if it would yield a different result. It made the algebra longer.

The plus & minus assigned to each component is a required for the MESH analysis. Once the loop direction is chosen then + & - symbols are placed on each component. As you move around the loop the symbol +/- is used to determine if you add or subtract that term in the equation. The plus & minus are only there to help write the equation. If your answer comes up negative, then you know that the loop direction was backwards.

Sorry for the confusion MyCleveland. There are many youtube videos that will walk you through a MESH analysis.