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

[LarryFine]

The debate seems to be whether a 120/240v supply is one 240v supply with a center tap or two 120v supplies that share a conductor.

I say the answer is that it's both, simultaneously. To a line-to-line load, it's the former; to a pair of line-to-neutral loads, it's the latter.

You guys are arguing over a distinction without a difference. It's both at the same time. I'm glad we're not arguing a 3-ph wye system.*

Did you guys read what I said in post #28? You could supply a 397v load line-to-line at the same time if the lines were at 180 degrees.


*Just to mention it now, it's the exact same thing, only with three lines and different timing. The timing is why the L-L voltage is lower.

 

[jselesk2]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

Yes. This is the question I would like to have answered. But thank you all for the informative posts.

 

[Besoeker]

Roger's diagrams, Larry's written explanation, and the video Jamesco posted all do a good job of telling us what the neutral does but none explain why. How do the electrons on an unbalanced MWBC coming from phase A know they are supposed to turn right and head down the neutral path, and the electrons from phase B know they are supposed to turn left and head down the same neutral? Why don't they go wherever they want on whatever copper wire they like the best?

For AC, they don't actually go anywhere. They wiggle back an forth a little bit. A tiny little bit. Not this answers the OP's question.

 

[Besoeker]

The debate seems to be whether a 120/240v supply is one 240v supply with a center tap or two 120v supplies that share a conductor.

I say the answer is that it's both, simultaneously. To a line-to-line load, it's the former; to a pair of line-to-neutral loads, it's the latter.

You guys are arguing over a distinction without a difference. It's both at the same time. I'm glad we're not arguing a 3-ph wye system.*

Did you guys read what I said in post #28? You could supply a 397v load line-to-line at the same time if the lines were at 180 degrees.


*Just to mention it now, it's the exact same thing, only with three lines and different timing. The timing is why the L-L voltage is lower.

Isn't it usually derived from a centre-tapped 240V winding giving 120-0-120 ?
I'd have called that a single supply in the same way I'd consider three-phase star (wye) as one supply, one source.

Not that that I think it matters all that much as long as we can clearly communicate what system we are discussing.

 

[JFletcher]

Yes. This is the question I would like to have answered. But thank you all for the informative posts.

That question has been answered. Without going into another 40-page techno Fest, they go down the neutral cuz it's the path of least resistance. That answer may be technically inaccurate, however it is basically true.

 

[gar]

180411-0800 EDT

jselesk2:

View your circuit with the AC voltage sources replaced by batteries (DC).

Current flow in a conductor is defined by the voltage between two points on that conductor divided by the conductor impedance (resistance for DC) between the two points. Does not matter what the circuit is outside of that conductor other than as it determines the voltage. Electrons only flow in one direction in said conductor at an instant of time.

There are not two or more flows of electrons in said conductor with some adding and others subtracting. There is just one flow and direction.

In an AC circuit at an instant of time the same thing happens as in a DC circuit except that we have a complex type of impedance and what the current does may not be in phase with the voltage.

.

 

[jaggedben]

Calling it two sources is the more helpful way to answer the OP's question, in my opinion. Load current goes back to the source it's connected to. Currents on the neutral happen to cancel out. The two sources are also placed in series and can be used in series.

In the case of the battery example, the sources are actually two separate pieces that can operate on their own. In the case of AC current, it is typically a center tapped transformer winding derived from a single primary source, I can understand why some might blanch at calling this two sources. But it could also actually be two separate sources capable of being reconfigured independently. For example, it could be two inverters with electronically synced waveforms (such as the Outback Radian Series). The behavior is essentially the same and the task for the electrician wiring the MWBC is exactly the same.

 

[Besoeker]

Calling it two sources is the more helpful way to answer the OP's question, in my opinion. Load current goes back to the source it's connected to. Currents on the neutral happen to cancel out. The two sources are also placed in series and can be used in series.

Well, the two "sources" are not really "placed" in series. They are physically connected in series, usually on the same transformer as far as I am aware. You can't NOT have them in series if you want 120-0-120 which is the system under discussion.
One source. That's my take FWIW

To directly answer the OP's question the neutral doesn't prevent a MWBC operating at 240V as well as 120-0-120.

 

[Jamesco]

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/single-phase-power-systems/#02168.png

Note the polarity markings on each side of the center tap split phase winding. + & -. (+ & - at neutral connection of the two 120V connected loads.)

Note the polarity markings on the two outer most leads of the 240V winding. One end + and the other end -.

I think the answer to the question has to be in there somewhere.

In a circuit the current moves from the source through the circuit from negative to positive back to the source.

If both 120V loads that are in series are equal then the current doesn't see the null + & - neutral connection and only sees the opposite polarity of the 240V winding.

If the load is greater on one of the two 120V series loads that imbalance of current will travel back on the neutral because its' polarity is greater than null equal + & - polarity.

Well???

 

[GoldDigger]

Yes. This is the question I would like to have answered. But thank you all for the informative posts.

The electrons go in the direction dictated by the electric field, which in turn depends on the voltage at the other end of the wire in question.
And the answer to what happens when you have equal but opposite currents flowing in the same wire is that there is no net electron movement in either direction, and it is not really important what individual electrons do.

Sent from my XT1585 using Tapatalk

 

[mbrooke]

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-10/single-phase-power-systems/#02168.png

Note the polarity markings on each side of the center tap split phase winding. + & -. (+ & - at neutral connection of the two 120V connected loads.)

Note the polarity markings on the two outer most leads of the 240V winding. One end + and the other end -.

I think the answer to the question has to be in there somewhere.

In a circuit the current moves from the source through the circuit from negative to positive back to the source.

If both 120V loads that are in series are equal then the current doesn't see the null + & - neutral connection and only sees the opposite polarity of the 240V winding.

If the load is greater on one of the two 120V series loads that imbalance of current will travel back on the neutral because its' polarity is greater than null equal + & - polarity.

Well???

Not to muddy that water, but this has me confused:

Wouldn't the bottom sine wave source be plus on the top minus on the bottom? As it is pictured would be 0 volts instead instead of 240.

 

[Besoeker]

Not to muddy that water, but this has me confused:

Wouldn't the bottom sine wave source be plus on the top minus on the bottom? As it is pictured would be 0 volts instead instead of 240.

Plus and minus doesn't really apply to AC.

 

[jaggedben]

Well, the two "sources" are not really "placed" in series. They are physically connected in series, ...

That is a distinction without a difference.

...usually on the same transformer as far as I am aware. You can't NOT have them in series if you want 120-0-120 which is the system under discussion.

The two halves of the winding are not in series with each other? Most certainly they are.

 

[mbrooke]

Plus and minus doesn't really apply to AC.

Over time- however at any one point in time there is polarity. This matters when hooking up/paralleling transformer windings, for example.

 

[jaggedben]

Not to muddy that water, but this has me confused:

Wouldn't the bottom sine wave source be plus on the top minus on the bottom? As it is pictured would be 0 volts instead instead of 240.

I agree it is confusing but don't overlook the phase angle being 180, which means at any given moment the polarity is opposite from the source on top. I prefer to think of the bottom source as you say, with plus on top and minus on bottom, but without the difference in phase angle. But mathematically, in the case of a split phase system it can be shown either as polarities in series and the same phase angle, or polarities opposite and phase angles 180deg apart. The graph of the negative sine 180 degrees apart just happens to be the same as the positive sin. sin=-sin(n+180)

(In a three phase system you don't have the same mathematical ambiguity.)

 

[mbrooke]

I agree it is confusing but don't overlook the phase angle being 180, which means at any given moment the polarity is opposite from the source on top. I prefer to think of the bottom source as you say, with plus on top and minus on bottom, but without the difference in phase angle. But mathematically, in the case of a split phase system it can be shown either as polarities in series and the same phase angle, or polarities opposite and phase angles 180deg apart. The graph of the negative sine 180 degrees apart just happens to be the same as the positive sin. sin=-sin(n+180)

(In a three phase system you don't have the same mathematical ambiguity.)

Makes sense- so in other words if it was series polarity (+-,+-) and pictured with 0 and 180 phase angles as like shown now, it would be zero volts?

Yahhh- confusing at first glance

 

[jaggedben]

...

Did you guys read what I said in post #28? You could supply a 397v load line-to-line at the same time if the lines were at 180 degrees.
...

180 degrees at opposing nominal polarities, or 0 degrees if nominal polarities are in series. Same thing, see last post.

 

[mbrooke]

180 degrees at opposing nominal polarities, or 0 degrees if nominal polarities are in series. Same thing, see last post.

Noted :thumbsup:


Good thing I asked- would have failed a test of it was like that.

 

[Besoeker]

The two halves of the winding are not in series with each other? Most certainly they are.

That's what I said.

 

[SG-1]

180411-0800 EDT

jselesk2:


There are not two or more flows of electrons in said conductor with some adding and others subtracting. There is just one flow and direction.

Thank you, I now know that when people say "the currents canceled each other out", I know they really meant to say " the currents canceled each other out mathematically "