Current in single phase neutral (120/240v)

[peetey]

I have a question about the current that the neutral carries under balanced 120v loads. I do realize that the neutral carries no current when there are 240v loads connected. I am pondering the 120v loads when they are exactly the same on L1 and L2. If I have 15 amps on line one and 15 amps on line two we say that the neutral carries zero current. I need to know what is going on when this condition occurs. I am assuming that the opposing currents cancel due to the math. At the molecular level is the currents still flowing but the devices we use to measure unable to discern the flow? Can we possible say that there is no current flowing in the neutral when there is definitely current flowing thru the hot phase? If I kill one of the loads the current magically appears. If the neutral carries zero current at balanced is it "NOT" fair to assume that the current bypasses the neutral altogether and flows through the companion load like what occurs in WYE under balanced three phase loading. I have an idea of the right answer but I need a more learned explanation.

Thanks Peetey

 

[dpc]

Since this is ac current, the phase angle of the currents must also be considered. If the currents are equal in magnitude and in phase angle, there is no current in the neutral.

If there is no current, it doesn't have to bypass anything, because it doesn't exist.

The currents do not "cancel because of the math". There simply IS no current.

Current in a conductor is the jostling of free electrons back and forth. If there's no current, there is not a flow of electrons.

This is an engineering forum, not a physics or philosophy discussion group. If you're not an engineer, better move on down the road.

 

[peetey]

I am a Journeyman Meterman by trade and an apprentice asked me this question and I normally get good responses in this forum. In order to sustain a current flow in the hot conductor there has to be flow in the neutral. If I have a current transformer and place two conductors with the same current and phase angle in an opposing fashion it will yield zero amps, it is not because there is no current flow but is due to the subtractive nature. It is due to the CTs inability to measure this subtractive nature that I think my answer lies. I think like my ct example there is current flow but my tong meter shares the CTs inability to measure this molecular friction. I appreciate any more answers.

peetey

 

[dpc]

If the two hot leg currents are exactly equal in phase and magnitude, there is no current in the neutral to measure no matter what you are trying to measure it with. It the same as three phase currents summing to zero at a wye point.

 

[davidbeach]

What dpc said. There is no current. Neutral current is the vector sum of the two phase currents, if they sum to zero there is zero current in the neutral. If you can exactly balance the two phase currents, you can remove the neutral from the point the two loads connect back to the source, no current, no need for a conductor. Any unbalance though and you'd better have than conductor.

 

[ausphil]

first up, molecular friction, that's up there with the flux capacitor in this example.

as davidbeach says, you can remove the conductor from the circuit, so it simply becomes a simple loop circuit. If you measure the voltage and phase angle of L1, you will find that it is 180degrees out of phase with the L2 current, so in fact, it is the same current in L1 and L2, it is just into the load in L1 say, and out of the load in L2.

if you can grasp the concept of 3 phase summing to zero in the neutral, then something that is 180degrees out of phase should be graspable.

the story would be different if you had an individual neutral for the L1 load, and another one for the L2 load, going back to a common point at the source. This would then still give the same current in the hot legs as the previous example, but the neutral current would be like your ct example if you tried to measure it. There would be current in each neutral, of the same magnitude, but in opposite directions (ie 180degrees out of phase). You would still measure zero current, however you would have current in each individual neutral connection.

essentially in the original example, you have brought the neutral point out to the load, rather than being at the source. But once you unbalance the load, you will have current in the neutral wire.

also, maths does not dictate the physics of it, maths interprets it for us to understand and predict.

the other technicality is that current is defined as the flow of electrons in a conductor, current itself doesn't flow. Otherwise it would be like saying a flow of a flow of electrons (sort of like a PIN number!), but we all get this wrong all the time.

but to quote DPC, this is not a philosophy forum.

ausphil

 

[NAZ55]

peetey,

I am going to try to take a stab at this question.

Current is a flow of charge which is generated by a difference in potential.

considering a +ve rotation.

A phase is at 120<0 from the neutral and hence the current only flows till it gets to the neutral.

The same arguement can be made for B phase with 120<240.

There is no difference in potential along the length of neutral wire itself, except for voltage drop due to the resistance of the wire itself, which can be ignored for the sake of this arguement.

 

[NAZ55]

I am sorry, the vector quantities I used are for a Y configuration, but the same concept of "potential difference" applies for the voltages in question

 

[pwrtran]

peetey

I like to add some comments based on zazmat's reply.

1) in static electrc field or DC, positive electrons flow from the high potential (+) to the low potential (-). If potential between two node is zero, there is no potential difference and thus no electric field to push the positive electrons to flow
2) in alternate field or AC, two things can cuase current flow - potential difference and power angle difference (as known as the torque angle). In the power grid, power can flow from one source that is at a lower potential magnitude but leading power angle to a higher potential source but lagging angle.

In your case, you have just one source, so if the potential magnitude between source neutral and the load common point are equal then there is no current flow.

 

[NAZ55]

scopidia thanks for your response and further clarification of my point. I should have made it clear that I was referring to potential as a vector quantity i.e. magnitude and angle. The rest is just math!

 

[roydm]

Scopidia,
Sorry to nit pick but according to theory the flow of electrons is from - to +, This is proven in an electron tube (valve)
I know we all think of it going the other way.
Roy

 

[jghrist]

I like ausphil's explanation on the individual loads L1 and L2, but I would add one thing. With the 120 volt L1 and L2 loads equal, you could keep the two neutrals connected, but remove them from the source. The current going in the L1 neutral flows out the L2 neutral. It would be the same as having the L1 and L2 loads in series. Each would have a 120 volt drop, with the voltage across the combined load of 240 volts.

 

[pwrtran]

RoyDMatson

You are correct in the real world. In theory the direction of current is the moving direction of "Positive Electrons" going from + to -, that is an imaginary reference direction and is used in all textbooks.

 

[waross]

Hi peetey;
You need a difference of potential to cause a current to flow.
Connect two 6 volt batteries in series for 12 volts. Connect two 10 ohm resistors in series. Connect the resistors to the batteries.
Now measure the voltage from one end of the batteries to the center connection. 6 volts. Now measure the voltage from the end of the batteries to the center of the resistors. 6 volts.
Measure from the center of the batteries to the center of the resistors. Zero volts. With zero volts, there will be no current flow to measure. Of course with no neutral conductor there will be no current flow, but even if you connect together two points with equal potential, (no potential difference between them), there is no voltage to cause a current flow.
That's DC. Now I am waiting to hear from you when your apprentice gets on an AC circuit with equal loads of different power factors. He may see something like; L1=9 A, L2=7 A, and neutral =5 A. Vector sum, not simple sum.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[UtilityDave]

I like waross' explanation. I'll add;

The load on each of the two leads in 120v rms with respect to the neutral. It is 240 v rms across both together. So when L1 is high, L2 must be low. So the current will flow from L1 to L2. The load on L1 sees it as the positive 1/2 cycle L2 sees it as the negative 1/2 cycle.

When L1 is high it is on the positive 1/2 cycle and current flows from L1 to the neutral point. L2 is low and so current flows from the neutral point to L2.

This is 180 degrees out of phase.

I can see why it is tempting to imagine that these currents "cancel" and we've heard that expression used many times. The math can indeed be worked to make it appear to cancel but nature and physics are too efficient to go through all they work. The current in the neutral just doesn't exist, for the reason waross explains.