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Does a potential coil carry current?

  1. Jan 16, 2010 #1

    foo

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    I have seen the potential coil go across two phases to measure voltage. But there is no neutral attached to the potential coil.
    If the two lines were 120v ea. the potential coil will have 240v of potential
    going across it.

    How is it that this coil can measure the voltage in both phases/lines without a return path? Like a complete circuit that needs a way back to the source. Or is it that the two phases of the lines are 180 degrees apart and it will use the -180 as a return?

    Or am I confusing that potential coils don't need to return any current? I'm not sure if current has to travel along potential coils. Wouldn't a tiny amount of current be necessary?

    I see phase to phase transformers and it makes me wonder how a transformer can have a phase connected to each side of the primary without the need of a neutral. Is this also because of the 180 degree phase shift?

    Thanks in advance for any explanations.
     
  2. jcsd
  3. Jan 16, 2010 #2

    vk6kro

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    The North American power system is similar to this diagram:
    120 volt power.PNG

    At the right you can see the transformer in your street.
    The secondary on this transformer is 240 volts with a center tap on it which is called a neutral.
    Either side of the neutral is 120 volts and this is what is used for house appliances etc.

    The 240 volts is very real though, and you could measure it with a voltmeter.

    So, you could run 240 volt equipment off it without even knowing the neutral was there.

    A potential coil for a power meter does draw a small current to operate. Ideally, any voltmeter should not draw current, but they all have to draw some current to work.
     
  4. Jan 16, 2010 #3

    foo

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    Thank you so much for your explanation. This would then mean that without a neutral, the two hot legs act as the complete circuit right? I mean, current is going to go in via the -180 degree leg, then come out of the load via the the other leg, the +180(or is it 0 degree) degree leg?

    See, it confuses me that this potential coil in the power meter can measure 240v since it only has two phases that it is connected to. I am trying to picture that everything must have a wire from the source of the power and a return to that source of power to complete a circuit. Being that these two phases are coming from a source and there's this little potential coil between them, I don't see where the completed path is. Unless, I'm guessing, is that one phase is going one way, and the other phase is going in the opposite direction. And that this is what completes the path for current to travel across the potential coil. The two hots are acting like a positive and negative where current will travel from one hot leg to the other. Then this switches in both phases at 60hz so that it still remains true in the opposite direction.
    Is that anywhere near reasonable?
     
  5. Jan 16, 2010 #4

    vk6kro

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    Yes, I think you have it right.

    The neutral doesn't matter if you are just connecting to opposite ends of a 240 volt winding. This is just a large coil of wire on a laminated iron core, so there is a return path through this.

    Relative to one end, the other end is swinging to +340 volts and -340 volts peak in a sinewave fashion. The RMS value of this sinewave is 240 volts (meaning it has the same heating power as 240 volts DC).

    So, anything placed across this voltage will get 240 volts on it.
     
  6. Jan 16, 2010 #5

    foo

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    Ok, then considering if a neutral was put in place. How does the power then travel?

    When it's said that the neutral carries the difference. The difference of amps in the hot legs is flowing through the neutral.
    What does this mean? So when the current is trying to use the other hot leg as a return, there is more coming back out than was put in and this excess is sent out via the neutral?
    I think I need a mapped directional of this flow between the split phase and neutral. splitphase.gif
    I'm not sure what is going to happen with the current here. I want to guess that the as power flows past X3 that it will go down the neutral. But then if that's the case, that means that as power wants to come back from X1 it will come up the neutral. This would mean there's a conflict and I am thinking this isn't right because both phases should be moving in the same direction as we've discussed; one will act as a positive leg and the other the negative(return). But if they are both traveling in the neutral in opposite directions, how can this be so?
     
  7. Jan 16, 2010 #6

    vk6kro

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    Ignore the earth connection as it doesn't affect the result.

    R5 will have 240 volts across it so it will have a current of 240 / R5.

    R1 // R2 will have 120 volts across them and there will be a current through the winding near the point marked X1 via the neutral wire of 120 / (R1 // R2).
    (R1 // R2 is the parallel result of R1 and R2)

    R3 // R4 will also have 120 volts across them and there will be a current through the LH winding near X3 via the neutral wire.

    Now, these currents in the neutral wire are out of phase with each other and will partially or completely cancel each other.

    The neutral wire provides an important stabilizing effect.
    In an extreme example, if the neutral wasn't there, you could have a room heater effectively in series with an electric toothbrush across 240 volts. This would be bad news for the electric toothbrush as it would get most of the 240 volts instead of the 120 volts it was designed for.
     
  8. Jan 17, 2010 #7

    foo

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    Ok, so the 120v part of this circuit is going to look somewhat like this right? The neutral is going to carry both opposing phases along it. Does this mean the neutral should always be thicker at this point?

    splitphase2.gif
     
  9. Jan 17, 2010 #8

    vk6kro

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    No, a wire can't really carry currents in two directions at the same time.

    What you get is the resultant of the two currents which could even be zero if the loads were balanced.
     
  10. Jan 17, 2010 #9

    foo

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    So then the left half should be reversed in flow?

    splitphase2rev.gif

    That would then make the neutral have flow in one direction since it can't hold opposite directional flows.
    But if so, what happens when the two flows hit R5 then?
     
  11. Jan 17, 2010 #10

    Averagesupernova

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    Your drawing in post #7 was correct concerning current direction. The neutral is usually smaller because it has to carry less current.
     
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