Basic AC Neutral Question

Discussion in 'General Electronics Chat' started by zachl1220, May 23, 2016.

  1. zachl1220

    Thread Starter New Member

    May 20, 2016
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    Hi Guys,

    This is quite a basic question regarding the neutral in an AC circuit, but for some reason it is still giving me a bit of confusion. In a simple AC circuit (not considering poly-phase neutral lines which are considerably different), the neutral is connected to Earth to maintain a potential of ~0V. Thus, the live alternates between a positive and negative voltage, supporting current flow throughout the circuit. What confuses me is that I naturally begin to think that tying the neutral to 0V instead of allowing it to alternate would dissipate some amount of energy and thus make the circuit less efficient than a circuit where the return is not bound to Earth. I'm sure this is a common misconception, but I haven't found any explanations that give enough detail to clarify this.

    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
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    Are you in 'Murica?
    If you are, the neutral is the center tap of the transformer on the pole. We can't let the neutral wiggle or the other half of the 240 VAC wouldn't like it.
     
  3. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,874
    369
    Have a look at the TT network diagram here: http://www.electrical-installation.org/enwiki/Characteristics_of_TT,_TN_and_IT_systems

    Current to the load flows through the live and neutral wires. In normal conditions (no faults in the system) no current flows in the earth to earth connection shown dotted in the diagram. Therefore the earth connection to the neutral makes no difference to any current flowing in the system and hence no change to power dissipation.
    HTH.
     
  4. zachl1220

    Thread Starter New Member

    May 20, 2016
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    I'm not in 'Murica at the moment, no haha. From a split-phase perspective, unless I am wrong, the voltage would remain near 0V even if you didn't physically connect it to earth, no? This is because a split phase system can (sort of) be modelled as two separate transformers that are in series, so the point between them is 180 degrees out of phase, so current and voltage are "cancelled" out. I may be wrong though.

    In a simple AC circuit with a single loop and a load, though, I see it differently in my head. If I instead think of it as two DC circuits with reverse polarities, I would have situations where either side of the load is at 0V and the other at "live" voltage levels. If I were to ground the "live" side of one of these circuits, then it would seem to me that I have lost my potential difference. I think this is where my understanding fails, though. I suspect what I am not getting is that even in that situation, the DC source would create a "negative" voltage to generate the necessary potential difference, but I'm still not 100% there yet.
     
  5. #12

    Expert

    Nov 30, 2010
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    Let's try going at it like this: Common is anyplace you decide it should be. The planet isn't providing any current as long as only one point of your power supply is grounded. At a certain point in time, the AC from a center tapped transformer can be modeled as two opposing voltages and a center, just like your DC example. If you ground the bottom of the supply, you have 120 volts at the center and 240 volts at the other end of the power supply. If you ground the center, you have two equal and opposing voltages. If you ground the top, you have negative 120 volts and negative 240 volts. It's just a matter of how you choose your Earth point. In the end, all the Earth point does is keep static electricity from building up on the wires. There is no load current in the ground wire and the Planet does not contribute either voltage or current.

    Part of the problem is that Neutral is a very specific point in a power distribution system. When you say, "Neutral" I know exactly where it is. It's the center of the power transformer secondary and it's also connected to the Planet. If you say, "common" that is a point you decided on and you have to explain where it is.
     
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  6. MaxHeadRoom

    Expert

    Jul 18, 2013
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    If not in N.A. you may be in a country where the neutral is already 'decided' namely the star point of a 3 phase transformer such as many parts of Europe & Australasia.
    Earthing the neutral does not change anything as far as potential difference is concerned.
    Only a situation where the ground itself was used as the return path, which is now only done on occasion with low voltage systems.
    Ground Practices
    Max.
     
  7. crutschow

    Expert

    Mar 14, 2008
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    The simple answer is follow the current.
    In a normal operating system there is no significant AC current flowing from the neutral to (earth) ground.
    And if there is no current then, since power equals voltage times current, there can be no power (lost).
     
  8. Stuntman

    Active Member

    Mar 28, 2011
    181
    47
    You are technically correct. By tying the center tap to ground you would theoretically be turning the earth into a plate of a physically massive, but practically insignificant capacitor; The hot wires being one plate, the ground being another, and the air being the dielectric. I say technically as the extra power lost in charging/discharging this capacitor would, in my mind, be orders and orders of magnitude less than the regular use resistive losses of your wires.

    Question: What if you were to hook the center tap of the transformer to a 1000V AC (WRT earth ground) signal? Would you blow the transformer creating a wormhole through time and space, or would you be able to run a 240V motor off of the two hot wires just as before??


    #12, I'm curious what you mean by this.
     
  9. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I'm north of the border and I got it!.
    Max.
     
  10. #12

    Expert

    Nov 30, 2010
    16,247
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    It's a probe to find several qualities of the Thread Starter I'm dealing with.
    It tests for ESL, sense of humor, tolerance for slang, mental agility (ability to visualize a scenario not previously defined) and the opposite of mental agility, like OCD or Asperger's Syndrome, or being way back in the early beginner stages of learning.

    If the next post comes back as, "Duh, whut?" I have to use a different approach.
    If the next post comes back as, "LOL". I know I'm working with a versatile and articulate person.
     
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