AC cycle how neutral stays neutral

Discussion in 'General Electronics Chat' started by brianl, Nov 3, 2007.

  1. brianl

    Thread Starter Member

    Nov 3, 2007
    11
    0
    Where to start? Right I have always had a problem understanding AC, mainly how the neutral stays neutral if the current is flowing in both directions. So I hope somebody here can steer me in the correct direction of thought.

    This is what I think I understand, in electrical (mains) circuit 50Hz, the phase/live wire has potential 25 times a second (when there is no load/return connected), for the other 25 times it is “looking ?” for potential from the neutral wire.
    Looking at generators the polarity changes on each wire as the magnet rotates within the coil, so one wire becomes + the other – and visa versa, I assume that one of these wires is referenced to earth at the generation station to give the neutral and I suppose this is where my problem originates from (does that + cycle go into the ground?)
    how does the neutral wire stay neutral if the potential changes polarity and current is flowing in both directions.

    I realise this could be the dumbest question ever….so I am ready for a toasting..
     
  2. GS3

    Senior Member

    Sep 21, 2007
    408
    35
    There are two different things which come together.

    A secondary of a transformer generates an alternating voltage and there is no "neutral". Anyone of the two can be neutral or none. It just depends on how you connect it and how you consider it. If you put one side to earth then that would be neutral. So, strictly speaking there is no neutral until you make it a neutral.

    In three phase systems you have three separate phases and if you connect them as a star then the common point becomes "neutral" although it would more correctly be termed "common".

    Neutral is just a reference

    Imagine a hydraulic cylinder which exerts force. It just pulls or pushes, it has no fixed reference. But if one end is fixed to something which would normally be considered the reference for that system, then that end would be the reference and the other end the variable.
     
  3. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    in addition to the above reply
    neutrals are grounded so they remain at that potential.
    additionally for a 3 phase system at balance there is no flow thru the neutral at the load and that at the source.
     
  4. brianl

    Thread Starter Member

    Nov 3, 2007
    11
    0
    Thanks people,
    Let me see if I can express my difficulty in understanding it a bit better, if we take the transformer example given above and reference one side as neutral and I measure the voltage across the terminals it produces a voltage that is referenced to the neutral that I understand. The output of the transformer is AC and I can see that the positive swing is positive with reference to neutral and I can see that the negative swing is more negative with respect to the reference, but what I cant seem to see is what that negative swing actually is, as if one side of the transformer was not referenced that is where the negative swing would manifest itself as a positive going cycle.
    Maybe I think I have a problem with a negative cycle in the sense of where is the work provided from on the more negative cycle.
     
  5. recca02

    Senior Member

    Apr 2, 2007
    1,211
    0
    the term -ve potential etc are a bit confusing since they are arbitrarily chosen.
    if u just imagine a coil rotating in a mag field the change in direction of current is due to change mag field thru it. the potential is so set up that the current caused due this change in mag field opposes this change.
    similar thing happens at the transformer.
     
  6. brianl

    Thread Starter Member

    Nov 3, 2007
    11
    0
    When the mag field collapses it induces a voltage in the opposite direction that set it up, yep I’m ok with that, connect a load to this coil and current will flow forwards and backwards as the coil rotates and produce work on the load, If this coil was producing 100V and I was to connect a meter to this rotating coil it would show a + 100V and then – 100V as the coil rotated. Now as one side of the coil is + the other side is – that’s my potential as referenced to the flow of current, if at the moment 100V + is reached on one side of the coil I reference it to earth at that moment I assume there is no longer potential, until the next rotation of the coil where I then will always have a positive potential across with reference to my now grounded side…..am I way off with this train of thought? Which brings me back to my original problem of where does the potential come from (or go) on the negative cycle……and as I grounded the coil on the + cycle where did that potential go?.

    Sorry if this all seems silly.
     
  7. chesart1

    Senior Member

    Jan 23, 2006
    269
    1
    The formula states that the instantaneous voltage across a coil is equal to the value of the coil in henries times the rate of change of current.

    Lets apply this formula to a simple circuit with a power supply, and a switch and coil. Lets assume that the negative terminal of the power supply is grounded. The positive terminal of the power supply is connected to the switch. The other side of the switch is connected to the coil. The other side of the coil is grounded.

    When the switch is closed, the coil will respond to a rising current with an opposing voltage. That voltage will oppose the voltage at the source meaning that the voltage at the coil will be positive with respect to ground (i.e. you are measuring across the coil).

    Now we open the switch. The coil reacts to a rapidly falling current with a negative voltage. If you measured the voltage across the coil, that voltage would be negative. So for an instant in time the open switch has a positive voltage on one side and a negative voltage on the other side. The voltage at ground is always zero.

    I hope this helps you understand the concept.
     
  8. brianl

    Thread Starter Member

    Nov 3, 2007
    11
    0
    Thanks for the replies, I only started looking at electronics as a hobby a few weeks ago so I have a bit to go before I get my head around it all, I think I will have a look at rectification to see if I can get to grips with the negative part of the cycle on AC as I have a problem seeing what it actually is and where it is, where one part of the circuit is always held at ground.
     
  9. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    "Negative" simply means the current is flowing in the other direction. Current flows out of the neutral for half a cycle, then into the neutral for half a cycle. One direction we arbitrarily call "positive." The other therefore becomes "negative."
     
  10. JohnBoy

    New Member

    Oct 30, 2007
    7
    0
    Brian,

    Check out Volume II - AC - Chapter 10. The whole volume is about AC, but Chapter 10 deals with single-phase and poly-phase ... and it has pictures (schematics) which are worth a thousand words.

    John
     
  11. brianl

    Thread Starter Member

    Nov 3, 2007
    11
    0
    Many thanks…..its finally clicked, bits and pieces from all replies and a bit more reading on the subject, my main mistake was to assume that because one side of a transformer was referenced to ground the potential for work on that side was lost as to when it was not referenced. I was looking at it as a case of half wave rectification with one cycle lost and not the potential reversed on the same line and with a load the current flowing in the opposite direction (as referenced to ground). I now realise that its like the case of conventional flow versus electron flow its just a way of looking at it, the work is still done. So again many thanks.
     
Loading...