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Alternating current and magnets question

  1. Jun 3, 2010 #1
    Hi everyone, ive been studying magnetic fields and alternating current generators and i have a question i need help answering. I have posted a few other questions in which i posted my generator design with magnet placement and plotted the sine waves on a chart but i have a question about the direction and length of current flow created by the magnets. i have drawn a diagram of the coils and magnets to try to understand whats happening in the conductor at a given point in time. I have read that an approaching north pole magnet creates a counter-clockwise current and an approaching south pole magnet creates a clockwise current. I also understand the alternating current moves from point A to point B and back in a determined amount of time. In the diagram below, which direction of flow is correct? I have read info on the internet that i dont know is correct so ill post it here also to show why im a little confused. here is what i read and the diagram of the north pole magnet with single coil applies to this quote :

    " I intuitively felt there was a problem. And this is where I had to do some learning about magnets and coils. And what I found out was that when the magnet passes over the coil, as it first approaches the coil the current in the coil begins to flow in one particular direction. As the magnet begins to pass over the coil, half of the coil's current is trying to move in one direction and in the other half of the coil, current is trying to move in the other direction. Nothings going no ware. Now as the magnet exist the coil, current begins to move in the opposite direction of when the magnet approached the coil.This means you only have current flow, and only a little at that, when the magnet is entering or exiting the coil. As it passes directly over the coil, when max output would be expected, close to zero output would be realized. "

    can someone try to explain this or point me in the right direction?

    Attached Files:

  2. jcsd
  3. Jun 3, 2010 #2


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    Your bottom diagram in the first picture seems OK, but the middle one shows the current going downwards regardless of magnet polarity. This can't be true.

    The second picture shows both sides of the loop passing over a magnetic pole and this would result in no output, as you show.

    Just use the bottom drawing in the first picture and see that the overall polarity will be reversed as the relative position of the coils and magnets reverses.

    You can check these with Fleming's Right Hand Rule.
  4. Jun 3, 2010 #3
    thanks for the reply vk6kro, i thought that might be the correct answer.
  5. Jun 4, 2010 #4


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    Don't forget that the field is not between magnets that are next to each other. It is between magnets that are on opposite sides of the path of the coil. Your diagram shows the bottom magnets of a matching upper set.
    Like this:
    [PLAIN]http://dl.dropbox.com/u/4222062/generator.PNG [Broken]
    Last edited by a moderator: May 4, 2017
  6. Jun 4, 2010 #5
    Thats a great illustration Vk6kro, my illustration skills just arent that good. I have to visualize the interactions to understand them. I understand what you illustrated in a 3-dimensional kinda way, i can only draw in a 2 dimension way. Thank you for taking the time to illustrate this the way you did to make sure i understood what was happening, I really do appreciate it.
  7. Oct 17, 2010 #6
    Hello everyone, i am back to this question because i cant grasp how to correctly place the alternating poles of the magnets in an a.c generator. In a three phase alternator theres a north pole placed over one side of the phase 2 and 3 coils while phase one is at 0 volts, then theres a south pole over the other side of the phase 2 and 3 coils while phase 1 is at o volts. Heres where im confused, Lens Law states that an approaching north pole magnet will induce an (anti) counter-clockwise current flow and an approaching south pole magnet will induce a clockwise current flow, im posting a couple diagrams to show why im confused. In diagram #1 : I have placed a north pole magnet over each side of the circular loop of wire and based on Lens Law i get a counter-clock wise current flow through the complete loop but i have read everywhere on the internet and here that you have to have a north pole on one side of the loop and a south pole on the other side of the loop like i have in diagram #2, based on Lenz Laws there would be no current flow with a north pole on one side and a south pole on the other side, the currents would cancel each other out wouldnt they? I believe diagram #1 is the correct way to make current flow, have a north pole over each side of the coil or loop of wire causing a counter-clock wise current flow, then have the 2 south pole magnets rotated over the coil or loop of the wire causing a clockwise current flow creating the alternating current flow, am i right? can somebody help me understand this ?
    Last edited: Oct 18, 2010
  8. Oct 17, 2010 #7


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    That was just about unreadable. Please put a blank line in where you start a new idea.

    The second diagram would illustrate the situation, but the current over the North pole is the wrong way around.

    You would not have a loop like that either, as it would be a short circuit. You would take power out by breaking the loop at the top or bottom.

    Try just following the right hand rule shown above, and the 3D drawing.
  9. Oct 17, 2010 #8
    Thanks for the answer vk6kro, the closed loop is just an example to show that the magnets are positioned over each side of a multi-loop coil wound through a stator like in an automobile alternator. Could you elaborate why the current in the second diagram is flowing the wrong way? Lenz Law states that an approaching north pole magnet induces a counter-clockwise current, which is why i drew the arrow in that direction, could you please explain ?
  10. Oct 17, 2010 #9


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    If you approach a shorted coil of wire with a north pole, the coil of wire will generate a current that opposes the incoming pole by creating a new magnet with a north pole facing the incoming one.

    But that is not happening here. The wire is passing across the north pole, not approaching it, and the whole coil is not approacing the North pole either.

    This is quite a different situation and you need to use Fleming's Right hand rule for conductors cutting a magnetic field.
  11. Oct 18, 2010 #10
    The generator im trying to build is a permanent magnet rotor alternator. I think ive complicated getting the explanation i seek by leaving out bits and pieces of information. The following is a diagram of the set up im trying to use. In my diagram:

    the north pole magnet is over side #1 of coil #1 of phase 2:
    the south pole magnet is over side #2 of coil #1 of phase 2:

    If you were looking from the perspective of the north and south pole magnets, the coil would be like a clock face with the north pole being at 9 oclock and the south pole being at 3 oclock.

    Based on Lenz Law:

    The north pole magnet over side #1 of coil #1 will induce a counter-clockwise current (into the page). Using the clockface analogy, the north pole would be at 9 oclock and current would be flowing counterclock-wise ( from 9 to 8 to 7 to 6).

    The south pole magnet over side #2 of coil #1 will induce a clockwise current (into the page) according to Lens Law. Again using the clockface analogy, the south pole would be at 3 oclock and the current would be flowing clockwise (from 3 to 4 to 5).

    It seems to me that with a north pole covering one side of the coil and a south pole covering the other side of the coil that the currents would cancel each other out and no current would flow?

    On the other hand, it seems that if each side of the coil were covered by a north pole magnet and each magnet inducing a counter-clockwise flow, the current in the coil would flow through each loop of the coil in a couter-clockwise direction through the entire coil until the 2 south pole magnets rotated into position over the 2 sides of the coil and reversed the current flow from counter-clockwise to clockwise through the entire coil creating alternating current.

    If im wrong in what i have just stated , can anyone please elaborate in a little detail why im wrong?

    Attached Files:

  12. Oct 18, 2010 #11
    Does anyone have any insight which answer is right?
  13. Oct 18, 2010 #12


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    You are trying to apply Lenz's Law where it doesn't apply.

    [PLAIN]http://dl.dropbox.com/u/4222062/generation%20of%20current.PNG [Broken]

    In the left drawing, the coil is going to pass down the sides of the magnet so the opposite sides of the coil cut magnetic fields in opposite directions, so they have opposite currents induced in them, producing the current as shown.

    In the right side diagram, the single wire is passing across the face of the magnet pole so it has a current induced as shown. This is what happens in a generator.

    Fleming's Right hand rule applies in each case, it is just that the magnetic fields are being cut in different directions.
    Last edited by a moderator: May 5, 2017
  14. Oct 18, 2010 #13
    Thanks for your patients vk6kro, im trying to fully understand and comprehendall the information im getting here.

    Using your second diagram, what if there were 2 magnets with their north poles facing a small coil of wire instead of a single straight wire, would the results be the same? a counter-clock-wise current flow as you show with the single wire in your diagram. Im trying to understand the magnet placement and position inside a permanent magnet automobile alternator, Lenz Law would have to apply to it the same as a axial flux alternator wouldnt it?
  15. Oct 18, 2010 #14


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    No, you would have to have a South pole facing the other side of the coil of wire if it was passing in the same direction as the vertical wire shown.

    This way, you would get a rotating current in the coil.

    Even in the second diagram, Lenz's Law applies. The generated current is as shown, but if you have a current like that travelling in a wire in that magnetic field, it would experience a force to the right, opposing the motion.

    However, it is more useful to concentrate on Fleming's Right hand rule. This tells you anything you need to know.
    Last edited: Oct 18, 2010
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