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Eddy currents, and induced current, can they both co-exist?

  1. Jul 23, 2015 #1
    Im a bit confused about one point.
    Can Eddy current, and induced current exist at the same time...? If a conductor is a part of a closed circuit, and there is a change in magnetic flux over time, both Eddy current and induced current would oppose that change?

    Let's assume a conductor that is a part of a circuit passes a magnetic field( a motor's armature), naturally, the conductor will slow down, is it due to the Eddy currents on the surface creating some form of magnetic braking? Or is it the induced-EMF that opposes the applied voltage causing the motion(and ultimately the change in flux)?

    Not sure what and when causes the opposition of change to the magnetic flux.
     
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  3. Jul 23, 2015 #2

    Philip Wood

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    Yes and yes, but the order of magnitude of the effect due to eddy currents in the conductor is likely to be much less than those due to the induced current in a closed circuit of (say) a few cm2 in area
    What do you mean by 'the applied voltage'?
    My recommendation would be to study a simple set-up, such as a closed conducting circuit with a uniform magnetic field from an external source at right angles to its plane. One side of the circuit is moveable, and, as it moves, cuts flux. Then you can study the induced emf, the current, the flux the circuit sets up, opposing the change in external flux, the Motor Effect force the moveable side experiences, opposing its motion and so on.
    .
     
  4. Jul 23, 2015 #3
    By applied voltage I mean the one supplied to the conductor making it move(like a motor's armature for example, the power supply supplies the applied voltage), and the induced-EMF act's as back EMF.

    Why would the Eddy currents be much less in effect Vs. Induced current(or induced back-EMF in relevance to a motor)?
    Also, does that mean that a motor does/ does-not experience a form of magnetic drag force? Only induced EMF that reduces the applied voltage? Because that is still valid with respect to Faraday & Lenz's laws.

    I've studied simple set-up, figured out a lot of important aspects, but didn't understand Eddy currents magnitude(yet, still studying the calculations) or role to the system, would it exist at the same time as the induced-EMF / induced-current(assuming no applied current i.e generator as an example), would this system(like a motor/generator) experience a form of magnetic-drag force...?

    Well, in the case of a generator it does, the induced current creates an opposing magnetic field, the resists the change.
    While in a motor, is there a magnetic-drag force from Eddy currents? I only know that it would decrease in torque(Lorentz force) due to the applied voltage& applied current from the power supply decreasing from the induced-EMF(or induced-motional EMF in that case).
     
  5. Jul 23, 2015 #4

    Philip Wood

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    I've now understood the context of your question better. Hadn't realised you were specifically concerned with electric motors. As you know, eddy currents are currents that 'find their own paths' through lumps of conducting material. In motors, transformers and so on, coils are wound on soft iron to increase the magnetic flux and to guide it round 'magnetic circuits'. The iron is laminated (cut into thin, insulated slices) to try and minimise eddy currents. An electrical engineer might be able to tell you more about the magnitude of effects due to eddy currents in a motor.
     
  6. Aug 2, 2015 #5
    @Philip Wood I figured out my confusion, and I will explain using some diagrams.
    Here is a proper set-up I'd like to reference:
    VzJUP.png
    A diagram that shows the Lorent'z force at play, the basic principle of an electric motor.
    Now, think of the conductor(copper) being of reasonable volume. Not narrow, where Eddy current's can be very small in magnitude(or negligible).

    That conductor, already has current flowing from a power source when it's placed in a magnetic field. When it being's to accelerate the only form of opposition here(acting on the conductor) to resist the change(motion ##\Delta A##) is induced EMF(i.e back EMF), that ultimately reduces the applied voltage and then the applied current. Let's now increase the number of conductors and assume a higher velocity(intial velocity ##V_i## ##\neq 0##) passing the magnetic field, the only change here... is a higher back EMF, I don't think there could be any induced Eddy current's in this system due to the applied current already existent. I believe in motor's the core's(of ferromagnetic material) would have such Eddy losses & Hysteresis.

    However, if we consider these diagrams:
    L0NESvC.png C68KYSM.png
    Things are different, the conductor does not have an applied current from a power source, and there is an induced EMF here that induces the Eddy's due to the change in flux.

    Sorry for not clearing my point earlier, as I was confused in explaining it, but this what I mean, is my analysis/assumptions correct?
     
  7. Aug 2, 2015 #6

    Philip Wood

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    A couple of remarks that may or may not be of use to you…

    Your lower two diagrams are classic cases where the effects of eddy currents are observed. It's important to note that the magnetic field through which the conductor is moving is non-uniform: only a portion of the conductor is between the poles of the magnet. If the conductor were moving through a uniform field, and completely contained in that field, there'd be no emf around any closed path, and no eddy currents.
    I don't think that a current already present would prevent eddy currents occurring - if they'd occur without that current. What you'd get would be a vector addition at each point of individual current densities due to the circuit current and the eddy current.

    I'm fairly confident that in 'ordinary' circuits, made of copper wire, eddy currents within the wire (as opposed to circuit currents along the wire) are usually negligible in magnitude and effects.
     
    Last edited: Aug 2, 2015
  8. Aug 3, 2015 #7
    Wait, I didn't know that... the uniformity of the magnetic field had a role... so If we used an electromagnet to create a uniform field, and only a portion of the conductor moves in it, the conductor enters/exists the a uniform magnetic field, the only form of opposition to the change is motional EMF? If connected to a circuit, induced-current that opposes it.



    Well, that was my intuitive assumption, that there is already current flowing within the conductor the only form of opposition that makes sense would be induced back EMF that opposes the applied EMF and reduces the current flow. But, that is an interesting and complex process to consider( vector addition).

    Hmm, lets use the 1st diagram I've attached, assuming it's large conductor(in volume) the Eddy's are still negligible?


    It was , thanks for the informative reply!
     
  9. Aug 3, 2015 #8
    Eddy currents are induced currents--currents induced by a changing magnetic field, so you presented something of a false dichotomy.
     
  10. Aug 3, 2015 #9
    That is true, however, look at the diagrams and the cases... it would make sense more as to the point I tried to get across.
     
  11. Aug 6, 2015 #10

    Philip Wood

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    Probably not. My first step in assessing whether or not they are negligible might be to imagine the loop (contained in the material) through which the rate of change of flux is largest or somewhere near largest. [That's why the flux density needs to be non-uniform: if it isn't, the flux through the loop will stay the same as the loop moves through the field.] Then we can estimate the emf in the loop. There's no guarantee that current will follow exactly the path of this chosen loop, but if, wildly, we supposed the current density vector, j, to be always tangential to the loop and the same in magnitude (j) all the way round, then [itex]j=\frac{loop \ emf \times conductivity}{loop\ perimeter}.[/itex]
    This might give an approximate upper limit on the eddy current density.
    Thanks for making me think about this. Someone else may have a more sophisticated approach.
     
    Last edited: Aug 6, 2015
  12. Aug 15, 2015 #11
    Glade to spark some thought there @Philip Wood , but I find it a bit confusing... see Eddy currents usually occurs in systems where there is a conductive slab like so:
    2000px-Eddy_currents_due_to_magnet.svg.png
    Which prior to the induced current there must have been some form of induced EMF that causes such circulation. But if we look in the case similar to a generator like so:

    current-generator.gif
    Assume those wires(where current is induced) is of greater volume, it's connected to a load or what not... I think the current flows in that direction as diagramed and because of that current flow it induces a magnetic field opposing the exterior, and creating a Lorentz force that opposes it. In such applications/configurations I doubt the Eddy currents are of great impact. You see where I'm going with this? Eddy currents is best demonstrated at it's highest effect with a large conductive slab(not connected to an exterior circuit). Generators/motors have losses due to Eddy currents relative to the ferromagnetic cores not the copper wires that move at high velocities.
     
  13. Aug 15, 2015 #12

    Philip Wood

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    Yes, that's what I think, too. See post 6.
     
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