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Strong electromagnet with a constant field

  1. Mar 22, 2009 #1
    Hi, so i had an assumption and wanted to see if my assumption was correct so here goes.

    If we have a rather strong electromagnet with a constant field, when a metal object is introduced to the field, eddies form in the metal creating a magnetic field of opposite polarity.

    If a second electromagnet were present, but with substantially more power, was pulsed so that the polarity of the magnetic field is the same as the first, would there be any noticeable force exerted on the metal object entering the first magnetic field?

    just a curiosity any thought and suggestions would be much appreciated.

  2. jcsd
  3. Mar 22, 2009 #2
    Re: Electromagnetics

    sry, to correct, the second magnet would create a pulse with polarity opposite to the first magnet. and would there be a repulsive force exerted on the object?
  4. Mar 22, 2009 #3
    Re: Electromagnetics

    I'm not sure of your proposed application or your level of knowledge, but there is a similar application that you might want to Google. NASA has long used magnetic suspension systems for holding test models in wind tunnels without having to use a sting. Some of their work should relate very closely to what you're seeking.
  5. Mar 22, 2009 #4
    Re: Electromagnetics

    Eddy currents are usually thought of as being only dissipative, like in transformer laminations. However what you are studying does have some real life applications. First, a little math (sorry).

    Suppose in cylindrical coordinates we have the field Bz and dBz/dt (sorry,I can't type partials) along z, and the other components of B being zero. Let's consider a ring of copper or aluminum in the r, theta plane (perpendicular to z axis). The dBz/dt is generating currents around the ring. There is no force along z, because The Lorentz force equation requires either a theta or r component of B to produce a force along z. The Bz field is trying to crush the ring. If the ring were tilted, then there will be a torque on it. However, if Bz = Bz0 - Bz1 z (decreasing with increasing z), then there will be a radial component of B (from Div B = 0) which combines with the induced azimuthal current in the ring that will force the ring upward (sometimes violently).

    Many years ago, before World War I, there were single-phase fractional-horsepower induction motors called repulsion-start motors, that had low-resistance eddy-current coils on the armature which were shorted by brushes on a commutator for high starting torque. The starting torque was superior to the modern capacitor-start induction motor, and had lower starting surge currents. But they were expensive motors to build, and were phased out as capacitors became cheaper. See attached photo of the radial commutator in a fractional HP induction motor (circa 1915).

    I hope this helps.

    Attached Files:

  6. Mar 22, 2009 #5


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    Gold Member

    Re: Electromagnetics

    Bob what I think you are referring to is a repulsion-start induction-run motor right? These started as a universal motor (series I believe) and when up to speed a centifugal switch shorted out the commutator so the armature 'appeared' the same way the rotor in a more modern induction motor 'appears'. Unless you are referring to something different? There are motors like this around that are much newer than WW1.
  7. Mar 22, 2009 #6
    Re: Electromagnetics

    so i think you guys may have mis understood me, lets take what i described in my previous post. that metal object that is being introduced into the magnetic field is free falling, much like if you've ever seen in a physics lecture, where an aluminum bar is dropped through a strong magnetic field and is slowed due to the eddy currents forming as the object passes through the lines of magnetic force. with the same principal in mind we add a very strong electromagnet that will pulse, with a field that is opposite in polarity to the field set up by the eddy currents forming in the falling object, effectively creating a repulsive force. I want to know if this is a plausible idea or if i've just crossed my wires...sry for the sick pun.

    See picture for basic setup of free falling object into first coils magnetic field.

    Attached Files:

    Last edited: Mar 22, 2009
  8. Mar 23, 2009 #7
    Re: Electromagnetics

    It looks like The Machine from the movie "Contact"
  9. Mar 23, 2009 #8
    Re: Electromagnetics

    Yes, the motor I pictured is repulsion start, induction run.
    The centrifugal switch on the motor pictured retracts the shorting (grounded) brushes on the radial commutator to disconnect the repulsion start when the motor approaches full speed.. The squirrel cage on the rotor are always connected. It is not an AC/DC (universal motor) that has powered windings on the armature. Century Electric I think stopped making repulsion-start motors about WWI, but other brands did continue into the Depression (and maybe until WW II). I happen to have a depression-era Delta Mfg. Co bandsaw that belonged to my granfather that has a monster 1/2 HP repulsion start motor.
  10. Mar 23, 2009 #9
    Re: Electromagnetics

    I think I understand the physics in your example, and I think I have reduced the principle to the simplest form. 2-S (commercially pure) Aluminum is best, based on ratio of conductivity to mass. Also a hollow tube is better than a solid tube because the eddy current induction depends on area, not diameter. The simplest of all is a very short tube (ring). If a conducting object is in a pulsed magnetic field, the magnetic fields will interact with the currents in the object, whatever they are. The pulsed field will always generate (more) currents, and the object will move or rotate to minimize the area of the induced currents exposed to the pulsed field. It certainly helps to already have eddy currents in the object when the field is pulsed. In my physics class, I saw an object (ring) hit the ceiling.
  11. Mar 23, 2009 #10
    Re: Electromagnetics

    what about any metal object? ferromagnetic, diamagnetic and paramagnetic, can they all be affected by this means of repulsion, the aluminum bar through a field was just an example.
  12. Mar 24, 2009 #11
    Re: Electromagnetics

    The short answer is yes. But they will just interact to different extents.
  13. Mar 27, 2009 #12


    Could u pls help me.
    I have modeled a multilayer waveguide using comsol/RF module/harmonics propagation/ I have solved it and everything is ok but I do not know how I can plot the electric field versus x, y or z. I have the results in eaxh of the tree plane but I need to plot the Ey vesrus x.

    thanks for your help.
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