Alternating Current Electromagnet

Discussion in 'The Projects Forum' started by ramsey, Nov 4, 2010.

  1. ramsey

    Thread Starter New Member

    Nov 4, 2010
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    Hi all,

    I'm working on creating a strong electromagnet for fun and I wanted to try something a little different by created an AC based one. I have most of the components worked out, its a fairly simple thing but one thing that I am unsure of is how to deal with back-emf. Now in DC based design one will have a diode placed in parallel to the coil. But this is an AC design so I'm a little confused on how to address back emf here since trying to use a diode will prevent it from being AC. Also I'm very far from being a pro so perhaps there is something important I'm missing conceptually. Any thoughts? I'm designing it to push about 1.5 amps from an 18 volt wall transformer through a set of higher wattage parallel resistors. Thanks in advance.
     
  2. ramsey

    Thread Starter New Member

    Nov 4, 2010
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    I was just thinking though, in contrast to a DC circuit, in this one there will usually always be a back-emf while in operation, switch closed, because of the nature of AC of course. So, if the switch is opened and the current quickly drops off to zero, the current field within the coil would simply break down with the current meaning worrying about back emf after power is cut pointless in this scenario? Am I in the ballpark here?
     
  3. wayneh

    Expert

    Sep 9, 2010
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    I think most electric motors (an application of AC electromagnets) in household items include nothing to mitigate back-EMF. Just on or off.
     
  4. wayneh

    Expert

    Sep 9, 2010
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    Parallel resistors? Maybe you mean serial? It would be helpful if you posted a diagram of what you're thinking of.
     
  5. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    U don't need diodes in AC magnets.
    Diodes are used in DC circuit with semiconductors and inductors in case of switching.

    In typical AC application to prevent arcing in the contact of switch when the current via an inductor or a magnet is cut off, a capacitor connected across the respective contact can absorb the EMF spikes or surges
     
  6. ramsey

    Thread Starter New Member

    Nov 4, 2010
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    Without making a drawing since its fairly simple, I basically intend to use 4-5 50 ohm 10 watt resistors in parallel so that I can achieve the range of 1.44-1.8 Amp current from an 18v wall transformer and also not exceed or get too close to the 10 watt rating of each resistor. Its what I came up with and I feel it should not make anything too dangerous. (the wall transformer output max is 2.2Amps). I hope my logic isn't flawed, lol.
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    You won't get any magnetism from resistors.

    You will need to use motor winding wire to create a coil with that impedance at 50/60Hz. The winding will then be the electromagnet.
     
  8. wayneh

    Expert

    Sep 9, 2010
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    So you're adding ~10Ω in series with your coil. Doesn't your coil have an impedance of its own? You probably have several ohms of DC resistance or more, and the inductance may raise the impedance higher than you think. No worries, you can always reduce the series resistance further if you want, possibly even eliminating the resistors as you gain knowledge about your device.
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
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    What is this electromagnet made of? There are some non-ideal features of magnetic materials that make an A.C. magnet more difficult than D.C. Thus A.C. magnets are typically made of laminated material, to prevent eddy currents (essentially that would be the metal of the magnet itself developing circulating currents as in a transformer). And the material itself needs to be a certain kind of alloy to avoid magnetic losses; look up "electrical steel" on Wikipedia.
     
  10. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    Choose wire gauge and length to give yourself the required DC resistance for the max current you wish to limit the circuit to, for the voltage that you are using. 120 ohms coil resistance on 120 VAC circuit would give you 1 amp(RMS) of current. The back EMF will lower that further.
    Also because of the EMF they induce in materials they come close to, AC magnets work for all metals. They are like regular magnets with iron material, but will also pick up non ferrous metals that conduct, like copper and aluminum. They are used on large cranes in metal junk yards quite often for picking up the scrap.

    Google "master magnet"
     
  11. John P

    AAC Fanatic!

    Oct 14, 2008
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    And 120 Watts of heat. Plus whatever you'd get from magnetic hysteresis and eddy currents, if you don't build it right.
     
  12. wayneh

    Expert

    Sep 9, 2010
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    Nope, it won't make more heat than the energy it draws as I^2*R. That'd be REALLY great, but alas, no.
    Of course you're point is correct, that this thing could get very hot without attention to shedding the heat.
     
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