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Electromagnet: Current or Voltage??

Discussion in 'Electronic Basics' started by Randy Gross, Sep 5, 2005.

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  1. Randy Gross

    Randy Gross Guest

    I'm trying to determine what gives an electromagnet its strongest
    power: high current or high voltage.

    I ask this question because of L-reactance in the coil when it is
    energized which limits the current through the coil.

    Should the initial power be more current with low voltage or, higher
    voltage with low current?

    rg
     
  2. BobG

    BobG Guest

    Its a resistor. A coil of wire. It has x ohms. Put V volts across it,
    it draws V/x amps.
     
  3. Bob Eldred

    Bob Eldred Guest

    Current creates force in an electro-magnet. Voltage develops current through
    resistance. You cannot have one without the other. So your question makes no
    sense. Reactance is not an issue unless in an AC circuit where the reactance
    does limit the current. But, that's not the usual case. It's the DC
    resistance that establishes the current by ohms law and the current
    establishes the force. The resistance of the wire will dissipate heat and
    that limits the amount of power that can be placed on a given magnet.
    Bob
     
  4. By power, I assume you mean flux. Magnetic flux, for a given core
    permeability, is always proportional to ampere turns (total amperes
    circulating around the core).
    Inductance limits how long it takes for the field to reach full
    strength but resistance limits the ultimate current. Are you in a hurry?
    Voltage gets current going, in spite of inductance, and keeps it going
    in spite of resistance. For a given coil volume and wire resistivity
    there is an optimum wire size that gets the most ampere turns in that
    volume for a given amount of total power (volts times amperes). The
    optimum combination will come out to about the same power, regardless
    of the voltage (neglecting the space taken up by wire varnish).

    If you are in a hurry (if inductance is still controlling the rise of
    current when time is up), then the optimum balance will be different.

    Are you ready for some math? My head hurts already.
     
  5. Ralph Mowery

    Ralph Mowery Guest

    The strength is determined by the amp-turns. That is how many amps and how
    many turns. The voltage will be determined by how many amps you want. That
    is you can use the same number of turns and large wire that will draw more
    amps for the same voltage or you can use smaller wire and draw less amps and
    it will be weaker.

    There is a short period of time that it takes for the current to reach its
    maximum. If you put a resistor in series with the coil and make the voltage
    larger so the final current is the same , it will reach the maximum strength
    sooner. Not much like a couple of miliseconds or so for a small coil.
     
  6. Randy Gross

    Randy Gross Guest



    Yes. I'm looking to get maximum flux density. The electromagnet will
    only be used in a 12 vdc momentary burst so I need maximum capability
    in an instant for an instant.

    The width of the coil, including Iron core, is one inch. The length can
    be up to one inch. I have 18 awg. magnet wire to wind the coil.

    rg
     
  7. vic

    vic Guest

    Current and voltage are linked by this equation :

    U = L di/dt + R i

    You can't have one without the other.
     
  8. Randy Gross

    Randy Gross Guest

    [email protected]

    I think you misunderstood. I know that for a given volt/amp, lets say
    12VA for demo., I have a range of choice. I can use 1 amp @ 12 volts up
    to 12 amps @ 1 volt. I was just trying to determine where along this
    scale I will get the best performance from an electromagnet and, amps
    and turns appear to be the answer.

    rg
     
  9. I'm not familiar with that measure of time, ;-)
    Can you put any number on how long an instant is? I don't understand
    if we are talking about a second or a nanosecond.
    If you can explain more about what you need this flux for, I could
    make better suggestions. After that it is a matter of writing and
    solving an equation.
     
  10. Randy Gross

    Randy Gross Guest

    Sorry about the ambiguity.


    This is an experiment in Torque.

    I'm building a motor from a different point of view. Instead of
    rotating the coil in a magnetic field, I'm mounting the magnets
    (1"x1/8" NEO disks) on the rotor and spinning the magnetic field.

    I want to use the coils, mounted in planetary fashion, like spark plugs
    in an engine with an adjustable "distributor" (armature) for energizing
    the coils mounted on the rotor. When the rotation of the magnets reach
    approximately 75 degrees to the coils, The coils (electromagnets) will
    energize wrenching the magnets toward 90 degrees but will de-energize
    before the poles try to cog on each other.

    At this time, I have no Idea what the RPMs will be so time for me is a
    best guess. I'm thinking low milli for about 13 degrees of rotation.

    The adj. feed is to time for maximum torque and to compensate for my
    shortcommings;-)

    rg
     
  11. I think the ideal place the magnets should be with respect to the
    coils is that the coil should be exactly half way between two opposite
    magnet poles when the coil current peaks. That way, one magnet pole
    is pulling toward the coil and one is pushing off it. The coil
    current is ideally zero as the magnet pole passes directly beneath it.

    If the coils have iron cores that extend toward the magnets, there
    will be cogging, just because iron is attracted to a magnet. This can
    be reduced if there is a different number of magnet poles and coil
    poles. For instance, you might drive a pair of magnet poles with 3
    coils. That way, all 3 magnets do not cog at the same time. The
    current switching arrangement gets more complicated, though. You
    might look into hall effect switches as a means of detecting rotor
    position to switch the coil currents. That is the way brushless
    motors are made.
    For first effort, since you don't have a speed goal, I think you
    should forget speed and try to maximize stall torque per watt of DC power.
    It is commonly used to advance the coil voltage timing to get the
    current peaks where they are needed in spite of the inductive delay,
    at rated speed. This is like adjusting the timing on a gasoline
    engine to produce maximum power. There is a best setting for each speed.

    You should be able to come up with a mathematical expression for
    winding resistance versus turns, based on the dimensions of the
    winding volume and your wire gauge. Ohm's law converts that to
    current. Pick a DC power and you have the current allowed for a given
    DC voltage.

    Current times turns gives you flux which is proportional to torque.

    Combining the ampere turns with the power formula gives you torque per
    DC watt.

    It should come out that above some number of turns, the coil diameter
    is so big that the resistance is going up faster than the flux, so it
    may not be best to fill all available space with coil.
     
  12. Bob Eldred

    Bob Eldred Guest

    So, you are reinventing the permanent magnet DC motor??? In a traditional DC
    motor, the communtator does the switching to direct the current to the
    appropriate winding as it passed the magnets. What you have described sounds
    similar to this. In a brushless DC motor, the switching is accomplished by
    transistors or FETs turned on and off by hall effect or optical means as the
    magnets or coils pass each other. It doesn't matter which moves, the coils
    or the magnets. Motors have been built both ways, rotating coils and
    rotating fields. The important thing is the magnetic path, it's reluctance
    and the air gap. Get a book on motor design and study how they work, you'll
    save your self a lot of frustration and avoid reinventing what was done 100
    years ago.
    Bob
     
    sam ciarametaro likes this.
  13. Randy Gross

    Randy Gross Guest

    I agree!
    I've debated the inclusion of Iron cores for this unit because of the
    attempts at cogging and reduced potential power. I may settle for a
    synthetic (resin/Iron powder) to reduce attraction an retain some
    benefit from the core.
    On the back burner for now. I'll center on torque since load support is
    expected.
    Understood!



    I think I have enough now to set me on the right path. I'm more than
    sure that before I finish this build, I'll have a question or two.
    Thanks for the help!

    rg
     
    sam ciarametaro likes this.
  14. Randy Gross

    Randy Gross Guest



    No Mr. Eldred. What I am doing is akin to building a replica of the
    KittyHawk or a 65 Mustang only with variations. It's the same soup,
    just a different bowl.

    I guess you could say that It is built in honor of those that gave us
    the technology. I for one am fascinated by what they accomplished and,
    the completed units, if not done hastily, are works of art.

    rg
     
  15. Bob Eldred

    Bob Eldred Guest

    I give up...If you argue with idiots you begin to sound like one...no
    thanks!
     
  16. Jasen Betts

    Jasen Betts Guest

    both...

    the more current you can get circulating close to the centre of the winding
    the stronger the electromagnet.

    if you use a high current you need thicker wire, if you use a low current
    you can use thinner wire but will need more voltage to push the current
    through the wire.

    Bye.
    Jasen
     
  17. Jasen Betts

    Jasen Betts Guest

    How long is an "instant", you could use 8awg wire (and a supply capable of a
    few hundered amps - like a car battery) if you only want to run the
    electromanet for a small fraction of a second

    hmm you'd get somewhere around 500" of 18awg in your coil
    so resistance 0.3 ohms. from 12V peak curent would be about 40
    amps so about 500 watts disipation... (and this is best case)

    a coil about 1" in size can't stay cool enough to survive and disipate more
    than about 5 watts (unless you have heastsinks and cooling fans etc)

    so I'd reccomend a duty cycle of 1% (1:100) so if you run it for 1 second
    rest it for 100 etc...

    as for how strong it'll be I have no idea, pretty strong I expect.

    I'm no engineer so wear safety glasses and have a fire extinguisher on hand
    etc...

    Bye.
    Jasen
     
  18. Randy Gross

    Randy Gross Guest



    This is always good advice, thanks Jasen ;-)

    rg
     
  19. Randy Gross

    Randy Gross Guest



    Pull your horns in Bob, you were heard. Think of this as building a
    scale model only, you can't buy a kit.

    rg
     
  20. Jasen Betts

    Jasen Betts Guest

    all else being equal thin wire 1200 turns and 12V x 1A will get you the same
    efficiency as thick wire, 100 turns and 1V x 12A
    turns take length of wire that means resistance and resistance reduces the
    amps. you can't have both.

    Bye.
    Jasen
     
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