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Solenoid operation

Discussion in 'Electronic Basics' started by R.Spinks, Jun 30, 2005.

  1. R.Spinks

    R.Spinks Guest

    I'm trying to understand basic solenoid operation a little better. I was
    under the impression that if I applied DC to a coil (inductor) and the
    inductor had a metal object (like a nail or something) say in it and
    slightly out to one side the current through the inductor would create a
    right hand rule flux in the inductor and the nail would move in the flux
    direction. Then if I switched the polarity the nail would move the opposite
    direction as the current direction would be opposite. I tried building a
    little coil out of magnet wire and put a nail in it - then applied 14V - but
    it didn't move (the nail did get a bit magnetic as I touched it w/another
    nail). Is my understanding of the theory right or is there still something
    missing (like does the nail have to be a permanent magnet or something
    instead of just a 'ferrite')? Also -- I read on line something about the
    coil always pulls to the center... but ... the right hand rule should apply
    so .. I think it should have a direction not just to center of coil.
     
  2. John Fields

    John Fields Guest

    ---
    The "handedness" only refers to the polarity of the field, (That is,
    which end is north or south) not its amplitude. If you put a ferrous
    material in the field it will be attracted by the north pole as well
    as the south pole and will come to rest where the forces on it are
    equal.
     
  3. ehsjr

    ehsjr Guest

    Generally, solenoids have a spring that pulls or pushes
    the plunger away from the center of the coil:COIL PLUNGER SPRING
    )))))))))=========ssssssss

    In the diagram above, the tension spring exerts a force
    toward the right. When energized, the coil exerts a
    larger force, toward the left:
    <<<<----- -->
    COIL SPRING
    )))))))))===== s s ssssss

    Note that the plunger has begun to move inside the coil,
    and the spring has stretched a bit. The arrow showing the
    spring's rightward force is now stronger, but the
    attraction of the coil on the plunger is strong enough
    to overcome it.

    <<<--- --->>>
    COIL SPRING
    =)))))))))= s s s s s s s s

    Finally, the force exerted by the coil to move the plunger
    toward the left equals the force exerted on the plunger
    by the spring to move the plunger to the right, so the
    plunger stops moving. When you de-energize the coil, the
    spring pulls the plunger back to the right.

    If you had no spring and started with this:

    COIL PLUNGER
    )))))))))=========

    And then energized the coil, you would end up with this:

    COIL
    =)))))))))=

    The plunger would move in to the coil and stop when the
    magnetic center of the coil and the magnetic center of
    the plunger lined up. Unless there is a force to pull
    the plunger out of the coil - another coil, air pressure,
    a spring, whatever - the plunger will stay there when the
    coil is de-energized.

    Ed
     
  4. Bob Eldred

    Bob Eldred Guest


    Your basic assumptions are wrong. The handed rules do not apply. A ferrous
    object, iron,or steel will always move toward the highest flux concentration
    which is in the center of the coil, length wise. The handedness determines
    magnetic polarity, north or south but not attraction. Just as both ends of a
    permanent magnet, north or south will attract an iron object, so does a
    solenoid attract regardless of the polarity of the current. The nail should
    be sucked right into the coil if there is sufficient flux density to move
    it. Based on what you said, there is very little flux density in the coil
    and the iron barely moves but does show a little magnetic effect. It means
    you do not have sufficient magnetizing force or amp-turns per length to
    create much flux. To get a healthy pull on an object like a nail you may
    need about 500amp-turns or more in an inch or so of coil length. The higher
    the number, the more the force. Also, the greater the crosssectional area of
    the iron, the greater the force. A nail is a bit small, try a 1/4inch bolt.
    In fact, the iron sould fill the area in the center of the coil as much as
    possible but still move.

    For example, a 14volt coil drawing about 300mA, the resistance would have
    to be 14/.3 = 47ohms. To get 500Amp-turns, the winding would have to be
    500/.3 = 1666 turns of wire. It would take about 450 feet of 30AWG wire to
    get 47 ohms. And, for 1670 turns, the average length of one turn would be
    3.2 inches, about 1 inch dia. A smaller diameter would give more turns and
    higher force for the same length of wire and iron size. I suspect you used
    far less than 1600 turns of unknown wire size and that's why the performance
    is less than you expected. Try getting at least 500Amp-turns and see what
    happens.
    Bob
     
  5. R.Spinks

    R.Spinks Guest

    Ok -- that makes sense, I think. Thanks. If the ferrite was instead a
    permanent magnet (ie. has it's own north and south pole) and I beefed up my
    example to have sufficient pull (like 500 amp-turns) as you have
    indicated -- would it then work that if I switched the direction of current
    in the coil that the magnet would move in opposite direction (as I would
    have changed the polarity of the coil magnet -- or would it still just pull
    to center)?
     
  6. Bob Eldred

    Bob Eldred Guest

    It might work if done right, however the forces on a permanent magnet are
    at right angles to the forces of attraction for a piece of unmagnetized
    iron. Most likely, a permanent magnet in a solenoid would filp around until
    in was "most happy" then it would stick to the side walls of the coil.
    Reversing polarity might flip it some other way. It's hard to analyze
    because the field lines and their interactions are very complex. It most
    certainly would not work like you want where it pulls in one way and pushes
    out on reversed polarity.

    The voice coil on a loud speaker moves in the way you have described. Here
    the current, the magnetic flux and the resultant force are all at right
    angles to each other and a hand rule applies. When the current reverses, the
    force also reverses. It is possible but difficult to arrange a solenoid and
    permanent magnet to simulate this action but the magnet would have to be
    arranged so that its flux lines cross the coil at nearly right angles as it
    moves. The armature of a permanent magnet motor does this as it rotates. To
    generate linear motion, a voice coil is the best way to go, like a speaker.
    Bob
     
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