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OT: How does and AC relay/solenoid work?

Discussion in 'Electronic Repair' started by David Farber, Dec 2, 2004.

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  1. David Farber

    David Farber Guest

    I was just wondering about the theory behind an AC solenoid/relay. How is that
    a steady electromagnetic force can be generated from an AC current, or, maybe
    something else is happening? My internet search for this answer hasn't turned
    up much in the way of an explanation.

    Thanks for your reply.
     
  2. The induced magnetic effect is always is opposition (thus attractive).
     
  3. NSM

    NSM Guest

    | I was just wondering about the theory behind an AC solenoid/relay. How is
    that
    | a steady electromagnetic force can be generated from an AC current, or,
    maybe
    | something else is happening? My internet search for this answer hasn't
    turned
    | up much in the way of an explanation.

    Same way an AC electric motor works - an eddy current is induced in a piece
    of metal and is attracted to the coil. Both fields reverse on each half
    cycle. Note that the moving part often has copper embedded in it or has
    slots cut in it to modify the induced field.

    http://home.howstuffworks.com is always a good starting point.

    http://experts.about.com/q/1473/2280116.htm may help.

    N
     
  4. Graham

    Graham Guest


    I used to wonder about how the AC solenoid in a central heating boiler gas
    valve worked,
    until I stripped a couple of them down and discovered there was a bridge
    rectifier embedded in them;
    so they were DC after all.
     
  5. And there is what's called a "shading ring" to delay the collapse of the
    magnetic flux on part of the pole piece during the period where the current
    is going through 0 A. Thus, the magnetic field is never zero and the
    attraction is more or less continuous.

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  6. Jim Adney

    Jim Adney Guest

    If you tear one down you will find that the pole piece on an AC relay
    (or solenoid) will be split into 2 halves. One half will have a
    D-shaped copper ring pressed down over it. This ring is a shading ring
    and that half of the pole piece is called a shaded pole.

    The current induced in the shading ring produces a phase shift in the
    magnetic field in that half of the core. This means that there is
    never a time when the magnetic field is zero thru BOTH core halves.
    This is what keeps the AC relay from buzzing.

    -
     
  7. Asimov

    Asimov Guest

    "Jim Adney" bravely wrote to "All" (02 Dec 04 22:09:09)
    --- on the heady topic of "Re: OT: How does and AC relay/solenoid work?"

    JA> From: Jim Adney <>
    JA> Subject: Re: OT: How does and AC relay/solenoid work?
    JA> Organization: University of Wisconsin, Madison
    JA> Xref: aeinews sci.electronics.repair:2031

    JA> If you tear one down you will find that the pole piece on an AC relay
    JA> (or solenoid) will be split into 2 halves. One half will have a
    JA> D-shaped copper ring pressed down over it. This ring is a shading ring
    JA> and that half of the pole piece is called a shaded pole.

    JA> The current induced in the shading ring produces a phase shift in the
    JA> magnetic field in that half of the core. This means that there is
    JA> never a time when the magnetic field is zero thru BOTH core halves.
    JA> This is what keeps the AC relay from buzzing.

    Right on! In a pinch one can use an AC relay with a DC input but it
    will tend to draw a lot more current and overheat. Using a DC relay
    with AC won't work well because even if it doesn't buzz it may not
    draw enough current to latch properly.

    A*s*i*m*o*v

    .... Old pinballers never die, they just flip out.
     
  8. David Farber

    David Farber Guest

    Subject: Re: OT: How does and AC relay/solenoid work?
    The phase shift was the part I didn't know about. Very ingenious indeed.

    Thanks to everyone for their replies.
     
  9. Jim Adney

    Jim Adney Guest

    Right, if you want to do this, you'll want to use a DC voltage that is
    significantly lower than the original AC rating.

    -
     
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