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AC relay theory

Discussion in 'Electronic Repair' started by Steve, Aug 17, 2007.

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  1. In terms of the OP's question, no. Magnetic suggests it will attract an
    armature etc without being externally energised.

    I was searching for a single word that means 'magnetically permeable' but
    if it exists it escapes me.
  2. Right, I messed that up, in trying to differentiate between the total
    magnetic iron loop and the crossectional area of the iron. I tried
    to point out that I did understand what you meant by a slug, and that
    it did not cover the whole length of the magnetic circuit.
    Ok but Ron did not say that these AC relays were actually an
    extensively BPO use if at all by them. That was not the question, he
    just mentioned just that they do exist,and he has seen them. What
    has happened is that the focus has been drawn in to this red herring
    path of BPO relays. This was because they were cited as a typical
    relay one could describe. However in the process the actual original
    question has been lost in a discussion about DC relays when the
    subject is (was) basically how do AC relays operate. So the
    question to be answered (which I believe I have largely done) was :
    If a discussion on 3000 type relays were involved, I have my well worn
    gram balances, contact adjusters, and armature bender at hand.

    You are right that the 3000 style as an AC type is rare, as there are
    much better designs used for AC, but still using the same principle of
    the divided magnetic path as Ron described.

    Peter Dettmann
  3. Ross Herbert

    Ross Herbert Guest

    On Tue, 21 Aug 2007 22:41:21 +1000, Peter Dettmann
    My initial post was to simply add a comment so that other readers
    would not gain the impression that the only type of AC relay was one
    with a "shading ring". It is only because of responses to my initial
    post that I added further comments to expand on it or to correct those
    responses but I did not plan for the thread to specifically become one
    about BPO relays. If no one had cared to respond then it wouldn't have
    gone further.
    Actually, I haven't found any description on AC relays which refers to
    a "divided magnetic path", or anything remotely resembling this term,
    so I doubt that it means anything at all. What I did find is a
    description of the various types of shading ring (as refered to by Ron
    and others) which might be found on AC relays and/or contactors meant
    for 50/60 Hz operation. See page 36.

    While operating on the same principle, the solid copper slug as used
    on the BPO 3000 type relay is far more effective for operation at
    lower than 20Hz while also allowing the tailoring of the delay period
    by varying the length of the slug.

    AC relays commonly used today are not necessarily "better designed"
    for use on AC than the 3000 type (and similar) relays. The AC relay of
    today only has to operate on 50/60Hz where the slugging effect of a
    relatively small shading ring is adequate, thus making the design much
    simpler and less costly to implement than for the 3000 type relay. In
    many cases the addition of semiconductors simplifies the design of
    relays used for AC applications so that shading rings aren't required.

    I might add that every relay manufacturer in the world making relays
    for telephony applications, would have produced similarly slugged
    relays of a perhaps a slightly different mechanical design - it wasn't
    just the BPO 3000 type which used this principle. In closing, this
    type of slugged relay would have been far more common than any other
    type of AC relay used in any other industry then, and even today.
    Every final selector in every strowger based switching system the
    world over used one. They had to be reliable and good for millions of

    I don't have an armature bender but I do have the pressure gauges and
    spring adjustment tools.....
  4. Ross Herbert

    Ross Herbert Guest

    As I said, I am guilty of omitting one word in my phraseology. If by
    omitting this word you were led to thinking that I was saying the core
    itself was a permannent magnet then you may be less intelligent than
    you imagined. If it were a magnetic material as you obviously imagined
    then the armature would be permanently held operated, so what then
    would be the purpose of the winding?
    Well, that's because there is no single word (technically speaking) -
    and I didn't infer that there was. There is however, a symbol for
    Magnetic Permeability, Mu - expressed in Henry's per meter, with which
    I am sure you are familiar. As in most fields of science there is
    usually a set of symbols to accompany it and we all know that symbols
    are basically shorthand representation or descriptor for a term or
    property. So it can be said that the single character U = Mu (sorry,
    can't type the actual character) is a one letter word meaning Magnetic
  5. JW

    JW Guest

    I GOT to remember that one...
  6. Ross page 36 does not seem to be relevant, however, your comment that
    you find no reference to a divided path would indicate that you have
    no idea what I have been talking about. You need to understand that
    on an AC relay, the pole face is divided, and one section has the
    shading ring around it, and the other section has no shading ring (so
    that unshaded section it is just like you find on a DC relay). The
    armature is therefore is attracted by the sum of the fluxes from each
    of the two pole faces. The idea is that while the the coil is
    energised, then even when the un-shaded pole flux is zero (twice per
    cycle), there is still flux from the shaded (lagging or delayed) pole,
    and so that there is no time during the AC cycle when there is zero
    flux pull on the armature.

    Peter Dettmann
  7. Ross Herbert

    Ross Herbert Guest

    I certainly understand the principle of operation as described on
    pages 36 and 37, and you seem to be saying very much the same thing as
    depicted in fig.a(3) along with the accompanying description. As I
    understand it the total flux in the core is divided where it passes
    through the pole face and the electromagnetic force on the armature
    will be the sum of the two forces produced by the two flux paths
    through the shaded and unshaded parts of the pole face. The
    electromagnetic force resulting from both flux paths never actually
    drops to zero throughout a full cycle of current so the armature
    remains held in.

    I thank you for your patience and your comments have been helpful.
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