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AC relay theory

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

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  1. Steve

    Steve Guest

    How does an AC relay work? I understand that the magnetic field of a
    DC relay coil attracts the contact arm and I assumed an AC relay just
    had a diode to convert coil current to DC - however when I tried to
    find an AC relay fault there was no diode. I thought 50 or 60 Hz
    alternating magnetic field cannot produce such a corresponding
    movement in a mechanical contact arm so I would have thought the net
    magnetic effect would be zero (no overall attraction or repulsion).
    Given the relay obviously operated before, I just can't see how. I
    did think the 'AC' rating just meant the contacts but I don't see why
    this wouldn't just be a current rating.
  2. Straw Man

    Straw Man Guest

    Enough latent magnetism in the core to somewhat negate the alternating
    field? Same goes for an AC solenoid as in a doorbell plunger I suppose.
  3. AC coil relays are common. A magnet of any type will attract a magnetic
    substance regardless of polarity - it's just if the second substance is -
    or gets - magnetized that they can repel. So you use a low permeability
    substance for the armature. Something like soft iron.
  4. No.

    An AC relay is just an electromagnet operating a set of contacts.

    An electromagnet attracts ferromagnetic material during both halves of
    the AC cycle. You could try it at home with a home wound electromagnet
    and a battery, the electromagnet will attract iron no matter which way
    round the battery is connected, or for that matter which way round the
    coil is wound, (same difference !) .

    The only difference is that the wound coil of a solenoid (esp. with an
    iron core) forms an inductor which has an impedance which acts to
    reduce the AC current through the coil for any particular AC voltage
    and frequency, it does this without causing electrical energy to be
    wasted (For the pedants I'm not saying it's perfect).

    This figures significantly in the design of AC relays and


  5. DaveM

    DaveM Guest

    I haven't seen anybody describe the real difference between AC and DC relays.
    A relay has a coil and the pole piece, or armature. Voltage applied across the
    coil causes current to flow in the coil, creating a magnetic field, which causes
    the armature to be pulled into the center of the coil, thus energizing the
    relay's contacts. Both types of relays operate on the same principle of
    The difference between the DC and AC relay is that the AC relay has a shading
    pole, or a heavy shorted turn imbedded into one end of the armature. Its
    purpose is to maintain a high flux level in the armature when the current in the
    main coil goes through zero. This acts to eliminate buzzing or chattering that
    is evident when you drive a DC relay with an AC voltage.
    There is no diode in an AC relay. A diode is commonly used across the coil of a
    DC relay to eliminate the high reverse EMF caused by the collapse of the coil's
    magnetic field. If a diode were used on an AC relay, it would create a short
    circuit every half cycle, something you want to avoid.

    An AC relay can be used in a DC circuit, but not vice versa. If you drive a DC
    relay from an AC source, the lack of a shading pole in the relay will cause
    buzzing, possibly allowing the contacts to bounce during zero crossings of the
    power source.
    If you drive an AC relay from a DC power source, the shading pole on the AC
    relay will cause the relay to be slow to release when power is removed.

    Dave M
    MasonDG44 at comcast dot net (Just substitute the appropriate characters in the

    "In theory, there isn't any difference between theory and practice. In
    practice, there is." - Yogi Berra
  6. You will usually find a copper "shading ring" wrapped around the end of the
    pole piece. The current induced in the shading ring delays the decay of the
    magnetic field long enough to smooth it out between cycles. There is no
    diode or smoothing cap!

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  7. Ross Herbert

    Ross Herbert Guest

    In the telephony industry these are known as "slugged" relays. Such
    relays have a solid copper slug of a specific length - eg. 1/2" or 1"
    depending on delay period required - the same diameter as the coil
    itself. The slug could either be at the armature end or the heel end
    of the coil depending upon whether a predominantly slow operate or
    slow release was required. For ac operation it hardly matters which
    end is slugged and if pushed for a part you could use a relay with a
    spare winding on it and simply short circuit this winding to produce a
    "slugging" effect.
  8. James Sweet

    James Sweet Guest

    They have a shorting bar like a shaded pole motor.
  9. No there is a difference here Ross, in the AC relay, the "slug" does
    not cover the whole of the magnetic iron path, it is typically only
    applied to about a quarter of the iron circuit. The process is to
    delay the decay of flux in that slugged path so that there is a useful
    magnetic pull during the time that the un-slugged path has zero flux,
    (and therefore zero magnetic pull). Using the DC relay slug is not
    really useful for the AC case as it covers the whole magnetic path.

    Peter Dettmann
  10. For completeness I should have added that we did extensively use
    relays on AC fed from a full wave bridge rectifier, and without
    capacitor for smoothing. This gave a tendency to chattering as there
    is still a pulsating current to the relay, however this chattering was
    overcome by the use of an armature end slug (as you describe) which
    was only about 1/16" long.
    A longer slug could be used, but fast operating speed was critical.

    Peter Dettmann
  11. Ross Herbert

    Ross Herbert Guest

    Hi Peter, there is NO difference.

    The principle is exactly the same even though the use of telephony
    relays is predominantly DC usage. However, they also were used in AC
    applications such as the detection of ringing voltage in ring trip
    circuits before the advent of semiconductor rectifiers. A relay which
    would chatter in response to 16-2/3 c/s ringing would not be very
    effective as a ring trip relay so the slug performed the same function
    as in modern AC relays at 50 or 60 Hz.

    In the telephone type relay I was referring to the slug does NOT cover
    the full length of the winding bobbin either. As I said the slugs are
    in different lengths eg. 1/2", 1", 1-1/2". The winding bobbin length
    is always a fixed length for the relay type and the slug was located
    either at the armature end or the heel end. A typical ring trip relay
    would have a 1" armature end slug.

    Here is a typical 3000 type telephony relay as used in UK and
    Australia in SxS.

    In fact this particular picture appears to show the winding bobbin
    with a copper front cheek which is the smallest armature end slug of
    all. In normal "donkey" relays the front cheek is always bakelite.

    For further study of the BPO 3000 type relay data see
    WARNING: Over 3MB. It will take some time to download.
  12. Ron(UK)

    Ron(UK) Guest

    The common GPO 3000 had the copper slug embedded into the armature. They
    would work on AC but were much happier on DC

    My father manufactured amusement machines and I entered the business
    even before leaving school.

    There are still many GPO type relays and boxes of contact blades kicking
    around in my mothers house. (Also Quite a lot of those wonderful
    uniselectors). The ability to strip them down and reassemble them in
    many permutations of contact arrangement, made them a wonderfully
    versatile device, and pretty reliable also.

  13. Ross Herbert

    Ross Herbert Guest

    I didn't say it did.

    I beg to differ. As others have described, A relay, whether DC or AC
    is simply a coil of wire on a core of suitable magnetic material
    (usually soft iron) with a closed magnetic loop which passes through
    the pivoted armature. It is only the inclusion of a delaying mechanism
    - in the AC case, a shorted copper turn or slug - which results in a
    delay to ensure the armature stays held while the AC curent passes
    through each half cycle.
    For DC operation an armature end slug produces a predominantly "slow
    operate" function because the slug produces an opposing magnetic field
    to that produced by the winding when energised. Only after the field
    which is set up by the winding has stabilised, and the corresponding
    field produced by the slug collapses to zero, does the armature pull
    in. While an armature end slug also delays the release of the
    armature, it predominantly affects the operate time. If you required
    the relay to operate as fast as possible the slug must be on the heel
    end and not the armature end. In most AC operation situations, unless
    either fast operate or fast release is required, it hardly matters
    whether the actual delay occurs on closing or opening the magnetic
    A longer slug simply increases the length of the delay period. As I
    said earlier, for the fastest operation the slug must be on the heel
    end (furthest from the armature) of the winding. In cases where fast
    operation and slow release (or vice versa) is required, a relay will
    often employ a secondary winding which can be shorted or opened as the
    case requires, to produce the slugging effect, rather than using a
    fixed copper slug.
  14. Ross Herbert

    Ross Herbert Guest

    No Ron, the 3000 type relay armature was fitted with a small brass
    residual stud to ensure the armature was not unduly held by residual
    magnetism in the core on releasing, NOT a copper slug. Impulsing
    relays used an adjustable residual stud (brass screw and locking nut)
    so that the residual gap could be set according to specific

    Here is a pic of a 3000 type relay with a 1" heel end slug and an
    adjustable residual stud.
  15. Ron(UK)

    Ron(UK) Guest

    You are probably correct copper/brass non ferrous anyway

    I imagined that was to adjust the throw of the armature to provide and
    also take care of the residual hang.

    I don't recall seeing one just like that, all the GPO types we used had
    the coil full length of the frame, some did have a D shaped shading pole
    set into the armature end of the pole.

    We also used a later type of GPO relay, exactly the same in design but
    slimmer frame and coil.

    Life was so much simpler back then: GPO relays, Bulgin microswitches,
    Crouzet motors, Honeywell timers... Selenium rectifie... erm well praps

    Amazingly, I have one of those slider rheostats in my workshop right
    now. I converted it into a dummy load for testing power amplifiers.

  16. Think 'magnetic material' is the wrong term. It is capable of being
    momentarily magnetised - but isn't actually magnetic.
  17. Here we have a communications problem.
    You referred to the normal slugs which do in fact enclose the magnetic
    path (not the magnetic loop or circuit) While there is some leakage
    flux outside the iron circuit. the major operating flux is through the
    iron core for the coil, which is path to which I referred. With the
    exception of Ron(UK) all the posts are still centered on DC relays
    whether slugged or not. Ron said "some did have a D shaped shading
    pole set into the armature end of the pole".

    There is a slot cut into the pole face (to which the armature is
    attracted). The slot divides the pole face into two sections with a
    ratio of about 1/3. In this slot is usually just a solid D shaped
    copper piece which forms a shorted turn on the smaller pole face
    section. What happens is that the flux attracting the armature is the
    sum of the fluxes from the two sections of the pole face. With an AC
    energised coil, the smaller pole face flux actually lags the flux in
    the un-shorted larger pole face section, so that even when either pole
    face section has zero flux (and zero pull) there is still flux in the
    other section. Therefore the armature always has some pull from the
    pole face while the coil is energised. For larger relays, the
    magnetic circuit is made up of laminations, but the same style of
    having two distinct sections of pole face, with one having a shorting
    coil around one section.

    I hope this is clear, and I would rather have shown a diagram, but
    with the painters in I have rather limited access to my library to get
    at the old basic theory books. I should add that I have had many
    years working directly with relay, protection and control equipment

    Peter Dettmann
  18. Ross Herbert

    Ross Herbert Guest

    You are being a bit nit-picking aren't you?... You know what was meant
    by the shorthand description. In this case "magnetic material" means
    "magnetically permeable material" to be precise.
  19. Ross Herbert

    Ross Herbert Guest

    Copper is too soft and quickly becomes thinned out requiring the
    armature to be replaced too soon. Brass is much harder and takes a lot
    longer to wear out.
    I don't understand the term "residual hang".

    The travel of the armature was adjusted by bending it in a armature
    bending tool. This was set to allow the specified travel and
    functioning of all springsets fitted tothe yoke. The residual stud
    adjustment on the armature was to allow release time adjustment
    without unduly affecting the magnetic force attracting the armature to
    the pole piece on operation while at the same time allowing periodic
    resetting to compensate for wear.
    GPO relays are exactly what I have been talking about.

    That relay in the pic was a 3000 type with heel end slug. The 3000
    type relay was the standard relay used in all BPO Pre-2000 (later
    issue), 2000 type SxS and SE50 exchange equipment from the 30's til
    late 60's. The smaller brother to it was the 600 type relay but these
    found only limited use in exchange equipment. None of the relays used
    in exchange equipment that I worked on from 1956 - 1993 were fitted
    with a D shaped shading pole. All 3000 type relays which required a
    slugging effect were fitted with a cylindrical slug as shown in the
  20. Ross Herbert

    Ross Herbert Guest

    What is the difference between "magnetic path" and "magnetic circuit
    or loop"? It is the same thing in my experience.
    But Ron, who claims to be familiar with "GPO relays", is wrong. Not
    one of the standard 3000 type relays used in GPO exchange equipment
    was fitted with a D shaped slug. He may have come across a relay which
    was obtained for use in a specialised piece of equipment but this was
    definitely not inthe standard library of relays used by the GPO. I
    worked on SxS exchange equipment of the same type as used by the GPO
    from 56 - the early 60's when it was replaced by LME ARF102 x-bar
    (Aust), and not once did I come across a relay with a D shaped slug.
    I am not saying you don't know your relay stuff. It just seems that
    you are referring to the types of relay found in general AC and DC
    usage. Telephony relays are far more specialised and have the ability
    to be critically adjusted to suit a particular application.
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