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AC relay latching closed mystery

Discussion in 'Electronic Design' started by P E Schoen, Feb 26, 2013.

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  1. P E Schoen

    P E Schoen Guest

    My friend had a problem with a 120 VAC heavy duty relay he connected to a
    sump pump motor. It would turn on OK, but it would not release. If he tapped
    the armature, it would open, and he suspected welded contacts, but they were
    fine. It was the armature itself that was latching. So he put a piece of
    paper over the armature, and it made a loud buzz, but it seemed to work
    better. Then, when he removed the paper, it worked normally, with a solid
    click on operate and release (where previously it would take a half-second
    to open).

    He had gotten this relay as surplus from an electronics company where we
    both worked 30 years ago, and they used the same style relay with 24 VDC
    coils. I think perhaps they had ordered these incorrectly and used them on
    24 VDC, which may have drawn much higher current than usual and magnetized
    the iron of the coil. When he added the paper, it created a gap and lower
    inductance so the AC current was higher, and enough to demagnetize it.

  2. A few of the older types had a thin non-magnetic anti-residual-magnetism
    shim between the pole faces. After many operations it would wear
    through and disintegrate. Check for the remains of a shim. If there
    was one, replace it with another of the same material (something too
    conductive might act as a shorted turn).

    If the magnetic circuit is in the form of "E" & "I" laminations, it is
    sometime permissible to file only the centre limb of the "E". The "I"
    will then be supported on the outer limbs of the "E" and will not touch
    the centre limb.

    Do not be tempted to make the air gap too wide. Not only does it make
    the contactor noisy (as you have already noticed), but the holding
    current depends on the width of the final air gap, so there would be a
    risk of burning out the coil
  3. Baron

    Baron Guest

    Bruce Varley Inscribed thus:
    Iv'e used a swipe of nail varnish to solve that problem !
  4. ehsjr

    ehsjr Guest

    Another possibility is sticky crud that got removed by the
    paper, restoring proper operation.

  5. josephkk

    josephkk Guest

    There is one issue, ordinary relays are not motor starters. If an
    ordinary wall snap switch cannot handle the load a proper motor starter
    should be installed/used.

  6. josephkk

    josephkk Guest

    As i was not there, i have no idea if they were relays, contactors, or
    motor starters. There are significant differences in construction that
    the unaware might not notice. Often larger contactors and motor starters
    do not have pivot points.

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