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AC relay latching closed mystery

P

P E Schoen

Jan 1, 1970
0
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.

Paul
 
A

Adrian Tuddenham

Jan 1, 1970
0
Bruce Varley said:
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.

Paul

Common problem with old contactors. The magnetic path gets tighter and
tighter over time as the repeated slamming shut smooths out the pole faces.
A solution is to roughen them up again with a coarse file, or apply some
deep scratches.

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
 
B

Baron

Jan 1, 1970
0
Bruce Varley Inscribed thus:
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.

Paul

Common problem with old contactors. The magnetic path gets tighter and
tighter over time as the repeated slamming shut smooths out the pole
faces. A solution is to roughen them up again with a coarse file, or
apply some deep scratches.

Iv'e used a swipe of nail varnish to solve that problem !
 
E

ehsjr

Jan 1, 1970
0
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.

Paul

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

Ed
 
J

josephkk

Jan 1, 1970
0
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.

Paul

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.

?-)
 
J

josephkk

Jan 1, 1970
0
<snip>

We had this at a place that I worked. Some of the big relays even took
two hands to pull off. We always felt that it was due to wear at the
pivot point causing the gap to decrease. Replacing the relay always
fixed the problem.

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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