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AC relay latching closed mystery

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

Reply to
P E Schoen
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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.

Reply to
Bruce Varley

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

--
~ Adrian Tuddenham ~ 
(Remove the ".invalid"s and add ".co.uk" to reply) 
www.poppyrecords.co.uk
Reply to
Adrian Tuddenham

Bruce Varley Inscribed thus:

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

--
Best Regards: 
                        Baron.
Reply to
Baron

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

Ed

Reply to
ehsjr

a

tapped

were

solid

half-second

on

magnetized

lower

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.

?-)

Reply to
josephkk

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.

--
Jim
Reply to
Jim

to a

tapped

they were

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.

?-)

Reply to
josephkk

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