Adrian Tuddenham wrote:

Michael A. Terrell wrote:

Adrian Tuddenham wrote:

Michael A. Terrell wrote:


All I've seen in use, use AC to prevent this problem.

...except that A.C. doesn't prevent it. I've seen enough stuck
contactors with A.C. coils to be quite certain of that.


They are not contactors. Some hold the door open for years between
operations. Contactors get beat to death by repeatedly slamming the pole
piece into the metal on the moving contacts.


You're comparing apples to water melons.

These D.C. magnets with flat pole faces are getting stuck because of
residual magnetism - A.C. contactors with laminated pole faces which
have become hammered flat, get stuck in exactly the same way.

This is not because the contactors have had to do a lot of work in order
to develop flat pole faces, it is because the residual magnetism is the
same, regardless of whether it resulted from D.C. or the last half-cycle
of A.C.

The old sliding fire doors on slanted rollers that used a rope and
weights were pretty much foolproof. They didn't need electricity, and
there was little you could do to screw them up. Once they were moving,
you couldn't hold them open.

Those laminated contactors take thousands and thousands of operations
to flatten into a solid face.

--
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