Solenoid plunger becomes magnetic!?

[blcpro]

I have seen over the years the plunger of a standard pull-type solenoid (D frame, 12V DC) becomes magnetic over time (sometimes after only a few days of intermittent use). I was wondering if this could be prevented by reversing the DC current, say, every other actuation? In other words, the solenoid is activated, then deactivated, then the next time it is activated, the reverse current is used to activate it. I suspect that the reverse current flow in the coil would have the opposite magnetic polarity as seen by the plunger, right?

As a side question, is this a common problem for DC solenoids? Do AC Solenoids have this problem?

Thanks!

 

[roydm]

I think all magnetic circuits will retain some magnetisim, the ones with Iron anyway, isn't that what the hysteresis curve is about?
Roy

 

[Noway2]

From the theoretical side, I think that there would be some residual magnetism, but it could be reversed to some degree. It would depend on the volt-seconds or amp-seconds, or something like that. It involves 'resetting' the hysteresis curve.

I think that this is a common tactic employed in certain topologies of switching supplies. You may want to investigate there.

From the practical side, trying to totally neutralize the coil sounds like it would be a real challenge. Wouldn't you be better off trying to find a way to make it a non issue for your application?

 

[itsmoked]

Why would this even be an issue? There are billions of DC relays running in the world. Not one of them care about this issue.. What problem is this causing you?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[blcpro]

basically, the problem I get is that when the solenoid is deactivated, it does not release the plunger, which means that it doesn't work as expected.
Thanks everyone for your responses. I have contacted the solenoid manufacturer and they have told me that the end of the solenoid has a metal base which is used not only as a stop for the plunger, but also to maximize the magnetic hold at maximum stroke. Apparently, allowing the plunger to travel to this limit can result in some residual magnetism. He suggested, and I am currently testing this, that a brief current reversal (less than 100ms) on the solenoid would help break this residual magnetic field, just before turning off the current to the solenoid. I don't have any concrete results yet, but so far with the one unit I am testing there has been no more problem...

 

[itsmoked]

Interesting.

No spring in the picture?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[desertfox]

Hi bicpro

What is the mechanism that returns the plunger ie:- a spring or it own weight under gravity etc.
I would have thought that the solenoid designer would have made the spring strong enough to return the plunger under the conditions you describe assuming it is a spring return.

desertfox

 

[waross]

Sounds like either a design problem or component deformation with age. All magnetic devices such as relays or solenoids that I have seen where permanent magnetism may be an issue have a non magnetic gap in the magnetic circuit sufficient to limit the flux inside the hysteresis curve. I have seen relays where a brass rivet was used to create a small air gap and over time repeated closing impact deformed the brass pin allowing the air gap to be reduced below the original design dimension. Permanent magnetism would then become an issue. Newer designs often incorporate an air gap in a part of the core not subject to closing impacts.
Possibly a resilient seal was used to maintain a gap as well as the primary sealing function and over age has been reduced in thickness. Possibly the coils have been subject to over voltages that drove the flux above the design value. Possibly it is a marginal design.

Bill
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"Why not the best?"
Jimmy Carter