Solenoid plunger becomes magnetic!?
Solenoid plunger becomes magnetic!?
(OP)
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!
As a side question, is this a common problem for DC solenoids? Do AC Solenoids have this problem?
Thanks!





RE: Solenoid plunger becomes magnetic!?
Roy
RE: Solenoid plunger becomes magnetic!?
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?
RE: Solenoid plunger becomes magnetic!?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Solenoid plunger becomes magnetic!?
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...
RE: Solenoid plunger becomes magnetic!?
No spring in the picture?
Keith Cress
kcress - http://www.flaminsystems.com
RE: Solenoid plunger becomes magnetic!?
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
RE: Solenoid plunger becomes magnetic!?
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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