Electrical – Electromagnet Polarity Reversal Doesn’t Matter


I am working on an engineering design project for school which uses the electromagnet linked below to "lock" a door. The problem is that the electromagnet remains energized or magnetized even after turning off power to the coil. I believe this is because the core of this unit is tool grade steel and not soft iron so a magnetic field is induced while the magnet is powered on and is retained in the material after it is powered off.

My plan to counteract this behavior was to reverse the polarity supplied to the electromagnet for a short period after powering off the lock which would hopefully "release" the other side of the door lock. Unfortunately, even physically swapping the leads to the electromagnet does not change the direction of the field and it is not reversible. I don't know if there is an internal rectifier in the magnet or if the physical windings in the coil do not allow for a reversal of the field. Is there something I can do with the circuitry to counteract this or should I shop for a dual coil electromagnet that can be reversed?

Thank you!


Best Answer

If you want to demagnetize something, you actually have to use AC current, rather than opposite DC. Assuming the problem you are experiencing is residual magnetism you should be able to test by sticking the magnet to other things. Unless they've put a diode in the 2 lead electromagnet you're considering, you can reverse it, it'll just immediately remagnetize the other way and keep holding. Why do you assume the core is of an unsuitable material?