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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!

https://www.amazon.com/uxcell-Electric-Lifting-Electromagnet-Solenoid/dp/B01NASKR8D

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  • \$\begingroup\$ Let's see, no datasheet, but plenty of 'suction' (500N's worth) Who knows what is on the inside of that device. You get what you pay for. \$\endgroup\$ – Voltage Spike Jul 26 '18 at 4:04
  • \$\begingroup\$ Door magnets I've seen have a small spring that pushes a stud out about 1 mm, this counteracts the residual magnetism. Perhaps you could invent a short, stiff spring to do this. \$\endgroup\$ – tomnexus Jul 26 '18 at 4:31
  • \$\begingroup\$ Soft iron would be more likely to stay magnetized than steel. \$\endgroup\$ – JRE Jul 26 '18 at 7:29
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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?

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  • \$\begingroup\$ Just from research. Electromagnets using a soft iron core apparently do not retain as large of a magnetic field after powering off as the one I have. The magnetic force of this unit is always in the same direction no matter if the leads are swapped OR the magnet is powered on or off. \$\endgroup\$ – granthworth Jul 26 '18 at 3:51
  • \$\begingroup\$ You are definitely correct about that. If you can confirm that residual magnetism is the problem, you may be able to drill out the core and replace it with ferrite or such if you don't have time to make or order a better electromagnet. Some amount of residual is also normal in the type of door holder you're describing. \$\endgroup\$ – K H Jul 26 '18 at 4:03
  • \$\begingroup\$ What method are you using to confirm that the magnetic polarity stays the same? They may have put 2 diodes in to prevent reverse connection for some reason but it seems odd to me. \$\endgroup\$ – K H Jul 26 '18 at 4:05
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Magnetic remanence is caused by magnetic saturation and, if you drive the electromagnet at a slightly lower level, the proportion of magnetism remaining (when the current falls to zero) compared to the driven level reduces significantly. See this from wiki: -

enter image description here

My advice is to put a resistor in series with the electromagnet to limit the saturation of the core and thus deliver a significantly lower magnetic remanence. This may be enough to achieve what you want. If not then I suggest that instead of the electromagnet forming a closed loop magnetic field when locked, you add (say) a 1 mm space to prevent the peak flux density reaching saturation levels. Clearly the electromagnet can start to pull with a gap of presumably much more than 1 mm so restricting the final 1 mm movement should not be a problem.

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