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I purchased suction electric solenoid cylinder electro-magnet from Amazon and I purchased neodymium magnets as well.

I connect the magnet to a 12V supply and give it ~560mA. I then put the small neodymium magnet (on the end of a plastic stick) close to the field of the electromagnet such that they repel. I am able to feel the field, but when the neodymium magnet gets somewhat close to the iron core it is drawn to it and sticks to it.

My confusion is, shouldn't the field of the electromagnet be strongest closer to the core and repel with even greater force? Why would it be drawn to it? Did the strong neodymium field realign the domains in the iron core when it got too close, and the get stuck to it?

Please let me know if my question doesn't make sense. I am an engineering student trying to understand the behavior I am seeing. My goal is to control the current of the electromagnet to crudely control the repulsion.

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  • \$\begingroup\$ The answer is basically to use ironless electromagnets. Yes they are weaker. But they won't stick to your Nd magnet. \$\endgroup\$
    – user16324
    Feb 28, 2020 at 12:09
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    \$\begingroup\$ by ironless electromagnet, you mean basically a solenoid coil (air in the center)? \$\endgroup\$
    – blueether
    Feb 28, 2020 at 18:43

2 Answers 2

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There are two kind of forces: the electromagnet VS. magnet, and the reluctance force of the electromagnet and the magnet. The reluctance force is the one which actually binds the two pieces together. This is what the electromagnet is designed for.

enter image description here

As the NdFeB magnet has a higher magentic flux density than your electroagnet, it simply binds on the armature, the repulsion force is smaller that reluctance force.

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The magnetic field of the neodymium part, when getting close to the constant field of the coil, is being overriden by this stronger magnetic field, and the small magnet becomes just a piece of metal which sticks to the electromagnet. In other words small part, with its small magnetic field, gives up resisting to the bigger part with stronger magnetic field when this strong field gets behind the opposite pole of the small part and "grabs" it. I guess doing this way you risk blowing magnetic properties of the small part.

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  • \$\begingroup\$ Is there a way to reduce or avoid the the smaller magnet being attracted to the electromagnet? Could a glue or screw the neodymium magnet into some aluminum yolk? \$\endgroup\$
    – blueether
    Feb 28, 2020 at 7:41
  • \$\begingroup\$ If you do it, you increase the mass of the assembly. Force will still be there, but factored down by the mass. Also note that big surface will cause currents in the metal (even in aluminium). The only way to mitigate the situation is to increase the field of the smaller part, or decrease the field of the big electromagnet (change the balance of fields and their mutual collocation). This is a physics, and this is life... Would you clarify what is the purpose/would-be end product of your investigation? \$\endgroup\$
    – Anonymous
    Feb 28, 2020 at 8:05
  • \$\begingroup\$ First, thanks for all the explanation, it is very helpful and I understand things a bit better now. The contraption in question is part of a project. What it involves is a seesaw with a permanent magnet attached to it and an electromagnet below it. By varying the current of the electromagnet I can control the angle at the fulcrum of the seesaw. I intend to use an h-bridge to control the electromagnet via PWM. \$\endgroup\$
    – blueether
    Feb 28, 2020 at 17:07
  • \$\begingroup\$ As described, one of my problems has been that when the permanent magnet gets too close to the electromagnet it goes from repelling to getting stuck. I intend to address this issue by limiting the range of the seesaw physically so it can not make direct contact with the magnet, but I would of course prefer to not have the problem at all. \$\endgroup\$
    – blueether
    Feb 28, 2020 at 17:09
  • \$\begingroup\$ Some others have suggested using a solenoid and thereby eliminating the core. This seems doable since I don't need to generate a huge amount of repulsion force to move the seesaw. Maybe I could bolt the solenoid into a thick aluminum yolk to further strengthen the repulsion force? \$\endgroup\$
    – blueether
    Feb 28, 2020 at 17:09

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