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I have a number of solenoids I would like to use in a project. The problem is that they're not quite powerful enough so I've been trying to come up with ways to increase the their pulling force without increasing the voltage.

I was thinking that I could add a permanent magnet to the bottom of the plunger so in addition to the the core being pulled into the coil the magnet would be repulsed by the coil and add to the force the plunger has.

Would this actually work? And if so what strength of magnet should I use? Should the magnet sit on the outside of the coil or just inside?

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  • \$\begingroup\$ Not a duplicate. That question is discussing increasing the pulling force by changing the core shape. This one is discussing making use of a permanent magnet. \$\endgroup\$ – RedHatter Jul 27 '18 at 22:42
  • \$\begingroup\$ Have you not found “how to design a solenoid?” On the web? aip.scitation.org/doi/abs/10.1063/1.1752067 \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 27 '18 at 22:43
  • \$\begingroup\$ @TonyEErocketscientist I have read a lot on solenoid construction but nothing discussed the possibility of making use of a permanent magnet. Now this might be because it doesn't work for some reason, but the logic seams solid to me which is why I'm asking here. \$\endgroup\$ – RedHatter Jul 27 '18 at 22:46
  • \$\begingroup\$ Once the core is in contact with the permanent magnet, what is used to separate them? \$\endgroup\$ – Solar Mike Jul 28 '18 at 5:42
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Are you hoping for more constant push/pull force or would more start of push/pull force be acceptable?

because of the exponential decrease you'll get in force as the rare earth magnet is pushed away, depending on travel, you will likely alter the force produced and/or acceleration for a portion of it's travel range, but because you are not actually altering the strength of the electromagnet, I don't picture too much difference in constant push force. The metal casing on top bottom and 2 sides of the solenoid is hopefully silicon steel or high permeability steel, and if it's doing it's job well, most of the magnetic field available to repel the rare earth magnet won't extend far past the casing. The permanent magnet may also interact with, for example, the metal of the solenoid's casing in an unexpected way, and it will definitely try to pull itself towards the casing, against the action of the solenoid up until the moment the coil is energized.

It might even pull hard enough to prevent the solenoid from actuating, but what I picture happening is the magnetic field of the solenoid pushing the field of the rare earth magnet at least enough to get it out of the casing(the field, not just the magnet), and possibly accelerating the solenoid faster up to that point, and also acting as a soft brake against forces pushing against the already actuated solenoid. If the fields interact too closely/strongly the rare earth magnet WILL noticeably demagnetize over time.

For the half shmeckel a 1/4" rare earth magnet costs at a hobby store, I'd try it. I have rare earth magnets but no solenoids or I'd test it right now myself and I may have a chance this weekend. Please write again if you do test it. I'm curious. Trying will only break your solenoid if you somehow manage to overcurrent it.

If you want you could also use a Finite Element Magnetic Modeler to check and also visualize the interactions.

Other ways to increase the strength of the solenoid:

1.) Increase voltage/current

2.) Improve magnetic circuit (increase permeability of core and or casing)

3.) Re-wind core with smaller wire and more turns

4.) Lubricate

I think that covers it. If you absolutely can't do #1, I think #2 is your best bet for a quick fix/test. Just surround the core with high permeability material, like iron or ferrite and see if it's measurably stronger.

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  • \$\begingroup\$ smaller wire won't make the solenoid stonger unless the smaller wire is more conductive. \$\endgroup\$ – Jasen Jul 28 '18 at 9:56
  • \$\begingroup\$ Smaller wires can make the solenoid stronger because you can fit more turns in a smaller volume. In this case, he wants to avoid increasing the voltage/current. It's not clear whether he is already at max, but if not, he can use smaller wire, which can carry the current he is using, but use a larger number of turns at that current level, which will strengthen the solenoid. \$\endgroup\$ – K H Jul 28 '18 at 19:01
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One limiting factor in a solenoid is the amount of iron, iron can only carry so much magneitc flux so there's a hard limit on the strength of a solenoid

if you're using the solenoid at less than rated vopltage then adding a permanent magnet could make it pull at full stength at reduced voltage by adding some extra magnetism top the system. on the down side you may need to reverse the voltage to make the solenoid let go.

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