Changing magnetic polarity at will with electrical current?

[k1acept]

If two touching planks of wood are kept in place with opposing magnets in each plank, can one set have their polarity changed by passing an electrical current through them, allowing the other plank to move easily, and when the electricity switched off the plank now remain firmly in a new position as the magnets once again attract each other?

 

[MagMike]

Generally an electromagnet in one of the planks would be required.

 

[IRstuff]

How do you want them to move "easily?" That sounds like you need one side to be electromagnetic, and the other side to be ferromagnetic, as it's done with with magnetic door locks.

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[k1acept]

I see what you mean about magnetic door locks.

But that would require constant power, and if interrupted then the two surfaces would float freely, which is the opposite of what I need. I would like to disengage the two (reverse the polarity of one set so that both push away from each other) when the power is applied, relying on normal magnetic attraction to keep the two together (this is not a security product).

However, like the electromagnet door example, where one element is just plain metal (or a plank of wood, peppered with precisely placed magnets) the base element would be connected to electricity, but the top must be free to move.

 

[EdStainless]

The dot flip signs that used to be common along highways worked by re-magnetizing a magnet in order to change.
There are two magnets in the assembly, a ferrite disc magnet attached to the dot (flag) and a Vicalloy pin magnet that is wrapped with a coil. But pulsing the coil they can change polarity and flip the dot.
But doing this in a system with real force is difficult, as the magnets get larger, stronger, and the required fields (and currents) get very big.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

 

[IRstuff]

Magnetic door locks do not need continuous power; otherwise, they would be pretty silly when the power gets cut.

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[sreid]

See Magnetic Chucks.

https://www.globalspec.com/learnmor...e_tool_components_accessories/magnetic_chucks

 

[k1acept]

IRstuff: Only having just looked into electromagnetic locks I didn't realise that there were fail-safe and fail-secure types, I see now how both exist. However, it seems that the constant electrical field option with the fail-safe system will give added ability to the magnets so that they have the power stay "locked" together, but I believe with enough magnets that (for my non-secutiry related purposes) the magnets alone will be sufficient to secure the platform.

EdStainless: I am unfamiliar with the "dot flip" signs along highways, but after a quick Duck Duck Go I get the picture. Am I correct in thinking that they employed the solenoid design where the electromagnet is wrapped with a single coil? The issue of power, size and even heat then comes into play, so let's talk specifics: lets say that the base wood piece was one metric metre in length, by half a metre width (and x height). While the slab on top was double the length at two metres, but the same width and height, and both on the horizontal plane. Strategically placed strong magnets would keep them in alignment and make it a real push to separate the two. Using a solenoid system on the base (which didn't need to move) applying a charge to flip the polarity would then free the top slab be free to move by hand with ease: e.g. offsetting the top slab by half a metre to the original position from the base, turning off the electrical current the magnets then keep the top wooden plank in place with natural forces.

If this is possible, then depending on the size of the magnets, will heat become an issue with the solenoid wrapped magnets in the base unit?

 

[handleman]

So... Is this not as simple as taking two permanent magnets and wrapping a coil around one of them, such that applying a current would oppose the permanent magnet?

 

[IRstuff]

Much depends on the magnets, the ones used on hard drives are pretty darn powerful, and won't need any sort of EM backup. Moreover, if the facing surfaces are high friction, there will be almost zero chance of slipping, unless you've failed to mention the gorillas attempting to separate them.

Moreover, there are obvious bistable mechanisms, such as those on ball-point pens that can be used to engage and disengage the magnets without continuous power.

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[aussiejohn2]

Hi K1acept,
Have a look at magnetic bases (as used for machine tools, dial indicators, etc.). These have a magnet that can be switched on and off mechanically. It is possible to mechanise/automate that "switch".
John.

 

[k1acept]

handleman: yes, it seems so, but I'm trying to find out if it's possible in the first place? Issues like proximity of the magnets need to be addressed, as they will be touching and I only need one set to flip their polarity, but in essence they should repeal, then on a rail like system the top most plank would be free to move and then set itself in a new position once the power is turned off. No more complicated than that really.

IRstuff: Thanks for the advice on high friction surfaces, I didn't consider an uneven surface, which could well reduce unwanted movement when in a set position, good idea! However, with bistable mechanisms it seems they go against what I am looking for as they require moving parts, and I am trying to create a system that does not.

aussiejohn2: That was I believe the advice given from sreid, but that system requires constant power and I want a low maintenance system of which only needs power to move it (i.e. not very often).

 

[IRstuff]

Well, your choices are either continuous power to overcome the magnet force, or to move the magnet away from the surface, which can be done with either continuous power or with a bistatic mechanism. So, the way I see it, it's either continuous power, or move the magnet, with/without continuous power. Two cases require movement of the magnet, and only one of those two case will there be no continuous power applied at any time.

Actually, there is another alternative, which still requires a mechanism, but to move a mu-metal shield between the magnet and its attracted surface. Mu-metal is pretty cool stuff and reduces magnetic fields by orders of magnitude. The same hard drive magnets are backed with mu-metal, and that side can barely attract a sewing needle.

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[k1acept]

IRstuff: I was looking for a system with no moving parts. I just want to know if polarities can be switched with a currents passed through a magnet. The idea being that the opposing poles repeal the two materials and then when the electricity switched off then once again naturally attract, But the electricity only need be one to flip the poles of one set of the magnets.