Unconventional Homopolar Motor

As a hacker, chances are that you have built a homopolar motor, as you only need three things: a battery, a magnet and some copper wire. There are zillions of videos on YouTube. This time we want to show you [Electric Experiments Roobert33]´s version. Definitely a fresh twist on the ubiquitous design that you see everywhere. His design is a bit more complicated, but the result makes the effort worthwhile.

Right hand rule for the Lorenz force. By Jfmelero, via Wikimedia Commons

The homopolar motor was the first electric motor ever built. Created  Michael Faraday in 1821, it works because of the Lorentz force. This force acts on any current-carrying conductor that is immersed in a magnetic field which is perpendicular to the current. These motors really have no practical applications, but are an excellent way to learn basic aspects of electromagnetism.

In this setup, there are two conductive rings placed above a wooden base, connected to the battery terminals. Neodymium magnets are connected by a conductive rod that pivots in the center of the rings, closing the circuit and allowing the flow of current. Then the Lorentz force makes its magic and pushes the rod and magnets in a circular motion.

Very clean and well-edited work, as are other videos by [Electric Experiments Roobert33]. You may want to replicate this nice motor, or you can also make the simpler version to start experimenting.

37 thoughts on “Unconventional Homopolar Motor

        1. That would need to be some mighty conductive rubber.

          This thing is essentially a direct short on that poor battery. The current limiting element here is either the internal resistance of the battery or those puny clip leads he’s using.

          1. Got some conductive rubber boots for the end of the fluorescent bulb backlights in a scrapped (to be repaired*) DELL U2410f 1920x1200p LCD found in the scrap bin at work (still at work unclaimed on bench).
            Literally the failure mode is the resistance goes high enough the boots carbonize and the safety kicks in… Sometimes too late.

            *I didn’t scrap this one as I would hard-wire the PCB to the bulbs and use replacement silicon boots instead of the conductive fire hazards… for highly improved safety!

            But as for practicality of this posts’ motor… Apart from extreme efficiency issues that become obvious at first sight. Conveyance of light loads is the first I think of: analogue-infinite resolution positioners of sorts?

          2. Isn’t there some back-EMF pushing against the battery, limiting the amount of current? Same way as an ordinary motor. Otherwise an ordinary brushed motor looks like a dead short too.

          3. “Isn’t there some back-EMF…”
            Yes, there is, but the homopolar motor is essentially a single very short turn with a very large conductor cross section — the length of the “turn” is the radius of the disk magnet here, and its cross section is the thickness of the disk. Compare that “turn”‘s resistance to the metres of small-diameter wire found in the many turns of a typical motor. Vastly different stall resistance and vastly different back EMF. The homopolar motor is an extremely low impedance device.

          4. I was waiting for the clip leads dropping their insulation during the video. :-) They have normally around 1 Ohm each and at around 5A the PVC is sometimes dripping quite fast. :-)

      1. I wonder if you can get it working the other way? Paint the conductive tracks on the underside of a record, bolt the magnet in place, and see if it spins round by itself. Super awesome if it does. Then for an encore, we increase the power for self-launching frisbee!

        1. Homopolar means it rotates about one axis (homo = one/same, polar = well, polar)

          This rotates AROUND one axis (the central pin), and ABOUT another axis (the axle itself) so it’d be bipolar at least, yet not fall under the general definition of a “bipolar motor”

          1. The homopolar motors (there are two) here are the two magnets themselves, rotating on the brass shaft. The magnets themselves are the classical homopolar rotor(s). The brass axle is just going along for the ride and carrying the current, but is not contributing to the motor effect.

            I wonder why he bothered to use two magnets? He could simplify it by omitting one and just injecting current at the central pivot.

          2. Ah, I’m not very well-versed in the topological nomenclature of motor mechanics, I’ll admit.

            As for using two magnets: Stability and repeatability. If both magnets are the same diameter, then they can both bear the load as opposed to the pivot pin doing so (in which case the tolerances on the pivot pin would need to be tighter)

          3. “The name homopolar indicates that the electrical polarity of the conductor and the magnetic field poles do not change (i.e., that it does not require commutation).” (Wikipedia: Homopolar motor)

          4. The rolling electrically conducting coating on the magnets are the brushes. It is simply a Faraday disc motor in reverse. “Homopolar” is sadly a pseudoscience term that has permeated popular culture.

          5. Matthew, go refresh your memory on the difference between a sliding contact and a commutator. True this device has sliding contacts (“brushes” you call them) where the magnet disk meets the shaft, but in no way does that perform any commutation. There is no circuit switching going on here. It’s simple DC current.

            And true, the homopolar motor or generator has attracted more than its share of the woo fringe crowd, but it’s certainly not “pseudoscience”. It’s a well-known device and name, in commercial use for well over a century. Even now it’s tough to beat it for large-scale pulsed power applications.

      1. Bigger magnets would help, but in this configuration it will be essential to use a material with high nuclear spin to adequately convert the angular momentum. Niobium-93 might be adequate, with a 9/2 spin (the highest normally available), and has the additional benefit of being superconducting at liquid helium temperatures. Niobium-90 would be even better with a spin of 8, but unfortunately has a half-life of only 14 hours so would tend to degrade quickly, and also be difficult to keep cool enough to remain superconducting.

        1. Problem is, when a parody is indistinguishable from the things the parody’s subject actually says, nobody can tell the difference. That’s why parody’s so hard to do these days. You make some ridiculous exaggerated ironic prediction, then a week later it comes true. The world’s gone mental. The thin veil of whatever it was that was preventing mob anarchy, has pretty much disappeared.

          I think it’s fair to blame mass communication. Facebook in particular, and Facebook-ness in general. Twitter too, you can’t put much reason into 120 characters or whatever it is. Before, people were quietly ignorant and stupid in their homes and families. Now nonsense rampages all over the world, and for some reason newspapers have shortcut the path between gossip and front page news. There’s nobody at the controls! In future when you buy a newspaper, they’ll print off a page of search results from Twitter, right at that moment.

    1. I was wondering just that. Would be nice to see a few more runs. AIUI it’s necessary for it to be moving, in order for the magnetic field to be “behind” it and push. And yup it should surely be able to run in either direction.

      Are the magnets just ordinary round magnets? Maybe they’re polarised in an N-S-N-S pattern around the rim.

    2. The direction it goes is determined by the polarity of the magnet(s), and the direction of current flow. Reverse either the magnet or the current and it will rotate in the opposite direction. Reverse both the magnet and the current and you’re back to the same direction.

      In this implementation, you should to be careful to get the magnets oriented so they produce torque to rotate in the same direction, not fight each other, though the outer one will win if you get them opposed.

      The magnets are normal simple disk magnets: north on one face, south on the opposite. Just like any other homopolar motor, when you pass a current along the radius from the hub to the periphery (i.e., the current flow is at right angles to the magnetic field) it produces the force (a torque) that tends to rotate the disk.

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