# EEs & electrohobbyists: I want a circular linear motor

I’ve got an idea I’m trying to prove/disprove and my meager recollection of Physics 201 won’t let me solve it analytically and my weak fabrication skills aren’t up to the task of solving it experimentally, at least not so far.

The basic idea is to take a solenoid or linear induction motor & extend it into a closed circle.

Imagine a tube 1/4" in cross section & say 12-1/2 inches long. Now bend the tube into a 4" radius circle & connect the ends, forming a torus. Before you close it up, put a ball of either steel or magnetic material in there. Wind transformer wire around the torus forming a toroidal coil.

In the case of a typical linear solenoid, simple DC applied to the coil will push the slug out. I think that’s because the coil produces a mag / B field down the core of the solenoid, pulling the ferrous slug with it. Reverse the polarity and it pulls the other way.

So for my coil, applying simple DC to the one toroidal set of windings ought to propel the ball bearing around the central toroidal tube either CW or CCW depending on the polarity.

True? Not true? If not, why not? Does the ball want to be magnetized itself, or will merely being ferrous do the trick?

Next iteration: replace the ball bearing with a curved rod (a “slug”) which fits the toroidal tube. Make it about 90 degrees of arc long. WIll the slug run around the tube under DC power? Will there be some interaction with the E field or something else which would cause the slug to try to rotate along it’s local central axis, thereby trying to bind in the tube?
What references (online or engineering handbooks, etc) would be good to look at to learn how to calculate windings vs field strengh vs voltage/amperage, choices & results of winding armature materials, energy requirements, coupling constants, etc. In short all the factoids I’d need to rough out a prototype by design rather than by luck.

Assuming it’s really possible, if anyone knows of a commercial version of this thing, let me know. All told I rather buy one than build one.

• I don’t think that this device of yours wouldn’t do much of anything. The slug would always be held in the center of the field, especially if you only had one single winding around the torus. But even if you used multiple coils and phased them, most of the field would still get conducted around the torus anyway. To be efficient the slug would have to be able to span as much of the distance (or put in other terms: as high of a percentage) as possible between the field poles–not held in the center of the field.
• If you wanted to make a “brushless DC motor”, then look up how to make an “eddy current motor”. I looked and couldn’t find any decent websites, but what it is is a circular coil of say 4-inches diameter. When you apply DC to this coil and hold a 4-inch diameter aluminum (non-ferrous!) plate near it, the plate will want to spin. The efficiency of these motors is very poor, they aren’t used for anything in the real world that I know of, only as classroom demonstrations.

…If you look up “lenz eddy current” on Google most articles you find are referring to eddy-current clutches, which is something like what the mechanical speedometers use–a spinning wire is drawn off the transmission (somewhere) and that runs to the speedometer in the dashboard. On the end if this wire is fastened one or more magnets. Near the magnets is an aluminum disk, and then the aluminum disk is what is connected to the indicator dial of the speedometer. When the magnets spin, they impart torque to the aluminum disk due to eddy currents that they produce in the aluminum disk. So to build a DC motor, you would just substitute a coil for the spinning magnets.
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It isn’t going to work as you think. First a magnet can only repel the opposite pole of another magnet. Second your toroidal magnet essentially has the two poles touching so there will be no circular direction to pull or repel the curved magnet/slug inside.

You could make it work by only wrapping sections of the torus and alternating current flow to them so it attracts the slug to a section until it moves enough the next section can pull it. What you’ll have is not very different from how an AC motor works but it will be vastly more difficult to fabricate.

You are not going to be able to do this practically because you need a shaft to develop rotational action.

Your core, or slug is going to be pulled along straight line, but it needs a second force to pull it inwards, whereas the shaft already does this, do your vector sums and you’ll see what I mean.

If you pulse the initial field to produce the linear motion, and another at an angle you might do it, but it would need to be in very small steps.

If it is possible to suspend the core in free air using a magnetic field, then it would be possible to move it, but the scale required and the amount of power would probably make it impractical.

The issue here is that once you actually get the slug in motion, the magnetic field that holds it from moving outwards would need to increase, the faster it goes, the greater this field would need to become.

The linear motor does not have this second moment, but bend it around to join at the ends and it certainly would.

You seem to have misunderstood what happens in the linear solenoid you observed. When you energize the coil, the solenoid tries to minimize the total energy by moving the slug into the coil where it will pass the greatest flux. This is true regardless of the polarity. A torus of steel would be equally permeable to magnetism all the way around, so the solenoid would not make it rotate.

And it’s a shame it wouldn’t. Electric motors would be much simpler if it worked that way.

About the best you could do would be to have permanent magnetic zones in the torus, and use an alternating current solenoid. Imagine making a rod by gluing together little bar magnets, north to north and south to south. They will fight you all the way, as you glue this thing together, but you struggle on somehow. Then bend this rod to make your torus, with a solenoid on part of it. Energize the solenoid with alternating current, and if you get the torus spinning it will keep spinning. Look up linear stepper motors and you will get the idea. Also see synchronous AC motors (the largest motors are built this way).

After more thought I realized simpler ways to build something like your motor. You could use an ordinary iron ring - indeed, even a ring of a non ferromagnetic material (though a magnetic one would be better).

What you need is multiple windings in the solenoid, that focus the flux at different points along the solenoid axis. Then you’d drive the windings with phase-shifted AC. It would work as an induction motor.

Or, for a different approach, using DC, you could have the iron ring pass through a permanent gap magnet, and arrange brushes on the sides of the ring so that they pass a current through the small diameter of the ring. You must arrange so that the motion vector of the ring, the current flow vector, and the magnetic flux vector are all mutually orthagonal. Then your ring will spin around due to the Hall effect, like circulating fluid in a Hall effect pump. This will have a power curve like a brush motor having permanent field magnets.

Thank you one and all. I was passing along somebody else’s arm-waving efforts at EM physics, and I was pretty sure it wasn’t gonna work, at least not as described.

I did recognize you could wind several separate coils around your toroidal tube and energize them sequentially to push/pull your slug around the toroid. Essentially that’d be a typical commutated motor turned more or less inside-out … sorta.

Obviously, there’d no way good to extract mechanical energy from this thing; it doesn’t have a shaft. The goal was simply to make drive the slug around the toroid. Another impracticality would be that at any meaningful RPM the slug would bind against the outside of the toroid due to centrifugal (I know) force.

I have a commercially made aquarium air pump using an oscillating compressor, diriven by an eddy current motor. Not much air output and gets hot as blazed within an hour. Marginally useful for an aquarium. More of a curosity piece.

Your idea sounds remarkably like a circular particle accelerator. I’m not a design guy, but I don’t know why you couldn’t just put a series of coils along the torus and fire them sequentially, repeating the sequence of coils every few inches or so. Depending on the power requirements, it might even be possible to use an off-the-shelf stepper motor controller to drive the set.

That said, good luck getting any energy out of the system. You might be able to wind the coils so that there is is an open slot at the inside of the ring by winding one loop of say the ‘A’ phase, then going back around for the first loop of the next ‘A’ winding and repeating ad nauseum, but I wouldn’t want to be the one doing it.

I remember one of my tutors at college mentioning something like this, he was trying to do the research for a very similar motor as part of his degree thesis.

In the end, it turned out that his team simply could not induce a large enough magnetic field in the rotor( his terms, even if it seems a misnomer in a linear motor)

The currents flowing in ductors are usually taken to have two components, the in-phase which relates directly to the power output, and the out of phase component, due to the inductive windings and is 90 degrees out of phase - and this component is responsible for setting up magnetic fields, EEs will understand this to be the X[sub]L[/sub] reactance.

His research found that he simply could not produce a large enough reactive current to generate the desired effect and with too weak a magnetic field, you don’t produce enough torque.

He could get the rotor to run on no-load, but that is of little practical use

Even when he increased the frequency of the supply, he found that it all got just too hot.

He did also mention that in effect, he needed superconductors, only at the time he simply produced a set of data that showed that it couldn’t be done, and superconductors were unheard of back when he did this.

This was a long time ago, dating back to the late 1950’s, and I would not be surprised if advances in materials would make it more viable, however, the telling thing is that AFAIK, it still isn’t commercially viable.