So I've been chasing down devices with unique operational characteristics looking for potential in energy generation and I ran into these things: homopolar motors.

About 9/10 of these things are a wire on a battery and a magnet and the professional variants are used as low voltage high current generators and use a disk spinning perpendicular to a static magnetic field to generate power, taking energy from the rim and axle.

Anyway, it got me thinking about it and I realised something about when these motors are running. After the stator or rotor has been powered for five RL time constants, there's no more reactive component to the engine and all the supply sees, no matter how much or how little work the rotor does, is the ohmic resistance of the coils.

With no current alternation and no reactive component, you could have 10,000 ampere-turns of magnetic field torquing the rotor and the coil resistance could be approx. 6 ohms and only need 0.5 amps supplied at 12 volts, if you were patient enough to wind 20,000 turns.

The rotor is torqued by the strength of the field and being torqued doesn't take field strength away or change the steady powering of the coil. 100% of the actual input power is turned to heat via resistive heating. The inertia in the rotor essentially isn't coming from your input power, it comes from the interaction of the rotor with the infinitely spiraling vortex shaped field of the stator.

The input power determines the strength of that field, but no conversion of energy is actually taking place between the input and the output. The input is 100% consumed by joule heating.

So normally people make these in science class with a battery, a magnet and a folded strand of wire or they copy Faraday's design using a disk rotor. But I figure there must be a better way to make these things and make them more practical and useful.

I figure that if a coil is used then the supply voltage could be kept low since wire resistance is not generally high anyway but the supply current could be dropped as well since a coil could make more efficient use of the amperage for field generation.

What do my fellow engineers think?

Any suggestions for how I should tackle this?

  • $\begingroup$ Paragraph 4 says, "6 ohms and only need 0.5 amps supplied at 12 volts ...". Check the maths on that! $\endgroup$
    – Transistor
    Dec 2, 2023 at 12:32
  • $\begingroup$ I never specified awg, but 22 gauge is only 16.14 ohms for 1000 feet. If you were willing to use 14 gauge wire, you'd have 2250 feet of cable to wind before reaching 6 ohm. But that's not the main point. The main point was that the motor runs while presenting the circuit with only wire resistance as the load after 5 RL time constants. That doesn't mean efficient, but it does mean not directly related. $\endgroup$ Dec 2, 2023 at 12:48
  • $\begingroup$ I just find it really interesting because the motor should basically look like a solenoid to the powering circuit $\endgroup$ Dec 2, 2023 at 12:54
  • $\begingroup$ If you have 250' of 22 AWG facing a 0.5 tesla magnet the force on the magnet will be "F = B * I * L = 0.5 * 1 * 75 m = 37.5 N" and the resistance of that length of wire, plus the parts of the wraps the magnet doesn't see should be about 12 ohms(750' total wire used let's say). Since the current is set at 1 amp, that means 12 V to supply that current. If the magnets are at a radius of 0.04 m on the rotor, that's 1.5 newton meters of torque for an input of 12 watts. $\endgroup$ Dec 2, 2023 at 13:17
  • 1
    $\begingroup$ You should approach this by not thinking you have discovered a great new way to generate power. 100% of the principles involved are known, and trying to put significant power through one would soon point out its shortcomings to you. These motors are used to demonstrate principles, there are better ways to generate power. $\endgroup$
    – Tiger Guy
    Dec 2, 2023 at 15:20

1 Answer 1


The first way to increase the homopolar/unipolar performance is to research possible methods of removing the friction from said brushes such as Tesla's own patent US406968, the "Dynamo-Electric Machine". using a flexible conductive belt between the rims of two such discs. Then imagine, as he did, multiplying thier output by interconnecting the axils of multiple such dualpolar discs.


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