Electromagnet question
 

Hello All,

I know that a lot of folks here possess expertise in numerous fields, including electronics and electrodynamics. I have a question about something that I’ve been unable to find any references on. The question is:

If one took a “shorted” electromagnet—that is, an electromagnet whose (+) and (-) leads were simply connected together instead of being connected to a battery or other DC power supply—and moved this “shorted” electromagnet back and forth past a coil (or another electromagnet) that was connected to a power supply that was turned on, would any significant magnetic force be induced in the “shorted” electromagnet? (I know that an induced *current* would flow in the coil [a closed circuit] of the “shorted” electromagnet.) Here is why I’m asking this question:

I’ve been reading about brushless electric motors, which are widely used to power R/C model airplanes and motor gliders (these motors have many more hobby and non-hobby applications, of course). The brushless electric motor, which has rare earth permanent magnets in its rotor, completely avoids the problem of brush wear because it has none, and because its rotor has no electromagnets, cooling is less of a problem (the electromagnets of the stator, which surround the rotor, are easier to cool with air flowing through the motor’s casing). Now:

I got to thinking that even the rare earth permanent magnets in the motor’s spinning rotor have to be cooled somewhat, or else they would lose their magnetism (this process likely occurs in brushless electric motors, but slowly, because rare earth magnets are more robust than iron magnets). But if a brushless electric motor that contained *NO* permanent magnets could be built, its ^only^ parts that would be subject to wear would be its front and rear rotor shaft bearings. (Even the bearing wear could be eliminated if the rotor shaft’s front and rear bearings were replaced with air jet shaft centering & suspension, or with magnetic [or electromagnetic] suspension and centering of the rotor shaft at what would otherwise be the front and rear bearing locations, although only larger brushless motors would likely be able to utilize such bearing replacements—or have any real need to do so.) Also:

An “all-electromagnet” brushless motor would have stator electromagnets that would probably be similar to those in existing brushless motors (a larger number of smaller ones might—or might not—be needed in such an “all-electromagnet” brushless motor). Its rotor could contain two, three, or more equally-spaced, “shorted” electromagnets, which would probably look like those in a brushed motor. As each “shorted” electromagnet passed by an energized electromagnet in the stator, the current induced in the “shorted” electromagnet’s coil (by its passage through the stator electromagnet’s field) would also induce a magnetic force in the “shorted” electromagnet, which would pull (or push) on an adjacent stator electromagnet, providing the torque to make the rotor spin. In addition:

I suspect that such an “all-electromagnet” brushless electric motor would have a lower torque—for the same electric power input—as a regular brushless motor of the same size, because the induced magnetic force of its rotor’s “shorted” electromagnets might require significantly more current to be run through the stator’s electromagnets in order to induce significant amounts of magnetic force in the rotor’s “shorted” electromagnets. This higher motor input current would also generate more heat, but since this motor would not contain any permanent magnets (which would be de-magnetized by being heated, if they were present), the motor’s only thermal limitation would involve the insulation on its electromagnets’ wires. With proper air cooling (which even existing regular brushless motors have), the “all-electromagnet” brushless motor could operate over a wide temperature range indefinitely. As well:

The “all-electromagnet” brushless motor would likely be ideal for applications where very long continuous motor running times, and very long motor life, are important. This motor would probably be preferable for applications in which high torque is not required (although this could be dealt with by using appropriate gearing). Also, this motor would be easy to manufacture, as it would not require any exotic and/or rare materials such as rare earth permanent magnets; it could be made using ordinary steel, iron (for its electromagnet cores), and copper wire. I’m not seeking to patent this electric motor (my description of it here on a publicly-accessible online forum would probably make it impossible, now)—it’s something that I’d like to see someone make (as in the old “Popular Science” magazine illustrated department titled “I’d like to see them make...” , which featured improvements to products—and new products—for which a need was apparent).

Many thanks to anyone who can help!

If I read your question correctly, the answer is an induction motor. Here is one site describing it.

As it describes, a key aspect is that "the rotor coils are short circuited", just like you describe. This allows the stator coils to induce a current in the rotor coils, which means that a magnetic field will be generated by the rotor coils, which will now react with the magnetic field in the stator coil, causing torque. (It's been many years since my physics degree, but I think that is right.)

There is also a reluctance motor which has no permanent magnets, but it does have magnetic material in the rotors. By arranging several donut shaped coils around the rotor (like in a stepper motor), the current in the donut shaped coils attracts nodes on the rotor, causing it to torque. But, that's a bit different than you asked about.

Why are you concerned about cooling? Model aircraft electric motors, for the most part, run at a constant speed. That is the lowest load/heat inducing situation an electric motor encounters. The motors in racing model cars experience the most severe heating problems because of low speed full current loads under acceleration and even higher heat loads under dynamic braking. How are they dealing with heat dissipation?

I think that summarizes it well. And I'm pretty sure that's how my favorite Aggie described it when I took EE415 at Kentucky circa 1982 :)

Doug

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Oh boy did my eyes glaze over on that one.

Electronics and electrodynamics. What?

Ask the aliens. :rolleyes:

Did someone say Aliens?!?!

Not to sound to flip, but....have you ever flown an RC airplane other than, perhaps, in a powered sailplane? Constant speed?

That said, you are right that RC cars probably are worse.

To Jason's original premise: Unless the motor gets so hot as to get above the temperature at which the magnets begin to degrade (which is pretty toasty, even for neodymium magnets and hotter for samarium cobalt magnets), they should last indefinitely. But the query has led to an interesting and educational discussion.

LMAO! :chuckle:

I fly quite a bit of R/C aircraft and the ONLY planes that fly at a constant throttle setting for the majority of a flight are Pylon Racing aircraft (WIDE OPEN).
Below is a pic of my latest that will do an honest 140mph in level flight and is rarely at full-throttle.
Phoenix 7 "Ballistic Pattern" aircraft.
It uses a full 8 channels of control in my Graupner MZ-24 2.4GHz system.
Ailerons, Flaps, Rudder, Elevator, Throttle, Retractable Landing Gear, In-Flight Mixture Control, GPS Vario Telemetry Sensor.
Futaba coreless servos throughout on flying surfaces. Micro servos on throttle and Mixture OS .61 FSR ABC/Hatori pipe/OS 7F Carb from a 65 VR DF fan engine.

Thank you. I had read up on those motor types, and they are rather similar, but they are AC motors. The one I'm thinking of is a DC motor.