My original theoretical consideration was using the electric motor's rotor as drive shaft in a car. It led me to this question: "What will happen if we place a couple of separate, small stators around a long rotor instead of 1 large stator around a short rotor?"

I am a biology teacher. A motor design like this seems useful to me when I consider the structure of skeletal muscles. A skeletal muscle is constituted of a lot of long cells/fibers. Our body controls the force produced by the muscle by controlling the number of contracting fibers at a time. When we want to pick up a pen on the table, a few fibers are recruited; but when we want to pick up the table all fibers are recruited to generate enough force.

I guess that via changing the number of the active stators, a motor like this can be more efficient.

Is there any electric motor design like this? Or, is it possible and if it is, does it make sense from the points of efficiency and weight?


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  • $\begingroup$ What kind of motors we are talking about? In induction motors with salient poles, the shorter the rotor axial length the lower the speed, and vice versa. Long rotors dissipate more power. $\endgroup$
    – user14407
    Dec 8, 2019 at 16:34
  • $\begingroup$ @SamFarjamirad, I don't have enough knowledge on electric motors to answer what kind of motors can be used or if this is important. From my [amateur] perspective, it can be any electric motor type which can be used to move a car. $\endgroup$
    – serekani
    Dec 8, 2019 at 18:30
  • $\begingroup$ The electric vehicles are run by induction motors, your diagram, shows a huge air gap, between the stators. We have to keep the air gap as minimal as possible, this design probably won't work. Even if you manage to turn the rotor, then it wouldn't be efficient at all. $\endgroup$
    – user14407
    Dec 8, 2019 at 18:47
  • $\begingroup$ This is how you built frequency and voltage converters in the days before all solidstate electronic converters. One worked as the motor, the other ran as a generator. Consider a backup power supply for a critical application. Mains power delivered to one motor. If the power failed, a genset started, powering a separate set of coils. While the genset was warming up, batteries ran the thing through a third set of coils, and the site power came out the generator end from a forth set of coils. Fun stuff. $\endgroup$
    – Phil Sweet
    Dec 9, 2019 at 23:00
  • $\begingroup$ @PhilSweet Really enjoyed the funny aspect that you brought in. :)) But, need a clear answer. Won't it work? $\endgroup$
    – serekani
    Dec 10, 2019 at 11:07

1 Answer 1


A genius by the name of Nicolai Tesla conjured the idea of a shaft of an engine capable of delivering a variable range of power/torque with variable speed all controlled by demand, from scratch in his mind.

He even thought of a system of delivery of electrical power suitable for this purpose, three phase AC.

These engines are called a variable frequency drive, similar to what you see in a battery drill. The more you push the trigger back the faster the drill spins.

Another good example is the engine of a Tesla care. By changing the frequency and current fed to the engine they control the RPM and torque.

Instead of adding stators to the shaft, they make that same stotor flexible that causes the rotor to turn faster and or deliver more torque.


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