I want to learn about electrical wheels, typically used in electric bicycles. But most of information I could find is related to marketing, and nothing on theory. This is probably due to "relative novelty of this field", and each manufacturer want to keep its secret. But to become one - one should learn motor construction theory at first.

Afaik, design of the vast majority of these wheels implies multiple coils on the inner stator, which (when fed by AC of rectangular form) forms magnetic fields, that pushes neodym magnets on the rotor, thus creating motion: enter image description here

But also I heard about (magnetless) squirrel cage rotor, with construction principle, illustrated by following image. Despite it is not a actual motor rotor, it has some similarities in its construction (bars, short circuiting rings) : enter image description here

Image from: pnpntransistor.com

So I have a question - why even bother with rare and expensive neodym magnets (that could easily break on the road), while always can use latter variant? I dont understand well, how latter works, so it would be nice, if you could compare and explain operating modes of these two types of motors, "side by side" (to show difficulty or impossibility of creating latter variant). Thanks.

  • \$\begingroup\$ " I dont understand well, how latter works" I suggest you learn that first, Otherwise it will be difficult for us to put the motor types side-by-side, not knowing what you understand. Also the answer would be an educational exercise and we don't do teaching here. Besides, there are many more motor types then just those two. (And if the above shown wheel breaks I doubt it will be the neodym magnets) \$\endgroup\$
    – Oldfart
    Mar 24, 2019 at 13:50
  • 2
    \$\begingroup\$ @Oldfart "we don't do teaching here" - that is falsy statement, I have learned a lot from SE sites. And many people did. So what you actually do - is actually a teaching process, or "sharing knowledge in a form of answering questions". \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 13:57
  • \$\begingroup\$ The NIB motor is notably more efficient. \$\endgroup\$
    – Digiproc
    Mar 24, 2019 at 13:57
  • \$\begingroup\$ @Digiproc So I asked - why and how? You may create a force by a constant magnet in a rotor. Or a winded electromagnet, that creates a field by running a current through rotor coils, that is opposed to stator field. That creates a force. But what about squirrel cage? There are no magnets in it - nor constant, nor electrical. So I wondered a simple question - "how does that even works?". Answerring that could also answer "why is it efficient/inifficient/impossible to use?" By "side by side" I mean you can show analogies and diffirencies, so that novice could easily understand on a given example. \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 14:01
  • 1
    \$\begingroup\$ Contrary to their name, most of the mass in a neodym magnet is actually iron. And the copper in a cage rotor is not that cheap either. \$\endgroup\$
    – Turbo J
    Mar 24, 2019 at 14:26

3 Answers 3


There are two concepts you are discussing here

  1. in-hub electric machine
  2. Asynchronous vs synchronous machines

The novelty here is the in-hub electric machines. By producing an inside out motor where the rotor is on the outside and rotates around the inner stator, the drive chain is simplified, maintenance is simplified etc.. There are technical challenges associated with this and hence where there are per-manufacture specific trade secrets.

As to the type of machine: Induction vs PMSM, horses for courses. The reason that there are about 6 fundamental types of machines ( How do DC motors work with respect to current, and what consequence is the current through them?) is because each one solves the need to convert electrical energy into rotational energy in their own way. Fundamentally all need to produce rotating magnetic fields

Squirrel cage induction machines are extremely robust and relatively cheap to produce. HOWEVER, they are not that efficient and equally their displacement power factor they draw puts a higher VA rating on the inverter. PMSM solve part of this problem by already providing a rotor flux.

If you are after power density you will be hard pressed to beat a PMSM. If you are after efficiency you will again be hard pressed to beat a PMSM complete assembly. If you want a cheap motor, robust motor and can handle the additional losses and oversizing of output stage then induction machines are viable

Below is a blog from Tesla as to why they chose induction machines over PMSM. They however were comparing Induction vs BLDC control. A BLAC would provide smoother torque ripple which


  • \$\begingroup\$ Thanks, You've provided some entry points to "where could I start studying the question". Good job! \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 14:32
  • \$\begingroup\$ I always felt like all that was made previously was "inside out". First people jump over head to push field inside, to rotate some inner axis. Then they put a gear on that axis, to bring rotation back to gear's exterior surface. And now they stopped doing double inversion, making exterior surface rotating directly (be it a gear, or a wheel surface in my case). This is just a point of view, but what I was asking for is actually "direct motor". And all motors with rotating shaft are "inside out motors" \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 17:58

Because of the limitations of this forum, it is only possible to provide a brief introduction to the types of motors used for vehicle propulsion.

Squirrel-cage induction motors have been around for a long time. Their main advantage is their simple construction. That makes them relatively inexpensive to manufacture and very reliable. The rotating part (rotor) receives power from the stationary part (stator) by induction. The magnetic field in the rotor is produce by the induced current rather than by permanent magnets. All of the electrical power that is converted to mechanical power is transferred to the rotor by induction. In order for current to be induced in the rotor, there is a speed difference called slip between the speed of the rotating magnetic field and the mechanical speed of the rotor. That slip causes quite a bit of power to be lost as heat.

Since the rotors of permanent-magnet motors are magnetized by permanent magnets, no current is required in the rotor and their is no slip. That means less power lost as heat and higher efficiency.

Although induction motors and simple in their concept, there are certain complexities in their design that make it difficult to construct induction motors with many magnetic poles. Speed is proportional to frequency divided by the number of poles. That means that a motor that operates at the wheel speed of a vehicle needs to have many poles or a very low frequency. That makes it quite difficult to make an efficient induction motor for a wheel motor.

Permanent-magnet motors can be designed in several different ways. They can be DC commutator motors, but those are not very good for wheel motors. They can have the magnets on the surface of the rotor to make surface permanent magnet (SPM) motors. They can have the magnets in the interior of the rotor to make interior permanent magnet (IMP) motors. They maybe called brushless DC (BLDC) motors or synchronous permanent magnet (SPM) motors. The difference between BLDC and SPM motors is somewhat semantic, but also a design subtlety.

As you can see, there is quite a bit to study to really answer your question.

  • \$\begingroup\$ Such a brief introduction is more than enough. You have successfully covered that principle of transferring energ from stator to rotor. Now I got the point. \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 14:30

It's an induction motor, invented by N.Tesla. Also the Tesla company uses this kind of motor in their roadsters. You can learn about this principle in many sites/books. It's not so efficient as permanent magnet synchronous motor (PMSM) or brushless DC motor (BLDC). It's also heavier than PMSM/BLDC but is much cheaper than permanent magnet motors.

P.S. : By the way, your second picture is a fan blower, not a squirrel cage rotor.

  • \$\begingroup\$ Thanks. I think that its design is similar. Just could not find good picture of it :) Feel free to edit and improve question, if you want. \$\endgroup\$
    – xakepp35
    Mar 24, 2019 at 14:27

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