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I was wondering if there is a motor/generator design where the rotor moves gradually in and out of the stator along the axis of rotation, or the stator moves over the rotor. If so, I'd like some resource(s) to read more about it. The application I have in mind is for a wildly variable load like a wind turbine. If this is a bad design, please briefly explain why. Thanks in advance.

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That actually exists and it's applied in magnetic levitation (maglev) trains. It's called linear induction motor (LIM). LSM if it's synchronous.

https://en.wikipedia.org/wiki/Linear_induction_motor

The rotor is addressed as "slider" here because it doesn't rotate.

More info here: https://e-archivo.uc3m.es/bitstream/handle/10016/13575/PFC_CARLOS_PEREZ_RODRIGUEZ.pdf

It's in Spanish, but use Google Chrome translator.

Download these scientific articles as well:

A Study of Non-Symmetric Double-Sided Linear Induction Motor for Hyperloop All-In-One System (Propulsion, Levitation, and Guidance)

Woo-Young Ji 1, Geochul Jeong 2, Chan-Bae Park 1, Ik-Hyun Jo 1, and Hyung-Woo Lee 1

And some formulas here:

Hyperloop Transportation System: Control, and Drive System Design

Jawwad M. Sayeed Deperatment of Electrical, Computer, and Software engineering University of Ontario Institute of Technology, Oshawa, Ontario jawwad.sayeed@uoit.ca Ahmed AbdelRahman Deperatment of Electrical, Computer, and Software engineering, University of Ontario Institute of Technology, Oshawa, Ontario ahmed.abdelrahman@uoit.ca Mohamed Z. Youssef Deperatment of Electrical, Computer, and Software engineering, University of Ontario Institute of Technology, Oshawa, Ontario mohamed.youssef@uoit.ca

And the Hallbach Array:

Hyperloop Transportation System: Control, and Drive System Design

Jawwad M. Sayeed Deperatment of Electrical, Computer, and Software engineering University of Ontario Institute of Technology, Oshawa, Ontario jawwad.sayeed@uoit.ca Ahmed AbdelRahman Deperatment of Electrical, Computer, and Software engineering, University of Ontario Institute of Technology, Oshawa, Ontario ahmed.abdelrahman@uoit.ca Mohamed Z. Youssef Deperatment of Electrical, Computer, and Software engineering, University of Ontario Institute of Technology, Oshawa, Ontario mohamed.youssef@uoit.ca

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  • $\begingroup$ Thank you for your answer, but I'm not sure this is quite what I'm talking about. Assuming this is a generator, I mean a motor design where the primary motion is rotary, and this rotary motion is moved into, or out of a stator slowly to a fixed position so that more or less electricity is generated by the rotation. $\endgroup$ – Dan Apr 17 '19 at 17:52
  • $\begingroup$ Maybe that exists, but I don't know of that. Controlling the gap you can control the energy generated, I reckon. $\endgroup$ – user20096 Apr 17 '19 at 20:10
  • $\begingroup$ I think you totally missed the point of linear motors: they are simply unrolled cylindrical motors, nothing new there. Both work on identical principles. The OP asked about a rotating rotor sliding in and out from the stator - a rather pointless exercise. $\endgroup$ – Kuba hasn't forgotten Monica Oct 11 '20 at 3:34
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It’s a pointless design since all you do is decrease the amount of magnetic coupling between the rotor and stator. You may as well just reduce the current - it’ll have the same effect.

You’d have to explain why you think that a wildly varying load would somehow benefit from having less of a motor available (when the rotor is not all the way in). What sort of a problem are you trying to solve? Those systems are constrained at the maximum torque, not minimum torque. It’s easy to generate minuscule torques. It’s the large torques that are the limiting factor, and for those you need the rotor to be fully in anyway. So I think you got something backwards somewhere in the way you analyzed it.

One benefit of this approach, though, could be to minimize the relative amplitude of cogging forces if they are of concern.

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