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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 over the rotor, but 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. 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, pleas briefly explain why. Thanks in advance.

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closed as unclear what you're asking by Chris Stratton, Finbarr, Warren Hill, RoyC, W5VO Apr 23 at 13:57

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  • \$\begingroup\$ So a motor where you only partially or fully insert the rotor inside the stator? Threading the needle, so to speak? I think you would get crazy stray flux lines a very large equivalent air gap. \$\endgroup\$ – Toor Apr 17 at 23:24
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    \$\begingroup\$ This isn't really on topic here or specific enough to be answerable, but you might consider things like motors where the rotor has a splined keyway and a keyed shaft can slide in it, or things like a car starter motor where an axially moving piece "spins" into position via a leadscrew type thread. And of course there are linear mechanisms where a stepper motor is built around a leadscrew nut rather than a shaft, or is built around a leadscrew on which other nuts ride. \$\endgroup\$ – Chris Stratton Apr 17 at 23:48
  • \$\begingroup\$ I've seen a (large) synchronous AC motor where the stator could rotate. At start-up, the stator would spin up quickly to the rated RPM. An operator would then apply a mechanical brake to the stator, and the controlled braking slowly brought the rotor up to speed. Eventually the stator would be locked. \$\endgroup\$ – W5VO Apr 23 at 13:56
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There IS such a motor design, but not for the purpose you are thinking of. In the crane industry, a company named Demag makes a conical rotor motor so that the act of energizing the motor creates axial (lateral along the axis) movement so that the motor disengages a mechanical brake assembly on the end of the shaft. This avoids needing to have a separate solenoid coil for the brake.

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If you want to experiment, you could buy one of these and remove the brake assembly, then rig up a way to move the rotor in and out of the stator. But honestly what you will likely learn is what Janka has already told you...

Another variant of motor designs, no longer used, was the "super-synchronous" motor, in which the stator and rotor could both spin freely. So at start-up, the STAOR would spin, because the rotor had the load connected to it. Then when the stator got to speed, an operator would tighten a band around the stator to stop it slowly, which would make it transfer its mechanical power to the rotor. If you used that design as a generator and rigged a way to only partially clamp the stator, you could control the output. It's hard to envision with words, so here's a cool video of one in action: https://youtu.be/fNuI6keQXYA (I call this "motor porn"...)

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What you are doing with such a construction is weakening the field mechanically. It only makes sense at all for a motor with permanent magnets for field generation. If your field is created by an electromagnet, you could simply lower the field current. Which is what you generally want, because your mechanical alternative is complicated and error-prone. If you wanted to push efficiency by using a permanent magnet field, you could have an additional electromagnet just for the controlable part.

In addition, weakening the field just means your machine will produce less current at higher voltage and speed. You still cannot control the amount of electrical energy produced this way, because the voltage needed is set by the grid and the speed is set by the wind. But you could use a simpler inverter stage with less voltage spread.

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