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Why are squirrel cage rotors used in induction motors, as opposed to just a solid conductive cylindrical shell?

As far as I understand, a solid cylindrical shell (with solid metal endcaps) should work just as well. There is nothing in the operating principle of the induction machine which strictly requires the bar construction, correct? If not, please explain why this is the case.

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As far as I understand, a solid cylindrical shell (with solid metal endcaps) should work just as well.

It will work, but not as well.

A copper shell would not have the permeability to get a good field through it. An iron shell would not have the conductivity to allow high currents with low loss. In either case, the motor would have to be significantly bigger than they are now for the same power, and would be less efficient.

If we could use unobtanium metal, with this combination of good magnetic and electrical properties, then we could use a single metal shell. We can't get unobtanium, so this combination of iron and copper is the best we can do from what nature provides.

A single metal shell would be cheaper to make than the present bi-metal construction. We can therefore assume that it's worth spending the extra money to fabricate the squirrel cage, that the extra performance and size reduction is worth the extra cost to make.

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All-aluminum rotors have been widely used in one unique type of motor. That is the rotating-disc, watt-hour meter. In that case, the rotor only drives the clockwork mechanism that mechanically records the energy use. The rotation is also retarded by a permanent-magnet, eddy-current brake.

The first linear induction motors developed by Professor Eric Laithwaite had an aluminum stationary strip and moving windings on steel cores on both sides of the strip.

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It's all about engineering the induction current path to get the maximum mechanical torque. A solid cylinder sheet will have eddy currents that do not travel along the axis to create large torque. The eddy currents will be circluar loops.

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