Torque Production in Eddy Current Motors

[shahvir]

Hi guys,

The purpose of this post is to understand the actual phenomenon behind torque production in eddy current motors. This query is based on theoretical principles of torque production in AC motors.

Consider a simple eddy current AC motor with a disc type or cylindrical rotor (Watthour meter principle). The general theory of torque production explained in textbooks is that two stator magnetic fluxes, out of phase with each other, induce individual eddy currents in the rotor which in turn react with these magnetic fluxes to generate torque. But since these stator fluxes are out of phase, they will also create a net magnetic flux which will be rotating (or shifting) in space depending on the stator poles arrangement (induction motor principle). This in turn will also induce eddy currents in the rotor producing torque.

My query is, which of the above two phenomena are actually responsible (or both) for production of torque in such motors.

Thanks

 

[shahvir]

check this out for induction disk relays: The Art and Science of Protective Relaying
start at the bottom of page numbered 22 (which is 32/257 of pdf).
INDUCTION-TYPE RELAYS–GENERAL OPERATING PRINCIPLES

Thanks for help, I greatly appreciate. The working principle and diagram in the pdf is the same as illustrated in the link I posted earlier. Two split phase magnetic fields induce individual eddy currents in the rotor which react with the stator poles to generate a resultant torque.
But there's another induction torque principle commonly related to induction motors wherein a resultant magnetic flux sweeps past the rotor conductors in space, cutting the rotor conductors and inducing currents in them. If instead of squirrel cage rotor, it were a solid conducting rotor as in the present case, eddy currents would have been induced in the rotor.
So my doubt is why induction motor principle of 'flux cutting' not applicable to the mechanisms in the present case. Thanks.

 

[MacGyverS2000]

I googled the title waross provided and found multiple locations in the first several search results where it can be had... if you don't consider that "readily available", I fear there's nothing else we can do to help.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[electricpete]

But there's another induction torque principle commonly related to induction motors wherein a resultant magnetic flux sweeps past the rotor conductors in space, cutting the rotor conductors and inducing currents in them. If instead of squirrel cage rotor, it were a solid conducting rotor as in the present case, eddy currents would have been induced in the rotor.
So my doubt is why induction motor principle of 'flux cutting' not applicable to the mechanisms in the present case. Thanks.

The same principles are at work. In the case of a motor there is a clear rotating magnetic field. I think in some relays there is not necessarily a complete rotating magnetic field moving uniformly around the whole disk, but there certainly is a moving magnetic field near portions of the disk. In almost all cases we discussed, the currents induced in the conductor by a moving field will produce a force that tries to move the conductor in the same direction of the moving field (Bill mentioned linear induction motor as an example).

There is also another wrinkle in the story when you look closely at conductors located within iron slots like the rotor bars of an industrial squirrel cage motor rotor. That's something for another day.

Personally I try to stay away from the concept of "flux cutting" and look instead toward Faraday's law

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(2B)+(2B)' ?

 

[shahvir]

Personally I try to stay away from the concept of "flux cutting" and look instead toward Faraday's law


Please see below links;

https://en.wikipedia.org/wiki/Faraday's_law_of_induction#Two_phenomena

https://en.wikipedia.org/wiki/Faraday's_law_of_induction#Two_phenomena

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

As is aware, "flux cutting" concept is very much part of Faraday's laws of EMI (EMF induced due to conductor moving in uniform magnetic field, Motional EMF)
This is also the induction motor torque principle which I was referring to. As per my original post, principle of torque production by reaction of 2 magnetic fluxes with individual eddy currents is different from torque production due to rotating or sweeping magnetic fields, although both are due to Lorentz forces. So why the latter principle is not mentioned in theory of induction type instruments or relays.

 

[shahvir]

https://maritime.org/doc/radio/chap7.htm

https://www.nagwa.com/en/explainers/792152624193/

https://physicsabout.com/motional-induced-emf/

Please check above links. They are self explanatory. Faraday's disk generator is based on this principle. Please review Dr. Feynman's comments in the wikipedia link of my earlier post in detail.

Two different phenomena are responsible for emf induction in a conducting surface; 1)Rate of change of flux density linking the conducting surface
2)Conducting surface moving in a uniform magnetic field (premise of Faraday's paradox of motional emf).
There's no conflict here.