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# Torque Production in Eddy Current Motors

## Torque Production in Eddy Current Motors

(OP)
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
Replies continue below

### RE: Torque Production in Eddy Current Motors

Consider a shaded pole motor which uses a similar action.
The winding creates magnetic flux in the main pole which induces an eddy current in the rotor.
The shading coil creates a second pole that is phase shifted or lagging the main coil.
The flux from the shaded pole interacts with the flux from the eddy current and creates torque.
The shaded pole also causes eddy currents. The magnetic field from these eddy currents interacts with the flux from the main pole and adds to the torque.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Torque Production in Eddy Current Motors

(OP)
Yes. This is two field reaction theory (induction wattmeter principle). But there is another methid of torque generation by induction motor principle I.e. due to rotating or linearly shifting magnetic field (as per stator pole arrangement)
My query is, why induction motor principle of torque generation not applicable to induction type measuring instruments.

### RE: Torque Production in Eddy Current Motors

Same principle. The interaction of two or more field that are out of phase.
In the induction motor the eddy currents are developed in the squirrel cage winding.
A linear induction motor may be the heavy duty industrial equivalent of the Wattmeter application.
The linear induction motors that I am familiar with developed eddy currents in an aluminum strip, backed by a steel strip.
They developed about 400 HP and were capable of speeds of over 70 KPH.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Torque Production in Eddy Current Motors

(OP)
https://www.electrical4u.com/induction-type-meters...

My only contention with reference to the above article is that why induction motor principle of rotating or linearly shifting magnetic fields not applicable to these types of mechanisms.

### RE: Torque Production in Eddy Current Motors

(OP)
From the article in the link it is evident that torque production in such mechanisms is due to mutual interaction between the magnetic fields and 2 separate induced currents passing under the individual poles. In induction motor, currents are induced in the rotor due to sweeping of the stator flux in space. In my opinion the method of torque production in both these systems are different although both are induction machines.

### RE: Torque Production in Eddy Current Motors

(OP)
I would politely disagree. The torque generation principle illustrated in the link and the induction motor principle are not exactly the same although they both operate on the principles of induction.
The above statement is also valid for a shaded pole stator because it is just a technique of phase splitting and in no way contributes to any similarities.

### RE: Torque Production in Eddy Current Motors

(OP)
I don't believe everything that is put up on the internet. Unfortunately, the same drawing is in all EE textbooks on the subject. It is not about mechanisms or job skills, it's about working principle in theory.

### RE: Torque Production in Eddy Current Motors

To rephrase:

#### Quote (shahvir)

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

1) 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.

2) 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.

At first glance - these two phenomena are not separable.

Answer 1a: If there was no stator magnetic flux there could be no torque.
Answer 1b: If the flux were in phase they would cancel and there would be no net torque.
Answer 1c: If the rotor could not support an induced current, such as a nonconductive material, there would be no torque.

So all of (1) is required.

Answer 2: If the rotor doesn't rotate then the motor doesn't move and there is no rotation of the induced magnetic field in the rotor - the stators aren't moving so their magnetic fields cannot move.

So all of (2) is required to move, though there would still be torque due to (1) which is what initially moves the rotor.

In linear motors the effective radius for the rotor is infinite; that's the primary structural difference.

Is there some importance to the mathematics that is of concern in the different forms of induced electromagnetic force?
Are you designing some other kind of motor that is not either of the two originally suggested and for which you think no theory explains how to analyze it?

### RE: Torque Production in Eddy Current Motors

(OP)
Ok then please provide some references or links representing the working principle you mentioned.

(OP)

### RE: Torque Production in Eddy Current Motors

Can you not simply Google the title waross suggested? Plenty of options there...

Dan - Owner
http://www.Hi-TecDesigns.com

### RE: Torque Production in Eddy Current Motors

(OP)
Tried but cannot find, it's chargeable.

### RE: Torque Production in Eddy Current Motors

I'm not suggesting you'll find a free copy, only that the information is readily available...

Dan - Owner
http://www.Hi-TecDesigns.com

(OP)
It's not.

### RE: Torque Production in Eddy Current Motors

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 RELAYSâ€“GENERAL OPERATING PRINCIPLES

=====================================
(2B)+(2B)' ?

### RE: Torque Production in Eddy Current Motors

Pete and Dan. This is pointless. I give up. Let's hit the pub. The first round's on me.

Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

### RE: Torque Production in Eddy Current Motors

(OP)
They won't come. They are busy helping me.

(OP)
Enjoy.

### RE: Torque Production in Eddy Current Motors

(OP)
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 RELAYSâ€“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.

### RE: Torque Production in Eddy Current Motors

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

### RE: Torque Production in Eddy Current Motors

#### Quote:

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

=====================================
(2B)+(2B)' ?

### RE: Torque Production in Eddy Current Motors

(OP)

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

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.

### RE: Torque Production in Eddy Current Motors

(OP)

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

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.

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