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quiz: torque-producing e/m force acts on bars or core?

quiz: torque-producing e/m force acts on bars or core?

quiz: torque-producing e/m force acts on bars or core?

(OP)
Quiz: In a squirrel cage induction motor, do you think the electromagnetic torque-producing force acts primarily on the rotor bars or on the rotor core.

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RE: quiz: torque-producing e/m force acts on bars or core?

Bars.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
That was quick.  You sure about that?

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RE: quiz: torque-producing e/m force acts on bars or core?

Hi ePete,

I will agree with David.

In my mind the core is a means of directing the flux to ensure that maximum flux linkages occur at the bars. The torque is produced by flux cutting the current-carrying rotor bar and transferred via contact into the core and thence to the shaft.

Are we both wrong?

----------------------------------
  Sometimes I only open my mouth to swap feet...

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
You're not wrong about the flux linkages. But surprisingly, you would be wrong to state the forces act on the bars themselves.  They do in fact act directly on the core.

"The Induction Machine Handbook" CRC Press 2002 ISBN 0-8493-0004-5

Quote (IMHandbook):

"The electromagnetic traveling field produced by the stator currents exists in the airgap and crosses the rotor teeth to embrace the rotor winding (rotor cage)?Figure 2.16. Only a small fraction of it radially traverses the top of the rotor slot which contains conductor  material. It is thus evident that, with rotor and stator conductors in slots, there are no main forces experienced by the conductors themselves. Therefore, the method of forces experienced by conductors in fields [F=q*(v x B) ] does not apply directly to rotary IMs with conductors in slots...

...According to the Maxwell stress tensor theory, at the surface border between mediums with different magnetic fields and permeabilities (mu=mu0 in air, mu <> mu0 in the core), the magnetic field produces forces. The interaction force component perpendicular to the rotor slot wall is...[lots of math

...So the tangential forces that produce the torque occur on the tooth radial walls. Despite this reality, the principle of IM is traditionally explained by forces on currents in a magnetic field. [F=q*(v x B) ]"

If you think about it, it is very true that the flux in the rotor bar is very small (flux takes the path of least resistance which is primarily from stator tooth top to rotor tooth top).

So if you take away F=q*vxB = L IxB, it seems a little tougher to get a simple intuitive picture of the physics principles  that produces the torque.   

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RE: quiz: torque-producing e/m force acts on bars or core?


 Bars! yes it is... david may be in a hurry. smile



 For clarity, Pete is precisely correct!


RE: quiz: torque-producing e/m force acts on bars or core?

My thought was that if you had a core of circular laminations you would have no torque; it is the current flowing in the bars that makes the torque possible.  Not sure what would happen if you had a bar cage without the laminations; but my guess is that it would develop at least some torque.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I'm glad to see questions and skepeticism. It's something that surprised me initially and I was hoping it would surprise others as well.  But again, if you don't believe it, the key proof comes from remembering that the flux generally takes the shortest path between the stator iron and rotor iron which does not go through the bars... so the majority of the flux doesn't go through the bars, so there is very little flux at the location of the current to produce any force F=q*(v x B) = L * (I x B).

"My thought was that if you had a core of circular laminations you would have no torque; it is the current flowing in the bars that makes the torque possible.  "

Yes, the current flowing in the bars participates in creating the field that helps makes the torque possible. (which is not to say that the torque/force is applied to the bars).   If the core was not electrically conductive and had no bars, there would be no torque.  (A quick mention that the induction cup relay can produce some torque directly in the copper cup based on F=q*v x B = L * I x B, but that is a complicating fact which is not directly related to this question which discusses slotted rotors).

"Not sure what would happen if you had a bar cage without the laminations; but my guess is that it would develop at least some torque."

Then the flux would take the shortes path to encircle the stator coils which would be tangentially across the stator slots.  There would be no significant flux in the rotor, no significant current in the bars.  Furthermore the fringing flux near the bars would be primarily tangential.  The cross product of tangential flux and bar current would be in a radial direction which is not a direction that produces torque.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
"Bars! yes it is"

I'm not sure I understood that comment.   That sounds like you are voting for bars as the answer to the original question but my answer is core.


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RE: quiz: torque-producing e/m force acts on bars or core?

Check out the Lorentz force eqn

F=q(E+v X B)  
where
F is force vector for a charge
q charge of particle
E electric field
v velocity of charge (moving due to current)
B Magnetic field vector

Charges moving inside a conductor perpendicular to
a magnetic field are induced with a mechanical force
F

RE: quiz: torque-producing e/m force acts on bars or core?

I see pete had allready included this eqn.
sorry must read more carefully before posting.

However i have to say it is the bars not the core.
If you could place microscopic load cells between the
bars and the core walls I would suspect you would
measure in the sum of all the spots where bar touches
core, the total torque developed.

If not, what is the mechanism for force being created
within the core itself if not due to electrical current.
Remember even permenant magnets repel each other due
to electrical current caused by the allignment of
the orbits of electrons.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I think it is important to emphasize there are two views.

One is the view shown in high-level (not-detailed) textbooks. It shows us uniform airgap flux interacting with current carrying bars to produce force  F = q*(v x B).   That is a very useful view for analysing and explaining many aspects of the motor.  Qualitatively it shows us how low power factor reduces our torque capability for a given rotor current.  I suspect quantitatively the predicted torque would be close to actual torque.  

But a more realistic view reflects the physical reality that the flux field is not uniform.  Flux very strongly prefers to travel in iron rather than air or copper.  It would much rather jump accross 0.1" of airgap from stator tooth-top to rotor tooth-top than to jump maybe 0.8" from stator tooth-top to the iron at the bottom of the rotor slot.  The result is that there is very little flux in the copper bars where the current is.  There is a much higher flux flowing in the teeth between the bars.

Quote:

However i have to say it is the bars not the core.
If you could place microscopic load cells between the
bars and the core walls I would suspect you would
measure in the sum of all the spots where bar touches
core, the total torque developed.
I say the total of the load cells would read close to 0  (much closer to 0 than to the force associated with full load torque).

Quote:

If not, what is the mechanism for force being created within the core itself if not due to electrical current.
I don't have a good description (I welcome other response to that question).  Here is my best shot at the moment but I'm still working on it: The field of a magnetized piece of iron resembles the field of a bound current flowing on the surface of the iron.  Force comes from interaction of the field with that bound current.  If there were no current in the bar, the force associated with interaction of the airgap field with bound current at the trailing edge of the slot roughly cancels the force associated with interaction of the airgap field with the bound current at the leading edge of the slot.  If there is bar current within the slot, that equality is destroyed and a net tangential (torque-producing) force is produced.            

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RE: quiz: torque-producing e/m force acts on bars or core?

I was thinking of an experiment.
Place two coils carrying current in the same direction
near each other and they repel.
Now slip seperate iron pieces around each coil with
the same seperation but each iron piece actually
extends beyond its coil in the direction of the other
coil, making the tips of the iron circuit come together
far from the coils but with a spacing between them less
than the coils themselves. Think E cores with the middle
leg trimmed. Now of course there is more repelling force
and this force is concetrated in the outside legs of the
E core. The copper doesn't care.
If this sounds like what would really happen if we built
it I am led to change my mind from above and say
there is force in the cores.
I would have to guess the force arises from the magnetic
dipoles in the metal itself.

a very interesting question eh ???

RE: quiz: torque-producing e/m force acts on bars or core?

A couple of suggestions;
1: A motor is sometimes described as a rotary transformer.
Even though the laminations are more tightly held than the windings, most often noise inducing vibration originates in the laminations rather than in the windings because of the greater magnetic forces in the laminations.

2: Although there are forces between busbars carrying normal current, the forces are quite small. It is only under shortcircuit conditions that the forces become great enough that there is a risk of damage.

3: Many of us have seen multiple, parallel cables connecting a large transformer at a mill or other industrial plant.
We have seen the cables swing together when there is an increase in the load such as a large log entering a gang saw.
The current increase may be several hundred amps, but the force is quite small.

I would suggest that there may be a small torque generated by the squirrel cage bars, but the greatest part of the torque is generated by the magetized core.
respectfully

RE: quiz: torque-producing e/m force acts on bars or core?

Core.

Start by simplifying to an ideal three-phase stator. By itself, it creates a rotating magnetic field of constant magnitude.

Next simplify the rotor to a loop with no core. It only sees the magnetic field perpendicular to its plane. If the loop doesn't move, the flux through it varies sinusoidally, the projection of the rotating stator flux onto the loop's axis.

If we make the loop a one-turn conductive coil, the varying flux induces a current in it.  The stator field exerts a torque on this current loop.  If we let the coil rotate, the torque makes it spin.  (As a complication, it gradually falls behind, eventually so far that it goes perpendicular and the torque disappears.  Real motors use squirrel cages so that some loops will always be perpendicular to the stator field, giving a steadier torque.)

Unfortunately, the coil has little inductance.  Low self-inductance means that the current readily increases, maximizing resistive losses.  Low mutual-inductance with the stator reduces the maximum power that can be coupled to the rotor.  Think of it as putting a shorted one-turn secondary on an air-core transformer.  Only a little power gets through, and most of it gets turned to heat.

Next we add a magnetically permeable core to the rotor.  Permeable means the core has zillions of tiny current loops of its own that align themselves with the external field.  They essentially act as a magnifier for external currents.  This increases the inductances dramatically, reducing the rotor current.  At equilibrium under a steady load, the tiny current loops' axis will be pointed somewhere between the rotor loop axis and the stator field axis.  Therefore the tiny magnets will experience a torque, in the direction of rotation of the stator field.

The proportion of the torque that goes to the core will be about equal to its relative permeability.  (At least that's what I come up with off the top of my head.)  For a good transformer-type silicon steel, that's on the order of 25,000.  So the lion's share of the torque goes to the core.  

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Good comments.  I agree with the clarifications of waross and BobbyNewmark (not sure I understood your geometry 2dye4).

I especially liked the idea of a core as a current multiplier given by BobbyNewmark. It was what I was stumbling to being to describe when I talked about bound surface currents.

Let's examine only the loop of rotor cage core formed by two adjacent bars and the sections of end ring between them.  Compare two cases: 1 = with and 2 = without the section of core they encircle (which happens to be a rotor core tooth).

First we'll give a generous assumption to the no-core case and assume that the current in the loop will be the same in both cases.

Without the core (case 1) as discussed by waross, the force is relatively weak.  With the core (case 2), that loop of current in the copper induces all the dipoles in the core to align with it. Each dipole is equivalent to a small loop of "bound" current within the core.  Many loops of bound current across the area of the core, all circulating in the same direction (for example clockwise).  In the interior, pick a single loop with bound current CW.  Each section of that loop is adjacent to another loop also with current CW.  If you look at two adjacent sections, current travels in opposite directions and cancels. Within the interior, all portions of those loops cancel with adjacent portions, and all that is left is the exterior portions of exterior loops which don't cancel. This acts like one big loop of current around the core (what I was calling bound surface current), just inside of the original copper loop (free current).  It magnifies the field of the original free current by the number mu_r as mentioned on the order 20,000.  So by adding the core, if the applied field didn't change, the total current (bound plus free) available to produce torque went up by 20,000.

But don't forget that generous assumption that we gave to case 1.  Without the rotor core, the free current in the bar for case 1 will be far lower than for case 2, so the difference is far greater than 20,000.

So in a way maybe we can say that the torque-producing torque is generated in a manner similar to F=L*(IxB).  We just have to keep in mind the two components of the I.  The free portion in the copper is very small. The bound portion of current in the core is very large.  The bulk of the torque is produced by the field interacting with the bound current in the core.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Correction: I mixed up my 1s and 2s. For consistency, revise it as follows: "Compare two cases: 1 = without and 2 = with the section of core they encircle"

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I should clarify that the "bound" current within the core is not an actual current (otherwise it would create a lot of heating), but it interacts with magnetic field in the same way as a current.

Those familiar with dielectrics may be familiar there is an analogous parallel discussion involving bound and free charge.  The difference is that the magnetic dipoles (associated with bound current) align to reinforce the external applied magnetic field, while the electric dipoles (associated with bound charge) align to oppose the external applied electric field.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
It would be an intersting idea to test some of my comments above with actual motor data.

Rotor data is somewhat inaccessible, but I have some stator data fairly accessible.  The logic for the stator should be similar (as long as we account for the total number of turns per coil the coil plays the same role as a rotor slot).  It would be interesting to calculate the force we would expect on the stator coils based on their lenght, winding configuration  and an assumed flux density.  Off the top of my head I am thinking the torque by that calculation will be a lot more than 1/20,000 of the motor full load torque.  I will try it this weekend if I get the chance.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I looked at Liwschitz/Garakl Electric Machinery Volume 2 Appendix 7, and I see he analyses torque-producing force using amp-turns (free current in rotor... approximately same as amp-turns in stator at full load) times flux times length with a few other correction factors.

So I have to withdraw my comments 19 Sep 06 20:32 (current-amplifying effect of the core) and stick with my comments in earlier posts such as my post 19 Sep 06 15:49

One item:

Quote:

"One is the view shown in high-level (not-detailed) textbooks. It shows us uniform airgap flux interacting with current carrying bars to produce force  F = q*(v x B).   That is a very useful view for analysing and explaining many aspects of the motor.  Qualitatively it shows us how low power factor reduces our torque capability for a given rotor current.  I suspect quantitatively the predicted torque would be close to actual torque."

This is true, and now I believe quantitatively the predicted torque is close to actual torque.

Quote:

I don't have a good description (I welcome other response to that question).  Here is my best shot at the moment but I'm still working on it: The field of a magnetized piece of iron resembles the field of a bound current flowing on the surface of the iron.  Force comes from interaction of the field with that bound current.  If there were no current in the bar, the force associated with interaction of the airgap field with bound current at the trailing edge of the slot roughly cancels the force associated with interaction of the airgap field with the bound current at the leading edge of the slot.  If there is bar current within the slot, that equality is destroyed and a net tangential (torque-producing) force is produced.

This is still my best attempt to explain how it is that the torque is the same as is predicted by interaction of conductors and the field, but does not actually act on the conductors.  The reason is that the bar produces a difference in bound surface currents on the two adjacent slot faces which is approximately equal to the bar current. At least that's my best explanation for now but I'm still open to comments.

The principle of the current-amplifying effect of the core I believe is correct but I misapplied it in my 19 Sep 06 20:32 post.  For one thing I looked at current in only one loop of adjacent bars.  That doesn't resemble the motor where adjacent bars have current flowing in the same direction.  For a more complete view we have to add together multiple loops and the current in each bar is the difference between adjacent loops.  There may be other errors in that post as well.

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RE: quiz: torque-producing e/m force acts on bars or core?

Quote (electricpete):

Off the top of my head I am thinking the torque by that calculation will be a lot more than 1/20,000 of the motor full load torque.

After more thought, I agree.  The force resulting from the squirrel cage current is amplified by μr ~= 20 000. However the inductance is also larger by μr, so the current is divided by μr.  Regarding the rotor torque, it's a wash.  The benefit is not torque but efficiency and practicality.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
The other benefit is flux. The flux from the magnetizing current of the stator is much higher than it would be caused from if the same magnetizing current were present (hypothetical) without a core (that current amplification effect).  And the rotor core is needed to carry that flux to the area of the rotor bars.

So the comment about relatively low forces seen on conductors in air is true... there is not a lot of flux created.

If you could place the bars in the airgap and still get the same current and flux, then the force would act directly on the bars and you would get roughly the same torque. But that would create a large airgap which leads to large leakage reactance, low power factor and efficiency etc.

By placing the bars in slots, they are in an area of no flux and no significant force occurs at the bars.  As I said it occurs in the bound current immediately adjacent to the bars within the core whose difference has the same magnitude as the bar current.

It's kind of tough to describe the origina of that but a rough analogy that shows how bound quantities can mirror free quantities would be a capacitor.  If we have +Q on the free charge on the positive terminal and -Q free charge on the negative terminal (these are the charges we are familiar with), then within the dielectric we have bound charge -Q on the surface of the dielectric immediately adjacent to the plate with +Q and bound charge -Q on the surface of the dielectric immediately adjacent to the plate with free charge +Q.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Whoops, now that I engage my brain the capacitor analogy was not exactly correct. Strike that.

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RE: quiz: torque-producing e/m force acts on bars or core?

electricpete, you could post a revised version of your post, then go back and Red Flag your original post to have it deleted, explain that you have posted a revision and wish the original deleted.  You can also, for minor revisions, Red Flag the post and in the comments specify what revision you wish made.  My experience is that site management is quite open to making those types of changes.

RE: quiz: torque-producing e/m force acts on bars or core?

Hello Electricpete.

My opinion, the force primarily develops on the boundary of two magnetic fields attracting or repelling each other. Then that force reacts against the media creating the field.

I can construct an induction motor without laminations, it will be very inefficient and with a huge size to produce a few Lb-FT torque but it is a motor. That eliminates your initial statement that the force primarily acts directly on the core.

The lamination core is the “magnetic circuit” to make an ease an economical path for the magnetic flux.

The magnetic flux created by the stator winding (anchored by the stator frame) reacts to the magnetic flux created by the current circulating into the rotor cage. That is consistent with the basic principle F=BLI.

The two magnetic fields are present too in a DC machine with a flux produced by the static fields and the other by the current injected into the rotor armature, or in a synchronous motor with an AC stator rotating field and a DC excited rotor field.

Since normally  on  motors  the rotor is free to rotate, the attraction or repulsion of the two fields result in rotor movement, but if you anchor the shaft and lose the stator ( as in some dynamometers) the stator frame will rotate.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)

Quote (aolalde):

I can construct an induction motor without laminations, it will be very inefficient and with a huge size to produce a few Lb-FT torque but it is a motor.

I agree.  That is consistent with what I said before.  If we removed just rotor iron, the vast majority of stator flux would not enter the rotor.  If we also remove stator iron, the stator cannot produce nearly as much flux to begin with.  We agree on this point that a motor of comparable size without iron would produce much less torque.

Quote:

That eliminates your initial statement that the force primarily acts directly on the core.

I don't agree with that at all.  I'm saying the torque producing force in a normal induction motor (with iron) acts primarily on the core.  There may be some small torque producing force on the copper, but it's much less than the torque producing force on the core. Hence the term "primarily".

Are you saying:
A - a small amount of torque producing force is produced acts on the the copper, but the vast majority of the torque-producing force acts on the iron (I agree)
  OR
B - All (or the majority) of the torque-producing force acts on the iron (I disagree)
?

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
CORRECTION:
Are you saying:
A - a small amount of torque producing force is produced acts on the the copper, but the vast majority of the torque-producing force acts on the iron (I agree)
  OR
B - All (or the majority) of the torque-producing force acts on the copper (I disagree)
?

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
The proof that the majority of the induction motor torque is produced in the iron comes (perversely) from observing that the incorrect assumption that the force acts on the copper leads to the correct quantitative result. (I know that sounds backwards but please read on).

If you assume (incorrectly) the rotor current is uniformly distributed around the rotor circumference and assume (incorrectly) that all of that current interacts with the airgap flux density, then you will calculate the correct torque. I suspect many here have seen calculations like this in a textbook.  (For example Liwschitz/Garakl Electric Machinery Volume 2 Appendix 7)

BUT, in a real induction motor, only a tiny fraction of the airgap flux flows through the rotor bar where the rotor current is.  So the total torque produced by interaction of field and current N*L*IxB IN the copper rotor bars must much less than the correct result predicted by the textbook. So the majority of the torque-producing force had to come from somewhere else.... the core.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I shouldn't say the torque is "produced" in the iron or "comes" from the iron.  As I have acknowledged before, both the copper and the iron play a role... a rotor with a core and no copper cage would produce very little torque.

But if you take a normal induction motor and put those tiny load cells between the copper bars and the iron core, the total force they record will be much less than the torque-producing force because the majority of the e.m. torque-producing force is acting directly on the iron.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
The following graphic may or may not shed any light.
http://home.houston.rr.com/electricpete/CoreSlot3.ppt

Conductor A represents the textbook situation - current sees full airgap flux.  The force on conductor A would be B = Phi/Area times Ia in the downwards direction.

Conductor B represents the real situation - current is shielded from airgap flux by virtue of being situated in a slot within the iron.

Hopefully it is obvious that conductor B sees less flux and therefore (assuming A and B are carrying the same current), conductor B experience less EM force than conductor A.  The balance of force to recreate the textbook case (which gives the correct quantitative result) has to come from somewhere else.

My contention in the drawing shown under static conditions (a simplifying assumption which still illustrates the point), the presence of the current B within those two teeth creates a difference in the mmf seen by the upper and lower tooth.  We can use the use integral H dot dL =0 around the dashed square path to prove the H in the upper tooth exceeds the H in the lower tooth by an amount equal to Ib (the left and right sides of the square contribute zero to the integral).  Now we model this difference as a difference in bound surface current and we will see it gives a difference in force of Ib x B (where B is the flux in the tooth).  If the idea of modeling magnetization as a surface current sounds wrong, I believe it is supported by the folliwng link below.  Slide 1 gives the reference. Slide 2 shows  that bound surface current affects magnetization in the same way the free current affect H.  In my mind that means we can replace the magnetized core with an equivalent metal with mu0 and with surface current enough to create the magnetization.  It also shows on slide 3 that a difference in tangential component of H across a boundary is equivalent to a surface current flowing at that boundary.   I believe all of this supports my explanation above.
http://home.houston.rr.com/electricpete/ExcerptRegardingBoundCurrent.ppt

Qualitatively, there are more lines of flux coming out of the upper tooth than the lower tooth because it has more magnetization (the flux from Ib is additive to the original flux Phi in the upper tooth and subtractive in the lower tooth).  When considering the fringing fluxes, we should be able to see that this creates a downward force on the right-most core leg because the magnetic force wants to pull the highest flux tooth closer to the center of the adjacent leg across the gap.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
And the bottom line, even if you don't believe me, you've gotta believe the author of "The Induction Machine Handbook".

The idea of free and bound current is something I don't see used in many of my textbooks.  (Even Engineering Electromagnetics linked above just uses it in a passing manner).  I learned about it from my Fields and Waves Professor 20+ years ago.  I didn't exactly understand it at the time, but I remember that viewing magnetism as equivalent surface currents significantly increased the intuition. Otherwise all you have is a bunch of math.  To go through that Maxwell stress tensor you have to use Stokes theorem and curls and math until you head hurts and no intuition remains.

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RE: quiz: torque-producing e/m force acts on bars or core?

Would A and B see the same or different back EMF?

RE: quiz: torque-producing e/m force acts on bars or core?

Back again from France. And what do I find? Complete chaos! Rotor bars not producing any force!? What crap!

So, I read through the posts. Thinking about d'Arsonval movements (surely the force acts on the conductors) and a DC armature (not so sure there), eddy currents in a variable slip coupling (only a conducting cup to produce torque). That Maxwell guy sure had his brain cells well in order - I think he and that text-book author are right after all.

I thought I should RF your post for being provocative, but there seems to be no such rule...

Thanks for keeping us alert!!

Gunnar Englund
www.gke.org

RE: quiz: torque-producing e/m force acts on bars or core?

It is all very simple, once you get rid of your prejudices.

Bore a hole in a magnetic core and put a conductor in that hole. Let there be flux and let there be current. Since the magnetic material shields the conductor from the flux - there will be no force on the conductor.

Now cut a trace tangential to the hole (to produce an air gap). Still no force on the conductor. But the flux distribution is disturbed and there will be a force between the two pieces of magnetic material.

But I have, so far, lived a long and reasonably happy life believing that the force came from the conductor.

Gunnar Englund
www.gke.org

RE: quiz: torque-producing e/m force acts on bars or core?

Seems a very big thread for the seemingly obvious.

RE: quiz: torque-producing e/m force acts on bars or core?

Seemingly obvious and contrary to the majority of published texts. I've learned something through this.

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RE: quiz: torque-producing e/m force acts on bars or core?

Really scotty?

RE: quiz: torque-producing e/m force acts on bars or core?

Yeah, my opening post was that I thought the force acted on the bars. That's what most of the classical machines theory tells us. This has been interesting because it challenges those texts and presents some sound reasoning why they are wrong.

Still, I can see why it would be boring to people who know everything... smarty

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  Sometimes I only open my mouth to swap feet...

RE: quiz: torque-producing e/m force acts on bars or core?

Scotty no electical machines text book that I can recal states that the force acts on the bars. Common sence says otherwise. Torque is prduced by magnetism, magnetic flux is in the iron so therefore so is the force. If you believe the force is in the rotor bars then you must belive the stator counter fource is in the windings and if that were the case it would require more than a coat of varnish to hold them in place.

RE: quiz: torque-producing e/m force acts on bars or core?

It is a good thing that you industrial guys are so knowledgable and know all these things. We electrical guys have been misled by text-books teaching about forces on a conductor in a magnetic field. And most electric machine theory is building on that knowledge. So it is no wonder that we are surprised to learn that we should rethink. That is what education does to you; make you think along false lines.

I wish our instructors had skipped all those equations and just shown us some wires and varnish. That would have made us understand everything a lot better...

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Gunnar – glad to see you join the discussion.  Welcome to the chaos.  That’ll teach you to leave us unattended.

Dave – My discussion didn’t address back emf or induction of current. I simply assumed current in A and B was the same.  

[quote cbarn] no electrical machines textbook that I can recall states that the force acts on the bars[/b]

In addition to the reference I cited before, take a look at the "Standard Handbook for Electrical Engineers" (S.H.E.E.), 13th ed, edited by Fink and Beaty.  Page 20-10 section 21 gives theory of operation of a synchronous motor.   An excerpt follows:

Quote (S.H.E.E.):

In the magnetic-field model of Fig. 20-7a, the stator windings are assumed to be connected to a polyphase source, so that the winding currents produce a rotating wave of current density Ja and radial armature reaction field Ba as explained in Para 27.  The rotor carrying the main field poles is rotating in synchronism with these waves.  The excited field poles produce a rotating wave of field Bd.  The net magnetic field Bd is the spatial sum of Ba and Bd; it induces an air-gap voltage Vag in the stator windings, nearly equal to the source voltage Vt.  The current density distribution Ja is shown for the current Ia in phase with the voltage Vt, and pf=1 [for simplicity].  The electromagnetic torque acting between the rotor and the stator is produced by the interaction of the main field Bd and the stator current density Ja, as a J x B force on each unit volume of stator conductor.  The force on the conductors is to the left (-Phi); the reaction force on the rotor is to the right and in the direction of rotation

It seems that the S.H.E.E. states the force acts on the copper conductors.  They are talking about a stator rather than a rotor but the situation is the same.  They specifically mention JxB integrated over volume which would be equivalent to IxB integrated over length.  Again this gives close to the correct answer quantitatively but not misleading in terms of  where the force acts.   

Like many others, I took those textbooks at face value and never considered carefully where the force actually acts until I stumbled across that passage in the Induction Machine Handbook.

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Correction: "Again this gives close to the correct answer quantitatively but is misleading in terms of  where the force acts"

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RE: quiz: torque-producing e/m force acts on bars or core?

Well I've never read that book, however surely its clear that the iron concentrates the flux and surely its clear the flux produces the torque. Surely at some point in your lives youve made an electromagnet, does a piece of iron stick to the coil? no it sticks to the core. Its not rocket science is it.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Yes, in that case the attractive force is on the iron.  That hardly proves that magnetic force always acts on iron and never on copper. Consider the induction cup relay described page 26 (13 of 22 of pdf) here:
http://www.geindustrial.com/industrialsystems/pm/notes/artsci/art02.pdf

Quote:

Induction-Cup and Double-Induction-Loop Structures. These two structures are shown in Figs. 11 and 12. They most closely resemble an induction motor, except that the rotor iron is stationary, only the rotor-conductor portion being free to rotate. The cup structure employs a hollow cylindrical rotor, whereas the double-loop structure employs two loops at right angles to one another. The cup structure may have additional poles between those
shown in Fig. 11. Functionally, both structures are practically identical. These structures are more efficient torque producers than either the shaded-pole or the watthour-meter structures, and they are the type used in high-speed relays.

So, in this device the rotor iron is stationary, and clearly the torque acts on the copper cup.

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RE: quiz: torque-producing e/m force acts on bars or core?

electricpete, sorry, I'll make myself a little clearer about the back emf: if there is a back emf, does that mean that there is an affect of the magnetic field on the conductor? So far, I think the question discussed is "Does the conductor have an effect on the magnetic field". Now think of it backwards. I think that since back emf is present, then there must be some influence of the magnetic field in the core on the conductor - but how can it if the field is only in the core, and does not pass through the conductor. Never-the-less, we know the final result is that it does happen.

RE: quiz: torque-producing e/m force acts on bars or core?

Hi Pete, I think it does prove the case. I never said its an all or nothing arrangement. Some flux, leakage, does cross the winding space and that will produce torque on the conductors in tha space. Magnetic forces can move a non magnetic conductor, Ive paid enough electricity bills to know that, but in the presence of a large magnetic component why would you assume that the torque would be acting mostly on the non magnetic part? That would seem counter intuative wouldnt it?

RE: quiz: torque-producing e/m force acts on bars or core?

Thanks electricpete for the subject.  I just assumed it was iL x B.  I've been playing with my FEA software since you started the post and can now see that the iron is the big contributor.  What's really wierd is that for the motor (DC series wound) we use, if I assume that all the flux density crosses the wires then I can predict the torque (and get good agreement with the actual motor)using iL x B.  I've been doing it wrong for all these years but getting the right answer!

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
DaveS - I'm still not sure I completely understand your question.  The flux induces current in the rotor conductor.  That function does not rely on flux passing directly through the conductor, only on the flux passing through a loop formed by the conductor.

Bob - I looked a Liwschitz-Garak again and I see you are precisely correct that L Ix B is used for many quantitative calculations of torque and is assumed to give the correct result.  On page 179 he starts with F= L I x B and derives several commonly-used expressions for torque.

cbarn - I'm glad it obvious to you.  To summarize a few conclusions:

#1 - In a normal induction motor with bars in slots, the torque acts primarily on the core.

#2 - If we took that same induction motor#1, filled the rotor slots with iron, added a thin highly-conducting copper cylinder around the outide of the rotor (assume the airgap is wide enough to accomodate it), then the e-m torque-producing force would act on the copper cylinder.  (Call this motor #2)

#3 - If we engineered motor #2 so that a given speed we had the same  airgap flux, same total rotor current in amps per length of rotor periphery, and same rotor power-factor angle as the original motor (#1), then we would have the same torque on motor #2 as motor #1, even though in one case the torque acts on the copper and in the other case it acts on the iron.

If all of this is obvious, I would appreciate any intuitive explanation you can offer, particularly on #3.  

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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
To clarify:
"#2 - If we took that same induction motor#1, removed the copper from the rotor slots,filled the rotor slots with iron, added a thin highly-conducting copper cylinder around the outide of the rotor..."

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RE: quiz: torque-producing e/m force acts on bars or core?

"Filled the rotor slots with iron."  Would that be leaving the slots in the stampings and filling that void with an iron conductor running the length of the rotor, or would that be not removing the iron from the stampings so that there is iron in each lamination where the slots would have been?

I think that the important thing is that there are rotor bars, whether they are copper, aluminum (more common that copper as I understand), or iron than the specific material.  If the rotor was solid iron (with enough insulating material to separate all the laminations), it wouldn't work as a rotor.  If you made that solid core and coated it with a uniform wrapping of copper, I don't think that would gain you anything either.

I think that what you have to have, even if it is only to set up the fluxes that act on the iron, is the "windings" in the rotor, conductors running parallel to the axis of the machine, cutting through the insulated laminations of the rotor.  Without those, there would be no rotor flux to interact with and be dragged around by the rotating stator flux.

RE: quiz: torque-producing e/m force acts on bars or core?

electricpete, I'm sorry, perhaps the points were too subtle. I'll start on a new tack: I'm not convinced by your arguments.
The copper conductor in the slot does not have much magnetic field because the iron conducts it alongside. But a copper conductor, with a current, sets up a magnetic field of its own, which rotates around the conductor. I think it is the interaction of those two magnetic fields that causes the force. On one side the two magentic fields are in opposite directions (repelling force), and the other side the magnetic fields are reinforcing each other (attractive force).
If the core was removed, and the same field strength applied across the conductor using a much bigger magnet, then the copper conductor should experience the same forces when current is passed through.
In this case, I think it doesn't make sense to say the force is created in the air alonside the conductor. But the air has a magnetic field similar to the iron, and the conductor current creats a field which interacts with the magnetic field in the air.
The function of the core is to conduct the magnetic field through what would otherwise be a huge air gap, and thereby cause the magnetic field to be as large as possible.

RE: quiz: torque-producing e/m force acts on bars or core?

Hi Pete, i'm not sure I can even follow your latest post however it would seem intuative that if you competly redesign a motor you have a diferent motor and not the same one. In an induction motor in both the stator and rotor you have a current carrying conductor wound around a lump of iron, an electro magnet. There maybe several of then all interacting but that doesn't change anything.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)

Quote (davidbeach):

"Filled the rotor slots with iron."  Would that be leaving the slots in the stampings and filling that void with an iron conductor running the length of the rotor, or would that be not removing the iron from the stampings so that there is iron in each lamination where the slots would have been?
Fill the slots with laminated core iron. (sorry I should have been more specific).

Quote (davidbeach):

I think that what you have to have, even if it is only to set up the fluxes that act on the iron, is the "windings" in the rotor, conductors running parallel to the axis of the machine, cutting through the insulated laminations of the rotor.  Without those, there would be no rotor flux to interact with and be dragged around by the rotating stator flux.

You talk about a conductor shielding a field.  I think this applies to stationary time-varying magnetic fields but not to rotating/traveling magnetic fields.   If you have a stationary applied magnetic field varying sinusoidally in time and space penetrating normal to a conductor, the peak induced current occurs at the spatial 0 of the field and the induced current tends to prevent the applied field from penetrating the conductor (or more specifically cancels the applied field).  But the current induced by a rotating/traveling wave peaks at the spatial location of the field peak (neglecting small power factor angle) and does not tend to cancel the airgap field.  

If the field did not penetrate the rotor conductor sheet, why would they put iron at the center of the induction disk relay?

Further consider the following statement about linear induction motors from The Induction Machine Handbook: "The secondary is either made of a laminated core with a ladder cage in the slots or of an aluminum (copper) sheet with (or without) a solid iron back core. "

Bars are more efficient than sheets.  The torque-producing force comes from the longitudinal current.  The circumferential end-ring current does not contribute.  In the sheet rotor, there are many circumferential paths which tend to reduce the effective length of the longitudinal paths.  Instead of a loop traveling longitudinally and circumferential at the ends (barred rotor), the loops are still centered on the axial center of the core but have many circumferential paths because the return path between bars is at the ends of the rotor.  

In case #2 part of the engineering for the same current would require the same total longitudinal amp-turns-meters per meter of circumference.

They key reason for introducing #2 is to emphasize that the torque depends on the airgap field, the rotor current, and the rotor power factor as if the current were acting directly on the conductor.  In case #1 the force  doesn't act on the conductor but produces the same torque that would be predicted from L*I x Bairgap.

Quote:

DaveScott electricpete, I'm sorry, perhaps the points were too subtle. I'll start on a new tack: I'm not convinced by your arguments.
I agree with most of the statements in your post but I don't agree they prove your conclusion.  To clarify, is your conclusion that the torque-producing force acts primarly on the rotor bar?   (would be seen by a load cell between the bar and the core?)

Quote (cbarn):

Hi Pete, i'm not sure I can even follow your latest post however it would seem intuative that if you competly redesign a motor you have a diferent motor and not the same one. In an induction motor in both the stator and rotor you have a current carrying conductor wound around a lump of iron, an electro magnet. There maybe several of then all interacting but that doesn't change anything.

cbarn - can you explain why the torque from an induction motor (case #1) can be correctly predicted AS IF the airgap flux level was present at the rotor bars and created a force F=L I x Bairgap directly on those bars (even though Bairgap is not present at the bars and the force does not occur on the bars)

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RE: quiz: torque-producing e/m force acts on bars or core?

Hi Pete, its because its an electromagnet, god how many times, calculate the flux on an electromagnet is it at the winding? no!!!! its at the poles, how hard is this.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
My question was not why the force acts on the iron. It is why we can correctly calculate the force as if it were based on direct interaction of the airgap flux and the copper current.

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RE: quiz: torque-producing e/m force acts on bars or core?

I just told you, maybe you should refresh your memory on basic electromagnetism, the kind of stuff they teach a 15 yr old.

RE: quiz: torque-producing e/m force acts on bars or core?

Pete

I think in your example #2 above with a continuous sheet of copper the force would still act from the iron. As the field moved relative to the copper/core the current induced in the copper would support the magnetic field inside the core. Or equivalently the copper could not shield the source field from the core. So the source magnetic field would primarily interact with the magnetic dipole current loops in the rotor core thus producing force in the core.

True or Error ??

RE: quiz: torque-producing e/m force acts on bars or core?

cbarn,

You are getting very unpleasant. No need to be that.

What they teach a 15 yr old is about force on a current carrying conductor in a magnetic field. No Maxwell at that stage. And I doubt if you have any calculus at all at 15 years. You can not deduce or understand anything about magnetic fields and/or forces without that background.

Pete is not arguing with you. Try to understand that. All he asks is how it can be that torque calculations in a motor give the right result if one assumes that the force is acting on the conductor.

And that is true. Many (most, I would say) texts on motors do tell us that the force comes from the current in the conductor. We all, I think, know by now that that is not the case. But we still calculate torque as if it was true. That is what Pete is asking about.

Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
2dye4 - I don't know.  Case #2 is strange, but apparently this is roughly the configuration used for induction cup relay and for some linear induction motors, so it must work.  If anyone has a better explanation exactly how this configuration works I would be interested to hear it.

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RE: quiz: torque-producing e/m force acts on bars or core?

Hi Skoggs, I want bieng unpleasant at all. A simple schoolboy experiment, wire, iron bar, paper and some iron filing should remind him where the flux lines are and are not on an electromagnet. Why do equasions work? because they are derived from experiment. You know, do an experiment, measure the results, formulate a formula that fits.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
cbarn - You are saying it's obvious that the force acts on the core, but yet obeys equations as if it acts on the conductor.  We'll have to agree to disagree on that point.

All - Here is a more mathematically rigorous explanation of why we can get the correct answer for torque by INCORRECTLY assuming the airgap flux acts passes directly through the rotor (or stator) conductors creating a force F = Ibar L x Bgap on each conductor.
http://home.houston.rr.com/electricpete/FORCE_ON_IRON_NOT_BAR.pdf
Note the above-linked document is not mine, but was provided by a colleague on maintenanceforums.com

One point that I objected to on the first read-through was the much larger airgap shown at 3:00 and 9:00 positions.  It didn't seem representative of an induction motor.  But I thought about it some more and I realize that we would get the same answer if we continued a uniform gap all the way around and put in any B(theta) distribution... for example a snapshot of a typical motor field B = cos(p*theta-w*t0).

The reason is given by examining the torque
Torque = mu0/2 * d/dgamma  {Integral   [B(theta) + b(theta,gamma)]^2  dtheta}
where:
b is a constant except for a step change at theta=gamma and a step change theta =gamma+Pi
theta is statinoary angular coordinate
gamma is position of the conductor

Careful inspection of this expression (looking at the derivative as the limit of small changes in gamma) shows that it depends only on the value of B(gamma)  and B(gamma+Pi).   So while it is proven for this simple case, it will also apply for all other distributions of B(theta) and the result will only depend on the value of B(gamma).  i.e. the answer depends only on B at  the location of the conductor.  So if we apply the incorrect assumption that airgap flux acts directly on the conductor, it will still give the right answer for any flux distribution.

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RE: quiz: torque-producing e/m force acts on bars or core?

No cbarn, what you describe is called curve-fitting.  

A deep understanding usually means formulating a theory, and that almost always means a mathematical relationship. In the case at hand, Maxwell formulated a theory about electro-magnetic waves (the Maxwell Equations) long before anyone had observed electromagnetic waves. Hertz and Helmholtz did their experiments after Maxwell had done his part of the work.

You are simplifying things a lot. And also ridiculing people that seek a deeper truth. That is not needed.

Gunnar Englund
www.gke.org
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RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
For those that might still be skeptical of torque-producing force acting on the iron core rather than the copper conductor, I have collected some more info on this at the link below.

If you have broadband, you might be very interested to see the video (7MB) of a lab demonstration that I did which visually shows to torque-producing force acting on the iron.  I have an iron core slot section supported from above and a copper coil inside... both hanging from above (like a pendulum) and free to swing independently.   I taped magnets above and below.  When the current increases, you can see the iron move, not the copper.  When I remove the iron slot and repeat, you can see the copper now moves in the same direction that the iron had moved.  The video is the 6th bullet here:

http://electricpete1.tripod.com/torque_web/index.htm

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RE: quiz: torque-producing e/m force acts on bars or core?

From observation of Linear induction motors used for mass transit, I believe that a solid rotor wrapped with a cylinder of copper or aluminum would work. One type of LIM uses a strip of iron or steel about 12" wide and 1/2" or 3/4" thick. This is covered with an aluminum cap that is about 1/8" thick. This is the Linear part of the Linear Induction Motor
In close proximity to this is the movable part of the motor, the core and windings that are suspended from the coach.
Back to the original question:
It has been stated that no flux passes through the conductor.
It has been stated that the equations work if it is assumed that flux does pass through the conductor.
Flux does pass through the conductor, but in the inverse ratio to the permeability of the iron. something like 1/20,000.
It may appear that major flux passes through the conductors, but it is more likely that the flux density is increased in the iron adjacent to the conductors. (Of course, if the iron saturates in this region, significantly more of the additional flux will pass through the conductor).
I suspect that motor design allows enough iron between the conductors to avoid saturation.
I believe that the force depends on the total flux. The ratio of iron flux to conductor flux depends not only on permeability but also on the relative effective areas of the conductors and the iron.
Would it be accurate to state that the force or torque in a motor is developed overwhelmingly in the iron, but some is developed in the conductors.
The force in the conductors will be related to both the effective areas of the iron and the conductor and the permeability of the iron.
If we assume the permeability to be in the order of 20,000, then the ratio of the conductor torque to total torque may be in the range of 1/5000 to 1/15,000.

And, on the practical side, when you consider the instances of damage to improperly secured stator windings from magnetic vibration, I believe we must accept the concept of some force on the conductors.

This is a delightful thread you have started electricpete. Thanks.
respectfully

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
Yes, I agree 100%.

1- overwhelmingly - yes.   The radial flux in the conductor is much less than the radial flux in the iroin and as a result the torque-producing force acts primarily on the core (not the iron).  

2 - Are there forces on the conductor, yes.  Note I said radial because radial flux is the one associated with torque.  The flux that does occur in the conductor in the slot section is primarily tangential. That results in a radial force on the conductor trying to move it up and down in the slot.  The result you may see on form-wound coils loose in their slots upon removal is a ladder pattern where coil sides have rubbed against slot sides, but no rubbining in the area of vent ducts (forming the rungs of the ladder).    Also in the endwindings are not enclosed in iron and there are forces on those although the flux is not as high in that area and the direction of flux and force varies.

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RE: quiz: torque-producing e/m force acts on bars or core?

Hello,

we have a circular laminations that build rotor and there are rotor bars short-circuted at both ends by rings...an emf induced across the rings causes rotor current and then a torque..so answer is bars...

Now comming why not in core??  This is simple...because of laminations there will be a eddy current and the there will be no torque produced by each lamination...

Hope I am clear.

RE: quiz: torque-producing e/m force acts on bars or core?

(OP)
I disagree.  Eddy current in the core is not required to produce torque on the core.  All that is required is a high permeability core.

I may have confused the issue with a discussion of bound current above.  Bound current is a way of describing the fields and interaction associated with magnetic materials, but it is not a physical current or an eddy current.  (it is the sum of dipole loops).   To avoid confusion, I would suggest to not even use the concept of bound current in this discussion.

The principles by which torque acts on the core are proven in the links above based on the permeability of the magnetic material in many different ways.  The simplest way is to remember that a high-permeability core slot will shield the conductor from flux (does not require the presence of eddy current).  

The radial airgap flux pattern produced by conductors sitting in slots is approximately the same as that by conductors sitting on the air-gap side of the core.  This can be proven by Ampere's circuital law, with very high mu the portion of the loop within the core contributes almost nothing, and any loop that encircles the two situations will have approximately the same flux in the airgap.

So far away from either conductor (one in a slot or one sitting on the airgap side of the core surface) the conductor produces a simple step in mmf.  Add together all of those steps and you have a staircase sinusoid.  Torque is predicted (from conservation of energy) as proportional to mmf_stator * mmf_rotor *sin(delta_sr).   The motor with conductors in the slots since it produces the same mmf (assuming same currents) will produce the same torque as the motor with the conductors sitting on the surface of the airgap.  But the force cannot appear on the conductors if they are in slots, the force appears on the core.

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