Rotation by Lorentz or by Faraday? 3

[Intermesher]

The following assumes the application of a constant DC current to rotors that have no rotational inertia to start with.

Some illustrations show the flux from the pole of a permanent magnet causing rotation by 'pulling' the laminate core of a coil wound electromagnet into alignment with the permanent magnet; before stopping.

http://www.unicopter.com/1904_7.gif

However, other illustrations show the flux from the pole of a permanent magnet causing a portion of a coil of wires to transition across the full width of the flux; before stopping.

http://www.unicopter.com/1904_8.gif

Are both of the above statements correct ~or~ where is my limited intelligence coming off the tracks.


Thanks,
Dave

 

[waross]

Your first link shows an external rotor. That may be causing some confusion. The rotation would be the other direction if this were an internal rotor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I think the first illustration is supposed to be a synchronous motor. The magnets on the outside represent a rotating field without depicting either iron or copper.

The 2nd illustration illustrates interaction of current located within a magnetic field and you might qualitatively see that it results on force on the conductor in this case.

The exact question of exactly how / where the torque-producing force is one that is near and dear to my heart. Not so simple as it may seem. For machines with conductors located within iron slots, the normal torque-producing force acts primarily on the core (not the conductor), even though you can come pretty close to calculating the correct torque producing force using a force on conductor equation (F=q*v X B = L i x B) under the incorrect assumption that the conductor is located in the airgap flux.

I have written a short and long whitepaper on the subject, along with video and some other assorted stuff.

http://home.comcast.net/~electricpete1/torque_web/

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[Intermesher]

Thanks gentlemen.

It's a small world. Peter, your short paper was read and printed out a while back. Then after entering the above posting, I took it out and read it again over coffee. Thank you for the informative and readable paper. Unfortunately, it does not appear to answer my weird question.

My query was not well presented. The following sketch includes the second sketch above (b), plus a third sketch (c).

http://www.unicopter.com/1904_13.gif

Without considering the mechanical aspects of the 'motors', there are two electrical questions.

~ In sketch (b), will the conductor at location A move to location B before it stops due to lack of force?

~ In sketch (c), there is a row of adjacent and physically bonded conductors that are aligned normal to the flux path. All of these conductors have their current flowing in the same direction. In addition, there is no cycling of the current direction in these conductors.
Under these conditions, will an endless column of these of conductors be continuously moving by electromagnetic force from top to bottom of the sketch?


Dave

 

[electricpete]

~ In sketch (b), will the conductor at location A move to location B before it stops due to lack of force?

I don’t believe it is necessary to talk about motion for the question if the currents and fluxes are specified. Instantaneous force is a function of instantaneous current and flux distribution and therefore the relevant electromagnetic results are captured by describing force as a function of position for the specified distributions. (if we were trying to determine voltage and calculate an induced current, then motion would be important). Also we assume the return path for this current is far away in a location of zero flux. Then the force on the conductor would decrease as conductor moved from point A to point B and continue decreasing as it moves further in same direction (because flux density B is decreasing as we move from point A to point B and beyond... F = q V x B = Length * I x B)

~ In sketch (c), there is a row of adjacent and physically bonded conductors that are aligned normal to the flux path. All of these conductors have their current flowing in the same direction. In addition, there is no cycling of the current direction in these conductors.
Under these conditions, will an endless column of these of conductors be continuously moving by electromagnetic force from top to bottom of the sketch?

You have to describe your physical problem better. But if the conductors are rigidly linked together so that their forces are summed, then there is a constant force on the assembly as it moves downward through the gap.

* I think it can sometimes be misleading to try to talk about partial systems without considering the complete loops for current and flux. Can you describe more completely the physical system you are evaluating? I am also guessing this will be a better question for the magnetics forum.


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[Intermesher]

Peter, the following link is to the proposed motor. It is intended for a 'backpack' coaxial helicopter. The physical system is to be an axial flux motor. It will consist of a central stator, plus a rotor on each side, which are to turn in opposite directions.

http://www.unicopter.com/1904.html

The primary question is if this concept motor can be driven in a phaseless or half-phase manner. OR, if it can even be driven at all.


Dave

 

[electricpete]

It is not a type of motor I am familiar with. You called it “axial flux”, but it looks to me as if the copper winding is formed as a toroid such that flux would flow circumferentially within the stator and not interact with the rotor magnets.

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[electricpete]

Also, is this a hobby project? If so, eng-tips is probably not the right place to ask. There may be hobby sites where you can get better answers.

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[electricpete]

Actually I think I understand now the reason for calling it axial. The flux is supposed to flow axially from the magnets, past half of the stator winding, to the core in center of the stator winding, then back through shaft to rotor plate to opposite side of magnet. It is a similar concept discussed here on the forum before here in this thread:
thread237-184398
(see my post 21 Apr 07 15:20 )

Sseems to me like it could in theory create a simple dc motor without need for mechanical commutation. I don’t think everyone in the forum agreed on that point. Maybe I am missing something.


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[Intermesher]

A little more on the objective;

It is to apply electric propulsion to helicopters. Helicopters are extremely inefficient vehicles.
The 'backpack' configuration was selected for development since it offers the best payload to empty-weight ratio.

This 'strange' motor was conceived as a possible way to provide CW and CCW torque from a single motor. The question now becomes; Firstly, will it even work and secondly, will it work efficiently.

The idea (or hallucination) started with the consideration of a Rotor-Stator-Rotor axial flux motor and then the consideration of how to get the rotors turning in opposite directions. This initial concept is here;

http://www.unicopter.com/1740.html.

________________________________________

Peter, I read your thread (21 Apr 07 15:20 ). It appears that my idea is a duplication of your earlier one. Did you ever come to a final answer?


The following drawing is an exploded version of a drawing on the web page given in a previous post. This shows the flux path.
this flux path differs slightly from that on the prototype.

http://www.unicopter.com/1904_14.gif

The flux flows from the magnets in one rotor, through the adjacent conductors, through a radial laminate core, through the return conductors, through the magnets in the other rotor, and then back via a very thin air gap to the originating magnets. The radial laminate in the core is intended to direct the flux from one side of the core (flattened doughnut) directly to its other side.

The hope is that the opposite flow of electrons in the conductors will drive the permanent magnet rotors in opposite directions.

 

[electricpete]

I have not done my prototype yet, but I have a renewed inspiration and an idea how to make my construction job a little easier. Maybe this weekend (or maybe not). I was mistaken to say mine is the same as yours. Mine was radial flux. Yours is axial flux. There are still similarities in the approaches. If successful, they would both be dc motors working without commutation (if such a thing is even possible). Yours has an interesting twist to take advantage of the opposite polarity of forces/torques produced on the two halves of the stator coil, since counter-rotating rotors is apparently something you can use.

I am pretty sure that if yours would work mine would work and vice versa.

Here is what makes me suspect they have a chance of working. If we draw the flux lines in the static condition, there clearly is a torque produced. That much I am sure. Next step... what would make that torque stop when the machine rotates? I can't think of anything.

Here is what makes me hesitate. The stumper question posed in the other thread: what is the mechanism for inducing voltage in the stator winding. It is not a straightforward question to answer for textbook homopolar motor (using brushes) and it is not a straightforward question to answer for these motors (no brushes). I think to answer it one way or another would require careful application of reference frame theory converting B to E based on v. I have not done that. I think it would be easier and more productive to try to build it.

Have you tried yours?

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[waross]

Pete, I took the original sketch to be stationary coils and an external rotor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

You're probably right. The annotation suggests some kind of mechanical commutation. Not a very descriptive or useful figure.

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[Intermesher]

Peter, yes they were tried.

The first test, shown in this picture

http://www.unicopter.com/1904_3.jpg,

did not rotate either rotor. Perhaps this was due to; the center of the stator being a plywood disk, plus the other three air-gaps are quite large. In addition, the permanent magnets are only ceramic or ferrite.

The second test is shown in this picture

http://www.unicopter.com/1904_10.gif.

The + and - magnets only cover about 20-degrees of rotation. The U shaped single wire moved about 10-degrees when the current was turned on, and it moved about the same amount in the other direction when the terminals were switched.

This resulted in the following considerations;

~ If a CONSTANT dc current flowing through a wire cannot create a constant force; what about using PULSE Width Modulation to put a non-reversing current through the wire?
~ Might PWM be required anyway, since it may be the best means of varying the speed and/or torque?
__________________________

waross, you are correct.

It is Rotor-Stator-Rotor and the hope is that the electromagnetic activity at one side of the stator will not detrimentally effect the activity at the other side of the stator.

Dave

 

[waross]

Yes, the experts are not always right. But I believe that they are right much more often than is suspected.
A review of the basics of magnetism may be in order.
An energized coil will exert a magnetizing force. The strength of the magnetic field is roughly the magnetic force divided by the weighted length of the magnetic path. The weighing is based on the magnetic permeability of the materials that make up the path. One inch of air or plywood is equivalent to about 800 Feet of core grade iron. The textbook drawing of a wire in a very large air gap does work but if you want more force than a fraction of the torque of a d'Asonval meter movement you should spend a few hours reviewing the basics of magnetism and then seriously shortening your air gaps.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Waross - Just to clarify - what is it that the experts say? Is it written somewhere that a brushless dc motor cannot be built? If it is written somewhere I'd like to read it?

On the other hand, I agree on is not built anywhere... that makes me suspicious there is a reason it is not built.

Alos, where is the large airgap you are talking about? Here is the flux path:

http://www.unicopter.com/1904_14.gif

There is no doubt the designer of this device is aware of the need to minimize airgap distance since all airgaps are minimized.



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[electricpete]

Waross - Just to clarify - what is it that the experts say? Is it written somewhere that a brushless dc motor cannot be built? If it is written somewhere I'd like to read it?

On the other hand, I agree on is not built anywhere... that makes me suspicious there is a reason it is not built.

Just to clarify the first part of my quote, it is not argumentative... it is what it says: asking if this configuration is addressed somewhere.

As per 2nd part, I certainly agree that there is strong evidence to be inferred from the fact that motors like this have not been built. I have zero doubt motor designers no boatloads more about their craft than me.

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[electricpete]

Also I put my linked file from my other thread onto an attachment of my post 25 Mar 10 12:03t since my ISP changed and link no longer works.

I have another way to think about my drawing that suggests why it might not work. Replace the stator core with a permanent magnet which produces the same flux pattern. In that case the stator-replicating permanent magnet would be toroidal magnetized in the circumferential direction (perhaps with an axial gap.). Now looking at orientations of the stator-replicating permanent magnet and the rotor permanent magnet, we don't see any attraction created even in the static condition. Hmm.

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[electricpete]

I have another way to think about my drawing that suggests why it might not work. Replace the stator core with a permanent magnet which produces the same flux pattern. In that case the stator-replicating permanent magnet would be toroidal magnetized in the circumferential direction (perhaps with an axial gap.). Now looking at orientations of the stator-replicating permanent magnet and the rotor permanent magnet, we don't see any attraction created even in the static condition. Hmm.

My comparison above is not valid for analysing static torque in my drawing..... Replacing stator core with circumferentially-magnetized toroidal permanent magnet does not properly capture the effects of the high permeability of the core which directs the flux from the rotor permanent magnets so that it passes by through only half of the stator winding loops. In contrast, the flux from rotor permanent magnets would pass thru both sides of a stator-replicating toroidal permanet magnet.


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[waross]

Hi Pete;
My comments were based on a picture of a toroidial coil on a plywood core, and magnets a long way away from the flux which could be expected to be mostly inside the toroid. One of the advantages of a toroidial coil.
The second link in the first post may produce an order of magnitude less force than a d'Arsonval meter movement. Only one conductor instead of many and no return path in the flux field.
The first picture posted shows a sensor to switch the coils on and off. Switching on and off rather than reversing the current as is done in a brushless DC motor may give the same performance as a broken brushless DC motor.
If Intermesher wants rotation with a steady DC current look here:

http://en.wikipedia.org/wiki/Homopolar_motor

and here

http://en.wikipedia.org/wiki/Homopolar_generator

Bill
--------------------
"Why not the best?"
Jimmy Carter