Stator shape 1

[Wedwin]

I have dabbled with the thought of making a simple sort of linear motor. This far it is merely a thing in my mind. But it raised a few questions about how to produce the components.

Main issue is the shape of the stators. As I have read here the faces to the magnets seem important.

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I have tried to visualize a C and U respectively. I hope You understand what I mean. I wish to know if any of them are favourable from a magnetic view.

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The C-shape has a distinct area facing the magnet, whereas the U-shaped one doesn't, but should be easier to make.

I could of course make blocks to fasten in the gap and make faces. Would I gain something from making faces that are bigger than the magnets, even in the C-shape?

Next up: How much of a loss will a hole for threads fastening the stator make? I guess most of the hole will be filled with a screw.

Not only for fastening, but making the stator out of straight blocks, would it be a bad idea to screw the pieces together instead of bending a solid stick into shape?.

I intend to use DC or rectified AC.

And as a finishing point. In the formulas there is no reference to where along the stator that the coild is placed. So, am I right in assuming I can make two coils on the 'arms' just as well as one on the 'backbone'?

 

[UKpete]

Hello Wedwin. Not too sure what you are trying to do, linear motors are ac not dc because you need 2 or more phases of ac to get a moving field and hence motion. Or are you talking about a suspension device?

If it is a dc device you can probably get away with using solid steel, but if it is ac you need to use laminations.

Re some of your supplementary questions:
-You don't really need to make the steel pieces of larger cross sectional area than the permanent magnets, because electrical steel isn't saturated at the maximum flux density of any PM.
-The holes shouldn't be a problem provided they are relatively small, especially if the bolts are steel.
-You can bolt pieces together rather than bending, but if it is an ac device and therefore laminated, you have to be careful not to short the laminations at the interfaces (otherwise eddy currents will occur), i.e. insulate the interfaces.
-To a first approximation the coil can be placed anywhere along the core, although there is always a small amount of leakage (flux finding a shorter path through the air rather than out of the pole faces) especially if the core is operating at high flux density.

 

[Wedwin]

Thanks!

Well current is alternating per se, but I see it more as DC applied the proper direction at the proper time. And, it won't move very fast either. If it's wrong to call it a linear motor, then I stand corrected.

Maybe a linear stepper motor is a better approximation.

I made a animated gif out of my thoughts. Basically, it is to pull the magnet in and then push it on it's way, turning the field off right in the middle since I figure it a waste. No need to 'hold' the magnet is there?

The magnets will be rigged on a long trolley/carriage. So there will always be two or three stators active

All the magnets are treated individually by the stator closest to them. the image is not to scale.

I really want to get away with solids, because laminates are not within my grasp.

If I can get away with a simple U-shape, life will be even easier.

Bear in mind, this is still a thought experiment.

 

[UKpete]

Yes I see what you mean, no reason why you can't call it a linear motor.

With a combination of permanent magnets and stator windings, you have two excitation sources making it a hybrid motor. The total force on the moving magnet will consist of two components:
- an excitation force due to field alignment (the magnet field will try to align with the field due to the winding ampere-turns)
- a reluctance force due to the magnet moving to reduce the total reluctance in its magnetic circuit i.e. it will want to move into then maintain its position centrally between the poles and will resist any movement outwards.

Superposition will apply i.e. you can simply add the two forces to find the total, provided the fields don't saturate the iron.

The problem you will have I think is that although the magnet will be attracted into the area between the stator pole faces, it will then encounter a force in the opposite direction so that it will want to stay between the stator poles (except for an inertial force wanting to keeping moving in the same direction). De-energizing the coil will remove the excitation force but the reluctance force will remain.

In stepper motors and other hybrid machines, the technique is to place another stator pole behind the first one, in fact to have a succession of stator poles, so that the moving magnet experiences a force moving it onto the next pole. What you are looking at may be called an electromagnetic launcher:

http://www.sussex.ac.uk/press_office/bulletin/22oct99/article1.html

There is also a good description of hybrid stepper motors (which have magnets and coils) on:

http://www.cs.uiowa.edu/~jones/step/types.html

Incidentally, the switched reluctance motor (which has stator coils mounted on stator teeth, but a simple toothed rotor made of steel - no magnets) works because the spacing of the rotor teeth is different to that of the stator teeth, so that with correct energization of the stator coils there will always be a majority of teeth forcing the rotor in the correct direction.

By the way, I have to ask - how did you do the moving graphic image? It is very effective in illustrating your point.

 

[Wedwin]

Thanks again! My confidence is strengthened yet another notch.

I have to learn all these expressions to sort things out. You call it reluctance that creates the field that makes a holding force on the magnet. I tought it was remanens. Maybe it is just a Swedish word.

Don't worry, I will hunt it down out there on the net.

The holding is supposed to be overcome by inertia indeed, and that in turn is supposed to be supplied by a few of the other magnets in the same carriage.

The one carriage is to have 10 magnets in all. And at least two or three should be in active duty with a stator at any given time. Maybe I will change the spacing, but it can't be much fun winding stators ...

Should I go for large faced not so strong but cheaper magnets or stronger ones? I suppose the saturation comes in here.

Does it do any harm if the stator faces are significantly larger than the magnets? Who knows what sort of material I may come by

Do I gain anything at all from extending the stator faces such that it 'greets' the magnet as it arrives and adds to the push as it leaves?

Actually, this whole experiment was brought on by me watching 'Space: 1999' and thinking it would be sooo cool to have sliding doors like in most SciFi movies/series. This story could become long, so I end it here. I did work for

http://besam.com/

but none of their stuff is cool. Nor would it exercise my brain.


The pictures for the animation was CAD'ed in QCad from

http://www.ribbonsoft.com/

It took me a full night to make all the layers copying the objects, moving them to their respective layer. But that was more due to my lack of skill. After that it was a mere matter of turning layers on and off. As the export function no longer works in my QCad, the images were made by screendumps instead. Finally, the images were put together in GIMP

http://www.gimp.org/

Any graphic manipulating software should be able to do animated gif's ... I think, I am no expert on that either.

For a brief moment I thought of adding my imagined magnetic fields, but even I have a limit. Though it would be good to learn if I am barking up the right tree.

If You have any use for it, feel free to download it. All rights abandoned. I have no need to take it down, but You never know.

I have another one describing a three phase rotary motor, driven totally digitally, two phases at any given time. But I guess that would be another thread.

 

[Wedwin]

I happened to click away a little and here are a few candidates:

Vote?

Am I right in thinking thee are too much loss in the first, and not enough benefit in the last?

 

[UKpete]

Hi Wedwin,

Regarding the reluctance force - the reluctance of the magnetic circuit is effectively the "resistance" of the
magnetic circuit, the full analogy with the electrical circuit being:

ampere-turns -> voltage
flux -> current
reluctance -> resistance

One of the forces exerted in a magnetic circuit will be in such a direction as to minimize the reluctance
in that circuit (hence a keeper is attracted to a magnet; it is related to the fundamental principal that
the force acts to minimize the energy in the circuit). Hence it is referred to as a reluctance force.

Remanance by the way is the flux density in a permanent magnet that is not subject to any external fields
(such as a magnetizing force).

Winding stators isn't so difficult provided you can wind them onto a bobbin that will slide onto the core.

Just an idea, I wonder if you even need to use permanent magnets, simple blocks of steel on the door would be attracted to the stator electromagnets. If you use permanent magnets, you have the problem that they will provide a retarding force when they have passed through the stator (as mentioned in my earlier post) - you can't switch off a PM. Of course the spacing of the magnets (or steel blocks) on the door must be different to that on the stator otherwise you will only get a small movement then the door is held in a fixed position. It is trial and
error really, unless you have access to some good finite element software.

If you do use permanent magnets, I would experiment with a single magnet/stator first - you won't get a very
linear force with displacement but if you could at least obtain a force-displacement characteristic then you
can estimate the optimum spacings for the magnets along the door. Unfortunately there are a lot of variables,
so quite a lot of experimentation may be required; I also suspect it will be difficult to get a smooth force
without using a lot of poles. Maybe your extended stator faces may help a bit in that regard; but initially
I would go for the middle of the three stators shown in your diagram - the first one is ok but only if there
is enough room for the coil.

 

[Wedwin]

Allright! UKpete, You have been a real help!

I think I have all my questions answered now (well, not by a longshot but I am covered).

I did find

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

and similar sources of info on the web.

The thing about the magnets, I don't want them to be switched off. By reversing the stators to shove them off once they passed through, I hope to make use of the magnetism twice.

Bare steel would indeed be easier, smaller and by far cheaper. But would it give the same performance? And would it not give the opposite effect of being shoved off when I energize the stator in reverse?

As for smooth running, my guess is that the more stators, the more evened out the travel will be.

Further pondering and calculating will now commence.

 

[UKpete]

Wedwin, you are probably right to use permanent magnets.

Neodymium magnets (Neodymium Iron Boron) are ideal if you can get them, one of the low temperature grades if you have the choice as they have the best performance of any magnet material at room temperature. Energy density per $ is highest of any type, and they are cheaper now than they have ever been.

 

[Wedwin]

I believe You have brought this thread as far as it goes UKpete. Thanks loads.

My ponderings are on the whole verified.