I A donut electromagnetic core comprises main section + movable section

[cairoliu]

TL;DR

pure magnetism problem

There is a short movable section of cylinder shape in a O-ring magnetic core with DC coil, of course, there are 2 air gaps between the movable section and stator main section. My question: if rotate the movable section, is there a bunch of twisted magnetic lines?
The rotation does not change the outlook of whole O-ring core, so gaps keep same volume.

 

[berkeman]

Can you upload a sketch? I'm having trouble visualizing what you are describing. Use the "Attach files" link below the Edit window to upload a PDF or JPEG copy of your sketch. Thanks.

 

[cairoliu]

 

[berkeman]

Thanks for the drawing. If the cylinder is ferrous, the B-field lines from the C-core will just go right through it, with a little fringing at the two air gaps. The cylinder will be in an unstable position, so will need to be supported against thrust in the vertical direction. I don't see anything that will cause the cylinder to rotate about its dotted-line axis that you show.

 

[DaveE]

The field lines will readjust to minimize their path length (i.e. straight) almost instantly. Unless your spinning that think near the speed of light, LOL.

edit: Maybe the worst sentence I ever typed.

 

[cairoliu]

Though undrawn in the sketch, I can use a motor to drive the cylinder via belt, of course bearings are needed to keep stable.

Both the stator main section & the rotatable short section are made of same magnetic material, e.g. silicon steel etc.

Are you sure the MagnetoMotive Force (MMF) lines will not twisted within the gaps?
I remember textbooks state there are lots of magnetic domains inside magnetic materials, and MMF lines go through magnetic domains; so if the said section is rotating, then supposedly MMF lines should be twisted.

It seems experiment can be done like this:
scatter some ferrous powder on a hard paper, then insert the paper in a gap, hold at hand, and make sure the paper not touch the stator or the rotor. watch the powder during rotation.

Not test, just by theory imagination now.

 

[cairoliu]

The field lines will readjust to minimize their path length (i.e. straight) almost instantly. Unless your spinning that think near the speed of light, LOL.

I remember textbooks state there are lots of magnetic domains inside magnetic materials, and MMF lines go through magnetic domains; so if the said section is rotating, then supposedly MMF lines should be twisted.

So I guess regular speed can render the twisting effect.

 

[cairoliu]

Thanks for the drawing. If the cylinder is ferrous, the B-field lines from the C-core will just go right through it, with a little fringing at the two air gaps. The cylinder will be in an unstable position, so will need to be supported against thrust in the vertical direction. I don't see anything that will cause the cylinder to rotate about its dotted-line axis that you show.

see my reply in #6

 

[berkeman]

It seems experiment can be done like this:
scatter some ferrous powder on a hard paper, then insert the paper in a gap, hold at hand, and make sure the paper not touch the stator or the rotor. watch the powder during rotation.

The ferrous powder will be attracted to the nearest pole of the C-core. If you want to keep it in place, maybe put it between two microscope glass slides separated by a small 1-2mm spacer around the periphery of the slides. I'm guessing the experiment will not show any movement/drag of the powder between the slides as you rotate the slides in the gap.

 

[DaveE]

TLDR: EM waves move fast. Really, really fast.

Any sort of anisotropy or inhomogeneity in the permeability of the core will create a corresponding difference in the flux distribution. So yes, if you prefer, they do twist and then readjust at almost the speed of light*. Also If you move the material the flux distribution will move along with it. In the frame of reference of the spinning part, there should be no changes based on rotational speed within realistic limits.

* 99.999% BS. I have no idea what happens in this case near the speed of light. But I know what happens at normal speeds.

 

[cairoliu]

The field lines will readjust to minimize their path length (i.e. straight) almost instantly. Unless your spinning that think near the speed of light, LOL.

Instantly minimize their path length between magnetic domains?
Does it mean the so-called magnetic re-connection that often refers to solar surface magnetic phenomenon?

 

[DaveE]

Instantly minimize their path length between magnetic domains?

Yes.

Does it means the so-called magnetic re-connection that often refers to solar surface magnetic phenomenon?

Let's leave the sun out of this. It's a gas/fluid/plasma that changes in response to the EM excitation, unlike your core. Neither you, nor I, are ready for magnetohydrodynamics. If you want to know about that, start another thread.

 

[cairoliu]

TLDR: EM waves move fast. Really, really fast.

Any sort of anisotropy or inhomogeneity in the permeability of the core will create a corresponding difference in the flux distribution. So yes, if you prefer, they do twist and then readjust at almost the speed of light*. Also If you move the material the flux distribution will move along with it. In the frame of reference of the spinning part, there should be no changes based on rotational speed within realistic limits.

* 99.999% BS. I have no idea what happens in this case near the speed of light. But I know what happens at normal speeds.

If I paste a copper wire on top surface of the movable magnetic core, then the conductor wire will cut MMF lines during rotation.

My new question: is there the induced voltage between the two ends of wire?

Of course, I know there must be an induced voltage, if keep the movable section standstill and let the wire independently cut through the gap.

But now the wire and the movable section are bounded together, and move together, so the induced voltage may no longer exist? or not render the same voltage value as scenario of everything standstill except the wire?

In new scenario, the movable section can be imagined as an infinite area plate, not have to rotate, but simply linear move.

 

[DaveE]

The "ends" of your wire will be part of a loop (circuit), for example including your voltmeter leads. Any change in the total magnetic flux passing through that loop, either due to a time variation or spatial movement, will induce a voltage according to Faraday's Law. So I'm a bit unclear about how the field lines in your experiment are distributed throughout that entire loop.

BTW, you often get more or better answers if you start a new thread when your questions change. The only people likely to see this are those who responded to your original question.

 

[cairoliu]

The "ends" of your wire will be part of a loop (circuit), for example including your voltmeter leads. Any change in the total magnetic flux passing through that loop, either due to a time variation or spatial movement, will induce a voltage according to Faraday's Law. So I'm a bit unclear about how the field lines in your experiment are distributed throughout that entire loop.

BTW, you often get more or better answers if you start a new thread when your questions change. The only people likely to see this are those who responded to your original question.

Only the wire is in B field, and motion direction is vertical to B, the wire is a part of a loop, and other parts of loop is outside the B field.
So what's the result in this scenario of wire going with the magnetic plate?

BTW I will follow your advice in next topic, new thread for new subject. thanks.

 

[DaveE]

If the flux within the loop doesn't change, there will be no voltage induced. The concept is relatively simple compared to actually knowing where the magnetic flux is. It's sort of a bookkeeping problem. Sorry, I don't know what "vertical to B" means. Nearly every time you see a problem like this it comes with a sketch of the geometry and field lines (flux). That's one big advantage to magnetic cores; you know where the flux goes and can usually ignore the flux outside of the core.

 

[Vanadium 50]

It seems to me that you are overthinking this. For the cylinder to start spinning, there needs to be a toque on it. A torque means that the energy at one angle φ is different from another angle. But, assuming isotropic materials, this is not the case. So no torque.

If you want to ask about anisotropic materials, draw how they were anisotropic. However, it is all but certain that the torque changes sign as the cylinder rotates, so it will rotate to some configuration and stop because of friction.

 

[DaveE]

It seems to me that you are overthinking this. For the cylinder to start spinning, there needs to be a toque on it. A torque means that the energy at one angle φ is different from another angle. But, assuming isotropic materials, this is not the case. So no torque.

If you want to ask about anisotropic materials, draw how they were anisotropic. However, it is all but certain that the torque changes sign as the cylinder rotates, so it will rotate to some configuration and stop because of friction.

Though undrawn in the sketch, I can use a motor to drive the cylinder via belt, of course bearings are needed to keep stable.

Both the stator main section & the rotatable short section are made of same magnetic material, e.g. silicon steel etc.

It won't stop unless his motor stops. I have no idea what the point of the apparatus is though.

 

[cairoliu]

It seems to me that you are overthinking this. For the cylinder to start spinning, there needs to be a toque on it. A torque means that the energy at one angle φ is different from another angle. But, assuming isotropic materials, this is not the case. So no torque.

If you want to ask about anisotropic materials, draw how they were anisotropic. However, it is all but certain that the torque changes sign as the cylinder rotates, so it will rotate to some configuration and stop because of friction.

From reply #13, then after, I assume the cylinder is replaced with an infinite area plate with thickness equal to original height of the solid cylinder, and no more rotation, but linear motion.

My only concern is whether the motion of wire (combined with plate), is affected by the plate in term of induction effect.

By my imagination, the contact area between wire and plate seems not to cut B flux, as the motion may stickily drag the MMF lines if not instant straightening (reconnect to nearest magnetic domains); only the wire upper portion causes flux change?

Following sketch illustrates the combined wire and plate moving towards reader.

If the plate is attached with the C core, and let the wire independently move, is there the same induction effect?

 

[berkeman]

Can we back up for a moment and ask you what your background is in Electromagnetics? Have you taken any university classes (either at uni or online) in EM yet? What EM equations and calculations are you familiar with so far?

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcon.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

 

[cairoliu]

Can we back up for a moment and ask you what your background is in Electromagnetics? Have you taken any university classes (either at uni or online) in EM yet? What EM equations and calculations are you familiar with so far?

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcon.html#c1

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html

View attachment 321630

university

 

[berkeman]

university

Then you should be posting equations in support of your questions. Please use LaTeX to post math here at PF -- see the "LaTeX Guide" link below the Edit window to learn how to post math using LaTeX. Thanks.

 

[berkeman]

Then you should be posting equations in support of your questions. Please use LaTeX to post math here at PF -- see the "LaTeX Guide" link below the Edit window to learn how to post math using LaTeX. Thanks.

BTW @cairoliu I don't mean my comments to be insulting at all. It's just frustrating when you throw out ideas that should be addressed in a straightforward way by writing the applicable equations and then asking questions about those equations. The more math that you can include with your questions and the more links to your reading about those questions, the better we can try to help you with understanding what will happen. Thanks.

 

[cairoliu]

BTW @cairoliu I don't mean my comments to be insulting at all. It's just frustrating when you throw out ideas that should be addressed in a straightforward way by writing the applicable equations and then asking questions about those equations. The more math that you can include with your questions and the more links to your reading about those questions, the better we can try to help you with understanding what will happen. Thanks.

I understand you.

Seeking answer for my question is not for my theoretical interest, but serve my engineering need, I'm trying to reinvent the "wheel" of DC motor with slip ring.

I hate to let the magnetic media move, but have to wind wire to a rotatable magnetic media.

Now I think the moving magnetic media seems not to affect conductor's induction, as DaveE says the MMF lines probably instant straightening.

 

[berkeman]

Seeking answer for my question is not for my theoretical interest, but serve my engineering need, I'm trying to reinvent the "wheel" of DC motor with slip ring.

What reading have you done so far in your re-invention research for electric motors? What are the different electric motor types and their advantages/disadvantages/applications? That's a good place to start, IMO.

 

[cairoliu]

What reading have you done so far in your re-invention research for electric motors? What are the different electric motor types and their advantages/disadvantages/applications? That's a good place to start, IMO.

All brushed DC motors only use commutators, which generate nasty sparks and EM interference.
If using slip ring in DC motor, then no spark no EM interference, everyone will be happy, but I cannot find a DC motor with slip ring in market.

 

[berkeman]

All brushed DC motors only use commutators, which generate nasty sparks and EM interference.
If using slip ring in DC motor, then no spark no EM interference, everyone will be happy, but I cannot find a DC motor with slip ring in market.

Please always post links to your reading on your questions in the technical forums. Otherwise we have to do our own searches:

https://www.moflon.com/showen127.html

Paging @anorlunda

 

[cairoliu]

Please always post links to your reading on your questions in the technical forums. Otherwise we have to do our own searches:

https://www.moflon.com/showen127.html

Paging @anorlunda

They manufacture AC motors with slip rings, not DC motor.
Control circuit is complex to generate multi-phase power supply.
another link:
https://www.vedantu.com/question-an...lass-12-physics-cbse-5feb209117b0716062784bca

 

[anorlunda]

I'm having trouble figuring out the question in this thread. @cairoliu, can you restate the question from scratch?

Instantly minimize their path length between magnetic domains?

That sounds reminiscent of a reluctance motor.

 

[artis]

@cairoliu

I believe what your asking in the first post of this thread is "do magnetic field lines move together with a perfectly symmetrical magnet that is rotated around it's axis of symmetry and that is also polarized along the same axis of symmetry.

The answer is NO! Magnetic field lines are a human made construct to help aide thinking, their not real, there is just a physical phenomenon called magnetic field.
Magnetic field doesn't move, it only has a quantity named "field strength" and this quantity changes if you change your distance to the field source.
If you are at a static non changing distance from a round disc magnet that is axially magnetized and that rotates around it's symmetry axis of polarization then you won't notice a single thing, the field won't change. There are no hairy field lines sticking out of the magnet disc surface that are dragged along as the magnet moves as if they were human hair dragged along the wind, no such thing exists.So if you take a magnetic core like you showed and insert a cylindrical and symmetric part in it and keep it perfectly steady radially but just spin it axially and the spinning material has an almost perfect structural isotropy then the "field lines" won't be altered in any way I think, the field in the core won't "notice" so to speak.