CR4® - The Engineer's Place for News and Discussion®

 Previous in Forum: Designing Molybdenum Heating Element for a Vacuum Furnace Next in Forum: 3 Phase AC Capacitor
Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40

# Shapes of Induction

09/11/2017 1:55 PM

Hi all. If a permanent magnet moves relative to a coil there will be a variation of magnetic flux, therefore there will be a current generated in the coil. OK.

My question is: assuming you have an air wound coil of, let's say 2k turns, what would be the setup that would generate more current?

I explain myself better: how would compare...

having a disc shaped magnet vibrating in the proximity of the flat side of a disc shaped coil? (vibrations along axis)

with having a ring shaped coil with a cylindrical magnet vibrating into the ring? (vibrations along axis)

Say that the magnet is the only vibrating element, like at 1000Hz, which configuration would provide more current?

And I repeat: the coil is air wound and the magnet can have any shape.

Maybe there are other shapes of the coil to consider, like toroidal?

Thanks.

Interested in this topic? By joining CR4 you can "subscribe" to

Check out these comments that don't yet have enough votes to be "official" good answers and, if you agree with them, rate them!
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#1

### Re: Shapes of Induction

09/11/2017 4:06 PM

The best design is the one where more of the coil intersects (cuts through) the flux lines of the magnetic field. Does that help with your homework?

__________________
If it ain't broke, don't fix it. Just build a better one.
Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40
#2

### Re: Shapes of Induction

09/11/2017 4:33 PM

Thank you James, it does!

That would make the disc magnet and the disc coil (aligned axially) the best solution, I think. In that way the flux lines would fully intersect the coil, especially if the diameter of the coil and the one of the magnet at similar.

Things would probably change a lot if the flux was "guided" by some other ferromagnetic material around the coil...

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#3

### Re: Shapes of Induction

09/11/2017 5:00 PM

Yes, that is why most transformers have metal (iron) cores made up of laminates.

__________________
If it ain't broke, don't fix it. Just build a better one.
2
Guru

Join Date: Apr 2010
Posts: 5906
#4

### Re: Shapes of Induction

09/13/2017 9:54 PM

Technically, a changing magnetic flux (think field lines) within a loop of wire generates a voltage. If you have 2000 turns (in series) you have 2000x the voltage of 1 turn. If the coil forms a closed circuit, the current will be the voltage divided by the resistance in the circuit.

The magnetic flux of a magnet passes through the magnet, expands at each pole and converges on the other pole. So as the magnet moves, more or less of the flux is inside each loop of the coil. The rate of change of the flux within each loop determines the voltage generated by that loop.

So the amount of voltage generated (and current) depends on the position of the magnet (how much the flux is diverging/converging) and how fast the magnet is moved.

I hope this is not too confusing...

Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40
#5

### Re: Shapes of Induction

09/14/2017 12:01 AM

Thank you Rixter,

If you have two disc magnets attracting each other but at a certain distance I would think that the magnetic lines are going from one magnet to the other before diverging and converting, right? Well, what happens if a flat coil would be moving inside that gap? I imagine not much if the movement is axial with magnetic poles, maybe I am wrong...or maybe it would generate more voltage if the coil moves perpendicularly to those axis. Another question about the two magnets with the coil inside: what would happen if the magnets move far apart and close to each other?would that be a good way to generate more voltage? Or maybe tilting one?

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#6

### Re: Shapes of Induction

09/14/2017 2:16 PM

The answer on both accounts is "yes".

You have to calculate/decide which configuration cuts the most magnetic lines/second.

__________________
If it ain't broke, don't fix it. Just build a better one.
Guru

Join Date: Apr 2010
Posts: 5906
#7

### Re: Shapes of Induction

09/14/2017 3:01 PM

If you have two separated, attracting magnets, the flux expands and then contracts between them. If you have a coil that moves with respect to the magnets, a voltage will be generated by the changing flux within the coil. So in the diagram below, due to symmetry, there would be no voltage when the moving coil is midway between the magnets.

https://en.wikipedia.org/wiki/Force_between_magnets

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#8

### Re: Shapes of Induction

09/14/2017 4:34 PM

Rixter: If the coil is moving there will be a minimum in voltage at the central point between the magnets, but lines are still being cut.

Consider a Faraday generator (little bit different animal), where a conducting disc (not ferromagnetic) rotates such that its periphery is maintained between magnetic poles.

Also works when the magnets are mounted to the disc! This is mind blowing stuff.

__________________
If it ain't broke, don't fix it. Just build a better one.
Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40
#9

### Re: Shapes of Induction

09/15/2017 1:17 PM

Thank you both.

I am trying to imagine/visualize what happens when one magnet moves relative to the other, (assuming they are attracting each others). As soon as the distance becomes greater the lines should migrate from the center of the magnets to the outside, right?

A coil in the middle should "experience" a lot of magnetic flux, I would think.

Not sure what would be the best shape and position of the coil in order to "catch" most magnetic flux variations...any comment of that scenario?

I hope I am explaining myself...

Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#10

### Re: Shapes of Induction

09/15/2017 1:43 PM

Have you considered what if the poles are not symmetric? For example, one could be a flat pole (disc-like) with the other one nearly a point or at least a cone.

Suppose the coil itself were disc-shaped, and rotated inside the space of two hemispheric poles (concave with a set gap distance). What happens then?

What if there were two cone shaped poles with very minimal gap distance between them and a thin rotating disc (Faraday generator plate)? Would pole size/shape matter as to the voltage-current characteristic of such a Faraday generator?

Could you set up a Faraday generator with sufficient discs in series to have an output voltage > 50 V? What would be the output current, and Thevenin resistance?

__________________
If it ain't broke, don't fix it. Just build a better one.
Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40
#11

### Re: Shapes of Induction

09/15/2017 3:39 PM

Interesting. I didn't consider not symmetric. I would think a ball much smaller then the disk magnet should behave similarly. The motions of one magnet relative to the other would create magnetic fields of different shapes. I should see what happens..

That coil rotating inside the hemisphere is also very interesting. that reminds me of a galvanometer, 3d version. No idea where to find magnets with that shape thus.

As for the two cone shaped idea I think two balls magnets should work. The thin rotating disc should react to the amount of magnetic field (and how close is to the disc), but I cannot imagine the shape would be as relevant.

About the Faraday generator: I would think it would be dependent on the mechanical energy applied...Since nothing is free, it could be possible to get more then 50v but low current and high resistance. What if some of the discs were in parallel?

I was looking at diagrams of phono cartridges (both moving coil and magnet): amazing how many designs went to this kind of transducers!

Guru

Join Date: Apr 2011
Posts: 6750
#12

### Re: Shapes of Induction

09/18/2017 8:37 AM

If you use two cylinder magnets (magnetized axially) aligned with like poles forced together and vibrate the assembly axially with a flat coil concentric with and just outside the touching like poles, the result is a radial flux that is very compressed and can cross many windings with a minimum of displacement.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Associate

Join Date: Jul 2006
Location: Austin, Texas
Posts: 40
#13

### Re: Shapes of Induction

09/18/2017 11:33 PM

Thank you so much!!!

Compressed radial flux, that's very interesting! So, the coil is going to be just outside the circumference of the two opposing magnets in the vicinity of the gap. Brilliant!

I wonder if it would be better to have the coil shaped like a thin disc (with a hole like in a washer) or a "tube" with thin walls like those in speakers.

Thanks again, that was a great help!

2
Guru

Join Date: Apr 2011
Posts: 6750
#14

### Re: Shapes of Induction

09/19/2017 12:06 AM

Most likely thin washer-like ring of windings, with axial thinness on the order of the range of motion/amplitude of the opposing magnets assembly.

__________________
Eternal vigilance is the price of knowledge. - George Santayana
Guru

Join Date: Mar 2011
Location: Lubbock, Texas
Posts: 14222
#15

### Re: Shapes of Induction

09/19/2017 9:47 AM

There you go designing his experiment (homework) for him.

I want this kokkoplus guy to start putting out some actual effort. Some wire, some magnets, a little cardboard, and some hot glue might go a good distance here.

__________________
If it ain't broke, don't fix it. Just build a better one.
Off Topic (Score 5)
Interested in this topic? By joining CR4 you can "subscribe" to