questions for simple magnetic experiment 4

[electricpete]

I am thinking of trying to build a quick/cheap demo regarding some aspects of interaction of current and magnetic field. I'd like to demonstrate two aspects:

First aspect: a current in an external magnetic field experiences a force.

Second aspect: if we place a permeable shield or slot around the current carrying conductor, the force now occurs on the shield or slot.

The first one is easy to demo: Something similar to this

http://www.exploratorium.edu/snacks/motor_effect.html

or this

http://www.coolmagnetman.com/magmotor.htm

would show that part.

Now the second aspect is tougher.

The easiest way is to repeat the first experiment with the shield in-place and show no movement of the conductor occurs.

But that's not really showing the force acting on the shield. To take it a step further I’m thinking it would be great to somehow measure the force on the shield, but I have no idea how to do that. Two challenges that I see right off the bat:
A – I’ll be dealing with a pretty small force I suspect unless I can create a large dc current and large field
B – I’ll need to restrain my shield against moving towards parallel to the field towards either magnet surface which will make it tough to measure force or observe movement perpendicular to the field.

My questions for starters:
1 – What would be the best magnet? I assume rare earth magnets (neodymium iron boron ).
2 – Is there any walk-in retailer where you can buy permanent magnets (I’ve seen a lot of the internet but I’m interested to buy some locally if I can). I asked at Radio Shack and the salesman looked at me like I was from outer space.
3 – I’m somewhat familiar with how to calculate the flux resulting from an electromagnet using the magnetic equivalent of ohm’s law: Phi = NI / Reluctance. I’m somewhat at a loss of how to calculate the flux associated with a given permanent magnet configuration. Can anyone shed light on that? For example if I have two rare earth magnets 1”X2”X0.5” separated by 0.5”, what is the flux density between them (under simplifying assumption of uniform field and no leakage). Would there be a tremendous boost in the flux density between the magnets if I built a permeable steel C-core connecting the back-sides of those magnets?
4 – Any other thoughts on ways to demonstrate the principle that the force on a conductor inside of a slot/shield acts on the slot/shield, not the conductor?


=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I did find specs such as this:

http://www.kjmagnetics.com/specs.asp

Residual Flux Density (Br) 11.7-12.1 KGs
Coercive Force (Hc) >10.8 KOe
Intrinsic Coercive Force (Hci) >12 KOe
Max.Energy Product (BH)max 33-35 MGOe

I'm still not sure how to convert these into flux density for a given geometry

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[IRstuff]

You could use strain gages to support the shield.

TTFN

 

[MagMike]

Hello electricpete:

There is a calculating tool for calculating the flux density for a given geometry (both for an isolated magnet and for a pair connected by a c-core):

http://www.dextermag.com/CalculationList.aspx

 

[electricpete]

That's some great info. Just as a sanity check - to compute the effect of two separate magnets (without the core), I should add the contributions from two single-block solutions? (I assume this is linear and superposition applies, but just wanted to double-check).

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Also I'm curious what method might be suggested to restrain the magnets.

I think I've read that it's not safe to drill them.

I could use superglue, but that's kind of permanent if I'm going to be doing some trial and error to get the best effect, possibly stacking them in different series and parallel configurations and changing the gap.

Maybe multiple secure wraps of twine?

Any ideas?

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[MagMike]

Hello electricpete,

In response to your first question, it is a reasonable enough approximation to calculate the magnets without the core as the addition of two single-block solutions.

Regarding fixturing the magnets: If you are just going to be stacking magnets in series _and_ you are somewhat careful about keeping your fingers away from the gap, magnetic attraction should be enough to keep the magnets in place. If you stack them in parallel, then wraps of twine or even cable ties might be enough. Two-sided tape may be enough if the magnets aren't too large.

If you do need to go with large magnets (larger than 1" x 1" pole face), superglues are the safest way to go because they'll provide the necessary strength and you can heat the assembly to weaken the glue & release the magnets. In this case I'd recommend using SmCo magnets, as they can tolerate heat better than NdFeB.

 

[MJR2]

Glass reinforced packing tape may work well for you. Make sure you cover any sharp corner. As soon as you have an edge the glass fibers can break and the tape is worthless.

Superglue as MagMike says is the best way. Clean and prime the magnets before glueing. They oxidize easily on the surface so keep them dry.

Working with anything larger than 1x1 pole face will make you wish you had started smaller.

If you describe the circuit when you decide on it perhaps I'll have time to run some FEA for it. We'll need geometry and magnet material.

Be careful and have some non-magnetic pry bars and a helper near by. Gloves would be a good idea.

Mike

 

[MJR2]

Your circuit could also be used to demonstrate eddy current forces. I helped my son do a science demo many years ago with a penduleum swinging a fiberglass or aluminum plate between two magnets. Neat demo. Not quite what you are doing but easily included. We used both ceramic 8 and RE magnetics as well. The RE had more dramatic effect.

Mike

 

[electricpete]

Based on low cost and high flux, I have my heart set on rare earth Neodymium magnets, even though I have never worked with them before.

Good tips about the safety. Also I understand goggles are a good idea in case magnets accidentally jump together and shatter.

I’m thinking about getting 8 Neodymium magnets 1x1x0.5” each and arranging into two slabs 2x2x0.5” separated by one inch to create a high-flux area between them.

I have a roll of steel banding 1.25” wide by (1/32)” thick which I can use to mount the magnets and provide a yoke to connect the backsides.

First assemble a steel backplate 2.5” x 2.5” by taking two strips of steel banding vertical on top of two strips horizontal, glued together between the two layers. Steel backplates shown on top of slide 1 here

http://home.houston.rr.com/electricpete/Experiment.ppt

Then put the a set of four magnets onto the backplate to form the 2x2x0.5 magnet on 2.5x2.5” backplate. Fasten securely with string or packing tape.

Then repeat to make another identical backplate with another four magnets.

Then add a yoke (slide 2). Fasten magnets/backplate assembly securely to the yoke with string or tape.

Then bend the yoke into position while keeping magnets apart somehow (either use nails into 2x5 on the inside-edges of the magnets, or put extra 2x4’s secured outside of yoke and tie to those).

Then do experiment 1 (slide 3) – current directly in conductors. Expect maybe 7.5 Newtons.

Then do experiment 2 (slide 4) with simulated slot. I expect the conductors will jump to the left due to cross slot (vertical) leakage flux, but the simulated slot won’t move due to friction because it is being pulled to the LHS.

Then do experiment 3 (slide 5)with circular closed slot (pipe). There will be no more cross-slot leakage flux due to symmetry of the circular slot so we won’t observe that. Friction problem is reduced since pipe may be able to roll up the side of the magnets. At least one question remains in both experiments 3 and 4 – can I add enough steel to the simulated slot to avoid satuartion of the slot?

If I don’t avoid saturation, then flux will flow to the conductor and make it move upward and will destroy the purpose of the experiment since we won’t be able to tell whether movement of iron is from force on iron or force on conductor pushing on iron.

I’m worried that to make my slots beefy enough to avoid saturation, they may be too heavy to lift. In that case I may take a closer look at how many amp-turns I can get.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

Revision 1 - suggested by a knowledgeable colleague:

Turn the whole thing on it's side to create vertical flux.

Suspend the slot/pipe by strings from above at each end
Suspend the conductors by strings from above at each end.

Then I don't need to worry about generating enough force to lift anything, just enough force to move it sideways slightly.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[prex]

Please be careful at the following points:
1) Magnets put side by side strongly repel themselves: I'm afraid it will be difficult to secure them with packing tape and, if there is a gap between them, you will loose flux. I suggest using blocks 2x2x0.5".
2) You'll get a lot of problems trying to bend your steel bands while the magnets are already mounted: please don't do that. You can build your C shape with two flat sides and a steel block 2" thick in place of the bend. This assembly will be stable, due to magnetic forces, even if not secured, but you can easily secure it with bolts.
3) Do you really mean that you want to use steel plates less than a mm thick to build your horseshoe? They will easily bend due to magnetic forces and your magnets will snap together. Also you need at least 3/4" thickness for the yoke to avoid saturation (that would make your horseshoe almost useless).
4) Can't really understand the purpose of experiments 2 and 3: you'll demonstrate that a ferromagnetic material captures all the flux lines that surround it, but this is also easily demonstrated with a gaussmeter or with the classical experiment using a sheet of paper and iron particles to visualize flux lines.

prex

http://www.xcalcs.com

Online tools for structural design

 

[electricpete]

1 – From my recollection of playing with normal ferrite magnets, I’m not familiar with magnets repelling each other when placed side by side with similar polarity. Is that common?

2/3 - I had intended to use multiple steel bands to achieve a thickness of 0.5” for the yoke. I had not planned for the steel bands to be the sole restraint although I may not have explained it well. Now that I’ve changed plans to provide vertical orientation of flux, I’m thinking I will tape/tie the magnets and banding yoke onto an upper 2x4 and a lower 2x4.

4 – It is a very widespread “error” (simplification with unstated assumption) in motor textbooks to state that the force acts directly on the conductors. In fact you get the correct answer for total torque by calculating the force the conductor current would experience IF it were placed directly in the airgap flux. IMO, the reality of force acting on iron rather than core is clear and the textbook explanation obviously a simplification to make the problem more understandable/solveable. (similar to the motor equivalent circuit) But others are not so easily convinced.

=====================================
Eng-tips forums: The best place on the web for engineering discussions.

 

[electricpete]

I started the experiment with some simple ferrite magnets. I did experience the problem that prex talked about in #1 - magnets repelling each other. If I go on to use the Neodymium, I think I will have to use a 4-step process to hold magnets in place:
1 - Put in place and hold with a clamp in the middle.
2 - Wrap twine around magnet and supporting 2x4 each side of the clamp.
3 - remove clamp
4 - tape directly over twine


=====================================
Eng-tips forums: The best place on the web for engineering discussions.