on the efficiency of magnets

[bwanaaa]

When current is passed through a coil of conducting wire that is insulated, the aggregate magnetic field is masurable and depends on things like - how many turns in the coil and how much current is passed. Obviously the resistance of the wire causes coil failure at high temperatures as the whole thing melts.

Copper is the winding of choice as it has the highest conductivity. (until we get a very long carbon nanotube that can be wound) Then, an iron core is used in the center to increase the field. ( by concentrating the field lines?)

Why dont we use a magnetic winding - like iron wire that has an enamel coating? This is conductive AND magnetic. (which should an even stronger field for a given current)

 

[TecHNooB]

nm

 

[bwanaaa]

nm

sry, i dont know what that means. (another reflection of my naive ignorance)

 

[TecHNooB]

if the skin effect causes the current to flow mostly through the outermost shell of the conductor, won't the enamel coating effectively make the resistance a lot worse? not only would the resistance be worse due to the material, but the fact that it's magnetic would amplify the skin effect in the enamel region?

 

[bwanaaa]

if the skin effect causes the current to flow mostly through the outermost shell of the conductor, won't the enamel coating effectively make the resistance a lot worse? not only would the resistance be worse due to the material, but the fact that it's magnetic would amplify the skin effect in the enamel region?

I thought skin effect is something you worry about at high voltages, not high currents. Without an insulator between the windings, wont you just get an instant short circuit? I can see the magnet field generated by the current creating an impedance, but this would happen in any conductor. If the conductor is magnetic as well, I would think the field lines would just get concentrated-and that's what the iron core does anyway in most modern electromagnets.

 

[Biftheunderstudy]

I'm not sure if this is a good answer or not, but the field is perpendicular to the current in the windings. If you made the windings themselves the magnetic core, I think you would end up with an induced current that would cancel the one you were trying to create....sort of like an eddy current or an inductor. I would really have to sit down and think about this more, this is kind of off the cuff before lunch sort of thing.

 

[PottedMeat]

why would you want increased field strength inside the wire?

 

[CycloWizard]

There are a few parameters you need to consider. Permeability is the property of the material which determines how well it "conducts" magnetic fields. The magnetic field strength may be set by the strength of the magnet, the induced magnetic field due to applied current, and using high- and low-permeability materials to shape the magnetic fields. As always, the correct approach depends on what you're really trying to do.

 

[bwanaaa]

why would you want increased field strength inside the wire?

good question....

if i did that, then maybe that's the opposite of using an iron core in an electromagnet. instead of packing the field lines more tightly around the core, you'd get a bigger 'hole in the middle'. The field lines would get concentrated around the windings. Dont know why i'd want that?! Also, the windings would get smushed together increasing the chance of failure. And of course, the increased resistance of iron...

sry, what a waste of bandwidth...if i had had thought it through, the solution would have been evident.

 

[PsiStar]

1st silver is the the most conductive element at room temperature. It has a conductivity that is 105% of Cu.

2nd skin effect is related to frequency only.

3rd An enamel coating is a dielectric which affects the electric field and not the magnetic field.

4th iron is very resistive relative to Cu. It has a conductivity that is 18% of Cu.

5th an iron core in a winding concentrates the magnetic field in the winding. This improves the coupling but is alway a composite with frequency dependent properties.

 

[TecHNooB]

1st silver is the the most conductive element at room temperature. It has a conductivity that is 105% of Cu.

2nd skin effect is related to frequency only.

3rd An enamel coating is a dielectric which affects the electric field and not the magnetic field.

4th iron is very resistive relative to Cu. It has a conductivity that is 18% of Cu.

5th an iron core in a winding concentrates the magnetic field in the winding. This improves the coupling but is alway a composite with frequency dependent properties.

Hm.. i clearly had motors on the brain as that's where I learned most of my magnetics theory

googled some ac based magnets. not completely off

 

[PsiStar]

I thought so ... here is a link to "rare earth magnet motors". I think that is what I googled.

After you read thru that, think about that most of world's rare earth material are in and exported from China.

 

[wuliheron]

I thought so ... here is a link to "rare earth magnet motors". I think that is what I googled.

After you read thru that, think about that most of world's rare earth material are in and exported from China.

They're mostly exported from China simply because they have the easiest to access deposits of rare earths.

There are now some 400 imported raw materials considered "vital to the national defense" of the US, like oil, and we can expect this situation to grow rapidly worse as the global economy and technology continue to expand. Just this year the US came to an agreement with Canada to import water if it becomes necessary. Rare earths for fancy magnets are pretty low on the priority list, but already a US mining operation has resumed.

 

[piasabird]

Solar vehicles use motors of varying types:
http://www.solarvehicles.org/

This person is a kind of a Professor of magnets but he is from Canada. He has some interesting ideas and is definitely Anti-Automobile:
http://solarvehicles.blogspot.com/

Here is another interesting link. It is to a chinese company that makes hub motors for electric bicycles. I have seen some versions on the internet that can make a mountain bike go 40 miles an hour. However that is illegal as a on-road bicycle due to some stupid laws. Lighter could be better.
http://www.goldenmotor.com/frame-company-info.htm

I am taking a course on building electric vehicles for my information. Dont know just how cost effective this is. However, it is an interesting concept not buying any gasoline for local travel.

Would you buy a solar trike for $4,999 or so?

http://www.solartrike.com/

Might also look at Lithium Batteries.

 

[pandemonium]

OT: I've had crazy dreams of using magnetic field strengths to balance and propel wheels on a cycle and always got stuck on the materials that would be appropriate to allow such a feat. My thoughts were something along the lines of spaced rare earth magnets around a hub that would have a pulsing charge at given intervals to perpetuate no-contact operation as well as momentum.

This person is a kind of a Professor of magnets but he is from Canada. He has some interesting ideas and is definitely Anti-Automobile:
http://solarvehicles.blogspot.com/

I liked the "but he is from Canada" part. As if that's a bad thing? Haha.

Here is another interesting link. It is to a chinese company that makes hub motors for electric bicycles. I have seen some versions on the internet that can make a mountain bike go 40 miles an hour. However that is illegal as a on-road bicycle due to some stupid laws. Lighter could be better.
http://www.goldenmotor.com/frame-company-info.htm

Meh, just get out on a bicycle and get some exercise. I used to get to work faster than driving in my car (7.5 mile trip) on my mountain bike. Also, I've hit a top speed of 38 mph on my mountain bike. My father, who road cycles a lot, averages 35 mph and has hit a top speed of ~50mph (unassisted). Granted, that's going down a hill for the top speeds, but I digress.

The counter argument is that a lot of transportation on a bicycle is dangerous since most cities/communities lack relatively accomodating locations to cycle as a commuter model. The same can be applied to electrically assisted bicycling.

 

[Ghiedo27]

Iron is used in the center because of its ferromagnetic properties. When you apply a magnetic field to iron it lines itself up in such a way as to reinforce the magnetic field that it is in (I should add that this is an internal rearrangement). You never have to conduct a single electron through the iron to get this effect.

If you want to dig deeper into it, I recommend this lecture by prof Walter Lewin of MIT. The math to calculate the strength and direction of the fields is also included, but you can ignore that and get the general idea without them. The majority of the content is conceptual. There's a bunch more videos on electricity and magnetism in that course on the play list.

 

[bwanaaa]

Reading about permeability, I discovered permalloy as the best core to use due to it's high permeability. I also saw that superconductors have the highest permeability. Even higher than permalloy.

 

[pm]

nm => nevermind (meaning: 'I deleted the post but they won't let me remove it totally, so I changed it to "nm"').

On the original post, it looks like it's covered.

On rare earths and China, 15 years ago, the US pretty much completely owned the rare earth minerals export market, and now China does. Since China imposed export controls on rare earths, it's been interesting watching I noticed that several US mines are starting up mining of rare earth materials. Molycorp is mining in Mountain Pass, Calif. They are starting to ramp production and last I heard they want to have ~30% worldwide marketshare. There are other companies looking at Lemhi Pass in Idaho and in locations in Montana and Pea Ridge in Missouri. One thing people don't mention much about rare earths is that it's not a particularly environmentally friendly mining operation. Mining usually isn't but the reason the US mines closed (and mines in Malaysia) is that it's a particularly messy form of mining... it's interesting to see attitudes change when it's deemed to be in the US's national interest. I'm not a hardcore environmentalist and I think it's unwise for the US to let China own 97% of the rare earth export market... I'm just saying it's funny how attitudes change over time.


On electric bicycles, I have a nice road bike but I also have an electric bicycle that is not quite street legal, although I generally stay under the limit. While it's possible to go 30/40/50mph on a bicycle, I don't think it's wise to do it on regular city streets - cars don't expect cyclists to be going faster than 25mph so commuting in heavy traffic going faster than 25mph on a bicycle is dangerous just because people will pull in front of you by accident, I know this from extensive personal experience. So even if it's theoretically possible to go 40mph on an e-bike, it's unwise... besides the braking system on a road bike isn't set up for effective stopping at 40mph.

I commute to work on the road bike most days, but the electric is nice for running errands and days when it's really hot and I don't want to show up wherever I'm going sweating all over. I'm using a 9-continent 2805 rear-drive w/ a 16-cell 1.8kWh LiFePo4 battery and an Infineon 25A brushless controller with regen.

I just ugraded my car to be a plug-in hybrid last month... but now that the weather is better, I'm back to riding the road bike again... until it gets too hot and then I'll switch to the e-bike.

 

[Lemon law]

Well my position is and remains, a permanently magnetic material is 100% efficient in causing a paramagnetic effect in other materials

 

[Mark R]

Reading about permeability, I discovered permalloy as the best core to use due to it's high permeability. I also saw that superconductors have the highest permeability. Even higher than permalloy.

Superconductors have zero permeability - i.e. they are totally impenetrable to magnetic fields, and therefore are repelled any external magnetic field.

Permalloy is also not the material with highest permeability - however, the materials with higher permeabilities, have other properties that make them less good for electronic cores.

High permeability is not always desirable for cores - it's useful for transformers, as it gives very good coupling and low-leakage (good for audio, precision measurement, etc.) However, high permeability is undersirable where energy storage is required (e.g. voltage regulators) because high-permeability concentrate flux so intensely, that they reach saturation having stored very little energy.

 

[bwanaaa]

Superconductors have zero permeability - i.e. they are totally impenetrable to magnetic fields, and therefore are repelled any external magnetic field.

...

tnx. yeah, i guess what happens is:
1)turn on the external magnetic field
2)a current in the superconductor is generated as the external B field is turned on (or moved closer)
3)an opposing magnetic field is generated by the superconductor

technically, the external magnetic field cannot penetrate the superconductor. but actually, it had to penetrate to initiate the current in it.

i wish i had a good understanding of that maxwell 3d software to draw a model that animates this. i have a hard enough time with it getting nice static pictures out of it.