Magnetized Wheel (A Theory Thread)

[safe]

I think that would make for one weak performer...

True... compared to having 360 degrees of coverage a single point magnet would be less powerful.

However...

As has been said, the pulse that you send through the coil could be amplified because it has an entire revolution of the wheel before it has to function again. So from a heat perspective you have less to worry about.

And don't forget that the leverage is impressive. A small motor might have leverage of about an inch or two, but this would be like a massive pancake motor using a lever of a length equal to the radius of the rim itself. (so it could be a radius of about 10") So it might not be so bad.

I was hoping someone would jump out with the math and get things moving on this, but I guess at some point I'm going to have to do it myself...

 

[John in CR]

Sorry if this is OT, but I believe it's pretty close. Could it be relatively easy to create an electronic brakes using strong magnets (probably multiple pairs needed) in place of regular bike brake pads, but with a larger travel when the brake is pressed; then place coils where the spokes meet the rim; and instead of a commutator just short the appropriate coils to each other? It seems like it should work, though I don't know how to figure out the winding and magnet layouts. A brushless motor with shorted phase wires becomes much harder to turn, and this seems the same, but instead of on/off shorting the coils to create braking, you close the gap so the existing short has a variable effect. You might even be able to wire some red LEDs in there and have a brake light of sorts.

If it's an ignorant idea don't hesitate to say so, but please explain why so I can learn something.

John

 

[Link]

You've basically just made a motor in reverse. Were you to apply power at the right times, you'd be able to power that thing. But, it won't work well at low speeds, and won't do anything at all at less than a few MPH. Plus, you'd be adding loads of weight to the rim, making it harder to get up to speed and slow down.

So, yeah, that would work, but you might as well just install a hub.

 

[mr_exon]

look at the pic and well..if any thing you could use this as a L.E.D. lighting system..right...I know it does not corispond to the thread, but if you have disk brakes on your bike and you use this system...you coule make a good 3-8volts with no friction added to the bike at all....best part is you just add maby 1-2 pounds of stuff t the bike to get literly free lighting power for 400 years(nerodium magnets that is) as long as your moving right?

 

[safe]

John in CR, yeah that does sound like something that is possible. (the idea of making a sort of "switched reluctance" brake) If you wired the coils on the rim in the right sequence then one coil would become energized while another would take that current and use it. That's a sort of "passive electrical brake" and would be even less powerful than what I was suggesting which was using a single point of contact, but with extra energy added.

All I know from the research I've done so far is that at high speeds the electrical brake can fuel itself, but at low speeds it needs additional energy to be supplied to actually get some serious braking action. (this is from electric train data) This topic is something that will probably pop up again in the winter when we all have lot's of time for it. (another good wintertime technical discussion)

The disc brake could also be used somehow.

At some point we will need to actually dig into the math.

 

[John in CR]

You've basically just made a motor in reverse. Were you to apply power at the right times, you'd be able to power that thing. But, it won't work well at low speeds, and won't do anything at all at less than a few MPH. Plus, you'd be adding loads of weight to the rim, making it harder to get up to speed and slow down.

So, yeah, that would work, but you might as well just install a hub.

I've got a hub, just no electronic brakes, which is what I'm after.

John

 

[Link]

I've got a hub, just no electronic brakes, which is what I'm after.

Geared or direct drive? Geareds won't regen/plug brake because they freewheel, but just shorting out a hub turns it into a brake. Best to to it with a large resistor instead of a dead short to keep braking power to a reasonable level.

Do a search for "plug brake". Should turn up some stuff about what you're looking for.

 

[John in CR]

Thanks Link. That's exactly what I've been looking for. That should be part of the noob info arsenal. Now to go wring Knuckles neck via internet, as I've been asking for 2 months how to do electronic brakes. My only concern is running the current from braking through my motor, since it gets hotter than I'd prefer already, but it seems that the real heat of braking is dissipated in the resistance wire. I gotta be careful not to make it too strong, since my non-geared motor is a front hub. From the sound of how well it works, the rig should be something we can buy, at least until quality regen braking becomes a reality. Is a low cost, high amperage variable resistor something that exists, or could be DIY'd, that could be useful in a plug brake?

Back to topic,
I still like the idea of something like wrapping magnet wire around my rear spokes at the rim. Then secure them in place with epoxy, and put a gang of magnets on a lever that puts the shorted coils in a opposing magnetic gaps. If only a pound or two of copper is necessary, I doubt it would affect acceleration much at all. Also, this isn't like creating eddy currents, so high speed rotation isn't a requirement. Just short phase wires together and see how much harder a hub is to turn even at low speed, though higher speed does mean more braking power potential. If someone could explain the winding/magnet geometry required to create a braking force, I'd probably give it a test. Is it as simple as having two windings shorted together that have turns in the same direction, with one fully in a N magnetic gap, just as the other enters a S magnetic gap? It seems like the hardest thing would be to make a reliable and accurate lever action for the magnets to create a strong field in a variable gap on demand.

John

 

[Link]

Is a low cost, high amperage variable resistor something that exists, or could be DIY'd, that could be useful in a plug brake?

Yes. Wrap some magnet wire around some PVC and there you have it. The less resistance, the more braking you get.

As for being variable, that's a little more of a challenge. Fechter actually came up with the idea of rectifying the 3-phase current from the motor into DC and then feeding it into a regular old brushed controller. This will work for a plug brake, but, apparently, it also can be hooked up to the batteries to provide regen.

 

[John in CR]

Is a low cost, high amperage variable resistor something that exists, or could be DIY'd, that could be useful in a plug brake?

Yes. Wrap some magnet wire around some PVC and there you have it. The less resistance, the more braking you get.

As for being variable, that's a little more of a challenge. Fechter actually came up with the idea of rectifying the 3-phase current from the motor into DC and then feeding it into a regular old brushed controller. This will work for a plug brake, but, apparently, it also can be hooked up to the batteries to provide regen.

PVC? I was just going to let my aluminum frame suck up the heat from the magnet wire. It's seems like a high powered brake light definitely needs to be worked into that circuit too. I understand the current is high, but at what voltage? Re regen, I have to wait for the experts to hand that to me on a silver platter. I'd think about a second switch to short the other phase wire, which would at least give me a bit of variability. With different wire lengths on the 2 switches, I could have off/brake/More Brake/MAXIMUM BRAKE. From the 2 threads I looked at about the topic, it seems to be a "must have" with any direct drive hub motor, so I'm really surprised it isn't discussed much, especially being so simple. I wonder what the risk is to the controller if you accidentally hit the throttle while the brake switch is on?

John

 

[Link]

PVC? I was just going to let my aluminum frame suck up the heat from the magnet wire. It's seems like a high powered brake light definitely needs to be worked into that circuit too. I understand the current is high, but at what voltage?

That'll work, too, but beware of shorts. Magnet wire has thin insulation.

The voltage will depend on how fast the wheel is spinning. The faster it spins, the higher the voltage. Anything above a few RPM should be enough for a small light, though.

I wonder what the risk is to the controller if you accidentally hit the throttle while the brake switch is on?

The controller will probably see it as a stalled/stopped motor and just give max amps to the resistor. Definitely not good, but it shouldn't be disastrous by any means.

 

[safe]

If someone could explain the winding/magnet geometry required to create a braking force, I'd probably give it a test.

That's what I want to see too.

At some point we need to stop talking about the math and actually do it. (might have to wait until winter) I suspect that once the formula's are found that applying them shouldn't be too much of a big deal because the number of parameters should be pretty small, much like an electric motor.

An electric rim brake is essentially an electric motor, but with a single point of magnetic interaction.

 

[safe]

Starting With The Basics...

I figured I might start into this a bit. There is a nice website that I've gone to before that makes a lot of these formulas more "accessible" for those who are not frequent users of them.

http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magcur.html#c3

...so with the old "right hand rule" we get:

Just plugging in a simple value like 10 amps into their online calculator you get a magnetic force from a wire to be 0.2 Tesla's. (at 0.1 meter) With every coil of the wire you would get that same value again and again and again. (I would guess) They have other online calculator tools there too, so depending on how you wanted to model it you could probably get close to a "ballpark" estimate of the forces involved. Just how much magnetic force is 0.2 Tesla? I suspect not all that much.

I'll keep fiddling with it... maybe I'll post something, maybe I won't... hard to say...

 

[safe]

Two Wires

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

This is another online calculator and it deals with two wires. If you plug in the same 10 amps and -10 amps and 0.1 meter you get 0.0002 Newtons/m as the force per meter.

So I guess that's getting closer if you used an electric brake that induced fields on both the wheel and the fork at the same time. :?

Pretty cool. 8)

0.0002 Newtons/m is very small. I know from my spreadsheets that I've calculated hub torque stress as being something like 50 Newton/Meters, but I'm not sure if this is the same thing. Of course many coils of the wires and maybe you start to get somewhere.

Oh wait... what you need to do is multiply this scaler with the radius of the wheel to get the comparable hub torque equivalent. Hmmmm... it's still going to take a lot of coils to produce much force... (probably as many as you could squeeze into the rim)

Reduce the distance the wires are apart and it increases the strength proportionally, so you would want the magnetized wires as close as possible.

Here's a thought... when the wires are running opposite to each other then all you need to do is have them connected flowing in opposite directions and they will act as you might want them to act. Once one of the fields is created then you can loop the current. So you need to "bootstrap" this I think.

 

[safe]

Magnetic Front Fender?

While the math is still easier with a single point source, in reality the forces from just one coil are so small that you would want to increase the width of the fork based electrical magnet. It seems like you could build a front fender that wrapped around the wheel and on the edges you could have the coils. That way you could increase the braking force by several orders of magnitude.

An Observation

All these force equations deal with current and NOT voltage. That's kind of weird because with motors we deal with voltage a lot. (more usually being better) So voltage is sort of missing in this model so far.

This presents the dilemma of more current meaning more magnetism, and then heat and loss of efficiency which defeats the whole purpose. So the dynamics of electric motors are pretty clear based on this simple beginning.

The easy way to get more magnetism is with more current, but that's bad for resistance losses.

You can increase magnetism with more coils of the wire, but that can mean a longer wire and more resistance.

The only "easy" easy one is that the closer the magnets are together the more strength.

Hey, one "easy" is better than nothing. :lol:

 

[safe]

The 1000 Amp Solution

If you change the numbers around and introduce 1000 amps into a single coil (which means it could be very thick wire to reduce resistance) and then use that induced magnetism to recycle the energy into a recycling loop you get:

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

I1: 1000 amps
I2: -1000 amps
R: 0.01 m

Resulting force is 20 Newtons per Meter... which is probably in the ballpark for a significant braking system.

So that should give some idea of the general forces required.

1000 amps is a lot of danger to deal with.

(but by backing away from this value and multiplying the coils you would be able to get the same effect)

 

[safe]

"Newton per Meter" verses "Newton Meter"

These are totally different metrics:

"Newton per Meter" - N/m - kg/s2 - Surface Tension

"Newton Meter" - Nm - kgm/s2 - Torque

...so in order to get the two to be compatible we need to (I think) convert the straight line force by multiplying by the torque vector.

So 20 "Newtons per Meter" (N/m) times the perpendicular vector of the wheel radius (we assume 0.5 meter) means we get an equivalent hub torque of only 10 "Newton Meters". (Nm) Which is still rather weak. My calculations on my small electric motored (1400 watts) bike show an equivalent hub torque of around 50 Nm. The Rholoff 14 speed hub boasts of being able to handle up to a maximum of 100 Nm by comparison, so that's the upper edge of simple pedal power. I remember from before that you need something like an equivalent of 6000 watts (8 hp) to actually stop (skid) an ebike wheel, so this isn't looking good. (you would need a stopping force on the hub of something like 200 Nm) If you need 200 Nm to cause a skid then you would need something like 20 times the current to make the magnetic field and that means you need something like 20,000 amps through a single coil to make it happen... obviously that's unrealistic.

Electric Brakes are not going to be easy without adding a lot of weight to the wheel or finding a very efficient design. (not to be deterred however... there may be tricks to make this more realistic... but it's not going to be easy) As you increase the number of coils in the design it will reduce the current required.

 

[Toorbough ULL-Zeveigh]

...so with the old "right hand rule" we get:

...a hairy palm?

really, you've subliminally symbolized the essence of your theoretical threads.

 

[Link]

...so with the old "right hand rule" we get:

...a hairy palm?

really, you've subliminally symbolized the essence of your theoretical threads.

I dunno, TZ. Looks like a pretty good technique to me?

And what's this about 1,000A being dangerous? I think not. It'll be at a pretty low voltage.

 

[safe]

There is a lot of shallow interest in electric motors and things like electric braking on this forum. All I'm trying to do is try to tap into the deeper fundamentals of electromagnetism and how it might apply for our ebikes. I can remember "right hand rule" jokes in college physics classes some 25 years ago. (yeah we chuckled back then too :lol: )

When we look at motors we are seeing very "distilled" formulas like "rpm per volt" which are easy to work with but give us no idea of WHY things behave as they do.

For something like a magnetic brake that doesn't add much weight we're thinking about how one might be able to do it... the results so far is that it's "hard to do", the forces required to give satisfactory braking are pretty high.

Formula One auto racing has declared this a significant thing for the future, so we ought to begin to know more about it.

Eddy Braking was on a TV show the other day... they were talking about electric trains and how successful that's turned out to be. Maybe some more research into that will shed more light on the topic? (it's been mentioned already and seems to be the same "train" of thought if you can pardon the pun :wink: )

 

[TylerDurden]

And what's this about 1,000A being dangerous? I think not. It'll be at a pretty low voltage.

Think watts.

 

[cerewa]

Think watts.

1000A * 1V = not very dangerous.

 

[TylerDurden]

Think watts.

1000A * 1V = not very dangerous.

Depends on where and how long, I suppose.

I've had a 9V battery short in the pocket of my slacks... (loose change)
I've bridged the posts of a 6V deep cycle flooded battery with a wrench...

 

[Link]

I've had a 9V battery short in the pocket of my slacks... (loose change)

Heh. We were shorting out AAs and poking people with them in summer school a few years back. Too bad they didn't last very long this way.

I've bridged the posts of a 6V deep cycle flooded battery with a wrench...

That had to be interesting...

 

[safe]

Eddy Braking

From what I can tell about Eddy Braking it's something that only requires a magnet on one side. The idea is that an alternating current creates a wave in a material (like steel or others) that as it manifests it's inductance behavior the magnet "fights" the field it just created.

You could have nothing but a rim that exhibited magnetism (something other than aluminum... probably steel) and then do this Eddy Current idea (magnet attached to the forks) and do it that way. (you need some fancy wave timing though)

It's no longer a simple calculation... part of the reason why "beginners" like us wouldn't figure it out just using simple magnetism, but if you timed the waves right could do well because there is only one magnet.

Ride the eddy current wave dude! Hang Ten!

No moving parts too, no brushes or anything, so that's good.