Just out of interest, why don't you control the speed of the motor?

I'm with you on this......it sounds too simple to be true.....

Suggest you use a flat permanent magnet on each side with a caliper mechanism to bring the flat magnets close to the spinning disk with an RPM counter in a feedback loop to change the gap. This needs close tolerance and the magnets must be flat and perfectly parallel to the surface. The closer they get the more brake action, but you do not want the spinning disk to wobble or the brakes to be angled. flat magnets from a dead hard drive would be ideal and they already have an armature you can improvise from. Do not shock the magnets or heat them.

You can also simply buy magnets.

Look here for tips.

http://www.google.ca/#hl=en&source=hp&q=%22magnetic+brake%22+%2B%22how+to%22&btnG=Google+Search&meta=&aq=f&oq=%22magnetic+brake%22+%2B%22how+to%22&fp=3297ab6c55cbe21d

http://www.google.ca/#hl=en&source=hp&q=%22magnetic+brake%22+%2B%22build%22&btnG=Google+Search&meta=&aq=f&oq=&fp=3297ab6c55cbe21d

What mechanism are you using for the braking? If you are you talking about using induction in the aluminum disk to create resistance to forward momentum and generate a braking effect then you have a lot of work ahead of you.

Here are just a few issues you need to deal with or balance out before you even think about the gauge of wire involved.

1. 230 RPM isn't really that fast and generating enough back EMF from that speed will take a very strong magnetic field. Additionally, that motor is geared down and will create a ton of torque. If you do generate enough back EMF to slow this disk down you are dumping all that power as heat. That disk is going to get DANG hot.

2. Balancing the back EMF generated by the magnets against the power of the motor is going to be tough. Calculating that balance point to be right at 200 RPM is even going to be harder. Without some other speed management system I don't think that there is an easy way to even calculate a stable point. Remember that as you brake the spinning disk the motor will draw more current. This will cause it to start heating up which in turn reduces the strength of the PM motors magnets and the power of the motor. This is an extremely dynamic system that will be almost impossible to control without some feedback loop and if you have a feedback loop why do you need EMF braking to keep it at 200 RPM.

3.......

I just realized that I could keep going for a long time on this and I may be off track for what you are hoping to accomplish. Could you give us a more detailed description of what you are trying to do and the techniques you want to use to accomplish it.

Doug

Sorry, I can't help.

I once had a formula linking Torque to surface speed (RPM, distance from axis), resistance of the moving Al (resistivity, thickness, size of magnetically coupled area) and Strength of field (No of turns, current, impedance of magnetic circuit, corrections for fringing). I tidied up and threw it out 20 years ago. Never again!

I'm now too stupid to derive it from first principles and anything to do with magnetics and small motors usually has a whole lot of unknown parameters, so I'd build a few models and test them.

Once you've got some results you can home in on a practical solution (it may not be pretty, but it's common engineering practice).

You've left out the most critical information, that being how the motor is driven. The motor can produce a large amount of torque, if supplied with ample current. None of this stuff works by magic, so you can expect a lot of heat to be generated, unless you have provided for low amperage to the motor, via, for example, a speed controller.

Why not just control the motor speed directly -- or are you planning to use this device to cook eggs?

If you are assuming 230 rpm is "maximum" because the motor is loaded (in your application) to 30 inch-lb, (and is fed by e.g., a pair of car batteries in series for 24 volts) then loading it to 210 inch lb would increase the amperage to 31.6 and drop the speed to 200 rpm. Your eddy brake would then be dissipating roughly 450 watts. Most of the energy feeding the motor would go into heating up the aluminum disc and draining the 12 volt battery.

I'd suggest experimenting a little to see if this is the route you want to take -- are you are building a dynamometer?

I agree with nearly everyone else here. But if you still want to go ahead, I'd follow Aurizon's advice and use permanent magnets, but, instead of moving the magnets in and out myself: I'd use magswitches.

It almost sounds like you are using this EM brake to govern the speed?

The first thing that came to mind when reading your question... trains. Wiki "Dynamic braking", it will give an overview of how that works. However, I am not sure that is what you are trying to do. The other posters here have some pretty good observations and ideas, probably closer to your design idea.

http://www.earthchain.com/case-studies/5axis/5Axis.htm

This will explain how they work. They are called magnetic holding devices

I took a look at that page and it is an interesting description, but the diagram they use to show how the device works is definitely off. The magnet configuration they show in the diagram wouldn't produce the effect they are describing. I am not sure if in the process of dumbing it down for the web audience they messed it up or if they are trying to protect trade secrets.

Here is a link to a patent describing how it works (in legal patent speak) http://www.google.com/patents?id=3nctAAAAEBAJ&printsec=abstract&zoom=4&source=gbs_overview_r&cad=0#v=onepage&q=&f=false

I was looking for something better, but that describes it the best I can find so far.

Doug

have you thought of a linear motor?