# Braking Force from Eddy Currents on a Rotating Disk

• Michael Sofroniou
In summary, the conversation discusses a project that aims to use eddy currents to apply a braking force to a rotating disk for physical therapy training. The team plans to use natural magnets on either side of the disk and adjust the distances to vary the resistance. The conversation also mentions various parameters that may affect the braking force, such as magnetic field, angular velocity, volume and mass of the disk, and the distance between the magnets and the disk. The person seeking help mentions their lack of knowledge in this area and asks for any general equations or resources. The response suggests starting with Wikipedia and following the links in the references, as well as a PF thread that discusses similar questions.
Michael Sofroniou
Hello! I'm currently working on a project that will utilize the phenomena of eddy currents to apply a braking force to a rotating disk. Some background on this project: My team was tasked to create a physical therapy training device to train patients in wheelchair propulsion at resistances lower than resistances experienced on level ground. We plan on doing this by supporting a wheelchair above the ground so wheels don't touch the ground. Our goal is to use natural magnets on either side of a large rotating metal disk (replacing the original wheels of the chair) and adjusting the distances the magnets are from each disk to vary the applied resistance.

I'm not well versed in magnetic force application and was wondering if there is any general equation that roughly approximates the braking force eddy currents apply on this rotating disk. I imagine a few parameters this problem would depend are as follows:

- magnetic field generated by each magnet
- angular velocity/angular acceleration of the rotating disc
- volume of rotating disc
- mass of rotating disc
- some magnetic property of the metal disc
- distance the magnets are from the rotating disc
- point of application of eddy forces in relation to the center of rotation

I've done some preliminary literature review however some of the studies seemed to be a bit over my head. Any help would be greatly appreciated!

## 1. What are eddy currents and how do they affect braking force on a rotating disk?

Eddy currents are circular electric currents that are induced in a conducting material, such as a metal disk, when it is exposed to a changing magnetic field. These currents create their own magnetic field, which opposes the original changing magnetic field and creates a braking force on the disk.

## 2. How does the speed of rotation impact the braking force from eddy currents?

The braking force from eddy currents is directly proportional to the speed of rotation of the disk. This means that the faster the disk rotates, the greater the braking force will be.

## 3. Can the strength of the magnetic field affect the braking force from eddy currents?

Yes, the strength of the magnetic field can affect the braking force from eddy currents. A stronger magnetic field will induce stronger eddy currents, resulting in a greater braking force on the rotating disk.

## 4. Are there any other factors that can impact the braking force from eddy currents?

Yes, the thickness and conductivity of the disk, as well as the strength and frequency of the magnetic field, can also affect the braking force from eddy currents. Thicker and more conductive disks will experience a greater braking force, while a higher frequency magnetic field will induce stronger eddy currents.

## 5. How is the braking force from eddy currents used in real-world applications?

The braking force from eddy currents is often used in mechanical braking systems, such as in trains and roller coasters. It is also used in electromagnetic braking systems in vehicles, where the motion of the rotating wheels creates eddy currents, which provide a braking force to slow down the vehicle.

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