Eddy current brake

[Paul1999]

I try to calculate brake force for a primitive eddy current brake. The idea is that when a magnetic flux passes through a conductor (copper tube), it excites eddy currents, which are the source of the magnetic field, which is oppositely directed with respect to the external field, thereby generating a braking force.

I found the equation for the braking force, which is calculated through power losses, and derived the expression for power losses. But I don't know how to determine the frequency (а) for calculating the skin effect (б), because I use direct current (eddy currents are excited by a quasi-alternating magnetic field). I also can't decide what to use as the characteristic length (lh) for the magnetic Reynolds number (Rm). I assume that it could be the diameter of the copper tube or the length of the area through which the magnetic flux passes.

[joigus]

Ok, I'm far too rusty on magnetohydrodynamics to be of more concrete help, but maybe I can be used as an "educated sounding board" of sorts here.

For problems involving wires it's often the case that you cannot "dimensionally close" the formulas because you're missing a fiducial length. (The theoretically well-defined limit is often a wire of indefinite length).

Other way of seeing it is that rather than force, Reynolds number, etc, the relevant quantities be force per unit length, Reynolds number per unit length, and so on.

Could that be just the nudge that you need?

 

[Paul1999]

18 hours ago, joigus said:

Ok, I'm far too rusty on magnetohydrodynamics to be of more concrete help, but maybe I can be used as an "educated sounding board" of sorts here.

For problems involving wires it's often the case that you cannot "dimensionally close" the formulas because you're missing a fiducial length. (The theoretically well-defined limit is often a wire of indefinite length).

Other way of seeing it is that rather than force, Reynolds number, etc, the relevant quantities be force per unit length, Reynolds number per unit length, and so on.

Could that be just the nudge that you need?

 

To calculate theoretical eddy current braking, the length of the conductor does not matter (an infinite plate between the poles of the magnet is usually used), but its thickness and the source of the external magnetic field matter, and in my case they have a finite geometry. So I think I can use "dimensionally closed" equation, but thanks for Your advice.