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# Homogeneity of magnetic flux and rotational eddy losses

## Homogeneity of magnetic flux and rotational eddy losses

(OP)
Hi:

I am building an application were a Disc magnet is placed closed to a copper plate. This magnet rotates about its axis. This magnet will not move with respect to the plate, only rotate.

I want to minimize eddy current losses due to this rotating magnet but maximize eddy current losses by translation in any direction.

In principle if the magnetic flux is perfectly  homogeneous  on the surface, meaning that along a given circle of any radius on the surface of the cylinder the flux is the same along that circle. If also the magnetic flux axis is aligned with the axis of rotation, this rotation should not produce eddy currents.

How accurate is my assumption?

Are magnets in real life this homogeneous?

Is the magnetic axis the same as the geometric axis of the magnet?

Is there a way to minimize rotational eddy currents without minimizing transnational eddy currents?

I am planning to enclose the magnet as much as possible with non/ferrous materials to maximize this transnational damping.

My final goal is to have a magnet/plate setup in which the magnet can rotate as free as possible along its magnetic axis, but be damped as much as possible by any translation movement.

Very similar to a magnetic bearing.

Thanks

Fernando
Replies continue below

### RE: Homogeneity of magnetic flux and rotational eddy losses

Your assumption is correct.  Your disc magnet will need to be axially magnetized (it could even alternate N-S-N-S with increasing radius, (i.e. concentric poles, like a target); it just needs to be rotationally symmetrical).

Practical axially magnetized magnets should be quite homogeneous in field strength and direction about their axis.

The magnetic axis can be unrelated to the physical axis--you want an axially magnetized magnet for their axies to be coincident.

A single pole axially magnetized disc magnet will minimize rotational eddy currents.

Increasing pole count concentrically will increase translational eddy currents, to a point--the pole width should be less than the air gap.

### RE: Homogeneity of magnetic flux and rotational eddy losses

I don't understand this part:

"This magnet will not move with respect to the plate, only rotate.

maximize eddy current losses by translation in any direction

be damped as much as possible by any translation movement"

If the magnet does not move with respect to the plate, how do you get any translational losses/damping?

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### RE: Homogeneity of magnetic flux and rotational eddy losses

(OP)
Well what I meant by that was that the magnet and plate should not move translationally, that is the movement we are trying to damp. But its not constricted physcially, very similar to a magnetic bearing...

### RE: Homogeneity of magnetic flux and rotational eddy losses

In my experience rare earth magnets, at least when big (e.g. 4"x4"), are not very uniform, so I expect quite high losses in rotation. However, as I don't see how you will close your magnetic circuit, you shouldn't experience large braking forces (though this is relative to what you expect of course).
Concerning the translation, you should have the disc magnet smaller in diameter wrt the copper plate, so that there is a field change when moving, but once again you won't experience a large damping and it depends also on the amplitude of the lateral movement you want to stop.

prex
http://www.xcalcs.com : Online engineering calculations
http://www.megamag.it : Magnetic brakes and launchers for fun rides

### RE: Homogeneity of magnetic flux and rotational eddy losses

I think your translational eddy current detection is mostly mutually exclusive with your lack of rotation eddy current, but you haven't stated how big a translation motion you're trying to control.  Is it slow, is it fast, is it large, is it repeating?

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