Eddy currents and slug pole pickups

[CRU JONES]

With a blade, the eddy currents follow the periphery of the blade, in parallel to and just under the coil windings. If one laminates the blade, replacing one thick sheet with multiple thinner sheets insulated from one another, the eddy current is greatly reduced. Again, the effect is proportional to the square of the lamination thickness. Likewise, a thin layer of varnish or epoxy-soaked epoxy is sufficient insulation.

Joe,
Excellent info, BTW

Can you clarify this? It seems to me that if the eddy currents in a blade flow the periphery; Eddy losses would be proportional to the blades' perimeter. Two blade's would have a greater perimeter than a single blade. Would there then be more eddy losses from multiple laminated blades when compared to a single blade?

I think it might be worthwhile to note that,as it relates to the direction of the magnetic field, Transformer lamination's are arranged differently than Blade pickups. A transformer, the lamination's are continuous in the direction of the magnetic field, but divided in the direction of eddy currents. ...Unlike a blade pickup in construction.
Cheers,
Ethan

[CRU JONES]

So, the answer to "which perpendicular" is "both" essentially? Mainly because the coil is in fact round?

Are the currents parallel to the coil as a rule, or is that just how it works in pickups? Some things I read said that they were (if I remember correctly) "perpendicular to the magnetic field created" and that effectively would be the same thing since magnets are typically perpendicular and under the coil. We also are creating our own magnetic field instead of letting one be created by the coil, so I guess that is another thing to get tripped up on.

Oh, and by the way.... thanks Joe!

Sooo....
One long bar magnet would have more eddy losses than multiple insulated sections! Might just try that idea

Thanks Funky! Ill let you know,
Ethan

[Mike Sulzer]

Joe,
Excellent info, BTW

It seems to me that if the eddy currents in a blade flow the periphery; Eddy losses would be proportional to the blades' perimeter.

The power lost in a loop is proportional to V^2 over R. V is proportional to the area of the loop (law of induction). If you have thin sheets, then making them half as thick leaves the resistance around each nearly the same. And so if you cut the thickness in half, doubling the number, each sheet has one quarter the loss, but you have twice as many so you cut the total losses in two. I think this is the reason for thin sheets; cutting it up in other ways reduces the loss, but not as quickly.

[Joe Gwinn]

Joe,
Can you clarify this? It seems to me that if the eddy currents in a blade flow the periphery; Eddy losses would be proportional to the blades' perimeter. Two blade's would have a greater perimeter than a single blade. Would there then be more eddy losses from multiple laminated blades when compared to a single blade?

It's true that the resistance is proportional to the peripheral length. But it's also true that the loop voltage is proportional to the area linked by the changing field.

In a thin sheet, changing the thickness has little effect on the loop resistance, but a large effect on area and thus loop voltage.

It's easier to think about a wire loop. If one pulls it into a ring (max area for a given perimeter), one gets max eddy current in the wire loop. If instead one flattens the loop into a very flat oblong shape, one gets minimum area, and very much lower eddy currents. If one in addition twists the oblong about its long axis, one can get zero eddy current, as there is voltage cancellation.

I think it might be worthwhile to note that, as it relates to the direction of the magnetic field, Transformer lamination's are arranged differently than Blade pickups. A transformer, the lamination's are continuous in the direction of the magnetic field, but divided in the direction of eddy currents. ...Unlike a blade pickup in construction.

Actually, no. Transformer laminations and laminated blades have the same geometric relation between fields, currents, and lamina.

The key is to follow the easiest path for an eddy current: the plane of the lamina is perpendicular to the plane of the windings in the coils, so the area linked by a varying field created by the coil is the minimum possible.