If you use metal strap edge-on to the field it's like you used wire the same diameter as the thickness of the strap, which is fine. If you use it crosswise to the field you've got an electric brake.
Even though the tubing is very small, hollowing it out and expanding it makes it present more area to the field and thus have more loss than the same cross-section of copper as a wire would have. So you get more losses. It also means you have less wire in a given volume, which means you get less power out from a given geometry and set of magnets. Finally, a narrow hole in the middle means a lot of fluid friction, and thus a lot of power required to pump the coolant.
What I'd do for liquid cooling is wind the coil with heavy wire and embed it in a coolant - or embed the portions of the coil that are NOT in the magnet gap in coolant, and let the thermal conductivity of the wire bring the heat to the coolant. (Most of the heat conduction is done by the same electrons that constitute the electric current you're generating, by the way. That's why good electrical conductors are also good thermal conductors.)
I'd use oil rather than something water-based for coolant. Oil insulates. Water-based coolants would promote electrolytic corrosion through any microscopic defects in your insulation. In addition to the corrosion just sitting there, any slight imbalance in your rectifiers would quickly lead to major corrosion of the wire at the averages-more-positive end of the coil, possibly turning it into metal sponge within weeks. Also: The conductivity of water-based coolants could produce still more eddy-current losses, heating, and electrochemical pathologies as the mag field moves through them.[ Parent ]
oh crap. recursion :)
allan[ Parent ]
Think of the magnetic field as a beam of light.
If the beam of light hits a broad surface you get a lot of eddy currents. If it hits a narrow one you get few.
Your wire has to be pretty much at right angles to the field lines one way. So if you imagine you've got a field "shining" onto the table in front of you, your wire might be lying, say, left-to-right and moving, say, front-to-back. So far so good.
But if your wire is a strap you still have a choice: Stand it on edge, or lay it flat.
If you stand it on edge the field "shines" on a very narrow cross-section. Very little eddy currents. If you lay it flat it "shines" on a broad surface. Lots of eddy currents. So when you wind with strip you line it up so the field goes along the broad surface rather then penetrating it - the "standing on edge" orientation in your table example.
Turning a given amount of round copper wire into a pipe makes the surface "seen" by the field broader. So you get more eddy current losses, while the amount of generation remains the same. But with that hollow in the middle you can pack less copper in your slots, so you get less current generated. Lower gen and higher losses at the same time - phoey! You have to cool it a LOT - and use stronger magnets or more of them - to make up for that.[ Parent ]
Large auditorium.
Big pendulum hangs from the celing over the lecturer's bench. Weight on the end is a disk of copper. On the desk is a very strong magnet (perhaps salvaged from a moderate-to-large magnetron tube.) One of those buggers with a couple quarter-donuts of metal as thick as your neck, growing up out of a strong, flat, metal plate, so the ends face each other across a few inches of gap. Magnet is positioned where the disk would swing through the gap at the bottom of its travel (if it could). Pendulum is held up at one end of its travel by a rope.
Comes demo time. The prof passes his hand through the gap ("Nothing up my sleeve - especially no mechanical watch!") Then he yanks the rope, untying the knot and letting the pendulum swing free. It starts toward the magnet, picking up a bunch of speed in its three-story drop. The leading edge starts to enter the gap.
WHANG!
The disk rings like a gong and stops nearly dead with the leading edge inside the magnet gap. Then it gradually eases into the gap until it's at the bottom of its travel, where it stops.
Eddy currents can produce a LOT of force.[ Parent ]
