Variable drive idea

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The E & M fields are powerful. Sounds neat.

One wants something like that in the oilfield - no sparking. Induced currents. I noticed a number of new designs in a magazine I get at work - Hope I have it still - the ends are speed control and all sorts of fancy stuff. Guess the industry has thought energy saving or something.

Martin

Not quite lossless; anywhere you have the same torque (and torque input supplied to the magnetic flywheel, must equal torque output from the copper flywheel) and a difference in RPM, power is lost. If torque were higher on the lower RPM shaft, then power would be conserved. But that requires gears or belts.

In this case, the power is lost as eddy currents in the copper plate. Same mechanical effect as friction in a normal clutch (with the exception that a real clutch, when tight, won't have slippage!). Neat application of it though.

Bit of trivia: that's exactly how they do tachs and speedometers. The needle is spring-loaded and has a copper disc on it; a spinning magnet (which is feed by whatever shaft) transmits torque to the spring based on RPM, thereby causing the needle to deflect (based on the spring constant).

Tim

-- "I have misplaced my pants." - Homer Simpson | Electronics,

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Sure sounds like boob bait. No energy consumed, eh? Were they asking for investors?

If you have any of those neodymium magnets around, try rolling one down a thick slab of aluminum, or dropping one down the middle of a close-fitting aluminum or copper tube.

Eddy currents in non-ferrous metals. Fun stuff.

Wanna baffle a lot of people? Drop a rare earth magnet down a piece of aluminum pipe. Have them explain to you why it drops the way it does.

The same effect is used to damp oscillations on beam balances. The faster the plate passes the magnet, the more resistance there is, and there is no effect once the plate stops. Used on a lot of reloading scales, among others.

Cheers Trevor Jones

OK, having seen this type of mention twice in this thread now, and lacking the ability to perform the "experiment" (Got some HDD magnets, but apparently nothing suitable to drop them down - they just slid down the aluminum flashing I tried. Big fat hairy deal...) I gotta know:

What happens? (Not the physics, electrical theory, etc, just the physical "What am I supposed to see when I do it?")

I worked on a small Renault in the early seventies that used a air gap clutch with electromagnets. It was an interesting affair with radio tubes in the control box. All kinds of devices have been designed using eddy currents including tachs as mentioned.... even dynomometers. Lots of new designs using old ideas. Also old ideas like perpetual motion/zero energy loss in new designs. Randy

Get a magnet 'set' out of an old hard drive - the type that has two magnets in a holder with a gap between the two. Saddle this assembly over the edge of an aliminum plate (an aluminum framing square is perfect for this). By hand, move the magnets rapidly along the aluminum. You will be able to feel the resistance. Regards. Ken.

I actually did this when I read about it on this NG. I bought a few rare earth magnets, took 3 of 'em and stacked them up (be careful, they pull together so hard it can pinch) and dropped them into a straight piece of

I took the whole shebang to my son's physics teacher and told him "here's a great classroom demo". He thought so too and I'll bet that gets shown every year at the local junior high for awhile.

GWE

D> >

It drops in slow motion. In a vertical tube, the magnet will flutter down at about half the speed of about any other object of similar density that you can drop down the tube. A magnet dropped down the side of a sloped sheet of aluminum will show some of the same effect, but it really only shows well when you can drop a washer or such, alongside the magnet.

Pretty comical to hear the explanations why, sometimes.

Cheers Trevor Jones

"Tim Williams" wrote: (clip) Bit of trivia: that's exactly how they do tachs and speedometers. The needle is spring-loaded and has a copper disc on it; a spinning magnet (which is feed by whatever shaft) transmits torque to the spring based on RPM, thereby causing the needle to deflect (based on the spring constant). ^^^^^^^^^^^^ Once the needle reaches the balance point, the angular velocity is zero, so the efficiency drops to zero. But, since the device is transmitting information to the driver, the power-in is being converted to intelligence--it ought to be possible to derive a conversion factor for energy to information .

Another eddy current effect is to put a small DC voltage on a 120 VAC shaded pole motor. The rotor will sort of ooze around when you turn it by hand. I made a great payout reel drag by using one of these driven by a swing arm potentiometer controlling an LM317 voltage regulator. Very non-critical, but worked much better than a friction brake.

Earle Rich Mont Vernon, NH

Nothing new about it except the use of rare earth magnets and the variable airgap. It is an eddy current drive that has been common in large industrial drives since the 50's. In the original design, they used a large variable electromagnet to provide the flux. They have been replaced in most industrial uses today by VFD's since eddy current drives are so inefficient in constant torque, large speed reductions. A plant I worked in had 4, 500 hp eddy current drives controlling the screw speed in some polyethylene extruders. As soon as VFD technology got cheap enough, they were replaced with VFD's with a huge reduction in drive system losses.

Randy

Try something thicker than aluminum flashing. (To be sure, I just tried it with a HDD magnet on the back a piece of aluminum t-track and it worked great.) The magnet should slide down abnormally slowly. Compare by sliding a coin or something. It's way cool!

- Owen -

I just recently acquired a exercise type bike that worked on this same principal. It would turn the one heavy flywheel without any physical connection by way of belts gears chain etc. It was weird until I finally discovered there was a series of nice magnets in the one wheel that caused the other wheel to rotate. Right now its in pieces looking to be turned into another home brew something or other.........

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