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Re: Unconventional Dynamo (TPT, April, 2002)

On Tue, 18 Jun 2002, Chuck Britton wrote:

The April issue of The Physics Teacher has an article that grabbed my
attention (once I got around to opening the copy)

I think that the Note on page 220 by Wojciech Dindorf deserves our attention.

It's a fascinating device, and I've encountered it before. It's called a
"Cylindrical HPG" (Homopolar Generator.) It's a variation on Faraday's
Disk. But where Faraday's disk has a dipole magnet, this device has a
tripole.

Think about it. If you cover the surface of a cylinder with inward-
pointing magnets, then the end-caps of that cylinder must serve as the
other magnetic poles. The result is a tri-pole magnet, with (say) north
poles on the cylinder ends, and a south pole on the curved cylinder
surface.

The topology of cylindrical HPGs is almost identical to two Faraday's Disk
generators melded together. Chose the magnet polarity so that both disks
can rotate in the same direction, then join the edges of the disks by
using a conductive cylinder:

"Morphing" two Faraday's disks into a Cyl. HPG
http://amasci.com/graphics/nmchcyl1.gif
http://amasci.com/graphics/nmchcyl2.gif

The advantage of a cyl. HPG is that there are no slip rings on the
fast-moving edge of the disk as there are with a Faraday's Disk machine.
The contact points can be placed on the axle (or even on axis) which leads
to very low wear and greatly reduced friction.

The device shown in the TPT article uses a U-shaped wire as the "rotor."
The current path is similar in a cyl. HPG, but in the cyl. HPG an
infinitude of U-shaped wires has been added (forming a conductive cylinder
with axial "brushes.")

As with Faraday's Disk, spinning the magnet has no effect on the rest of
the device, and if the device is run as a motor, the magnet does not spin.
All the torque appears between the rotating conductor and the unmoving
external circuit.


I need either a better constructed and operated device OR a
reasonably clear Maxwellian explanation of the 'effect' to become
convinced.

Or do it empirically: try making a "high power" version as depicted in
the 2nd GIF above. Form a 3-pole magnet by gluing two cylinder-shaped
neodymium supermagnets so their alike-poles are facing each other. Place
this inside a slip-fit copper cylinder, then place the whole thing on a
pair of parallel nonferrous metal rails (such as bronze welding rod.)
When operated as a motor (using a 30-amp low voltage supply) the little
cylinder flys off the end of the rails. Also, it's direction is
determined by the power supply polarity, but flipping the cylinder endwise
does not change its direction.

I messed with this around 92-93, but at the time I didn't understand the
physics, and it didn't occur to me to try measuring microamperes between
the rails as I pushed the roller along. I was running it as a motor.


For those who have access to the April issue - go to it! Let's hear
YOUR analysis.

For the REST of you - well - I'll TRY to explain the mechanicals of it.

Create a 'tubular magnet' with north poles outward.

It might be easier to use stacked ceramic disk magnets. Use the ones from
Radio Shack which have a central hole. Make two stacks, then force them
together with alike-poles adjacent (so you have one magnetic pole in the
middle, with opposite poles on either end.)


PS, I sent the following letter to the author's email address, but haven't
yet received a reply.


(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-789-0775 sciclub-list freenrg-L vortex-L webhead-L




From billb@eskimo.com Tue Jun 18 21:00:28 2002
Date: Tue, 4 Jun 2002 01:12:03 -0700 (PDT)
From: William Beaty <billb@eskimo.com>
To: wdindorf@interia.pl
Cc: physics teaching apparatus <tap-l@listserv.appstate.edu>,
list physics teaching <phys-l@lists.nau.edu>
Subject: "Unconventional dynamo" = cylinder HPG


Hi Wojciech!

This regards your article "Unconventional Dynamo" in TPT April 2002 p220


Very interesting device you have there. I've seen that demonstration in
the past, but not such a simple version. (Actually, you can make yours
even simpler. Replace your array of magnets with a tripolar magnet: south
on both ends and north pole in the center, using a stack of disk magnets.)

This sort of device is closedly connected with Faraday's Disk, also known
as the Homopolar Generator or "HPG." Some hobbyists call these by the
name "N-machine." Another variant is called "Alexander's Wheel."

A few years ago I built various versions of these and made some GIF
diagrams. Take a look:

Two Faraday-disk generators
http://amasci.com/graphics/nmchcyl1.gif

Merge the two generators to create a cylindrical HPG
http://amasci.com/graphics/nmchcyl2.gif

Slotted-rotor self-excited HPG motor or generator (no magnet used)
http://amasci.com/freenrg/chevron.gif

The magnet feels no torque. The external circuit is the "stator."
http://amasci.com/freenrg/N_MACH1.GIF

Faraday's disk, but where's the rotor? Where's the stator?
http://amasci.com/freenrg/N_MACH2.GIF



Here is a book about this topic:

The Homopolar Handbook
http://www.amazon.com/exec/obidos/ISBN%3D0964107015/


Here are other things I tried back in 1992:

Rather than a wire loop, enclose the whole magnet within a metal
cylinder. The generator still works. This was suggested in
Charles Yost's hobby journal ESJ, "Electric Spacecraft."

Rather than using bearings and two brushes, place the cylinder-assembly
upon two rails. Connect your ammeter across the rails. When you
force the cylinder-assembly to roll along, the meter detects the current
being created.

Instead of a generator, RUN THE DEVICE AS A MOTOR. Connect a power
supply to the rails mentioned above. Up to 30 amperes is required. The
cylinder-assembly will begin to roll along. Reverse the current, and it
rolls the other way.

Something I did not try: make TWO rail assemblies and TWO cylinder
assemblies. Connect the rail assemblies together electrically. Now,
when you roll one of the cylinders, it generates a large current, which
should run the OTHER cylinder like a motor! Push one, and the other
one moves. Or push the second, and the first one moves. (Does this
actually work?) It's a motor/generator pair.

In my version, I glued two 2cm neodymium supermagnets together so alike
poles were face-to-face. This creates a cylinder magnet with one pole on
the side of the cylinder, and the other two opposite poles on the ends.
This I placed within a copper tube, then rolled the copper tube on
parallel metal rails.

These devices are very strange. When operated as a generator, motions of
the magnet do not create any DC. When operated as a motor, the magnet
does not spin. The torque appears between the "rotor" conductors and the
connecting wires. If you hold the "rotor" still, and instead rotate the
brushes and the ammeter around it, you will create just as much current as
when you hold the brushes and ammeter still and instead turn only the
rotor.

(((((((((((((((((( ( ( ( ( (O) ) ) ) ) )))))))))))))))))))
William J. Beaty SCIENCE HOBBYIST website
billb@eskimo.com http://amasci.com
EE/programmer/sci-exhibits science projects, tesla, weird science
Seattle, WA 206-789-0775 sciclub-list freenrg-L vortex-L webhead-L