On Jul 12, 2007, at 9:53 AM, Harry Veeder wrote:
However, I just realised the shield in my drawing
http://web.ncf.ca/eo200/generator.html
is also subject to its own weight -- without the magnets it will
tend to
orient itself so the gaps are aligned horizontally.
If built symmetrically as shown, i.e. well balanced, it will no more
have a preferential position than a propellor or fan.
So with gravity and the
right combination of magnet strength and shield weight distribution
you
can minimise the energy required to keep the shield rotating.
The main energy required to keep the shield rotating can be viewed as
coming from the need to compress flux. Magnetic flux has a magnetic
pressure:
P = B^2/(2 mu_0)
When the hole is cut off the flux is compressed on the magnet side of
the shield. When it expands into the core the energy from that
expansion is reduced by the counter emf, i.e. the current, in the
coils producing an opposing field. Similarly, by Lenz law, the energy
required to close the hole and compress the flux is increased by the
current induced in the coils.
If you have no core at all, the shield in your diagram will
experience a strong cogging effect when it rotates, and the net drag
is due to heating of the magnet and or shield via induced eddy
currents that also, by Lenz law, oppose the motion.
Horace Heffner
http://www.mtaonline.net/~hheffner/
