Jerzy
It is not that simple. If you put a coil on the spinner it will
pick up the
magnetic field from the permanent magnet or electromagnet
mounted
outside.
However, the induced voltage will be always AC. The direction
of
current
driving the motor will be defined by the direction of diodes in
the
rectifier (mounted in the spinner) and it will not depend on
flipping of the
magnetic field of the outside magnet or electromagnet. Flipping
the
external magnets would change the phase of the AC only.
If you want to reverse the voltage driving the motor in the
spinner,
you
would have to use synchronous rectifier made of transistors. The
system
would have a bunch of permanenet magnets to deliver power, and a
small
electromagnet that would "tell" the synchronous rectifier which
way
to
rectify, positive or negative.
A little more complicated but doable.
Fred Osborn wrote,
Thank you Jerzy. I was beginning to think I missed something really
remarkable in
my many years as a manager, a true commutatorless DC generator. I
swore
that I
could remember that Electrical Machinery prof telling us "there are
no
DC
machines, only AC machines with commutators".
You can make a DC generator (I think) by "commutating" the
electrical
excitation
on the stator (engine) side with solid state electronics but as you
point out you
need a shaft angle sensor of some sort with electronics to tell the
"commutator"
when to switch. That way the electronics can all be on the stator
side.
The wound stator of a multipole pancake brushless DC servo motor
would
probably
make a good generator stator but I do not know if you can find a
corresponding
wound rotor. The rotor should be mounted directly on the prop shaft
with
the
stator concentric around it. The less the runout the smaller the gap
can
be.
Given Paul's guess of 100 to 150 watts, which seems high to one who
knows
absolutely nothing about it, I would guess that a minimal generator
could be
driven to that level for a few seconds, depending on how hot it
normally
would
be.
Fred O.
Excellent points gentlemen. As I said previously, I'm a mechanical
type,
not too good with electrons (My wife refers to my experiments in
household
wiring as "arc welding"). AC it is. Much more complicated it becomes.
Brushes are beginning to look better all the time.
Tom Kendall
A nit picking point - but there is at least one case 0f a true DC genrator
it is known as magneto hydro dynamics or MHD . The conductor is
a flow of ionized high temperature gases, and there is never a revesal
of polarity. Vance
The AirCraft Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site. Copyright 1998-2002 All world wide rights reserved.
http://home.earthlink.net/~rotaryeng/ http://www.linux.org