On Wed, 27 Aug 2003 11:15:57 -0400, Gary Coffman
wrote:

On Wed, 27 Aug 2003 06:52:42 -0700, Eric R Snow wrote:
On Wed, 27 Aug 2003 00:39:31 GMT, clare @ snyder.on .ca wrote:

On Tue, 26 Aug 2003 14:47:48 -0700, Eric R Snow
wrote:

I thought about using a big stepper I have but it saturates at the rpm
that it would be running at. At least I think that's what happens. If
all the wires are twisted together and the shaft spun the resistance
increases and then at a certain rpm drops dramatically. I suppose if
there were less poles in the rotor this wouldn't happen.
ERS
You are getting AC out of the servo - and poly phase at that. You need
some diodes to make them work properly. Tying wires together makes the
phases work against each other.
If I tie the wires in pairs, so there are 4 pairs in an 8 wire motor
the effect is the same. I know AC is coming out. That makes no
difference when measuring the output when used as agenerator. The load
was resistive and the meter set to AC.
ERS

How are you combining the outputs of the pairs? Since each pair produces
a different phase, you can't just combine them across a resistive load. You'll
have phase cancellation if you try that. But if you diode rectify each phase,
you can then combine them.

For maximum output, you need to use a bridge rectifier for each phase
(each winding), then combine all the DC + and all the DC - leads from
all the bridges to form the net output to feed to the resistive load. (If
you're sure you know the winding senses, you could star connect the
windings and use individual rectifiers, but the bridge is insensitive to
phasing sense, and produces 1.41 times as much output voltage too.)

Alternatively, if you don't want to rectify to DC, you can use a separate
resistive load for each pair. Each load only gets a fraction of the total
output, but at least you won't have phase cancellation. This would require
4 bulbs for your lighting application, but that's not necessarily bad. It
gives you redundancy.

Note that saturation isn't what would limit current at high RPM (you
can't saturate a PM alternator). What could is the increase in reactance
of each winding as the frequency (RPM) is increased. Inductive
reactance increases proportionate to frequency. This reactance is
effectively in series with the load, and will limit the maximum current
the load can draw. But that shouldn't be an issue for any reasonable
RPM.

Note that conventional auto alternators are 6 phase (2 more than
you have), and they work up to 10,000 RPM. So I don't think that
the number of poles is an issue.

Gary
Well, I wanted to use a stepper but couldn't get one to put out enough
juice. But, a drill motor from HF draws 11 amps when loaded. It spins
a gearbox with a 36 t0 1 ratio with 500 rpm out. So the motor spins at
18,000 rpm. Spinning this same motor at 10,000 rpm gives 9 volts @ 1.5
amps for 13.5 watts. Since I was looking for 12 watts that will be
O.K.. If LEDs are used with a six volt battery,( the one from the
drill), then that should be plenty of light. I need to figure out how
to mount the motor with a belt drive to get the proper rpm and seal
the motor so it lasts more than a week exposed to salt air.
ERS