On Thu, 27 Aug 2009 07:46:24 +0000 (UTC),
(Andrew Gabriel) wrote:


All I actually need is a reasonably stable shaft spinning at up to
around 3000 rpm and able to apply a little torque while it does so. I

3000 RPM (being 50Hz * 60secs) is the synchronous speed
of a 2-pole induction motor on 50Hz. With slip, it will
probably be rated around 2900 RPM for max torque.

And that should be fine as it should allow me to test the system
(stator, flywheel, CDI and regulator) at a typical 'in use' RPM. If I
wanted to test it right up to 6k then I might need to think again. ;-)

initially considered using a Picador shaft / bearings but then I would
also need a motor and coupling.

And whilst some grinders do have variable speed control they are more
expensive than I can afford for this experiment / project.

I don't know what sort of motor bench grinders typically use,
not having one myself.

Variable speed control for induction motors requires decreasing
the frequency and the applied voltage by the same amount, but
the torque available drops off quite significantly as speed is
reduced. This tends to work for things like fans where little
torque is required at lower speed, but not for anything which
requires any additional starting torque. Indeed, some sort of
check is required to make sure the motor overcomes its own
bearing torque, such as starting at full power and then ramping
down the the required speed, or having a feedback tacho or
similar to ensure the rotor starts moving. This only gets you
a small speed variation because of the rapid drop off in torque.

Ok.

If you want to go still slower, I think you have to drive the
induction motor more like a pulsed stepper motor (which
requires a multi-phase motor, but you are probably using that
anyway if you're using variable frequency control).

No, I think the natural inertia of the flywheel should allow it to
spin down from whatever revs it can manage back to zero and I can
observe the spark all the way down.

As I said, initially I'm happy for any revs that actually show the
thing to be working at_all (so FWIW it could even be hand cranked) and
anything outside that could be considered a bonus.

*If* my pillar drill could hold the flywheel rigidly enough to ensure
it can't touch the stator coils and spin the flywheel at sufficient
speed to generate a spark (so 10 rpm probably g) then that would
certainly be good enough to offer my BIL some way of bench checking
the stuff he has with the minimum of effort and cost.

Cheers, T i m