T i m has brought this to us :
For an induction motor which uses a start capacitor on a single
phase supply, reducing the value of the start capacitor will
reduce the starting torque.

Ok, then that may be easier (if needed at all from what you say etc).

However a bench grinder motor is not intended for capacitor start.


For any type of induction motor, a series resistor will reduce
the starting and running torque, but this isn't normally done
because...

An induction motor must normally be allowed to reach its sync
operating speed (minus the designed slip for many types).
If you don't allow an induction motor to reach the right speed,
it will generally overheat (unless it has been designed with
such a bizzare operating mode in mind). For this reason, you
generally don't want anything that limits the starting or
running torque, because this risks having the motor fail to
reach its design speed.

Ah, ok, understood.

So not reaching synchronous speed means lots of heat generated inside
the motor, so not a good idea at all.


FWIW I was thinking that even a hand cranked bench grinder would be ok
for this project as initially it's a go / no-go test. However, if the
cheapo bench grinder idea worked then it could be further used to
stress / heat / cold test, especially the CDI unit (with the aid of
the hot air gun etc).

Now we know what you are trying to do....

A motor as used in an electric drill is more suited to have its speed
controlled and many have a built in speed control, but put a heavy load
on one a very low speed and even they will over heat due to poor air
flow. You can/could get in-line speed controllers for these.

Auto washing machine motors have various windings which do allow quite
low speed running.

Could you perhaps use a pillar drill which has various gearing?

He does have a few scooters that might also get
upgraded this way (also allows for 12V lighting) hence the 'universal'
approach. To be fair the company he bought the kit from have offered
to look at it all for him if he was to get the scooter and parts up
there but I sense he would rather sort it himself if at all possible
... hence my test jig idea. ;-)

3000RPM is around the bottom end of the rev range of a 2T engine, so
why can't you use a bench grinder?

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk