On Wed, 26 Aug 2009 21:21:32 +0000 (UTC),
(Andrew Gabriel) wrote:


Am I right in thinking you can make induction motors 'soft start'
(reduced starting torque) by the inclusion of a series resistor?

Not really enough detail about what you're trying to achieve.

Well as I said it's just a idea at the moment g. BIL is into
Lambretta scooters and decided to 'update' his latest restoration
project with the electronic ignition system they tend to be made with
these days (well, in India anyway). Due to the nature of the beast the
parts are typically bits used on other makes and models (I think the
regulator is from a Vespa and the CDI unit as used on some Ducatis
etc) and whilst this isn't unusual in the motor game they generally do
at least come fitted and working from the factory and fully (and
generally correctly) documented.

It would seem that the paperwork he was supplied with on this 'kit'
seems to be at odds with my understanding of the use of the charge,
exciter and trigger coils but in spite of a re-wire it still doesn't
work (as in not spark, let alone actually 'running'). He's already
replaced the CDI unit but again nothing (but he could have killed them
both). Sooo, I was thinking of a simple jig that could allow the
mounting of the stator and flywheel (by the use of a 'universal'
stator back plate and replaceable 'noses' for the flywheel / grinder
spindle) and a simple mounting for the CDI unit, spark plug,
regulator, dummy battery (or real one), lights and kill switch etc.

Induction motors don't generally need soft start until you get up
to the sort of power where you'd use a 3-phase motor, and then you
do the soft start using a star-delta switch which starts the motor
on 240V per winding, and switches to 415V per winding when the
rotor has got up some speed. Generally, this is only done to limit
the inrush current with a stationary rotor, and not because you
want the motor to run up slowly -- induction motors tend to do
that anyway because they naturally have poor starting torque.

Ok. I was just thinking how fast my bench grinders spin up (admittedly
from memory) and the potential of the shaft spinning within the
flywheel if not fitted with a keyway or tightened onto a proper taper
as it would on the crankshaft. I was hoping to be able to just 'nip it
up' with a nyloc nut and a suitable self centring taper [1] on the
grounds that might be more flexible (allowing a greater range of
flywheels etc).

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).

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.

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). 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. ;-)

Cheers, T i m


[1] Or unique collar / spacers turned to accept different flywheels.