In article ,
T i m writes:
On Wed, 26 Aug 2009 21:58:49 +0100, Bob Minchin
wrote:

I am tinkering with the idea of building a test jig and a basic bench
grinder could make the ideal platform.


Why bother with soft start? No grinder I know has or needs it.

Wellyousee, it's not going to be used as a grinder. ;-)

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.

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.
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).

--
Andrew Gabriel
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