harryagain wrote:
"Cursitor Doom" wrote in message
...
On Wed, 16 Jul 2014 13:01:48 +0100, Bob Minchin
wrote:

Cursitor Doom wrote:
Hi guys,

I have a 0.5HP 230V single phase motor that I want to use to make an
electric hacksaw. I'd like to vary the speed of it if possible (in
additon to conventional gearing) and as simply as possible. Could I
get away with using a light dimmer for this or is the inductive nature
of the load likely to cause an early "phut!"? If so, what's a workable
alternative?

cheers,

cd

Single phase induction motors are designed to run at full voltage and
constant speed. Causing the speed to reduce by any means. changing
voltage, running on part cycles or reducing the frequency will all cause
the starter winding to kick back in (usually with a centrifugal switch)
which will cause problems.

For a hacksaw, the best option is to seek out a 6 pole motor which will
run around 950 rpm and reduce the speed with a belt drive.
A four pole (1425rpm in uk) will be Ok with a smaller motor pulley but
the 2 pole (2850 in Uk) will be just too fast for the application.

The very successful Kennedy saws use a flat belt drive giving good grip
on a small diameter crowned motor pulley and a flywheel around 6-9"
according to model to drive the crank.
Modern source for flat belt drive is to use a toothed belt running on
its back. My kennedy model 90 has been running like that for several
years perfectly.


Well, it seems I've got a half horse 230V single phase 2850RPM
induction motor and will have to make the best of it. Since electronic
speed control seems to be out of the question, I'll just have to
fabricate a gear train to get the reduction necessary. Two strokes of
the blade per second ought to be about right so the maths is simple
enough (damn sight simpler than cobbling up the gear train but that's
life). Fortunately I have a lathe and a mill so things could be worse.
:-/

You can't vary the speed of and induction motor by varying the voltage.
You can vary it by varying the frequency.
You can buy a motor speed control that does this.
But be sure it is a frequency changer.

They are quite common on new cheap lathes for example, saves having a
gearbox.


This is ******** Harry. You should know better
A single phase motor running on reduced frequency will switch in the
start winding. The start winding and capacitor are designed for 50hz.

You can only change the speed of a 3 phase motor with a variable
frequency supply.