RE: SRSG motor modification

["Colin Dancer" <CMD-at-datcon.co.uk> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi Garth,

Could you give a bit more detail about your setup?  The key factors are the
width of your flats, the diameter of the rotor, the motor's speed and power,
and the dimensions of your disc.

I've modified a couple of bench grinder motors which sound like they have a
similar winding pattern, and found that I needed 2 x 15mm flats on an ~40mm
dia rotor. In terms of power, my 1/4HP motor will easily lock a 5 1/2"dia x
1/16" disc at 3000RPM with 4 x 4mm bolts as flying electrodes.  For a
thicker 6" x 1/2" disk it will only lock if I initially boost the voltage to
260V using a variac and increase the capacitor to 4uF from 2uF.  I've tried
making the flats a bit wider (I've gone up to 22mm in 2mm increments) but
the torque is if anything slightly lower.  

Last week I modified a 1/2HP 3000RPM bench grinder, again with 15mm flats,
and this locks with no problem on a 6" x 1/2" disk including 4 rather hefty
6mm bolt flying electrodes.  

It's amazing how much extra drag the 1/2" increase in diameter and thickness
generates, but I guess it makes sense as the formula for drag is ^2 for RPM
and ^3 for d.

I know running reducing RPM isn't an option with this motor, but have you
tried a smaller disk or even running the motor without a disk and looking
for lock using a mark on the shaft?

Hope this helps,

Colin.

P.S. There's a good photo of my altered rotor at
http://www.pyrochrome-dot-net/tesla/

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: 06 August 2000 21:57
To: tesla-at-pupman-dot-com
Subject: SRSG motor modification


Original poster: "Garth van Sittert" <garth-at-mediasupplies.co.za> 

I recently modified a 1/4 HP capacitor start; approx. 2800 rpm motor to make
it
synchronous.  I ground two flats across the 'dead poles' on the armature
according to motor1.gif etc...  However I must have done something wrong as
it
is not synchronous.  Using a neon as a timing light the blur only comes into
focus a couple of seconds after power off when it is coasting down.
 
The motor only has 16 poles so I was't 100% sure on the modification
compared
to the instructions referenced above which I have seen mentioned a couple of
times before on this list.  The starter windings seemed to ocupy much more
pole
space than the ones shown in the diagrams and are interwoven with run
windings
as well, so I thought rather than grind too much.....