RE: better sync motor mods?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi John,

After digging one of the rotors that I had been experimenting with out of
the scrap pile, and removing the rotor from the first SRSG that I built, and
looking at them without the cloud of several months of faded memory, I find
that I have not actually cut all the way through any power bars. I now
remember how the story went, as it were. I was going to build my second
SRSG, and not really sure just how I wanted to do it, and the idea having
been reported that one might be able to adjust the firing angle by using a
variac to adjust the voltage to the motor, and having had the displeasure of
trying to find the optimum firing angle on a SRSG that could only be
adjusted with the power off by moving the disk in relation to the rotor via
slots in the disk, I thought "what an interesting Idea", and proceeded to
try to figure out just what the best way to determine the optimal amount of
material to remove from the rotor in order to achieve that goal. That is
when I thought, having heard that someone used a motor as an adjustable
ballast, and thinking that a variable inductor could be made by moving a
core in and out of a coil, and reasoned that removing material from the
rotor would in effect lower the inductance of the motor winding thus
increasing the current, began to think on how to divide the rotor into
"poles" without removing so much material. Thus began a thought experiment,
that clouded by memory, resulted in that statement. Sorry about that.

My first sync rotary has a rotor approx. 3.125" in diameter. It is a six
pole motor, PSC, 1200 RPM modified. The flats I cut are approx. 3/4" wide,
the uncut "poles" approx. 7/8" in width. I have not tried it with a variac
to see how much the firing angle could be changed by changing the voltage.
At rated voltage, 240VAC, with the disk and six electrodes installed, it
pulls just under the 1.3 FLA. on the nameplate. It started life as a 1/4HP
condenser fan motor.

The second SRSG I built from a 1/3HP 3450 RPM A.O. Smith motor. It is split
phase. This motor was used for years as a power source for a bench mounted
wire brush. This is the motor I tried the slots on. I used a 1/4" solid
carbide end mill to cut two slots as deep as the aluminum band around the
edge of the rotor. Put it all back together and tried it for sync. Did not
work. Cut the slots a little deeper, still didn't work. Then I cut the slots
into flats, and it worked. Went into sync at 95V. Then I faced a pulley from
an old air compressor to use as a hub, drilled and tapped holes in it to
receive mounting screws, and installed the 7" dia. by 1/2" thick G-10 disk
with two 1/2" by 1 1/4" brass flying electrodes. It required 140V to lock.
Fearing that I had removed too much material, I took my first amp draw.
Surprised to find it barely registered in the meter. Then I looked at the
nameplate, and discovered that the 115/230 volt motor was set to the high
voltage tap. I had put the power wires back in the same place they came
from, it had been running on 120 for years, and I was not sure whether to
trust the nameplate or not. I switched the wires to the LV configuration per
the nameplate, and reapplied 120V. It spun up like a rocket ship. Amp draw
was well under FLA. I switched the wires back to 230V configuration and got
out the little 240V variac. Then I checked the thing out, and found that it
was well under the FLA for that mode as well. Next I took the motor back
apart and cut the flats deeper, tested, and repeated the process until I had
gotten the motor to pull 80% FLA at 240V in and the disk with the electrodes
installed. It would lock at just under 120V from a dead stop. This motor
will give 40 degrees of variation in the firing angle over the available
range of 140V min. to 260V max. of the variac. Stop at 140 for safety from
going out of sync. and 160 for not exceeding FLA. I can't give you any
dimensions off this motor as I would have to take it apart to make the
measurements.

I have not tried your phase shift trick on this motor yet, but am planning
to soon. I am guessing that the optimal cap value will be 1/4  of that
required for a motor running on 120V. I am also giving thought to combining
the method, as I have a couple of extra variacs. One to adjust the voltage,
the other to shift the phase. Do you have any thoughts on that?

deano

> >
> >  I had a thought along the same lines a while back. This was
> probably in May
> >  of last year, anyway just before I built my second sync rotary. I had
> >  thought that rather than cutting a flat, why not mill a groove
> in the rotor
> >  that is deep enough to cause the magnetic field which is
> induced in it to
> be
> >  separated into "poles" without removing so much material as to
> cause the
> >  current to increase as much as it would with the flats. I also
> thought that
> >  the torque might not be diminished as much. I tried this on
> one motor, but
> >  it would not lock in sync. once I had turned the grooves into
> flats of the
> >  same depth, it worked as expected. I don't know if it would
> have worked if
> I
> >  had made the grooves deeper. As far as cutting the current
> bars goes, it
> >  seems to me that some have to be cut to get the thing to sync. I had
> >  wondered if this might not be more the mechanism than the iron removal.
> >
> >  later
> >  deano
>
> Deano,
>
> Thanks for that report.
> Whenever I've modified motors, I've never had to cut all the way
> through the current bars.  But it sounds as though you have?
> How wide were the grooves you cut, relative to the uncut
> portions?
>
> John Freau
>
>
>