new sync motor mod tests

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

Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <FutureT-at-aol-dot-com>

All,

I modified still another motor to try the V cut idea.  This is now
an 1800 rpm motor (from a typewriter).  Rotor is 1 15/16" dia.  I first
filed four flats 7/16" wide, which is rather narrow.  I tested the
motor with a load disc rotor, and it locked at 93 volts, and unlocked
at 62 volts.

Next I changed the flats to V cuts, by filing them using a half
round file, a round file, and the edge of a flat file.  I gave the
rotor the cloverleaf look, like the commercial motor I looked at.
I made the V cut about 1/8" deep (a little deeper actually).
Unfortunately two problems occurred which makes the results
questionable; first, I could not remove the plastic fan, so I had
to break it off, second I slipped a couple times with the file, and
made the cuts wider than they were before in a couple of spots.
Anyhow the motor now locked at 98 volts, and unlocks at 35
volts.  So it took more voltage to lock, but this may be due to
the inadvertent widening, rather than the depth of cut... no way
to know now.  The lower unlock voltage may be due to the
absence of the fan.... no way to know now.

Next, I widened the cut by simply filing across the cusps of the
V cut, to form a flat area that was a full 1/2" wide.  Still narrow
for the rotor diameter.  Now the motor needed 108 volts to lock,
and unlocked at 45 volts.  It was definitely weaker now, and
this test is valid.  I didn't cut anymore because I figured the
motor would get even weaker.

Conclusions:  These tests suggest that even a little widening
of the flats/cuts can have a large effect on these small motors.
The deepening seemed to offer no advantage, but I did not try
an even narrower cut.  We don't really care much at what voltage
the rotor unlocks, but only where it locks.  This is because once
the rotor locks, there is nothing to make it unlock since the load
is constant for sync gap use.  Although, the use of the phase
shifter does alter this, since certain variac postions give a lower
voltage.  But in a normal setup, we want to maximize the lockup
torque, to permit the use of heavier rotors.  It is possible that
larger motors which have dead poles, require somewhat wider
proportional flats?  This small motor had no dead poles.  I did not
cut all the way through the conductor bars.

We will see what other's find when they try the V cut method.

The thing that really stood out in these tests though, was
the need for rather narrow flats or cuts.  Narrower than normally
used.  Again, the narrow cuts may be best only for small motors.

Maybe when the cuts are deeper, they *must* be even narrower?

I'm beginning to think that the flats/cuts should be the same
width whether the motor is 1800 or 3600 rpm?  (at least on
small motors with no dead poles).

John Freau