Starting circuit for synchronous motor

Hi All:

I have been determined to use a 3600 RPM synchronous motor that I pulled
from a Teletype machine for a rotary spark gap.  The motor is physically
smaller than most used for RSG's, about 3.5" x 3.5".  It just never seemed
right to have to have to use those quarter horsepower boat-anchors that
others use, just to make and break a connection.   Initial attempts to spin
up a 3/8" thk x 6.25" dia lexan disk with two copper pipe electrodes with my
small motor were unsuccessful.  The motor would fail to get up to speed and
just get very hot, hunting for but never achieving the 3600 RPM speed.

It helped somewhat if I boosted the applied voltage with a Variac, but still
it didn't always sync up, and the motor would get quite warm with the higher

I pulled out the original starting relay.  This device would energize the
start winding through the starting cap until the current through the main
winding fell below some threshold.  In place of it I built a circuit that
energizes a new 3-pole relay after a small, adjustable time delay.  One pole
energizes the start winding, still through the original starting cap, until
the relay energized.  The other two poles toggle the polarity of 120VAC
applied to a 24VAC stepdown transformer.  The secondary of this transformer
is wired in series with the 120VAC supply to the motor, so that it boosts
the voltage by 24V initially while the start winding is energized.  Then
when the relay switches, the voltage is reduced by 24V.  I've found that the
longer starting time plus the higher starting voltage allow me to
successfully spin up a larger disk/electrode assembly than the original
starting circuit would.  Plus, by reducing the voltage when it's running,
the current draw has dropped from its nominal 2 Amps to only 0.85 Amps,
while remaining in sync.

I will document this circuit on my web page in the coming weeks and I'll
post a note to the List when it's ready.  But first I want to get this gap
into action...

Regards, Gary Lau
Waltham, MA USA