Re: Starting circuit for synchronous motor

to: Gary

3/8th inch is too thick for these small motors.  I use only 1/4 inch x 7 in
dia max.


-----Original Message-----
From: Tesla List <tesla-at-pupman-dot-com>
To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
Date: Friday, August 13, 1999 7:10 PM
Subject: Starting circuit for synchronous motor

>Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
>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
>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
>it didn't always sync up, and the motor would get quite warm with the
>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
>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
>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