# Re: [TCML] SRSG question:

Austin,

Welcome to the list.  Yes, you probably need maybe a 40uF to 60uF cap,
or more, just guessing some here.

Yes, I use the resonant rise to find a suitable capacitor value.
I look for a 5V to 10V maximum resonant rise across the motor, over
line voltage.  To find the maximum resonant rise, it's necessary to
rotate the variac knob through it's range and monitor the motor
voltage.  At some point along the variac's range, the motor voltage
should rise, then it will fall again as you continue to rotate the knob.
But the maximum voltage should not be more than 5 or 10 volts
over the line voltage.  The system will also work with 0 rise, or with
15volts rise, but I like a 5 to 10 volt rise. If the capacitor is too small, you'll never get the resonant rise. If the capacitor is too large, you'll
get too much resonant rise.  So it's a trial and error process to find
the correct capacitor value, by connecting various capacitors, then
looking for the maximum amount of resonant rise across the motor
as the variac knob it turned.

I assume you have a 3450 rpm motor which gives 3600 rpm when
modified.  I assume also that you're using 2 spinning electrodes on
the rotor disc, for 120 bps.

Just using the variac without a capacitor may give a small phase shift
also.  The amount of  phase shift even with the optimal capacitor
will be only 90 degrees electrical and mechanical, so the motor will
need to be rotated (or the rotor rotated), to get the system in
the ballpark first, so the electrical shifting process will have enough
range.

http://futuret.110mb.com

Cheers,
John

-----Original Message-----
From: austin feathers <kartkid1@xxxxxxxxx>
To: tesla@xxxxxxxxxx
Sent: Wed, Aug 5, 2009 2:09 pm
Subject: [TCML] SRSG question:

Greetings from a new guy! I'm working on a 900 watt coil and recently
constructed a spark gap using a 3.5 amp, 1/6 hp general electric single
phase squirrel cage motor. I ground two flats in the rotor for salient-pole
operation, and it locks fairly well from 100v to 140 v on the variac. As
I've heard about in these lists, I tried making a phase controll circuit
with my 6 amp variac. I tapped the hotline input to the end of the winding on the variac, then took the output from the brush contact to the motor. I added a 250vac 20mfd capacitor in paralell between the motor and the variac
(the neutral line goes straight from the motor to the wall, with the
exception of the tap for the capacitor) I hooked it up, and i got the same
result i had when the motor was just hooked up to the variac, with the
acception that the variac now makes a mad buzzing noise when the motor is hunting, and a low hum when the motor locks. I'm assuming I need a larger capacitor? And I've heard about determining capacitor value from resonant
rise? Thanks!

Austin
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