SRSG Dwell Controller 1/4HP-1800RPM

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

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi All,

I have been working on a version of John Freau's sync gap dwell 
controller.  This is pretty much what I have decided on:

http://hot-streamer-dot-com/TeslaCoils/Misc/SRSG-Dwell-Controller.gif

It is based on John's design at:

http://members.aol-dot-com/futuret/page3.html

Some things I am doing differently are:

F1 - I put in a 5 amp slow blow fuse to protect everything and break the 
circuit if the variac is in a bad spot when the power is applied (starter 
windings stay on too long) or if no motor is connected (L1-C1 60Hz 
resonance).  The fuse value may vary depending on the varaic, cap, and 
motor, but this is what worked for me.  I am using this fuse in place of 
the capacitor fuse in the original design now.

K1 - I am using a 25 amp 480VAC zero crossing solid state relay to control 
power to the circuit.  This eliminates big current spikes at turn on due to 
the cap suddenly charging.  I could get close to 150 amp spikes without it 
if I played with the power switch.  That causes bad arcing at the variac 
and switch.  The relay also stops voltage spikes at turn off since it waits 
for the current to drop back to zero.  the relay only needs a very small 
heat sink at most.  Turn on current now looks like this as the motor spins 
up and then the starter windings fall out:

http://hot-streamer-dot-com/temp/030607-01.gif

The motor starting current is pretty high at about 25 amps RMS but that 
does not hurt anything.  The relay needs to be rated at 240VAC+ since the 
cap may keep the voltage high during turnoff allowing twice line voltage 
across the relay. Unfortunately, these relays run about $55 new :-p  So one 
may have to use just a big high current 240VAC switch if you can't spend 
that much.

Z1 - This is just an 1800V MOV that keeps the voltage across the variac at 
a controlled level if the motor is started with the variac turned up a 
bit.  I blew my 27XT meter up figuring this one out :-(  The high current 
at start up can drop a lot of voltage across a few winding of the variac 
and auto transformer action can transform that voltage way up at the far 
end of the variac.

R1, Z2 - If the fuse blows due to no motor connected, C1 can hold a very 
high charge.  R1 is a big power resistor that will drop the voltage in 
three seconds no mater what.  Z2 limits the voltage on the cap and clamps 
it if the L1 - C1 goes into 60Hz resonance.  It also helps the fuse to blow 
in that case.

C1 - I found that the value of C1 seems to depend on the varaic.  Both my 
motors (1/4HP 1800RPM) run with the same cap value even though they have 
pretty different current draw.  As in John's design, the cap value should 
be selected so that the output voltage stays pretty stable throughout the 
variac's adjustment range.

Right now, the actual circuit is a little rough ;-)

http://hot-streamer-dot-com/temp/030607-02.jpg

But all seems to be working well and things stay "tame".  Resonant circuits 
like this can be pretty goofy, but I "think" I have it under control ;-))

Cheers,

	Terry