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Re: question about driving a DC sync spark gap

Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>
The Scitech coil that I upgraded uses a DC motor on around 200V. In fact they did blow a couple of bridges to the motor if I recall. I would install MOV's across each diode leg and snubber resistors/capacitors rather than straight capacitors. And use a mains ground shielded line to completely enclose the wiring to the motor but expect significant RF currents to pass down this which may capacitatively couple to your DC line. Mains ground the motor too. And I would use cheaper diodes. If you blow them it will be due to voltage spikes not overcurrent unless you have big caps after the diodes. I would be careful with chokes unless you can dampen any oscillation with suitable resistance in the circuit. Terry has clearly shown that simple chokes may aggravate a situation and are not protective hence the "Terry filter" doesn't contain them - just MOV's, R and C. 

Peter



Original poster: Esondrmn@xxxxxxx
In a message dated 10/11/2006 8:19:28 PM Pacific Daylight Time, tesla@xxxxxxxxxx writes: 
Dear All,

Is anyone using a DC motor to drive an async rotary spark gap? If anyone is
doing this, how is your variable DC power supply built? We are having
consistent CATESTROPHIC failure of the power supply diodes. Not the motor.
The motor is always fine, and the variac is always fine. We are using huge
giant diodes (300V @400A) and they are frying instantly. We have tried
chokes in series, caps in parallel, caps in series, but nothing seems to
help.

Please help before we shoot the DC motor with a 1000 watt CO2 laser, and
rebuild our spark gap using a variable AC motor.

Background:. This is an 11" OD coil 57" tall #18 wire, 15kva pig, 11 to 15
foot arcs.
I have used a variable speed motor to run an async rotary gap. It was a series wound motor (similar to an old sewing machine motor but much larger). Just connect the field and armature windings in series and use a variac - running off AC, no diodes required. 

Ed Sonderman