RE: question about driving a DC sync spark gap

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Leigh Copp" <Leigh.Copp@xxxxxxxxxxx>

Miles:

Iif you reduce your motor speed too quickly, the back emf from the 
motor as it coasts down will most certainly be higher than the line 
voltage from your variac, and can cook your diodes. This will also 
occur to some degree due to the motor inductance if you turn up the 
voltage too quickly. A series inductor will exacerbate this effect.

Consider a freewheeling diode across the rectifier output to help 
with the latter effect.

A 300V rating on your diodes is not a very large safety factor 
either. You need to be at the very least, twice the peak voltage from 
your Variac, which will be 169 volts at 120 volts RMS input. Your 
current rating is huge overkill. Personally, I use 1000V diodes at 
the very least just to supress 24 volt solenoid coils, because they 
are cheap and readily available. A 30-40A, 1200V bridge rectifier is 
usually easy to find.

For example:

http://www.irf.com/product-info/datasheets/data/gbpc2502a.pdf

This part is in stock at Newark and costs $3.31 USD. It is rated at 
35A, and 1200V. Not too sure what your motor current is, but at that 
price, it would be a very cheap experiment. Larger bridges are 
usually three phase, but you can build one from 100A studs at 1200V 
if you need to (probably much more expensive however).

BTW: Another suspect in your failures, are the spikes due to the 
commutator on your DC motor. As the armature rotates and the brushes 
sweep across the commutator segments, you have some current 
discontinuities that will ring up your dc supply voltage. It depends 
on how many poles the armature has, and the overlap on the brushes. 
In any case, this further emphasizes the need for a higher voltage 
rating on the diodes. Matt's 400V diodes mentioned below are probably 
the minimum voltage rating you can get away with, at a 120V line input.

Cheers,

Leigh



-----Original Message-----
From: Tesla list [mailto:tesla@xxxxxxxxxx]
Sent: Thu 12/10/2006 10:18 PM
To: tesla@xxxxxxxxxx
Cc:
Subject: Re: question about driving a DC sync spark gap



	Original poster: "Matt Whitman" <teslacoiler@xxxxxxxxxxx>

	Hi, I use a DC motor on my async rotary spark gap. It is a 110v 10,000RPM
	1/2Hp motor. I am just driving it using a variac and a bridge rectifier (not
	sure the rating but probably somewhere around 25A 400v) with 200v 470uF
	filter cap. Power levels on my coil are somewhere between 8-10KVA and on
	average I get 8' arcs.  http://72.228.95.56/whitek/8inch.htm   I do my best
	to keep arcs away from my power feed lines. I don't run my coil that much,
	but I haven't had any problems with the rectifier.

	Matt Whitman
	teslacoiler@xxxxxxxxxxx
	www.electronsgonewild.com
	KC2IEV



	----- Original Message -----
	From: "Tesla list" <tesla@xxxxxxxxxx>
	To: <tesla@xxxxxxxxxx>
	Sent: Wednesday, October 11, 2006 11:08 PM
	Subject: question about driving a DC sync spark gap


	> Original poster: "miles waldron" <mileswaldron@xxxxxxxxxxx>
	>
	> 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.
	>
	>
	>