Thanks James. Don't think it was noted in my original post that the rectifier is inside of my cabinet, so only the motor is under the coil. To date, I've not observed and strikes to the components under the coil. Induced voltages is another thought as you've noted, and one reason I shielded the power cord going to the DC motor.
Terry
From: James <oldmbkr@xxxxxxxxx>
To: 'Tesla Coil Mailing List' <tesla@xxxxxxxxxx>
Sent: Tuesday, December 12, 2017 6:36 PM
Subject: Re: [TCML] ARSG bridge rectifier failures
I had the same problem. Took a bit of sleuthing to run it down. Spell that a bunch of dead rectifiers. One problem was flashover to the motor housing, more distance solved this one. Next was corona strikes on the primary. Raised the toroid, did some good. Finally put pexli on top of primary. This pretty much fixed the rectifier problem. Unless you had 500KV diodes, They are going die from hits. Oh yes, don't forget about induced voltage also. lol James
-----Original Message-----
From: Tesla [mailto:tesla-bounces@xxxxxxxxxx] On Behalf Of Terry Oxandale
Sent: Sunday, December 10, 2017 21:22
To: Tesla Coil Mailing List
Subject: Re: [TCML] ARSG bridge rectifier failures
I'm having an issue with the bridge rectifiers I'm using to power a variable speed DC motor for the rotating spark gap. When the coil is not energized (but motor is spinning), the diode stays cool, and appears to be reliable. As soon as I energized the coil sufficiently to operate with breakout, the rectifier gets a little warm, but appears to remain reliable. With more coil performance, I notice more heating of the rectifier. It appears, after some testing, that the lifespan of the rectifier is inversely proportional to the power put into the coil. I have a voltmeter and ammeter to monitor the power to the DC motor, and there is nothing to indicate a problem (but these are analog meters, and may not be sensitive enough to see everything I'm looking for).
So far, the only variable is the coil's output (performance), so that leads me to believe something is back-feeding or being induced into the motor's power cord and back to the rectifier. The motor only draws about 1 or 2 amps through 90 VDC at speed, yet the rectifier is rated at 30 amps and 600 volts. I don't run the motor at the full 90 VDC because the coil operates best at a lower RPM, and hence a lower voltage to the motor. The rectifier is fed through a small variac, whose output then passes through the rectifier. The rectifier does have an LC filter to dampen the full wave output of the rectifier. I don't know if it's the cheap Chinese Ebay bridge rectifiers or not, but am considering building one out of some robust diodes that I've had for years. Any thoughts please?
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