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Re: ASRG blowing diodes

Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>

Mike,
Do you have any filtering on the output of your bridge? Was the bridge hot when it died? It is likely that you are seeing induced spikes on the leads of your 24V bridge which are in excess of its 200 volt nominal rating. These may come from the motor itself, as it slows down and acts as a generator when you reduce the input voltage, or the noise may be picked up from the TC itself. I'd suggest putting a beefy electrolytic filter cap on the output of the bridge, also bypassed with a parallel high frequency rated cap as well, for the RF transients. Perhaps a single spare MMC type cap would do the trick. A similar RF rated AC filter cap on the input side of the bridge wouldn't hurt, either, to limit RF picked up between the transformer and the bridge. A beefy, stud mounted reversed biased diode across the motor leads (like the reverse protection diode on a DC relay coil) would shunt the current generated by the motor when it winds down. I know that the bridge also acts like a reversed biased diode, but you wild have a 1.2 volt forward drop for the bridge diode pair, as compared to a 0.6 volt drop for the dedicated shunt diode. This would move some of the waste power dissipation away from the bridge, and into the (hopefully well heat sunk) shunt diode. It would not dissipate no power during acceleration or constant velocity running. I wonder if you could just run the DC output through a Corcom type EMI filter, to block the high frequency spikes. I know that these are intended for 60 Hz AC use, but it is possible that their internal inductors might cause interesting oscillations. Perhaps some of the EEs on the list can comment on this.
Your bridge may also be somewhat underrated. Though the rated running current of the motor is 1.5 amps, it looks like a dead short to the bridge when it is at rest, and you could easily be pushing more than 20 amps through your bridge to accelerate the motor and its rotor from a dead stop. In the old days, big DC motors in industrial and transportation applications had a motor starting device that would progressively remove series current limiting resistors from the motor supply lead as the motor came up to speed. Those design principles still apply today. It would be instructive to watch a DC ammeter in series with the bridge output to the motor. You might be surprised at how much current you draw when you start up the motor. An 800 volt, 40 amp bridge wouldn't cost a lot more than your current unit, and might last longer, but you still have to watch the startup current. 

HTH,

Dave

Tesla list wrote:

Original poster: "MIKE HARDY" <MHARDY@xxxxxxxxxx>
I recently put a AR propeller gap on my 6", 4 kVA coil. It performs very well, Had some 9 foot ground strikes tonight. I run a 28V, 1.5A, 3600 rpm dc motor. Powered by a 6A variac into a 120 to 24 V 5A step down tranny into a full bridge rectifier (20 A, 200V). This should easilly run this, and it did for many hours. Varying the speed really does some interesting things to the character of the streamers! I decided to run the 24 line to the motor thru some flexible conduit that I grounded to RF ground (in case of streamer strikes). After a short time on the 4th, it poped the 2 A slowblow fuse, then stopped all together. I found that half the bridge shorted, overheating the tranny some. Replaced the bridge, ran it for 3 hours (not hooked to the coil) at nominal line voltage, and all was fine. Ran the coil again tonight, and after a few minutes it blew again! Oh by the way, I didn't ground the flex conduit this time. How do others deal with this, is my problem unique?