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Re: ASRG blowing diodes
Original poster: David Speck <dave@xxxxxxxxxxxxxxxx>
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.
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?