Original poster: "David Rieben" <drieben@xxxxxxxxxxxxxxx>
Hi David, Mike,
From what Mike has said, it sounds obvious that he is experiencing
some kickback transients from his coil primary circuit. From my per-
sonal experience in this matter, it seems that the most draconian efforts
to suppress this effect are still sometimes thrwarted by the kickbacks
from the primary circuit, especially in large, powerful "pole pig" type
coils. Both I and list member Cameron Prince have experienced this
with our large coils and blew out a 1000 volt, 50 amp FWB rectifier
in this manner! Cameron also blew out one of the "legs" legs of his
3-phase, 440 VAC, 80 amp line filter from this. When I tried to run
my ASRG with 2 pair of stationary electrodes set 180 degrees apart,
the coils output was EXTREMELY choppy (probably actually firing
less than 20 % of the time!) and I ended up smoking my 1000 volt,
50 amp FWB rectifier brick from this. And I did have a small Corcom
line filter at the unfiltered rectified DC input to the motor but still blew
out the rugged FWB rectifier. Once I removed one pair of the station-
ary electrodes and operated with just one pair, the operation of the
coil dramatically improved and I haven't smoked any more rectifiers
although the one I now use is actually smaller (600 volt, 30 amp I believe).
I too have since added a 450 volt, 3900 uFd 'lytic cap in parallel with
the input to the motor to smooth up the input voltage a bit and I'm sure
this is also contributing to additional line filtering.
Also, more recently, I fired up my coil for the first time since redoing
my control panel and adding about 1.25 Ohms of ballasting resistance.
The cooling fan motor for the hot resistors had been working fine while
firing my big Jacob's ladder with the same control panel but when I
fired my coil with it, the motor in the cooling fan almost immidiately
started shorting out with a loud "popping" sound that tripped my 100
amp breakers! ARRRGGGH!! Anyway, I ended up having to run it
without the benefit of the cooling fan for July 4th but it still worked ok
and the resistors never really got to full incandescence even though I
fired it for several minutes non-stop at nearly full power for the "grand
finale" ;^0 There were several friends who came over to film it and I
may be getting a hold of this footage in the near future ;^)) Also, I
located 2 more fans on ebay (the only differnece between them and
my old defunct one is that they are 115 volt rated instead of 230 volt
rated). I will try to install both of these once I get them and we'll find
out if these new fans can hold up.
David Rieben
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Friday, July 07, 2006 8:12 AM
Subject: 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?