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RE: Rotary Spark Gap - Construction Questions
Ross,
A "Universal" motor has brushes. When searching for a used one look for
large power tool motors, washing machine motors, etc.
If you are going for a new one then check out Dayton's 1/2hp model
http://www.grainger-dot-com/cgi-bin/item_detail?1847044Y2M145
They also make a 1-hp model
http://www.grainger-dot-com/cgi-bin/item_detail?1847044Y2M191
Regards,
Brian B.
-----Original Message-----
From: Ross Overstreet [mailto:ross-o-at-mindspring-dot-com]
Sent: Thursday, July 27, 2000 6:02 PM
To: Tesla List
Subject: Rotary Spark Gap - Construction Questions
Hi Gang,
I have a new project on the drawing board. I want to build a 10"x45"
secondary, PT powered coil that will be operated primarily in DC mode. I
would also like to run it as an AC coil, but I'm willing to do several hours
of work to reconfigure (I don't care to do all the initial design work so
that it will be an easy swap-over. I don't mind having 2 sets of caps, 2
rsgs, etc).
My main design challenges right now are the rotary spark gaps for a large
system.
----ASRG----
I'm looking for nice speed controlled gap from 0 to about 800 breaks per
second. I'm half-way basing my desgin Kevin Ottalini's DC coil that does
well all the way out to 700 bps. After reading lots of TCML archives, it
appears that the ideal motor for such a task is a 1/2 to 1 HP "Universal
Motor" that apparently can be run on either AC or DC. Chip mentions that he
uses one of these on his gap
http://bhs.broo.k12.wv.us/homepage/chip/details.htm
I don't know anything about this type of motor. Can someone suggest
sources/costs for such a motor? I will probably wind up using a phenolic LE
rotor with 3/16" tungsten rod electrodes. Looks like 6000 RPM with 8
electrodes can get me the 800 bps I desire. Don't worry with sending the
usual safety alerts - all will be cut on a good mill, statically balanced,
contained, etc. I can use any advice/links on high speed ASRGs and
universal motors.
----SRSG----
I also plan to build the typical SRSG using a design similar to the ones
shown in the links below. This design seems to be what many of the
professionals and big-coil hobbyists have settled on.
(TTR design) http://www.ttr-dot-com/rotary_spark_gap.htm
(*) http://www.foxtail-dot-com/Teslathon/D1/DSCN0007.JPG
(*) http://www.foxtail-dot-com/Teslathon/D1/DSCN0006.JPG
Brian Basura design: http://fp2.hughes-dot-net/brianb/rsg.htm
(*=TTR design on display SoCal Teslathon 2K, photo by Jody Kravitz)
There is an abundance of info on the web about this type of design, but
there does seem to be one magical problem - what sort of dwell time works
out best. (ok, ok, I don't want to start the mechanical dwell not equal to
electrical dwell argument so from here on out I'll just call it electrode
overlap) I know that this is power throughput dependant, but maybe we can
agree on some ballpark figures for coils driven from PTs or small pigs.
This question comes up after seeing one of my buddies almost attack his SRSG
with a sledge hammer. The initial design used .375" electrodes all the way
around. This made the electrode overlap a little over 1000us. The coil
would barely run. After changing his rotating electrodes to 1/8" and
offseting the stationary electrodes a little, he got the overlap down around
500us. This made a world of difference in his coil! Now Brian's coil is
one of the hottest 6.5" sec, PT powered coils out there!
Has anyone else stumbled across electrode sizes or overlap times that worked
really well on this size system? Are there upper/lower limits that should
be used early in the design stages? I have read that 1/8" stationary
electrodes don't work too well on 5KVA or above coils due to overheating.
Unless I'm doing my math wrong, most of the well performing gaps that I
reverse engineered from the photos on the web all have electrode overlap
times around 500us. Ideas?
Making arcs in SoCal
Ross-O
http://users.better-dot-org/roverstreet/