Re: building an rsg.

Hi Alan,

	I have made a few rotary gaps lately.  So heres my 2 cents...

At 01:51 PM 8/21/99 -0400, you wrote:
>Greetings all,
>I am considering building a rotary spark gap.
>I've got a sync AC motor out of an old LP turntable.
>I'ld have liked something a bit more powerful but I
>think that it will do.

Are your sure????  I would try and stall to it with a leather glove to see
really how much torque such a motor would have.  The spinning disk in a
rotary gap needs to push some air around for cooling and that takes a fair
amount of torque.  I tried a very small disk with small electrodes a few
months back that needed low torque but the electrodes quickly overheated
and failed.  Also your motor is probably NOT salient pole.  So if you want
it to be synchronous you will have to reset the timing every time it is
powered up.  Believe me the salient pole versions are worth the extra
effort right there...

I converted a 1/4 horse induction motor to a salient pole sync which works
great and I highly recommend it.  I simply used a electric angle grinder to
put flats on the rotor.  I "tried" to remove equal amounts of metal and
keep it balanced and was able to do very well with the grinder.  A fancy
machine shop could do a great job too but I don't think it is worth the
time, effort, or money if you have a grinder and try not to mess it up.

I would definitely stay with 1800 RPM and not go the 3600 RPM.  At 1800 RPM
things are much more forgiving and easier...

>I was going to make  disks from two 1/4" thick paxolin
>boards. I was considering using meccano parts for
>the spindle, using pulley wheels to secure the disks
>to the spindle. I would probably space the boards about
>1/2" apart to give a bit more rigidity.

I use replacement power drill chucks to mount the rotor.  Just find a big
bolt that fits in the end and clamp it to the motor shaft.  Mine seems rock
solid and has worked perfectly.

>I have several quite basic questions.
>1) how much difference will it make to the coil? At present
>I can get 4 foot arcs at 2kW with a static gap.

A sync rotary will make a BIG difference in my experience.  You can also go
the lager primary caps in the LTR configuration for even greater power.  My
humble 15/60 NST will do four foot sparks with 860 watts in.

>2) Is the project doomed because I only have a press drill
>and basic tools?

I hope not, that's all I have ever used. :-)  For the rotor I just made a
really good circle with a permanent ink pen taped to a compos.  Then I used
a belt sander with fine abrasive to grinder right up to the line on the
disk.  I was really amazed at how well that did.  I ended up truing it up
with Chip's nice lathe but it would have worked fine without that too.  I
think you really need a drill press But they have those drill attachments
to that could be lined up really good with a square...

>3) Can an RSG work with a horizontally spinning disc?


Plans for converting a motor to a sync motor are at:

Plans for a nice strobe light for playing with sync gaps is at:

A bunch of pictures of my salient pole sync rotary gap are at:
Perhaps you can get some good ideas from it.  It seems to work perfectly
for me...



>have fun,
>Alan Sharp.