Re: DC rotary gap? - A Working Example

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Kevin Ottalini by way of Terry Fritz <twftesla-at-uswest-dot-net>" <ottalini-at-mindspring-dot-com>

Greg:
    I found that the simplest DC setup for the ASRG is also the most robust ...
especially when you are throwing millions of volts around!
 
I use a small 24v DC motor that I found surplus and a small 48v transformer
followed by a small potted bridge rectifier (the kind with spade lugs).
(extensive testing proved that I can reliably run this 24V motor at up
to 50v with such light loads).
 
A picture of the motor and ASRG box (with a two-pole brass rotor) is here:
 <ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/rgap3.JPG>ftp:
//ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/rgap3.JPG
 
A plot of the measured BPS vs Motor voltage is here:
 <ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/motorBPS.jpg>f
tp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/motorBPS.jpg
 
I found that I didn't need to use any filtering on the DC output at all.
 
I run the mains AC through a small variac, then to the 48Vac transformer
for speed control.  The output from the rectifier goes directly to the
motor.  The total power to run the motor at max speed is about 40 watts.
 
Although it might be possible to run the rectifier directly off the
variac, I personally feel that the isolation is worth the $15 for the
transformer!
 
Here is a pic of the transformer and rectifier:
 <ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/motor_powersup
ply.jpg>ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/motor_po
wersupply.jpg
 
At stall speeds the motor will hum just a small amount, but the DC ripple
actually seems to help get the motor spinning at low RPM.  With a filter cap,
the motor took a little more juice to start running at minimum and would run
slightly faster (about 8 or 10 BPS) whereas without the filter cap it will
tick over at about 2 or 3 BPS.
 
I actually like the humm sound since it gives me a warning that I am
at the minimum RPMs ... and it isn't good to drive the coil at full
power with such slow BPS since it is almost guaranteed that the
coil will power arc during the slow dwell time, which can lead to
overheating the sparkgaps.
 
I suspect that light dimmers don't like inductive loads, and will tend to
eventually puncture in the presence of HV without protection.
 
The disk itself is very simple, just a 1/2" polycarbonate disk 4.5" in
diameter that has 8 or 16 holes drilled 3/8".  8 of the holes (every other
one) are pressure fitted with a hardened, 1.25" long stainless-steel wrist
pin which is available in bulk for under $1 each.
 
You can see a picture of an 8-hole 3/8" rotor here:
 <ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/newrotor.JPG>f
tp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/newrotor.JPG
 
and here is a pic of an earlier 8-hole 1/4" dia pin version but complete:
 <ftp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/oldasrg1.JPG>f
tp://ftp.mindspring-dot-com/users/ottalini/highvoltage/rotgap/oldasrg1.JPG
 
Ideally, the disk would be G10 or Phenolic and the pins would have an
allen set screw to lock them in place, but other than one accident I
haven't had any problems even with extended runs at 4kw. (I accidentally
powered up the coil one time without spinning the ASRG and the power arc
across one pin eventually softened the Polycarbonate around the pin
before I realized what was happening.  Then I just moved all the pins
to the alternate holes and was back up and running again in a few
minutes).
 
With heat, the pins swell slightly so the net effect is the warm pins
are actually held in place better than cold pins.  Nonetheless, it
is important that the pins are tight when pressed into the holes.
 
I balanced the rotors with a magnetically-suspended jig that they sell
at hobby shops for balancing propellers.  Two magnets suspend the disk
on a shaft with no contact, so the heaviest part of the disk will settle
at the bottom.  Then I just drill a little material out at the bottom
until the disk settles somewhere else and repeat the operation there.
 
When the disk is balanced, a light spin of the disk will run for many
minutes and will eventually stop, but at random locations.
 
To make all the pins coplanar, since all the pins are the same length,
I just set the rotor on the pins on a flat table top and use a small
hammer to tap the center of the rotor until all the pins are even.
Then (if needed) turn it over and do the same thing, but to center the
pins in the holes.
 
Best,
    Kevin
        
----- Original Message ----- 
From: "Tesla list" <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>
To: <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>
Sent: Wednesday, April 25, 2001 4:14 PM
Subject: DC rotary gap?

> 
> Hi, I am going to be working on a DC mot-powered coil soon, what 
> motor specs should i look for to power my rotary gap? I will be 
> looking through a surplus shop for my motor. Do i just throttle the 
> motor with a variac or dimmer, or is some electronic controller 
> required? I am assuming many electrodes are also used on the disk to 
> enable several thousands of bps. I seem to remember Kevin O. saying 
> his rotary gap did a few bps to thousands of bps.
> Here's to keeping the smoke in,
> Greg
> -- 
>