Spark Gap Construction Details
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Motor
The motor I chose for this SRSG is a 1/3 HP 3450 RPM split phase (capacitor start/induction run) made by A.O. Smith. It is rated to run on 115/208-230. I use the high voltage setting. The nicest thing about the A.O. Smith motor is that it can be taken apart and put back together rapidly and easily. This proved to be helpful while doing the conversion to synchronous operation. It allowed me to take a small amount of material at a time, test the motor under operating conditions, take a little more material off, then retest etc. till I was able to get the best compromise between torque and lockup. I was able to get the motor to
lock at about half rated voltage using this method. This results in about 40 degrees of adjustment to the firing angle by the simple method of adjusting the voltage to the motor with a variac. The motor is mounted in a cradle with resilient bushings at each end. This allows the position of the motor to be adjusted
mechanically with respect to the firing angle.
Hub
The hub I am using is actually a pulley for one of those ribbed flat belts. It came off of a 5 HP air compressor motor that came from Sears. It is cast iron, and was a bit wobbly, so I had to face it in the lathe. I then bored three holes 120 degrees apart at a 1" radius to mount the rotor disk to. The holes are tapped for 5/16" - 18 bolts.
Rotor
The rotor is made from 1/2" G-10 and is 7" in
diameter. It is one of two disks cut from a 12" X 14" piece of G-10 I purchased from Regal Plastics
of San Antonio, Texas. This piece came to less than $50 including shipping. The disks were cut using a "fly cutter" designed for cutting wood. G-10 is hard stuff, and I had to resharpen the cutting bit several times during the long slow process. The original 1/4" hole at the center of the disk was used to reference the three 5/16" holes at a 1" radius 120 degrees apart for mounting bolts, and the two 3/8" holes at a 3" radius 180 degrees apart used to mount the flying electrodes. The center hole was then enlarged to accommodate the 5/8" motor shaft. The rotor was then mounted to the hub with 5/16"-18 X 1 1/4" grade 5 (automotive grade) bolts with flat washers. The bolts were torqued to 20 Ft Lbs. The rotor runs true, and there is very little end play in the motor.
Flying electrodes
The flying electrodes are machined from 1/2" brass bar stock They are 1 1/4" long and have a 3/8-16 hole in the mounting end, and are rounded to a hemispherical shape on the business end. The flying electrodes are mounted to the rotor by 3/8-16 X 1 1/2" silicon bronze bolts. There are copper flat washers on both sides of the G-10 rotor. The flying electrodes are connected together electrically by a copper strap 1" wide and .030" thick. The copper strap goes between the copper flat washer and the rotor on the electrode side.
Hub Cap
A hub cap is cut from HDPE and depressions milled into it to fit over the heads of the bolts that mount the rotor to the hub. The hub cap is glued on to the rotor with clear silicon RTV. The hole in the center of the hub cap is
filled with the same RTV sealant. Once the RTV has cured, the copper strap is placed over the hub cap and the electrodes installed. The copper strap should be tight
enough to hold the hub cap in place without the RTV which is used for its electrical insulation properties only.
Frame
The mounting frame is made of angle iron and flat plate. A plate is provided with slots matching the holes in the motors mounting cradle which allows end to end adjustment for setting the gap clearance. Additional mounts are provided for the attachment of an explosion shield and the G-10 plates that are used to mount the stationary electrodes. A mechanical lug is provided as a convenient way to attach a ground conductor.
Stationary electrodes
The stationary electrodes are made in the same manner as the flying electrodes with the exceptions that they are 2" long and 3/4" in diameter. The business ends are rounded to the same 1/4" radius as the flying electrodes giving a 1/2" diameter ball end. There are longitudinal grooves around the periphery of these electrodes 1/8" wide by 1/8" deep providing "fins" to improve cooling. This seems to be entirely unnecessary, however, and will not be repeated in the future. The stationary electrodes are mounted to G-10 plates left over from cutting the rotor disks.
Operation
The first step in operation is to set the gap clearance. I set mine as close as possible without touching when the motor is pushed toward the stationary electrodes in its cradle mount with the resilient mounts. There is not much
movement and so I am able to set the gap to 0.020" each for a total gap spacing of 0.040". Then the position of the motor needs to be set. I use a variac to adjust the firing angle of the gap during operation. I start out with the variac set at 160V as this seems to be the middle of the adjustment range. I then use trial and error to find the
best spark at a reduced power level. One nice thing about using a variac to adjust the firing angle is that at this stage of the game you can tell which way the motor needs
to be turned by adjusting the variac and noting which way (advance or retard) gives improvement, then moving the motor in that direction. Once the optimum firing position has been set, it is time to crank up the juice!