Tesla Coils
JOHN'S  TESLA  COIL HOMEPAGE

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An example of a small synchronous rotary spark gap as used on my TT-42 Tabletop Tesla coil.
SRSG Electronic Phase Controller
Paul's TSSP Tesla Coil Research Website.  This site is the greatest for true Tesla Coil research.  Much of the theories at various websites is incorrect, confused, or simple fantasy.  Paul's website cuts though the nonsense, dispells the myths, and gives only the best proven theoretical models, backed up by extensive testing and measurements.
Above is a schematic for my new sync gap electrical remote phase controller.  The series variac gives a 90 degree range of electrical phase adjustment with the twist of a knob. (An 1800rpm motor will show a 45 degree mechanical phase range.) No more fiddling with mechanical phase adjustments, and trial and error phase adjustment runs... WOW. This circuit makes it a snap to adjust the sync gap phase while the coil is running. The phase should be first set mechanically to a ballpark setting, then the variac will provide a nice adjustment range.  This controller is for the new sync gap below, but it will work for other sync motors too.  It will be necessary to use a different sized capacitor for other motors. The variac must be set at minimum inductance when the motor is started, or a relay can be added that does this automatically at start up.  A  2 amp fuse should be used in series with the capacitor, and a 50k ohm, 1/2W resistor should be placed across the capacitor, as suggested by Terry Fritz (a larger motor requires a larger fuse).  To select the proper capacitor size, measure the voltage across the motor and look for a 3 to 5 volt resonant voltage rise maximum at some point in the variac's range. If the resonant rise is too great across the motor, then use a smaller phase shift capacitor.  If the resonant rise is too little or nonexistant, then use a larger phase shift capacitor.  Do not run the circuit without the motor in place.  The fuse prevents runaway resonance in case the motor open-circuits or becomes disconnected.  If your motor is supplied with a capacitor, keep it wired in place and add the phase shift capacitor also, as shown in the schematic.  Larger motors of 1/2HP or more may exibit a dangerous type of self-braking action, especially if the capacitor is too large.  However the circuit has been used successfully for motors up to 5HP, and perhaps larger.  The self-braking is just something to be aware of.  It's possible that the folks who reported this were using capacitors that were too large.  I'll research this more and update the information here.                                      Continued to the right, below,
Small sync motors such as the tiny Oriental brand motor require only a small phase shift capacitor of maybe 3 or 4uF.  Large 1/2 HP motors require a larger phase shift
capacitor of maybe 160uF.  The exact capacitor value required can be quite critical.  Try different capacitor values until the correct amount of resonant rise is obtained when measured across the motor as described above.  Be sure to find the maximum resonant rise by slowly turning the variac knob until this maximum is found.  The maximum resonant rise will not occur at the maximum variac knob setting.
To the right are two photos of my latest 120 bps sync rotary gap, which features a 1/20HP motor, four series gaps, and copper electrodes and holders.  The rotor is G-10, and the 3/8" copper electrodes are press-fitted in place.  The fixed electrode holders are 1/2" copper bars which hold the fixed electrodes by clamping action.  The electrode holders are supported by phenolic pedestals.  The base is oak with a pine platform for the motor.  The motor can be rotated in its cradle for a coarse adjustment, but fine phase adjustments will be made via the electrical phase controller described above.
24" spark staccato VTTC parts list and details for the schematic at bottom of page:

Reverse grid leads if coil fails to oscillate.

L1 primary form is 6" dia x 5" high, wind 23 turns 12awg pvc ins wire at one end of form.

L2 grid coil is 20 turns 24awg pvc ins wire, wind onto top portion of primary form.  Leave 1/2" spacing between the coils.

Plate chokes:  10 turns 14awg magnet wire wound on 30 ohm 20W resistor.

MOT: remove shunts.

R1  6.3k ohm, 75W.

C1  0.01uF 1kV mica

C2  0.001uF  3kV mica
To the left is a small spark gap Tesla coil I made a few years ago for a friend.  It's powered by a 9/30NST, and gives 9" sparks.  It uses a 4-gap static gap with silver electrodes.  The gap spacing is adjusted with wingnuts.  I keep the gap spacing close to protect the capacitors.  The primary is 14awg wire, about 17 turns, covered with a sheet of lucite which holds the primary in place.  The secondary is 3" by 9" wound with 28awg on a pvc form.  The toroid is 4.5" x 1.5" covered with aluminum foil (This was made before I started making spun toroids).  The capacitor is (2) 0.0025uF, 5kVAC doorknobs in series.  The coil was designed to be very low cost.  All of the parts were items I had lying around the house.  There are no protective circuits.
24" Spark Staccato VTTC Schematic below
My new zero-crossing staccato
controller circuit schematic and
Steve's staccato VTTC work.
Schematic notes to the right --------