First tesla coil building / Questions before ignition

[Nicolas]

Hi everyone,

After a Wimshurst machine long time ago, and more recently a Van der Graaf self inducted generator (32 cm sparks for now, but still in tests and improvements, I can post some pictures in the dedicated forum), I begin to gather all needed components for a Tesla Coil.

I would like to check with you if everything seems correct, and will ask also few questions…

Power supply
NST 10 000 V 100 mA with external GFI that I have easily removed. 50 Hz 240 Volts input current. Tested already with a Jacob Ladder, works fine.

TC dimensions
-   12 cm diameter PMMA tube for secondary
-   1367 turns, AWG28, 0.42mm diameter with insulator , gives 57cm secondary length
-   Primary : 6mm copper tube, 30° angle, JavaTC gives me resonant circuit for 6.2 turns, I will put 10 turns (is it enough or too much ?)
-   Topload : toroid diameter 50cm, thickness 12.5cm


Teslamap and JavaTC calculations
-   MMC : optimum larger than Resonate Capacitance with Sync rotary gap (but I perhaps think using an async motor, still right to use this Cap ?) : 83nF
-   Resonant frequency (both primary and secondary) : 154 kHz

Can you confirm this design is OK ?


Other choices :

-   Spark Gap
I own a lot of high performance brushless DC motors with ESC (electronic speed controllers) and adjustable speed, both inrunner with gear or outrunner with low kV (250 RPM/volt). My Van der Graaf generator is powered by one of them. They are very small (compared to synchronous motor often used for spark gaps) but powerful (500 to 1000 watts with efficiency above 90%).
So I could use one of them, with ESC adjustment possibility, in order to find the good RPM and keep the possibility to adjust and experiment.
It seems this motors are not very common for Tesla coil spark gaps. Is it a wrong choice ? Too much difficulty for a first TC ? Is there any risk to run the spark gap at too low or too high RPM ? I read risks of beating and surging, but I do not understand the exact meaning. So perhaps better to use a synchronous motor for rotary spark gap first ?

-   I also own a VARIAC (2000 VA – 0 to 250 volts) for the NST. Can I use it to manage the energy sent to the tesla coil ? What will be the difference between the effects of spark gap RPM adjustment and VARIAC effects ?  Both solutions are compatible ?

-   For the MMC, I plan to use 17 (serial) and 3 strings of Kemet J46 series (https://api.kemet.com/component-edge/download/specsheet/R76TR347050H3J.pdf) : 0.47uF 1600 V
Total 27.2 kV and 82.9 nF MMC cap

Is it a good choice ? I do not see too much of this caps in possible choices for tesla coil…



Thanks a lot for your help and advices.
Nicolas

[Mads Barnkob]

Hi everyone,

After a Wimshurst machine long time ago, and more recently a Van der Graaf self inducted generator (32 cm sparks for now, but still in tests and improvements, I can post some pictures in the dedicated forum), I begin to gather all needed components for a Tesla Coil.

Hi Nicolas and welcome to HVF! I will answer some of the questions. SGTC and RTG are not my expertise. Looking forward to see your post about the Wimshurst machine

I would like to check with you if everything seems correct, and will ask also few questions…

Power supply
NST 10 000 V 100 mA with external GFI that I have easily removed. 50 Hz 240 Volts input current. Tested already with a Jacob Ladder, works fine.

TC dimensions
-   12 cm diameter PMMA tube for secondary
-   1367 turns, AWG28, 0.42mm diameter with insulator , gives 57cm secondary length
-   Primary : 6mm copper tube, 30° angle, JavaTC gives me resonant circuit for 6.2 turns, I will put 10 turns (is it enough or too much ?)
-   Topload : toroid diameter 50cm, thickness 12.5cm


Teslamap and JavaTC calculations
-   MMC : optimum larger than Resonate Capacitance with Sync rotary gap (but I perhaps think using an async motor, still right to use this Cap ?) : 83nF
-   Resonant frequency (both primary and secondary) : 154 kHz

Can you confirm this design is OK ?

You could double check the calculations with https://kaizerpowerelectronics.dk/calculators/helical-coil-calculator/ and https://kaizerpowerelectronics.dk/calculators/mmc-calculator/

-   I also own a VARIAC (2000 VA – 0 to 250 volts) for the NST. Can I use it to manage the energy sent to the tesla coil ? What will be the difference between the effects of spark gap RPM adjustment and VARIAC effects ?  Both solutions are compatible ?

Spark gaps are usually set for a firing voltage, so lowering the input voltage on a variac might make the RSG fire in a unstable manner. RSG coil builders are welcome to correct me on this

-   For the MMC, I plan to use 17 (serial) and 3 strings of Kemet J46 series (https://api.kemet.com/component-edge/download/specsheet/R76TR347050H3J.pdf) : 0.47uF 1600 V
Total 27.2 kV and 82.9 nF MMC cap

Is it a good choice ? I do not see too much of this caps in possible choices for tesla coil…

I have another similar Kemet capacitor in my good capacitor list: https://kaizerpowerelectronics.dk/tesla-coils/drsstc-design-guide/good-mmc-capacitors/ and the one you linked is actually MUCH better. It is closer in performance with the CDE 942C capacitors than the other Kemet. You should be safe and sound to use that!

[Nicolas]

Thanks Mads for your answers !
I will put pictures of my Wimshurst Machine and Van Der Graaf generator soon in dedicated forum.
For the Cap, thanks for the confirmation that the ones I chose seems perfectly good.
The calculations seems OK too with your online tools, but I still have to double check some values.

For the VARIAC, it seems some coiler use one for their primary, am I wrong ?

Thanks again
Nicolas

[MRMILSTAR]

Using a variac to adjust the primary circuit voltage with a RSG is common. I use one. You can do it with a RSG because the electrode spacing is set as close as mechanical tolerances allow without collision. The firing voltage can then be as low as that minimum distance allows. The firing point is then determined almost exclusively by the point at which the electrodes line up and not by the voltage. You can then vary the transformer input voltage and thus the power by using a variac.

This is different with a stationary spark gap. The firing times are set by the gap spacing and the voltage. They interact. For a given gap spacing you set the voltage so that it ideally fires at the sine wave peaks. If you change the voltage with a variac this can cause erratic firing because you are then changing the firing points on the sine wave.

I've only used a synchronous RSG with my SGTC and its powered by a pole transformer. I have read that using an asynchronous RSG with a NST is not good for the transformer but I don't know the reason. I don't want to speculate and spread false information.

Trying to find the "sweet spot" by adjusting the speed with a non-synchronous motor will only work for a short amount of time. The motor will drift off of the "sweet spot" and will revert to asynchronous operation. The only way to do this with a non-synchronous motor is with some sort of phase-feedback electronics with closed-loop motor speed control. I don't recommend trying this. SGTCs are generally not kind to sensitive electronics. Its easier just to get a synchronous motor or to make one from a normal induction motor by grinding flats into the armature.

[davekni]

The only way to do this with a non-synchronous motor is with some sort of phase-feedback electronics with closed-loop motor speed control. I don't recommend trying this. SGTCs are generally not kind to sensitive electronics. Its easier just to get a synchronous motor or to make one from a normal induction motor by grinding flats into the armature.

Closed loop is possible with proper ground plane on circuit construction.  Because I like experimenting, I made two drives for my SRSGTC.  One is a DC brushed motor with servo feedback to lock to line frequency/phase.  Other is a synchronous motor.  Servo makes phase adjustment easy.  Phase adjustment potentiometer is with servo board and remote from coil, so can be adjusted during operation.  Synchronous motor phase can be adjusted mechanically, but that is harder to do safely during operation.  Phase can be adjusted remotely using a variac as an adjustable inductor and/or a capacitor and power potentiometer.

[Nicolas]

Thanks you all for your answers !
I also read that using a asynchronous RSG with a NST seems not being a good idea, but I never saw any explanation.
And because this is my first coil, and I do not want to add too much difficulties, I will just keep the following options :
- use a synchronous motor with a RSG
- use a Variac for the primary

Thanks again, I have to find a synchronous motor now ;-) !
Nicolas

[Nicolas]

Looking forward to see your post about the Wimshurst machine

Here some pictures of my Wimshurst Machine and my recent Van De Graaf generator :
https://highvoltageforum.net/index.php?topic=2017.0

Nicolas

[55#sofcopper]

I too have heard that asrsg is not good with nst.  I still had to try in in the process I ruined 3 different nst. Don't know the exact reason but that's my experience.