Introduction

Having already built a medium and a small DRSSTC, I feel that I have the experience and want the challenge of building a large DRSSTC system.

Safety

WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site. Read this document about safety! http://www.pupman.com/safety.htm

Considerations

I decided to design a high impedance system to run longer on times at a lower primary peak current. The average power flowing in the primary circuit will be the same as a low impedance system, but total cost of the system will be much lower. A MMC is typically much more expensive for a low impedance coil as a large capacitance is needed to work at a resonant frequency around 40 kHz. A high impedance primary coil also have a lot more surface area and will thus not require as much cooling.

When drawing power in the term of up to 10kW, power factor will become a problem.

Since this project have been running over a long period there have been many changes to the core design of the Tesla coil.

Secondary form went from 200 mm diameter to 300 mm diameter.

IGBTs went from CM300 to CM600.

MMC went from 400 nF at 12 kV made out of CDE942C capacitors to a 800 nF at 15 kV made from 5 heavy GTO snubber capacitors that is specified for over 5000 A peak currents.

Specifications

Bridge 2x CM600DU-24FA IGBTs in a full bridge configuration Bridge supply 4x 6000uF 350V filtering capacitors, two in series, two string in parallel for 6000uF at 700V. Primary coil Flat primary. Inner diameter 375 mm, Outer diameter 855 mm. 8 turns 10 mm copper tubing, turn spacing 20 mm

tapped at 6.75 turns. MMC 0.8uF at 15kV. Made from GTO snubber capacitors. 5 in series. Secondary coil 315 mm diameter, 1500 mm long, 1800 windings, 0,75 mm enamelled copper wire. Resonant frequency Around 38 kHz. Topload 7 rings of 25 mm aluminium tubing forming a toroid. Input power 3x400VAC 16A / 11kW Spark length 3.6 meters

Schematic

The driver is a variation of Steve Wards universal driver

Construction

24th December 2011

Work on the secondary coil began on Christmas day and lasted until the 6th January 2012 where is had its 7th layer of varnish. The first winding rig fell apart after the 4th layer of varnish and we had to build a better one.

The winding of the secondary coil was done on the first day, in about a total of three hours. The of the time was spent on varnishing. Took 16 hours for each layer to harden before a new layer could be applied.

With the frequency converter set at 70 Hz, the gear ratio and small to large diameter gear gave a winding speed of about 0,5 m/s.

29th December 2011

The primary supports was made from acrylic from the back light panel in computer monitors. It felt somewhat different to work with than new acrylic and it is our theory that the great tensions that it saw doing milling resulted in the catastrophic cracks that eventually led to the death of these supports.

On the 7th of January 2012, we gave up using these. Many hours of work was wasted here and we are now searching for a better material for the supports.

14th May 2012

When I first found these pieces of scrap PVC, my first thought was to use the already made holes to put the leads through and distance the capacitors that way. But recalling that Finn Hammer made a fish bone like skeleton for his MMC, I tried something like it and ended up using all the plastic, even the cut outs that is used for the backbone. The end terminals of 20×10 mm copper is overkill, but was what the scrapyard had at hand that day, they do however make a good connection in the sides for the capacitor strings.

The result is half of the MMC is completed. 0,2uF at 12kV. The completed MMC will be 0,4uF at 12kV.

Upgraded: This MMC will not be used, MMC is upgraded to heavy GTO snubber capacitors, scroll down for further information. This MMC will be used in a smaller Tesla coil.

17th and 18th May 2012

New primary supports was made from scrap PVC, I found some good pieces of 20 mm thick PVC to use for these.

The enclosure have been put together with wheel mounted, the primary platform is raised from the enclosure to allow for taking the complete Tesla coil apart, make connections of cables and possible water cooling easier and to gain distance to metal objects in respect to the primary coil.

The enclosure and primary platform is entirely put together with only glue and wooden nails, nearest metal in the current construction is the wheels at the bottom.

6th October 2012

Copper bus bars for the bridge and spacers for capacitor mounting machined and milled. 260 mm² copper bars are used for the DC connection with the capacitors.

20th October 2012

Material for topload toroid construction was bought cheap from the scrap yard. 7 large rings of 25 mm diameter coaxial antenna cable, aluminium shield with PE foam filling and copper tubing core.

Started construction of full bridge

IGBTs mounted on heat sink, support for capacitors constructed and capacitors mounted on IGBTs. Construction of 3 phase rectifier bridge on heat sink and preparing mounting on the bridge supports.

3rd November 2012

Finished installation of wheels on the enclosure and secondary coil was sawn over in both ends to get the right secondary tube length.

10th November 2012

Winding of the two GDTs, etching of gate PCBs and soldering of the gate PCBs and the GDTs.

Mounted 3 phase bridge rectifier, DC cables, snubber capacitors and TVS strings.

24th December 2012

Installation of fan in the enclosure, installation of bridge module in the enclosure and finding a solution to keep the GTO snubber capacitors in place.

29th December 2012

Driver PCB almost completely populated, only a few components missing that are currently in the mail.

Winding of the two current transformers for feedback for frequency tracking and over current protection.

All the bus-bar work have been done, in 30×10 mm copper and brass.

The new MMC consists of five 4 uF capacitors at 3 kVDC in series for a 0,8 uF MMC at 15 kVDC, here it is installed with the bridge.

1st May 2013

Bench test of driver was carried out. This is Steve Wards universal driver 2.1 converted to single sided board for through hole components only. Still some bugs and misroutings that was worked out during the test. The pulse width limiting network was bridged over as it caused some oscillations that made the driver turn the output on despite no input from the interrupter.

21st August 2013

The primary lead and tap is constructed from three pieces of 35 mm² cable connected to a heavy distribution net clamp through home made cable lugs made from 10 mm copper tubing, all soldered together using a gas camping stove.

28th August 2013

Video of the driver behaving very odd. Very glitchy switching can occur but it can also run fine. As it can be heard in the video this is not a switching sound you want to hear in a feedback regulated circuit that should be stable. 225VDC on the bus and switching 400A in the primary circuit.

Steve Ward found out that the problem is residual charge in the tank capacitor in the primary circuit that, depending on it being positive or negative, can influence on the driver and give it a bad start that can last for several cycles before it gets back on track.

30th August 2013

At first I thought that the problem was to be found on the driver board and thus have tried several improvements to limit the amount of noise that could be inflicted on the driver from the switching of huge currents. 10uF Tantalum capacitors was added close to the 74HC08 AND gate IC and the MAX913 comparator IC in the feedback circuit. Ferrite beads was added to the positive and negative rails for the same ICs. Pull-up resistors was added to the outputs from the MAX913 comparator IC.

None of these improvements helped in regard to the glitches in the switching.

The solution to this problem is adding a large power resistor across the output of the inverter to burn off this small charge between bursts. A resistor in the range of 10K Ohm and a suitable wattage according to inverter output voltage is needed, normally a bleeder resistor for a capacitor would be placed directly across the capacitor terminals, but we would then need a high voltage resistor that can withstand the tens of kV that the tank capacitor sees.

This video shows a static load test with 225VDC on the bus and primary circuit switching 1000A.

20th April 2014

I installed a bleeder resistor across the inverter output to take care of the residual capacitor charge as described above. The resistor is made from three 2KΩ 75W resistors in series for a 6KΩ resistor.

DC bus voltage on multimeter to the left. Yellow trace on oscilloscope is primary current, blue trace is inverter output voltage.

The load in place instead of the secondary coil is a metal container with water in it, primary coil got hand warm from a total of 4-5 minutes run time at 1500W input power, IGBT and MMC stayed cold.

This test is limited by my mains supply available, it is only 230VAC at 6 Ampere.

In the first clip the primary current peaks at 1440 Ampere at 300 uS on time at 120BPS. 320 VDC on the bus and load on mains varies from 4 to 6 Ampere at 250VAC.

In the second clip a low 4BPS gives a peak current of 1640 Ampere at 300 uS on time. DC bus voltage is 370VDC.

12th July 2015

We built a quick solution for a motorized tubing roller, the coaxial antenna cable outer aluminium shell is very soft and can be bent easily with a moderate force. A few pictures of how it looked when picked up at the scrap yard.

After the first run all the circles are almost in a good enough shape to mount on the arms of the topload skeleton. Only a few of the rings was run through twice to get them round enough.

The rings are mounted with rivets from inside of the arm. Unfortunately the thin walled aluminium shell of the cable was not strong enough to resist a bit of handling and came loose at a few points. We had to use cable ties to secure all of the tubes at each point, by making a cross it does not look THAT terrible if a better solution is not found.

The ends of the secondary wire was secured at the bottom earth connection and to the topload insert mount. A nylon plug in the bottom is used to secure the secondary coil to the base of the primary coil with a simple knock in split for easy dis- / assembly.

The massive and towering look of the topload placed on top of the secondary coil. The driver and power electronics box and elevated primary platform will raise the coil even further so it will stand a total of 2,8 meters.

31st March 2016

The first time it was possible for us to make a test on full power from a 3×400 VAC / 16 A supply. To initially see if things would run smooth, a 3 phase 6 A variac stack was used to ramp up the voltage.

To our big surprise we were able to produce up to 3.6 meter sparks from a supply that we thought would be inferior to the coils power demands. We did however not trip any breakers or even have the OCD at 1500 A blink at all.

The reason why the OCD never tripped once when set for a modest 1500 A is because of the built in over-current protection in the IGBT bricks, a small RTC element shuts the gate off at 1200 A. I will have to open up the IGBT modules and cut the wires going from the RTC element to the gate in order to run these at much higher peak currents.

These ground strikes was achieved with a settting around 350 BPS at 200 uS on-time.

With the raised breakout point the total height of the coil is now up at 3 meters.

10th September 2016

After cutting out the bonding wires to the RTC circuit of the CM600DU-24FA IGBT bricks, which we thought could be one of the reasons that we were not able to trip the 1500 A OCD setting, we had a short test run to witness performance. I made a video with more details of the real-time current control removal.

While it might have limited the operation a little bit, it was nowhere near hindering performance, this coil is just so high impedance that it runs long on-times instead of high peak currents.

Fed with 3×400 VAC through a variac resulted in a 0.6 power factor. After roughly 8-10 test runs at up to 2 minutes, with peak power consumption hitting 14 kW at 500 BPS, 200uS, the total power consumption over all the tests was 0.281 kW/h, 0.331 kVAr/h and 0.438 kVA/h.

First video shows the coil running 120-500 BPS at somewhere around 200 uS on-time. Peak power consumption from the 3×400 VAC supply was around 14 kW. Sparks are 3 meters to ground and somewhat shorter to the ladder.

Second and third video show tests with a static load, peaking at about 10-14 kW depending on MIDI or interrupter is used.

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