2A Variac for Phase Shifting

Last week I took delivery of the first item needed to create the phase shift module. I was going to rely on mechanical adjustment for phase shifting but after a lot of reading up I have decided that the ability to adjust phase remotely while the tesla is operational is a must. If you are pretty new to Tesla coil building (like me) the subject of phase shifting can sound quite complicated. In simple terms, the synchronous motor rotation is locked to the sine wave of the AC power supply meaning that the spark gaps always present themselves at the same point on the sine wave. Without any phase shift the same AC sine wave is applied across the capacitors meaning that the spark gap may not present as full charge is reached in the capacitors. Phase shifting allows you to advance/retard the sine wave to the synchronous motor so you can present the gaps when full charge occurs in the capacitors. Without phase shifting you can only alter this timing by physically rotating the motor in its mount or rotating the spark gap disc relative to the motor shaft. Both of these being impossible (or very dangerous) to do while the TC is in operation and can be very difficult to quantify adjustments.
To make the phase shifter we use a variac wired as a variable inductor, a motor run capacitor and bleed resistor.

First to arrive was a 2A panel mount variac. I bought it from a seller on eBay called Wattbits. This is the second variac I have bought from these guys, the first was a 10A version that is the main power source for the Tesla coil.

Here's the 10A version in the power supply module.

The 2A version is physically about half the size. I want the phase adjust module to look similar in design to the power supply module so I have ordered a length of 185mm diameter clear acrylic tubing and a couple of 185mm acrylic discs for the top and base.

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Primary Coil Clamp Pin

I spent sometime yesterday trying to figure out a good way to make the required electrical connection to the innermost end of the primary coil.

As you can see in the picture the connection needs to clamp onto the end of the 1/4" copper tubing and then pass through the 12mm perspex disc underneath the primary coil. In this pic the disc is still covered with protective plastic coating.

This is a side on pic taken level with the primary coil. You can see the anti-strike perspex disc above the primary coil and the 12mm thick disc below. After long consideration I decided that I needed a fitting that would clamp on to the end of the primary and have a protrusion that would pass through a clearance hole in the 12mm disc below. This "protrusion" would need to be threaded to allow a copper lug to be fitted on the underside. I had bought a drawing app for my ipad called iDraw and thought I would throw a quick design together. I've used this app before for designing the toroid mounting pin, it's not the most technical drawing app but it matches my abilities.

I started with a small length of 16mm diameter round copper bar. I turned down 25mm of it to a diameter of 8mm and then the next 25mm to 14mm diameter.

I cut the turned section off with a hacksaw then cleaned up the cut end in the lathe finishing the 14mm diameter section to a length of 22.375mm.

Here's the finished blank. Next I drilled the hole for the 1/4" copper tube (6.75mm drill bit used) and the holes for the clamping M3 grub screws. Decided to deviate from the plan and have 2 clamping grub screws at 180 degree to each other.

Using an M8 X 1.25 die I cut threads on the end of the 8mm diameter section (about 15mm of threads) and then cut the M3 threads for the grub screws.

Here's the finished article fitted with an M8 stainless nut and copper electrical lug.

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Connecting the Variac to the mains

Today I finished the wiring to the SRSG (synchronous rotating spark gap), added IEC female connectors to the two outputs from the variac and slapped o a 13amp plu onto the input to the variac. Although I'm not yet ready to test the Tesla coil charging circuitry I can now test the variac up to full voltage.

Here the variac is plugged into a 230v mains outlet. The SRSG 240v feed from the variac module is connected to the correct input on the Tesla coil base. The main Tesla coil feed (0v -110v) is left unconnected but tapped by the volt meter.

The variac dial was set to zero and the on/off switch thrown. The synchronous motor spun up immediately and the meter continued to read zero. Rotating the variac knob resulted in an increase of RMS AC voltage on the meter.

The reading were as expected and hoped. The printed scale around the variac knob isn't that accurate, but I didn't really expect it to be, it's really just a guide. Think I will mark on a couple of accurate markers.


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Checking the Variac with the Oscilloscope

Started on a little more of the wiring today. My task was to add a feed wire from the variac unit which would supply 240v to the SRSG feed transformer. This wire would just tap into the input feed after the on/off switch within the variac housing. I thought it would be a good idea to test the variac electrical connections before adding anymore wiring. The safest way to do this was to use the function generator on my oscilloscope to supply a low amplitude (about 5v) 50Hz signal across the input terminals of the variac and test the output with one of the channels on the oscilloscope.

Here's the setup I used. The function generator is plugged into the inputs of the variac and also channel 2 of the oscilloscope. The oscilloscope probe is connected across the output of the variac and feed into channel 1 on the oscilloscope. With both channels set to the same volts/div and the scope set to dual trace (shows both channel traces on same screen) it's very easy to see to compare input and output of the variac. Far safer than slapping a plug on the variac, plugging into the mains and hoping all is OK.

You can see from the video when the variac control knob is set to zero there is no output. As the knob is rotated the flat trace rises till it reaches the same amplitude as the output of the function generator. This is the required result and shows that the variac is functioning correctly. When in use with the tesla coil I will only need to use about half the range of the variac as the NST is rated at 110v. Tomorrow I will add the wiring for the SRSG. 

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Spinning up the Spark Gap Motor

Finished the last bit of wiring on the synchronous motor and the step down transformer supplying it. Annoyingly the little ceramic 3 pole terminal block cracked when re-mounting it, on closer inspection it really is a horribly made piece of crap from B+Q. I have ordered a similar looking unit from eBay tonight, hopefully it's better quality and will fit the mount holes already drilled, if not I may create my own from a block of perspex. Anyway time to plug in the the spark gap module. Fingers crossed.

Well that went pretty well. The spark gap disc seems to be well balanced and the motor is very quiet, not that that will make any difference once the gaps are firing. The motor took more than 40 seconds to come to a complete rest after unplugging. There is a fair bit of weight involved in the spark gap disc but I still think this is a good indication to the quality of the bearings in this little motor.

Hers's a close-up of the cracked terminal block. I had to remove it when I was wiring in the cable in the pic as I had trouble tightening the cable clamp screws. When I checked the screw heads non of the slots were formed correctly. I used the 2 best formed clamp screws to secure the new wiring and refitted the block, this is when it cracked. Two steps forward, one step back.


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230V to 110V for Synchronous Motor

Did a little more wiring today. It consisted of connecting one of the female IEC socket in the front panel to the step down transformer on the second level of the tesla coil. I used a length of standard 3A 2 core round black flex. The synchronous motor is rated at 7.5W at 110V so by the equation

                  Power (Watts) = Current (Amps) X Voltage (Volts)

            so   Current (Amps) = Power (Watts)
                                                ------------------
                                                Voltage (Volts)

                                             = 7.5
                                                ----
                                                110

                                             = 0.068 Amps

So, as you can see it draws very little current and even the lightweight 3A cable is a bit of an overkill.

The top IEC socket will supply 240V to the step down transformer which in turn will supply 110V to the synchronous motor. The transformer was sourced from Newlec and came in panel form (uncased) but mounted on a pressed steel bracket. I removed the bracket so I could mount the transformer on a perspex base. This meant that I had to remove the connection strip. This task was undertaken several months back and I thought it would be wise to check that the connections were correct and the transformer is doing what's expected, also a good reason to play with my Xmas present.

The transformer has inputs for 230V and 240V with an output of 110V. I decided to supply a 50Hz AC voltage across the 230V inputs and measure the output of the terminals marked as 110V. I used the function generator built into my oscilloscope to supply this signal to the transformer at a very safe amplitude of about 3 volts. The same signal was feed into channel 2 on the oscilloscope.

The oscilloscope probe was plugged into channel 1 and then connected across the 110V output terminals. With the same volts per division set on both channels and the scope set to dual I could view the input and output waves at the same time. I did take a pic to show you the result but my camera only got an image of a few dots on the oscilloscope screen due to the high speed of the trace. So I took a quick video with my Sony Bloggie.

As you can see, the terminal connection are correct and the transformer is stepping down the voltage by  a factor of about 2 so 230V would be output at about 110V.

I can now finish the wiring to the transformer with confidence. Very pleasing to see actual inputs and outputs on the oscilloscope screen also interesting to see how the 2 signals stay in phase.

Wiring in the Spark Gap

Had a spare hour today to do a bit of wiring on the tesla. I'm glad I put a bit of thought into the design of the main base unit as access will always be needed for tweaks and modifications. Access to the main base can be achieved by removing any of the side panels.

 For this job I removed the right hand side panel which gave me easy access to the safety gap end of the Terry filter. I needed to connect the left and right terminals to the left and right terminals on the synchronous spark gap module situated above on the next level.

I had previously drilled the 2 6mm holes for the passage of these wires from one level to the next so it was just a matter of making up the two wires and then threading them through.

The wire used is high quality GTO (Gas tube only) wire rated at 15,000V. This wire was sourced from Teslastuff.com and ordered from their eBay store. Click here if you want to take a look at the store.

I "sealed" the ends of the wires by coating in solder before fitting. This prevents repeated clamping from damaging the ends of the wires.

I tried to keep the wires reasonably short so they didn't pass to close to other components. This type of wire is almost all insulation but it won't do any harm to try to keep separation to a maximum.

Clamp on strike rail earth connector

Today I fabricated a mock up clamp to earth the strike rail. I started with one of the copper electrical lugs that I bought from Teslastuff.com.

You will not find these listed anywhere on the website or on the Teslastuff eBay shop you will have to email the owner Alan and ask for a price. They are far better quality than anything I have found available in the UK. If you know different drop me an email. I used a length of aluminium 1/2" square rod to make the pieces that form the clamp.

These pieces started off as one block which I drilled with a 12mm bit to form the internal curve and a 6mm bit to form the bolt hole. I used a file to produce the curve on the other end. The block was then cut down the middle using a slotting saw on the milling machine. The two pieces were then clamped back together in a vice on the milling machine and then a 4mm hole drilled exactly on the interface of the two pieces. Using a perfectly flat surface and a piece of fine glass paper I worked the adjoining surfaces this makes the 4mm hole slightly smaller so it clamps tight on the 4mm strike rail. 

Here are the assembled pieces. Straight away I spotted an improvement that could be made when fabricating the actual unit, if I put threads on the bolt hole on the top aluminium clamp plate I can dispense with the nut and use a shorter bolt. Also the actual unit will have copper clamping plates to reduce resistance (aluminium 2.7, copper 1.7ohm.m).

I will mount the finished clamp the other way up than shown above. There will be no need to move this clamp once it is in position on the strike rail. But, obviously, access to the grub screw will be needed to clamp the grounding wire.

With the improvements mentioned I think this clamp will do very nicely. Off to eBay to get some copper square rod.