Design and implementation of a Tesla coil Christopher Rutherford 20 April 2012 To insert this slide into your presentation Save this template as a presentation (.ppt file) on your computer. Open the presentation that will contain the image slide. On the Slides tab, place your insertion point after the slide that will precede the image slide. (Make sure you don't select a slide. Your insertion point should be between the slides.) On the Insert menu, click Slides from Files. In the Slide Finder dialog box, click the Find Presentation tab. Click Browse, locate and select the presentation that contains the image slide, and then click Open. In the Slides from Files dialog box, select the image slide. Select the Keep source formatting check box. If you do not select this check box, the copied slide will inherit the design of the slide that precedes it in the presentation. Click Insert. Click Close.

Introduction Scope and context Tesla coil theory and operation Tesla coil design Lumped circuit equations JavaTC calculator Measurements Conclusions

Scope and Context Objectives Constraints Remarks Learn about Tesla coils Make system that works Satisfy an interest Constraints Use easily available parts and equipment Keep costs to a minimum Carried out in spare time Remarks Wheeler’s and Medhurst’s formulas are empirical

TC Operation Resonant transformer Lumped vs distributed Spectrum analysis Spherical artefacts

Secondary envelope and coupling

Tesla coil Implementation

Tesla coil parameters Rotary spark gap NST Protection filter Asynchronous 2 breaks per rotation 50Hz (3000 RPM) Average power proportional to break rate Synchronous Counter rotating 2 * 8 contacts per disk Breaks per second is 16 * rotational speed <1000 bps NST Protection filter Remove any RF that could damage NST Series MOV, series resistors, series capacitor

Tesla coil parameters Multiple miniature capacitors(MMC) (20s, 3p) 220nF @ 1.5KV = 33nF @ 30KV NST Power Supply 4 * (25mA * 10KV) = 1KVA Coils (Wheelers formula, inches) Primary, Spiral coil, 10cm to 24 cm L = ( N*R )^2 / ( 8*R + 11*W ) =14uH Secondary, helical coil, 5cm * 72cm L = ( N*R )^2 / ( 9*R + 10*H ) =26mH

Tesla coil parameters Secondary capacitance Resonant frequencies Self, Medhurst equation (cm) , est 1940s C/d = 0.1126(l/d) + 0.08 + 0.27√(d/l ) pF/cm (0.81 + 0.08 + 0.1)*10 = 9.9 pF Top load C = (25.4*R) / 9=12 pF Total 24pF Resonant frequencies F = 1/( 2 * PI * ((L * C)^0.5) Primary 14uH and 0.033uF gives 234Khz Secondary 26mH and 22pF gives 210Khz V secondary Vs= Vp(Cp / Cs)^0.5=387Kv

JavaTC coil design for comparison

JavaTC Ls=23.7mH Cs=16pF Lp=12.9mH Cp=0.033uF Fp=243Khz Fs=256Khz Accounts for variations in inductance's and capacitance's due to the non-uniform current distribution  at F0

Measurements Experiments carried out 6 years ago, Email Equipment in storage Email “Measured F0 by connecting oscilloscope and signal generator to base of TC, fo=250KHz. These figures in JAVATC produce fo= 252.9KHz so I'm happy with the 1% error.” Can also see on F0 photo of secondary envelope, which appears to show same resonant frequency

Measurements Email Primary capacitor later upgraded to 33nF “Pri, C=10nF, RSG short circuited, one side of pulse cap to ground and sig gen and osc on other side I see a resonant frequency at 285KHz. 9 turns, pancake, start radius=10 cm and radius=26 cm cable dia= 0.5cm. These figures in JAVATC produce fo= 282.31KHz so I'm happy with the 1% error.” Primary capacitor later upgraded to 33nF Used “Have JAVATC tune Primary Coil” to compensate for new capacitor. Striped away insulation at 5.8 turns (can see on photo)

Measurements

Measurements

Measurements Assume 20us / div (could be wrong) F0=1/(20us/5)=250KHz Total energy transfer time 20us/0.55=36uS (javatc 22.98) Half cycle energy transfer 5*2 half cycles / 0.55 cycles =10 ( javatc 11.24)

Measurements Output power proportional to ark length ~50% losses P=(L/1.7)^2 = (35/1.7)^2=423W ~50% losses Primary spark Resistance Radiation

Conclusions Observation Performance Improvements Tesla coils follow known principles (To a certain extent) Performance Improvements Higher/Tight coupling (k=0.6) Lower height to width ratio Lower losses in secondary

Conclusions Future investigation More analysis at low power Solid state Tertiary coils Higher break rate (power proportional to break rate)

Questions / Answers