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Working Principle of Induction Heating Induction Coil Equivalent Circuit Inverter Configurations Power Control Techniques Induction Cook-tops Calculation of Power & Frequency Requirements Advantages of Induction Heating Major Passive Components of An Induction Heating System : Matching Transformers High Power Capacitors Induction Coils Applications of Induction Heating

- 2. Topics Working Principle of Induction Heating Induction Coil Equivalent Circuit Inverter Configurations Power Control Techniques Induction Cook-tops Calculation of Power & Frequency Requirements Advantages of Induction Heating Major Components Matching Transformers Capacitors Induction Coils Applications
- 3. Working Principle of Induction Heating Work-coil acts like primary of transformer and generates alternating magnetic field Workpiece acts like single turn shorted secondary and eddy currents flow in the workpiece Induction heating has two mechanisms of energy dissipation for heating Joule heating Heat power due to eddy currents induced in conducting material placed in changing magnetic field Ф=μo Ic nπ ro 2 , E = -N(ΔФ/Δt), R= ρl/A, P= E2 /R Sole mechanism of heat generation in Non-magnetic materials like aluminium, copper, stainless steels, carbon steel above Curie temperature Primary mechanism in ferro-magnetic materials below Curie temperature ( eg carbon steels ) Magnetic hysteresis loss Secondary mechanism in ferro-magnetic materials below Curie temperature ( eg carbon steels )
- 4. Induction Coil design as per the heating requirements of the load – Depth of Heating => frequency – Temperature & Duration => Wattage => current, voltage – Shape => Inductance Load Matching Capacitor to correct Coil Inductive Reactance and get unity power factor at resonance frequency Matching Transformer for isolation and matching with standard voltage levels Inverter for frequency control – typically square wave voltage, sine wave current due to load resonance Rectifier for power / voltage level control Inverter Block Diagram of Induction Heating System
- 5. Rp – work coil resistance Rs – secondary eddy current path resistance in workpiece reflected to primary Xlp – work coil reactance Xls – secondary eddy current path reactance in workpiece reflected to primary Xlg – secondary air gap reactance between coil and workpiece reflected to primary P= I2 x(Rp +Rs ) Under no-load only power to overcome leakage losses is drawn from the supply. When a lossy work-piece Rs is inserted in the work-coil the system is damped and draws power from the source Max Impedance at fr Min Impedance at fr
- 6. Inverter Design To have high and varying current in the work- coil, an oscillatory circuit ( resonant tank ) is formed by inductor and capacitor in series or parallel. Inverters used are load-resonant Inverters for series tanks are Voltage Fed Series Resonant Inverters VFSRI Inverters for parallel tanks are Current Fed Parallel Resonant Inverters CFPRI
- 7. Work-piece Power Angle betn Inverter V & I Capacitive Side Inductive Side Zero phase shift between Inverter output V & I at resonance so no reactive power is drawn from the inverter
- 9. At Resonance Frequency – Inverter Output Voltage and Current are in phase
- 10. Capacitive Switching fsw < fres Diode hard turn-off results in large reverse recovery current that creates voltage spikes that increase EMI, losses and destruction of semiconductors All authors recommend that Capacitive switching be avoided. However due to transient or fault conditions the` system may operate in this region -work- piece touches the work-coil shorting a few windings and thereby reducing L and increasing fres . Solutions : Use fast recovery diodes ` Leading Current at Inverter Output
- 11. Newer MOSFETs have fast diodes incorporated and may not need this circuit
- 12. Contrary to the previous case Diode turn-off and Switch turn- on is soft. Diode turn-on and Switch turn- off is hard. Inductive Switching fsw > fres Inductive reactance dominates Inductive integrating effect on the Square wave voltage gives Triangular wave lagging current Lagging Current at Inverter Output
- 14. Multiple Coils have mutual coupling due to proximity resulting in power re- circulation between the coils making individual current control difficult. By Synchronizing currents their values can be controlled accurately Application : Silicon wafer heating with multiple coils for precise temperature control
- 15. Inverters with 3 element tanks (LCL) Drawbacks of Series Tank VFSRI Current in the semiconductors same as the load Matching transformer needed between inverter and load thereby increasing cost and decreasing efficiency Drawbacks of Series Tank CFPRI Current in the semiconductors Q times lower than the load But over-voltage protection systems needed – voltage depends on load, due to the current source Advantages of 3 element tank Voltage Fed inverters can be used Current in the semiconductors is lower than the load by factor Ls/L Short Circuit Currents limited by Ls Stray Lead inductance becomes part of Ls enables inverters to be located at a distance from the work-coil In case of operation in the Capacitive Current region due to sudden de-tuning ( due to transient / shorted work-coil ) special commutation circuit does not allow switches to turn-on while current is flowing through their opposite switch's anti- parallel diode
- 16. Frp Frs (operating point) Two resonant frequencies Frp for parallel ckt Frs for union with Ls Non-Zero Phase shift between voltage and current at resonance Converter provides reactive power during normal operation, increased switch current and commutation losses Red Trace = Voltage across tank capacitor (Uc) Green Trace = Current through matching Inductor Ls (itank) Phase w.r.t. Inverter Output Voltage
- 17. Matching Inductors Prevent circulating currents Ensure even distribution of load Limit current in case of faults Parallel Connection of H-bridge VF LCL Inverter for High Power Applications
- 18. Series Connection of H-bridge VFSRI for High Power Applications
- 19. Power Control Methods Varying the DC link voltage Most suitable for square wave inverter Varying Duty Ratio ( Deadband ) of devices in the inverter Max power at 50% duty ratio Heavy commutation losses with high commutating currents due to hard switching at other duty ratios Varying the frequency of the inverter De-tuned to operate in the inductive region Current lags in phase and diminishes in amplitude Lagging power factor ensures that devices turn on with zero voltage across them and there are no free-wheeling diode recovery problems However de-tuning on the inductive side means operating at higher frequencies, so need to ensure that switching losses are within limits Increases reactive power drawn from the inverter
- 20. Adv Inverter commutating close to resonance frequency so commutation current and losses are minimum DisAdv Power levels not continuous Care to be taken to avoid saturation of matching transformer Typically used in Induction Cook-tops Power Control Methods : Pulse Density Modulation
- 21. T1conducting Lrcharging T1OFF LrdischargingCrcharging CrdischargingLrreversecharging LrdischargingthroughD1 T1conducting Lrcharging Parallel Tank Single Ended Quasi Load Resonant Converter For Induction Cooking upto 2.2KW Losses in Lr contribute to Heating ResonantReversal-->
- 22. Induction Cook-top needs ferromagnetic vessels to concentrate the magnetic field through the vessel and concentrate the current to the surface ( skin effect ) so the concentrated current sees a narrower path of higher resistance and produces more heating Cu and Al are have less permeability so they do not concentrate the magnetic field resulting in larger skin depth and broader current path, also they have better conductivity so they have less heat losses that are actually needed here. By increasing the operating frequency, non ferromagnetic materials like aluminum and copper can also be used For lower losses in the coil and optimum use of Copper – the coil is made of Litz wire consisting of a bunch of individually enameled insulated stands of less than skin depth in diameter. Strands are twisted so that they are alternatively on the inside and outside of the wire
- 23. Temperature Measurement Thermocouples Radiation Detection – Optical, IR Ultrasonic – velocity of sound depends on material's elastic modulus and density and these properties depend on temperature Eddy Current detection using a second low power inspection coil – to sense reflected workpiece resistance at various depths by varying the inspection coil's excitation frequency Load signature analysis – measuring V, I, phase angle, f of the main work coil and estimate workpiece resistance and temperature
- 24. Calculation of Power Requirements Power needed to heat workpiece P1=W*C*ΔT W = weight to be heated C = specific heat of the material ΔT = required temperature rise Heat loss due to radiation P2=Aeσ(T2 4 -T1 4 ) Power loss in Induction Coil P3=I2 *Rc At high frequency Rc should incorporate Skin Effect Total Power required = P1+P2+P3 Overall Heating Efficiency = P1/(P1+P2+P3)
- 25. Electrical Power in the workpiece Req calculation in next slide Considering the voltage across resistance is the first harmonic of Utank Thus power delivered to the workpiece can be controlled by controlling the DC link voltage (Ue ) Overall System Efficiency depends on Conversion efficiency of the Power Supply Matching of Coil+Load with Power Supply Tuning of Heating Coil and Power Factor Correction Capacitor Coupling of Coil and Workpiece
- 26. Effective Depth of current carrying layers is given by Reference Depth or Skin Depth (d) depends on : frequency of the alternating current f electrical resistivity ρ relative magnetic permeability of the workpiece μ Induced field strength and current has reduced to 37% of surface value Power density has reduced to 14% of surface value Effective resistance of superficial resistor Rs = ρ/d
- 27. For Ferro-magnetic material (eg iron ) Permeability μ ~ 100 below Curie Temperature Permeability μ = 1 above Curie Temperature, so inductance decreases. So heating depth d is low below Cuire and high above Curie Temp For Ferro-magnetic materials, the Control Circuit needs to sense the operating temperature and adjust the frequency above the Curie point so that accurate heating depth is maintained
- 28. Operating frequencies range from utility frequency (50 or 60 Hz) to 400 kHz or higher, usually depending on the material being melted, the capacity (volume) of the furnace and the melting speed required Smaller the volume of the melts, the higher the frequency of the furnace used; this is due to the skin depth which is a measure of the distance an alternating current can penetrate beneath the surface of a conductor. For the same conductivity, the higher frequencies have a shallow skin depth—that is less penetration into the melt. Lower frequencies are used for larger volumes and can also generate stirring or turbulence in the metal.
- 29. Power Calculation Graphically from IH equipment supplier data-sheets 1. Determine Energy Absorption Rate (Kwh/kg) for given material and target temperature 2. Multiply Energy Absorption rate by desired production rate (kg/hour) to get Power Requirement (KW) 3. Divide the Power Requirement from step 2 by the material specific efficiency to get the Total Power Requirement (KW)
- 30. Frequency Selection Chart for 'Through Heating' from IH Equipment Manufacturer
- 31. Advantages of Induction Heating Fast start-up & Quick heating Energy Savings – can be turned off often as restarting is quick Efficient as heat is generated inside the workpiece Non-contact, heated material not contaminated High Production rates Ease of automation and control Quiet, safe and clean environment Low maintenance Less scale loss
- 32. Advantages of Non contact Infrared Thermometry Speed, lack of interference, upto 3000 deg C Radiation maximum moves towards shorter wavelengths as temperature increases Invisible part of the spectrum contains more energy than the visible part
- 33. Active Transformers only active power transferred from primary Decrease current through the semiconductors Reactive Transformers Both Active and reactive power transferred from Primary Used for low impedance inductors – capacitance and capacitor current is reduced Matching Transformers Despite drawbacks like decrease in efficiency most modern IH systems use Matching Transformers Also provide isolation to the work- piece besides impedance matching Typically water cooled transformers are used – windings are water-cooled copper tubes – tubes for skin effect
- 34. Impedance Matching Work-piece and Work-coil takes large current while Power Source (Inverter) typically operates at higher voltage and low current. Matching is done using Step-down Transformer Auto-transformer, LCL tank Coil inductance is matched by Capacitor at resonant frequency to give unity power factor and maximum heating power to the workpiece Inductor coil 100kW, 40V, 10,000A ,10KHz Power Source 100kW, 440V, 350A, 10KHz Use isolation transformer 440:40 ie 11:1 Current drawn from the Power Source = 10000/11 = 909A beyond capacity of source Addition of Capacitor in load circuit to achieve unity power factor would result in a current requirement of 100KW/440V = 227A
- 35. A huge current flows through the work-coil and capacitor but inverter has to supply only relatively low current Placing the Capacitor nearest to the work-coil reduces the circulating currents in the system - reduces transformer VA Placing the Capacitor in the Primary of the transformer - reduces capacitance and capacitor current but increases the transformer VA Capacitor are specified in KVAr KVAr = VI = V2 /(XL x1000) = (2xπxFxCxV2 )/1000
- 36. Capacitors in Induction Heating A very demanding application for Capacitors Capacitors in the tank circuit must carry 100s ~ 1000s of Amperes of current at 10s~100s Khz with full voltage reversal every cycle Operation at high frequencies causes losses due to di-electric heating and skin effect in conductors Typically used polypropylene or mica Conduction cooled or water cooled
- 37. Work-piece Heating and Work-coil design involves Solution of Electromagnetic and Heat Transfer problems Numerical Computation of the Process Methods used : Finite Difference Method FDM Finite Element Method FEM Better flexibility for non-standard work-shapes Mutual Impedance Method MIM Boundary Element Method BEM
- 38. Coupling of Electromagnetic and Thermal Problems
- 39. Computer Simulations for heating coil design
- 40. Larger the air gap => higher the Q => more Compensating Capacitors needed
- 41. Work-coil design Power => Current requirements Geometry of work-piece
- 42. Flux addition due to same direction of current Flux cancellation due to opposite direction of current
- 44. The induced eddy currents form a complete circuit by flowing around the back of the tube and then along the open v-shaped edges to the point where the tube weld ends The currents are highly concentrated at this point resulting in more heat developed here, this makes it possible to weld the edges together without wasting a large amount of energy elsewhere Coil length and distance between the coil and the weld point should be equal to the internal diameter of the coil
- 47. Induction Shrink Fitting Scan Type Induction Hardening
- 48. Induction Straightening of Construction Beams and Ship Decks
- 51. Our 100KW Induction Hardening Machine Mr Solomon Talkar Mr Nereus Fernandes
- 56. Controls Incomming & Interlocks Converter Ac to Dc Inverter Dc to Ac Capacitor Bank Dc & Ac Matching Transformer & Work Coil Red-Hot White Hot!
- 57. References 1. Doctoral Thesis : Induction heating converter’s design, control and modeling applied to continuous wire heating - Guillermo Mart´ın Segura, Barcelona, June 2012 2. http://www.richieburnett.co.uk/indheat.html 3. http://www.efd-induction.com/ 4. http://celem.com/ 5. Handbook of Induction Heating - Valery Rudnev, Don Loveless, Raymond Cook 6. Elements of Induction Heating Design, Control and Applications S. Zinn, S. L. Semiatin 7. Induction Heating Coil and System Design P. G. Simpson 8. www.raytek.com 9. Induction Heating System Topology Review - AN9012 Fairchild Semiconductor 10. A Fundamental Overview of Heating by Induction - Nathan Rhoades, April 22, 2006 11. https://www.infineon.com/cms/en/ 12. https://www.tinycad.net/ This presentation is a compilation of the work from the above references and many more. Only the 100KW Induction Hardening Machine Circuit Diagrams represents my own work
- 58. Thank You