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

1. Induction Heating
Nereus Fernandes

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

49. Induction Brazing

50. Induction Hardening

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