SOME STUDIES ON HIGH

FREQUENCY RESONANT INVERTER

BASED INDUCTION HEATER AND

THE CORRESPONDING CHOICE OF

SECONDARY METALLIC OBJECTS

ATANU BANDYOPADHYAY

Reg.No-2010DR0139, dt-09.11.2010

Synopsis of Thesis submitted to

INDIAN SCHOOL OF MINES, DHANBAD

For the award of the degree of

DOCTOR OF PHILOSOPHY

in

ELECTRICAL ENGINEERING

October, 2012

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1. Basics of Induction Heating

Induction heating is the process of heating conductors (usually metals), by inducing an

electric current to flow in the object to be heated. Current is induced into the object in the

same manner that current is induced into the secondary of a transformer. A high frequency

source is used to drive a large alternating current through a coil. This coil is known as the

work coil. The passage of current through this coil generates a very intense and rapidly

changing magnetic field in the space within the work coil. The work piece to be heated is

placed within this intense alternating magnetic field. Depending on the nature of the work

piece material, the heat is produced in it.

Fig.1. Principle of Induction Heating

The alternating magnetic field induces a current flow in the conductive work piece as

shown in Fig.1.The arrangement of the work coil and the work piece can be compared with

an electrical transformer. The work coil is like the primary where electrical energy is fed in

and the work piece is like a single turn secondary that is short-circuited. This causes

tremendous currents known as eddy current to flow through the work piece. In addition to

this, the high frequency used in induction heating applications gives rise to a phenomenon

called skin effect .This skin effect forces the alternating current to flow in a thin layer

towards the surface of the work piece. The skin effect increases the effective resistance of

the metal to the passage of the large current. Therefore, it greatly increases the heating

effect caused by the current induced in the work piece. An induction heating device can

seem mysterious as it creates heat while work coil remains cool to touch it. Induction

heating is a process of heating metals where there is no contact between the heat source

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and the materials to be heated. The electromagnetic waves transfer the heat from the coil so

it doesn't become hot itself. High-frequency current drives alternating current through a

work coil. An intense and rapidly changing magnetic field is created within the area of the

coil.

A work piece to be heated is placed within that space. The magnetic fluxes cut the work

piece. As a result an induced emf is generated due to which eddy current is developed in

the work piece. Finally this eddy current creates heat in the object. The efficiency of

induction heating system depends on the frequency of alternating current, switching loss

and selection of metal for secondary objects.

Induction heating equipment must create alternating currents at frequencies from 4 Hz to

over 450 kHz. In the beginning, spark gap oscillators, motor driven generators and

vacuum tubes were used to create the alternating current. With the advancement of

semiconductor technology from the year 1960 to 1980 SCR (Silicon Controlled Rectifier)

based power supplies technology advanced and soon SCR-based power supplies were used

to replace older generators. Very large and fast switching semiconductor switches like

MOSFET, GTO, IGBT, MCT (MOS Controlled Thyristor) are now used in power supplies

for the generation of high frequency current in induction heating equipment.

2. Induction Heating Principle

(a) Domestic (b) Industrial

Fig. 2: Induction Heating System

Induction heating works on the principle of electromagnetic induction. In this process a

copper coil through which an alternating current is made to flow surrounds a disc of metal.

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The disc has a finite diameter and thickness. It is placed at a given distance from the coil

and concentric to it as shown in Fig. 2(a) & 2(b).Secondary current (Is) is induced in the

disc and this induced current circulates around the outer surface of the disc resulting

heating effect. The most important characteristics of such induction heating are:

The current flows mostly through the outer surface of the metal disc and heats the

surface.

The current flow is restricted to the metal surface contained within the heating coil.

The heating coil, however, may be of a single turn or of multi-turns.

The heat energy is transferred to the metal surface at an extremely rapid rate and

the rate is faster than any conventional method of heating metals. This is due to the

fact that heat is developed directly within the metal rather than being in the work

coil as in resistance heating.

The heat is generated without any physical contact between the source and the

metal being heated. Besides, the magnetic field can penetrate any non-metallic

medium placed between the heating coil and the material being heated. This non-

metallic medium is used for creating platform as well as reducing the leakage of

magnetic field.

Unlike conventional methods, the disc surface would attain extremely high

temperature if eddy current continues to flow.

3. Components of Induction Heating

Theoretically there are three major sections in induction heating. They are:

a. “A source of high frequency electrical power supply”.

b. “An induction coil (work coil)”.

c. “An electrically conductive work piece”.

“A source of power supply” generates the high frequency current in induction heating

system. The induction heating power supply sends alternating current through the

induction coil, thus generating a magnetic field .When a work piece is placed within the

coil and enters the magnetic field, eddy currents are induced within the work piece,

generating precise and localized heat without any physical contact between the induction

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coil and the work piece. The power rating of supply and frequency of power supply

determines the speed at which a work piece can be heated.

“Induction coil” is made by a copper coil which is wrapped around object to be heated.

Since the coil creates the magnetic field for induction heating, so the heating coil design is

important. The coil will be custom-fitted around the work piece so the coil can be of

varying shapes. The custom fit is needed because there is a proportional relationship

between the amount of current and the distance between the coil and work piece. Closer

the coil and work piece, generation of heat in work piece is more. Losses due to skin effect

and proximity effect are the major factors to be considered in designing an induction heater

operating at such a high frequency. To reduce them, litz wire-conductors made up of

multiple individually insulated strands twisted or woven together have come out to be

promising.

“Work piece” with appropriate resistivity & permeability is chosen for high frequency

induction heating. For ferrous metals, like iron and some types of steel, there is an

additional heating mechanism that takes place at the same time as the eddy currents

mentioned above. The intense alternating magnetic field inside the work coil repeatedly

magnetizes and de-magnetizes the iron crystals.This rapid flipping of the magnetic

domains causes considerable friction and heating inside the material. Heating due to this

mechanism is known as Hysteresis loss and is greatest for materials that have a large area

inside their B-H curve.

4. Justification of Present Work

After long search in the above mentioned field, the author has decided to perform some

experimental research work in the field of core less high frequency induction heating for

industrial appliances like fluid heating in non-metallic pipe lines for medicinal plants ,

sterilization process for surgical instruments, automobile industries etc. Induction heating

is already proved to be very effective in domestic household applications like hot plates,

steamer, gauzier, rice warmer etc. The process of core less induction heating has been

selected for the present project to minimize iron loss in the material of the core. Here in

this work the author is prompted to use helical shaped coil as the primary coil as shown in

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Fig.3 for the high frequency resonant inverters because this type of coil is very effective

for industrial applications. Therefore it is necessary to investigate the output of the current

scheme using different metallic objects one at a time which indicates the appropriateness

of each metal for a specific application.

Fig.3. Schematic diagram of helical coil with internal dimensions.

Author has also decided to experiment with composite metallic objects like double layer &

triple layers of different metals. This type of test is essential to perform on both software &

real time hardware environment as it is desired to obtain similar type of results during both

experiments.

After having made thorough literature search about induction heating and its applications,

following area are found unattended.

(a) No attempt was made to optimize the litz wire construction for induction heater. Thus,

there is still a need for optimal design of a litz wire construction for the induction heating

system. Also the shape of the heating coil suitable for industrial induction heater was not

clearly specified.

(b) No particular method was proposed to check the performance of high frequency

resonant inverter based induction heated system. Therefore, it was not possible to

anticipate the response of the induction heater with different secondary metallic objects.

Also, no comparison was done between the performances of different high frequency

resonant inverter based induction heated systems.

(c) Construction of secondary load circuit was not performed for optimum result out of an

induction heated system. Moreover, nothing was suggested to reduce the conduction loss

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during transfer of heat from the source to the fluid in the non-metallic pipeline of industrial

medicinal plant.

In view of the above shortfalls in this area the author has decided to work on core-less

induction heater with focus on the following objectives during the research work:

I. To establish the required expressions to calculate desired values of inductance & a.c

resistance for primary heating coil made of litz wire when the high frequency resonant

inverter is loaded with some metal.

II .To check the suitability of different secondary metallic objects during the on-load

experiment of above scheme and also to verify the response of induction heated system

with different number of layers of metallic objects.

III. To verify the performance of high frequency resonant inverters based induction heater

in both software & real time environment with different metallic objects as secondary.

IV. To compare the performances of two different configurations of high frequency

resonant inverters based induction heated systems.

5. Calculation of Heating Coil Parameters:

Parameters of heating coil and secondary metallic objects have been determined after

formulating the required equations. For industrial application of induction heated system,

stainless steel and galvanized iron are preferred as secondary metallic objects. From the

point of view to keep the strand diameter of the primary heating coil of the induction heater

as high as possible for industrial applications, the strand size with 24 AWG is preferred.

Due to the inherent advantages, induction heated system based on high frequency resonant

inverter are equally effective for industrial applications as it is for domestic equipments.

Computation of AC resistance and inductance of a litz wire is complex. In the present

work, an attempt is made to design a litz wire for industrial application. Inductance is

calculated for four different litz wires, 1-layer-4-stranded, 2-layer-7-stranded, 3-layered-

19-stranded and 4-layered-37-stranded. For two different high frequency resonant inverters

AC resistance is calculated for two different frequencies.

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Variation of AC resistances with the number of twist per feet and operating frequency have

been analyzed in the present thesis. From this study, it is noticed that number of twist

results in less value of AC resistance but at the same time inductance is reduced. However,

keeping in mind the physical constraints of constructing a twisted litz wire, 100 numbers of

twists per feet are considered. Moreover, AC resistances are found to be increasing with

the increase in operating frequency. Therefore, a lower value of operating frequency may

be preferred.

The sample calculations are presented for 24 AWG stranded coil through table 1 to table 5.

Table1. Dimensions of the twisted litz coil for the physical setup.

Physical parameters

(Layers, Strands)

(1, 4) (2, 7) (3, 19) (4, 37)

Radius of a strand, (m)

0.000255

0.000255

0.000255

0.000255

Number of helical turns, N

200 200 200 200

Coil radius of the spiral coil, R (m)

0.04

0.04

0.04

0.04

Twisted Bundle dia. of the Litz wire, (m)

0.00146

0.001951

0.004217

0.007487

Intermittent space between the winding of spiral coil, P (m)

0.00146

0.001951

0.004217

0.007487

Packing Factor () 0.686413

0.777778

0.76

0.755102

Height of helical coil, H (m)

0.583951

0.780392

1.686864

2.994895

Total length of twisted helical coil,(m)

50.26601

50.26643

50.26989

50.27937

GMD of the coil, (m)

0.00044

0.000556

0.000968

0.00136

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Table .2 Inductance of the multi-layered, helical shaped twisted litz coil .

(Layers, Strands)

(1, 4) (2, 7) (3, 19) (4, 37)

Inductance for strands per

unit length of the coil,

/

1.545933 1.499041 1.387962 1.320001

Total inductance of strands

for the entire length of coil

= /

77.70788 75.35142 69.7727 66.36883

Inductance for helical coil,

406.4327 308.6366 146.2498 83.13337

The total inductance of the

heating coil is,

µH

484.1406 383.988 216.0225 149.5022

Table 3. AC resistance of the multi-layered, helical shaped twisted litz coil with skin effect .

(Layers, Strands)

(1, 4) (2, 7) (3, 19) (4, 37)

30kHz 33kHz 30kHz 33kHz 30kHz 33kHz 30kHz 33kHz

Untwisted d.c

resistance ,Ω 1.05873 1.05873 0.60499 0.60499 0.222891 0.222891 0.114457 0.114457

Twisted d.c resistance

,Ω 1.485429 1.485429 0.98156 0.98156 0.608274 0.608274 0.502341 0.502341

AC resistance with

skin effect, Ω 1.462065 1.802808

2.1979

2.7060

3.481048

4.267161

4.156278

5.078483

Skin depth δ ,m 0.000382

0.000362

0.000382

0.000362

0.000382

0.000362 0.000382 0.000362

AC resistance with

skin effect & skin

depth,Ω

0.49537

0.52216

0.370676

0.390725

0.171497

0.180773 0.096613

0.101839

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Table 4: Equivalent inductance & AC resistance at 30 kHz.

Secondary

Metallic

Objects

Equivalent Parameters of Heating Coil

Skin Depth,

m

Effective,

Resistance,

,Ω

Effective

Reactance,

, Ω

Mutual

Inductance,

M,µH

Equivalent

Resistance,

,Ω

Equivalent

Inductance

, ,µH

Equivalent

Capacitance for

resonance, , µF

Aluminum 0.00048211 2.281637307 2.28163730 12.1 1.237432 143 0.197

Copper 0.000379057 1.793926821 1.79392682 9.52 0.993577 144 0.195

Brass 0.000717192 3.563898617 3.56389861 18.9 1.878563 140 0.202

Galvanized

Iron

0.000275804 13.05273489 13.0527348 69.3 6.622981 114 0.246

Stainless Steel 0.002466868 11.674721 11.674721 62 5.933974 118 0.239

Table 5: Equivalent inductance & AC resistance at 33 kHz.

Secondary

Metallic

Objects

Equivalent Parameters of Heating Coil

Skin Depth,

m

Effective,

Resistance,

,Ω

Effective

Reactance,

, Ω

Mutual

Inductance,

M,µH

Equivalent

Resistance,

,Ω

Equivalent

Inductance,

,µH

Equivalent

Capacitance for

resonance, , µF

Aluminum 0.000457372 2.4050448 2.4050448 11.5 1.304361 143 0.159

Copper 0.000359607 1.8909554 1.8909554 9.03 1.047317 144 0.158

Brass 0.000680392 3.7566602 3.7566602 17.9 1.980169 140 0.163

Galvanized

Iron 0.000261652 13.758721 13.758721 65.7 6.9812 116 0.197

Stainless Steel 0.002340289 12.306174 12.306174 58.8 6.254926 120 0.191

6. Simulation of High Frequency Resonant Inverter based Induction Heated Systems:

Through investigations are performed to analyze and optimize the performance of two

different configurations of high frequency resonant inverters used in the induction heated

systems for industrial applications. The response of the high frequency resonant inverter

based induction heated system is checked for different secondary metallic objects. Using

P-SIM & MATLAB-SIMULINK real time simulator detailed analysis is made with the

reflected values of heating coil a.c resistance and inductance for different materials on

secondary side. From this software experimental work, it is seen that stainless steel and

galvanized iron are the most suitable material which are employed as secondary metallic

objects for high frequency resonant inverter based induction heated systems.

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Appropriateness of switching frequencies for both the inverter circuit configurations is

confirmed by varying the frequency over wide range. Thereafter this process is verified by

real time experimental results obtained through a prototype for induction heated systems.

The efficiencies obtained with 33 kHz for mirror inverter and with 30 kHz for hybrid

inverter are been found to be maximum.

Selection of semiconductor power switch is made also for both the inverter circuits using

P-SIM simulator. It is found that IGBT as a power semiconductor switch in high frequency

mirror inverter and hybrid inverter is advantageous for induction heating purposes for

frequency below 50 kHz and highly acceptable. IGBT offers highest rms value of coil

current among all the probable configurations using different power semiconductor

switches. For a frequency range of above 50 kHz, MOSFET is a better option due to high

switching speed with its low switching and conduction losses.

For the sake of convenience, related waveforms are presented here for mirror inverter

configuration only. The main power circuit of radio frequency mirror inverter & its circuit

in P-SIM platform are shown in Fig. 4 & Fig. 5 only. Simulation waveforms are shown for

stainless steel in Fig.6.

Fig.4. Main power circuit of radio frequency mirror inverter.

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Fig. 5 Circuit configuration in P-SIM platform using IGBT.

(a)

(b)

Fig.6. (a) Current & (b) voltage waveforms for stainless Steel

Suitability of 33 kHz as switching frequency is confirmed through simulation work &

latter on by real time experimental work. Due to resonance the system efficiency is

maximum at 33 kHz as seen from table 6.

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Table 6: System efficiency at different operating frequency of induction heater using mirror inverter

Temperature Set Semiconductor

Switch ON-Time

(µ sec)

Semiconductor

Switch OFF-Time

(µ sec)

Time Period

(µ sec)

Frequency

(kHz)

% Efficiency

1-ph

input

3-ph

input

Warm 2 23 25 40 69.5 70.2

Set-I 4 22 26 38 79.8 80.5

Set-II 8 22 30 33 93.5 94.3

Set-III 12 21 33 30 84.7 85.5

Set-IV 16 21 37 27 82.7 83.4

Set-V 23 20 43 23 78.9 79.6

Set-VI 30 20 50 20 72.1 72.7

Simulation experiments are also conducted to validate the choice of IGBT as appropriate

semiconductor switch over other available switches like GTO, MOSFET etc. Through

results it is quite clear that a peak to peak symmetrical current is produced with IGBT only

& therefore heating effect becomes very prominent using IGBT as semiconductor switch

compared to GTO, MOSFET etc.

7. Real Time Hardware Experiments:-

Fig.7 shows a schematic system configuration of developed scheme in the present work for

electromagnetic induction fluid heating appliance based on high frequency resonant

inverter. A specially designed electromagnetic induction fluid heating assembly with a

metallic package to achieve eddy current heating in the pipe line system is incorporated

into the non-metallic vessel.

Fig.7: Schematic of induction heated electrical energy conversion in non-metallic pipe-line for industrial set-

up.

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The performance of the above system is checked and verified by real time hardware

prototype using both mirror and hybrid inverters for single phase and three phase input

power. For the sake of convenience the results are presented for mirror inverter only.

7.1 Efficiency calculation:-

The temperature response for different temperature settings with respect to time for single

phase input is indicated in Fig. 8 and detailed experimental data using both single phase

and three phase input with industrial set-up at different temperature setting are listed in

tabular form in table 7.

Fig. 8 Temperature response of the industrial set-up at different temperature-set

0

20

40

60

80

100

120

0 500 1000 1500 2000 2500

Te

mp

era

tute

in

ce

nti

gra

de

Time in sec

Warm

Set-I

Set-II

Set-IV

Set-V

Set-V

Set-VI

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Table 7: Experimental results for mirror inverter based induction heated system

Exp Qn. => Experimental Quantities; (A) => Input no load current Io (Amp); (B) => Input load current IL

(Amp); (C) => Input voltage, VI (Volt); (D) => Initial temperature of fluid, T1 (0C); (E) => Final

temperature of fluid, T2 (0C); (F) => Initial mass of fluid, mI (kg); (G) => Final mass of fluid, mf (kg); (H) =>

Mass of evaporated fluid, mev (kg) ; (I) => Total time taken,T (sec) ; (J) => Switching Frequency (kHz) ; (K)

=> Heat power transferred to the fluid, Po (Watt) ; (L) => Input power drawn from source, PI (Watt); (M) =>

Efficiency, ηI (%) .

7.2 Selection of Secondary Metallic Object:-

In view of the properties of different metals, experiments are carried out by choosing a

secondary metallic object with single metallic sheet, double metallic sheet and triple

metallic sheet for different temperature sets. Every time the input currents and input

voltages are recorded for each temperature set using both single phase and three phase

input power. Fig.9 shows the structure of load for induction heated system using different

layers of metallic sheet. In Fig. 10, Fig.11 & Fig.12 few samples of variation of the input

Sl

No

Exp

Qn.

Warm Temp.

Set-I

Temp.

Set-II

Temp.

Set-III

Temp.

Set-IV

Temp.

Set-V

Temp.

Set-VI

Type of the

Input

1-φ 3- φ 1-φ 3- φ 1-φ 3- φ 1-φ 3- φ 1-φ 3- φ 1-φ 3- φ 1-φ 3- φ

1 (A) 0.05 0.06 0.05 0.06 0.05 0.06 0.05 0.06 0.05 0.06 0.05 0.06 0.05 0.06

2 (B) 0.59 0.183 0.85 0.267 1.56 0.496 2.67 0.80 4.17 1.31 5.53 1.81 7.35 2.35

3 (C)

230 440 230 440 230 440 230 440 230 440 230 440 230 440

4 (D) 30.2 30.2 30.4 30.4 30.5 30.5 30.4 30.4 30.6 30.6 30.7 30.7 30.5 30.5

5 (E) 50.5 50.5 75.2 75.2 85.5 85.5 90.2 90.2 94.6 94.6 96.8 96.8 99.2 99.2

6 (F) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5

7 (G) 1.5 1.5 1.488 1.488 1.412 1.412 1.376 1.376 1.346 1.346 1.324 1.324 1.302 1.302

8 (H) 0.0 0.0 0.012 0.012 0.088 0.088 0.124 0.124 0.154 0.154 0.176 0.176 0.198 0.198

9 (I) 1500 1450 2200 2100 1800 1700 1400 1350 1050 1000 900 850 800 750

10 (J) 40 40 38 38 33 33 30 30 27 27 23 23 20 20

11 (K) 84.9 88 140.2 147.7 302.2 321.1 468 468.8 713.7 749.4 902.5 987.5 1097.8 1170.9

12 (L) 122.1 125.5 175.9 183.2 323 340.2 552.7 548.7 863.2 898.5 1144.7 1241.5 1521.5 1611.8

13 (M) 69.5 70.2 79.8 80.5 93.5 94.3 84.7 85.5 82.7 83.4 78.9 79.6 72.1 72.7

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current with different temperature settings are shown for single phase & three phase input

power. After performing real time experiments on the developed schemes, it is observed

that the input current is maximum for G.I sheet and stainless steel as compared to other

conventionally used metals like brass, copper & aluminum.

So it is better to employ a metallic sheet made of stainless steel or G.I sheet in induction

heated systems.

(a) (b) (c)

Fig. 9: Structure of load for induction heater using (a) single metallic sheet (b) double metallic sheet & (c) triple metallic sheet.

Fig.10. Plot for input current - temperature set using different metals for single layer metallic sheets

Fig.11. Plot for input current - temperature set using copper / aluminum combination for double layer

metallic sheets

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Fig. 12 Plot for input current - temperature set using copper /aluminum / brass combination for triple layer

metallic sheet

Following the real time experiments on both the high frequency inverters, it is inferred that

for given input power supply the efficiency is much better for hybrid inverter based

induction heated system as indicated in table 8

Table 8: Comparison of efficiencies between mirror and hybrid inverters with single and three phase power

input

Sl No

Type of the

Inverters

Warm Set-I Set-II Set-III Set-IV Set-V Set-VI

1-φ input

3- φ input

1-φ input

3- φ input

1-φ input

3- φ input

1-φ input

3- φ input

1- φ input

3- φ input

1-φ input

3- φ input

1-φ input

3- φ input

01. Mirror Inverter

69.5 70.2 79.8 80.5 93.5 94.3 84.7 85.5 82.7 83.4 78.9 79.6 72.1 72.7

02. Hybrid Inverter

83.3 83.8 84.6 84.9 93.5 94.2 91.1 91.6 89.5 90.0 87.2 87.6 --- ---

.

8. Conclusion & Future Scope of Work:-

In the present work IGBT has been employed as switching device. Litz wire with 100

numbers of twisting of enameled copper conductors of 24-AWG strand size is used to

construct a helical coil. With the simulation work the performances of the induction heated

systems using both types of high frequency resonant inverters are optimized. The

efficiency of the system is calculated for single phase and three phase input power supply.

The temperature response for each temperature setting with respect to different frequency

is obtained. Structure of load circuit is determined with different layers of secondary

metallic objects. The high frequency resonant inverter based induction heated systems

employed for industrial applications can be further developed and improved to meet the

requirements in different areas like in medicinal plants, sterilization plants, heat treatment

plants, auto mobile manufacturing plants etc.