SOME STUDIES ON HIGH FREQUENCY RESONANT INVERTER BASED INDUCTION HEATER AND THE CORRESPONDING CHOICE OF SECONDARY METALLIC OBJECTS

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SOME STUDIES ON HIGH FREQUENCY RESONANT INVERTER BASED INDUCTION HEATER AND THE CORRESPONDING CHOICE OF SECONDARY METALLIC OBJECTS ATANU BANDYOPADHYAY Reg.

1 SOME STUDIES ON HIGH FREQUENCY RESONANT INVERTER BASED INDUCTION HEATER AND THE CORRESPONDING CHOICE OF SECONDARY METALLIC OBJECTS ATANU BANDYOPADHYAY Reg.No-2010DR0139, dt 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

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.

2 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 2

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.

3 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. 3

4 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 (I s ) 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 nonmetallic 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 4

5 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 5

6 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 6

7 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. 7

8 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 Radius of a strand, (m) Number of helical turns, N (Layers, Strands) (1, 4) (2, 7) (3, 19) (4, 37) Coil radius of the spiral coil, R (m) Twisted Bundle dia. of the Litz wire, (m) Intermittent space between the winding of spiral coil, P (m) Packing Factor ( ) Height of helical coil, H (m) Total length of twisted helical coil, (m) GMD of the coil, (m)

9 Table.2 Inductance of the multi-layered, helical shaped twisted litz coil. Inductance for strands per unit length of the coil, / Total inductance of strands for the entire length of coil = / Inductance for helical coil, The total inductance of the heating coil is, µh (Layers, Strands) (1, 4) (2, 7) (3, 19) (4, 37) Table 3. AC resistance of the multi-layered, helical shaped twisted litz coil with skin effect. Untwisted d.c resistance,ω Twisted d.c resistance,ω AC resistance with skin effect, Ω Skin depth δ,m AC resistance with skin effect & skin depth,ω (Layers, Strands) (1, 4) (2, 7) (3, 19) (4, 37) 30kHz 33kHz 30kHz 33kHz 30kHz 33kHz 30kHz 33kHz

10 Table 4: Equivalent inductance & AC resistance at 30 khz. Secondary Equivalent Parameters of Heating Coil Metallic Objects Skin Depth, m Effective, Resistance, Effective Reactance, Mutual Inductance, Equivalent Resistance, Equivalent Inductance Equivalent Capacitance for, Ω, Ω M,µH,Ω,,µH resonance,, µf Aluminum Copper Brass Galvanized Iron Stainless Steel Table 5: Equivalent inductance & AC resistance at 33 khz. Secondary Equivalent Parameters of Heating Coil Metallic Objects Skin Depth, m Effective, Resistance, Effective Reactance, Mutual Inductance, Equivalent Resistance, Equivalent Inductance, Equivalent Capacitance for, Ω, Ω M,µH,Ω,µH resonance,, µf Aluminum Copper Brass Galvanized Iron Stainless Steel 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. 10

11 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. 11

12 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. 12

Table 6: System efficiency at different operating frequency of induction heater using mirror inverter Temperature Set Semiconductor Semiconductor Time Period Frequency % Efficiency Switch ON-Time (µ

13 Table 6: System efficiency at different operating frequency of induction heater using mirror inverter Temperature Set Semiconductor Semiconductor Time Period Frequency % Efficiency Switch ON-Time (µ sec) Switch OFF-Time (µ sec) (µ sec) (khz) 1-ph input 3-ph input Warm Set-I Set-II Set-III Set-IV Set-V Set-VI 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 setup. 13

14 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. Temperatute in centigrade Warm Set-I Set-II Set-IV Set-V Set-V Set-VI Time in sec Fig. 8 Temperature response of the industrial set-up at different temperature-set 14

15 Table 7: Experimental results for mirror inverter based induction heated system Exp Qn. => Experimental Quantities; (A) => Input no load current I o (Amp); (B) => Input load current I L (Amp); (C) => Input voltage, V I (Volt); (D) => Initial temperature of fluid, T 1 ( 0 C); (E) => Final temperature of fluid, T 2 ( 0 C); (F) => Initial mass of fluid, m I (kg); (G) => Final mass of fluid, m f (kg); (H) => Mass of evaporated fluid, m ev (kg) ; (I) => Total time taken,t (sec) ; (J) => Switching Frequency (khz) ; (K) => Heat power transferred to the fluid, P o (Watt) ; (L) => Input power drawn from source, P I (Watt); (M) => Efficiency, η I (%). 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) (B) (C) (D) (E) (F) (G) (H) (I) (J) (K) (L) (M) 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 15

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.

16 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 16

17 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 01. Mirror Inverter 02. Hybrid Inverter. Warm Set-I Set-II Set-III Set-IV Set-V Set-VI 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 input input input input input input input input input input input input input input 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. 17

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