Experimental Studies of Series-Resonant Inverters Using PDM for Induction Hardening Applications

Transcription

1 Experimental Studies of Series-Resonant Inverters Using PDM for Induction Hardening Applications S.Arumugam 1 E.L.Karthikeyan 2 1. Professor, Ganadipathy Tulsis Jain Engineering College, Vellore, India s_arumugam@rediffmail.com PG scholar, Anna University, Chennai, India elkarthikeyan@gmail.com. Abstract: This paper deals with a high-power (50 kw) high frequency (150 khz) voltage-fed inverter with a series-resonant load circuit for industrial induction heating applications, which is characterized by a full bridge inverter made of insulated-gate bipolar transistor and a power control based on pulse density modulation (PDM). This power control strategy allows the inverter to work close to the resonance frequency for all output-power levels. In this situation, zero-voltage switching and zero-current switching conditions are performed, and the switching losses are minimized. An additional improvement of inverter efficiency is achieved by choosing appropriate values of the modulation index. The inverter system is designed and the simulation is done using Matlab. The simulation and Hardware results of are presented. Results are verified experimentally using a prototype for induction hardening applications. The induction heater system uses embedded controller to generate driving pulses. The objective is to develop an induction heater system with minimum hardware. Keywords: Induction heating, pulse density modulation (PDM) control, series-resonant inverter (SRI),. I.INTRODUCTION INDUCTION heating generators are resonant inverters in which the resonant tank is formed by a heating coil and a capacitor, in a series-resonant inverter (SRI) [1] [3] or in a parallel resonant inverter [4] [7]. They are used to heat metals to be welded, melted, or hardened. The use of SRIs that are fed with a voltage source represents a cost-effective solution; however, it does not have the ability to control the output power by itself when a simple control circuit is used, so that the output power of such an inverter has to be controlled by adjusting the dc input voltage. A thyristor bridge rectifier having input inductors and a dc-link capacitor has been conventionally used as a variable dc-voltage power supply. This causes some problems in size and cost. In order to overcome these problems, an inverter with power control by frequency [8], [9] or phase-shift [10] [12] variation is normally used to regulate the output power and, using a diode bridge rectifier, as a dc-voltage source. These power control schemes, however, may result in an increase of switching losses and electromagnetic noise because it is impossible for switching devices to be always turned on and off at zero current. Therefore, in high-frequency induction heating applications, only MOSFET inverters can be used. Nevertheless, insulated-gate bipolar transistors (IGBTs) are preferred in high-power industrial applications (availability, cost, etc.), and it will only be possible if a low-loss power control scheme is found. This paper describes an induction heating system of 50 kw and 150 khz for industrial applications that use a novel low-loss control scheme. The induction system consists of a three-phase diode rectifier, a single-phase voltage-fed inverter using four IGBTs, and a series-resonant circuit with a matching transformer. The working frequency is automatically adjusted close to the resonance frequency in order to allow zerocurrent switching (ZCS) inverter operation for any load condition. In fact, the inverter performs as a quasi- ZCS because the transistors are always turned off at almost zero current. The output-power control based on pulse density modulation (PDM) maintains this condition in a wide range of output power. The blanking time of the inverter transistors is designed to maintain zero-voltage switching (ZVS) mode [13], [14].With this circuit, an important improvement of the inverter efficiency is expected at high-frequency working conditions. In addition, this paper presents a study to determine the most appropriate values of the pulse density and the output current in order to obtain a further improvement of the inverter efficiency for highfrequency working conditions. Under these conditions and with fixed transistor losses, the total output power of the inverter will be increased, improving significantly the relative cost of the induction heating generators without reducing its reliability. Many practical work pieces in induction heating applications have a cylindrical shape and are heated by being placed inside of coils with one or more turns. The magnetic field, induced in the coil when it is fed with an alternate current, causes eddy currents in the work piece, and these give rise to the heating effect. A theoretical analysis demonstrates that most of the heat, generated by eddy currents in the work piece, is concentrated in a peripheral layer of thickness δ given by δ = (ρ/πµf), where µ and ρ are the magnetic permeability and electrical resistivity of the work piece, respectively, and f is the applied frequency (skin effect). Induction heating loads (heating coil and work piece) can be modeled by means of a series combination of its equivalent resistance RL and inductance LL. These parameters depend on several variables, including the shape of the heating coil, the spacing between the work piece and coil, the work piece temperature, its electrical conductivity and magnetic permeability, and the frequency. work piece temperature, its work piece. A large number of topologies have been developed in this area. Current-source and voltage-source inverters are among the most commonly used types. The advantages 87

2 of this inverter are high switching speed, short-circuiting protection capability, superior no-load performance because of its currentlimiting DC link characteristic and low component count. signals of transistor Q1 and Q3 are also represented. Signals Q2 and Q4 have been omitted since they are the complementary of Q1 and Q3, respectively. The PDM inverter repeats run and stop in accordance with a control sequence to adjust its rms output voltage. Fig.1. Induction Heating System II. THE PROPOSED CONVERTER Fig.2. shows the typical system configuration of a series generator for induction heating. The output-power stage consists of a single-phase voltage-source inverter using four IGBTs. The output of the inverter is connected to a series-resonant circuit with a matching transformer. The dc power supply for the inverter is a three-phase diode bridge rectifier connected to a 400-V 50-Hz power line. The working frequency is 150 khz, the maximum rms value of the output voltage is 450 V, and the maximum output power is 50 kw. Watercooled load is used [16]. The output current is limited by power losses in order to ensure the inverter reliability. Fig.3. Switching modes (a) Mode I. (b) Mode II. (c) Mode III. B.0utput power (d) Mode IV. The dotted shape in Fig Shows the envelope of the resonant current ie that exhibits a first-order response. The time constant of this envelope is τ. The active output power of the series resonant PDM inverter is obtained by following the procedure explained in Fig.2.System configuration III. ANALYSIS OF THE PDM INVERTER A. Switching Scheme Fig.3. shows the equivalent circuit of the voltage-fed series resonant PDM inverter and its switching modes. A conventional SRI changes between modes I and II in Fig. 3(a) and (b) to produce a square-wave ac voltage. In addition to modes I and II, the PDM inverter adds modes III and IV to produce a zero voltage State at its output terminals, as shown in Fig. 3(c) and (d). During mode III or IV, a gate turn-on signal is provided to either lower or upper leg IGBTs, respectively. As a result, both one IGBT and the diode connected in antiparallel to the opposite IGBT remain turned on. Fig. 3 shows the principle of the PDM-based power control. νo and io are the output voltage and current of the inverter, respectively. The gate T is the period of the PDM sequence; Ton is the time where the inverter is running ; Pmax = (2/π)VdImax cos θ, where θ is the phase shift between the output voltage and the output current; Vd is the dc voltage; and Imax is the maximum amplitude of the inverter current in the case of Ton = T. If T _ τ, the output voltage and current become discontinuous waveforms, and the output power can be written as If T _ τ (high values of the resonant load quality factor), the envelope of the output current is limτ ie = Imax(Ton/T ); thus, the output power is given by 88

3 reaches the value Vd just at t = 0. It is obtained by substituting into (4) νo(0) = Vd The term Ton/T is defined as modulation index or pulse density, and its value determines the output power. One differentiating characteristic of this work consists in the fact that the maximum output power of the inverter, when working at high frequencies, is obtained for modulation index values less than one, due to a major improvement in the inverter efficiency. This time is also used to define the minimum value of the blanking time td. A working frequency of 150 khz, a maximum amplitude of inverter current of 300 A, and a dc-link voltage of Vd = 520 V are assumed in the design of the experimental system. The total equivalent capacitance of the inverter switches, including the snubbing capacitor and the output capacitance of the IGBT, is approximately 20 nf. Since the toff min obtained from (5) is 385 ns, toff is set up as 550 ns for the experimental system. The blanking time td is set as 400 ns. A phase-locked loop (PLL)-based switching frequency control discussed in the following section makes the PDM inverter achieve ZVS condition. Fig Switching pattern in PDM. C. ZVS Fig.3.2.shows a detail of the voltage and current at the output of the inverter in mode I or II. The vertical dashed lines represent the times when the IGBT are turned off (toff) and turned on (ton), and td is the blanking time when all transistors are turned off. The origin of time is set at the zero crossing of the inverter current io = I sin(ωot). In order to prevent undesirable off switching of diodes, the turn-on of transistors must occur necessarily before the zero crossing of the inverter current. This means that the switching frequency should be larger than the resonant frequency. On the other hand, the ZVS is achieved only if the +Vd (or vice versa) is achieved just before of the turn-on process. To ensure this condition, the following calculations must be done. When an IGBT is turned off at the time toff, the inverter output voltage νo starts to rise up from Vd, as given by the following expression: Fig.3.2.Waveforms of the inverter in the switching process. IV.SIMULATION RESULTS The PWM inverter circuit of IGBT series-resonant inverters using pulse density modulation is shown in figure 4.3. This AC supply is converted into DC then it give as input to inverter. Then the ac is boosted by using a high frequency transformer then this boosted ac voltage is passed through load. The AC input voltage waveforms are shown in figure 4.4(a). The switching pulses for Q1& Q2 is shown in figure 4.4(b).The Switching pulses given to Q3& Q4is similar that of Q1&Q2. The gate voltage and drain to source voltage waveforms of the switches is shown in figure4.4(c). The primary voltage waveform of the transformer is shown in figure4.4 (d). Similarly the secondary voltage waveform of the transformer is shown in figure4.4 (e). The output Where Ce is the equivalent capacitance of the inverter (output capacitance of the IGBT, snubbing capacitors, and layout). The minimum value of the time toff corresponds to the time on which νo 89

4 voltage waveform of the circuit is shown in figure4.4 (f). Then the output current waveforms of the circuit are also is shown in figure4.4 (g). Finally the output voltage and current are shown in single scope in figure 4.4(h). Fig.4.4. (d)-transformer primary voltage Fig.4.4. (e)-transformer secondary voltage Fig.4. 3-Circuit Diagram for proposed circuit Fig.4.4. (a)-input waveform Fig.4.4. (f)-output voltage Fig4.4. (b)-gate pulse Q1&Q3 Fig.4.4. (g)-output current Fig.4.4. (h)-output voltage & Output current V. Hardware Results Fig4.4. (c)-gate voltage and Drain to source voltage Laboratory model is fabricated and tested. Top view of the hardware is shown in fig.5.a.the hardware consist of control board and power board. The pulses are generated using microcontroller. Dc input voltage is shown in fig 5b. Drain current waveform is shown in 5c. Driver output waveform is shown in fig 5d. Output of the Inverter is shown in Fig.5.f. Output of the Inverter is not a pure sine wave due to the resistance present in the circuit. 90

5 Fig.5.a. Top view of hardware kit Fig.5.e.Output voltage VI.CONCLUSION Fig.5.b.DC Input voltage This paper has proposed a voltage-source series-resonant PDM inverter for induction heating industrial applications. This power control strategy allows the inverter to work close to the resonance frequency for all output-power levels. In this situation, ZVS and ZCS conditions are performed, and the switching losses are minimized. Therefore, IGBT transistors can be used for an optimum design of the power stage. A 50-kW 150-kHz PDM inverter prototype with IGBT has been tested successfully in order to meet the industrial application requests. Tests made with the presented prototype using the IGBT module FZ600R12KS4. This quantitative comparison concludes that the PDM inverter would be more suitable for various induction heating applications, particularly those of high frequency when IGBT transistors are applied. The converter using full-bridge inverter is simulated using mat lab simulink. The proposed circuit is implemented using microcontroller based driver circuit. The response of micro controller based system is faster since Atmel controller operates at 12MHz. The experimental results are in line with the simulation results. This work deals with open loop system. The closed loop implementation is beyond the scope of the work. This inverter system can also be used for dielectric heating. ACKNOWLEDGMENT Fig.5.c.Drain current waveform The authors are acknowledging the support given by Power Electronics division and Simulation Laboratory in Ganadipathy Tulsis Jain Engg College, Vellore, Tamilnadu, for conducting the d Hardware and simulation studies during July 2012 to November REFERENCES [1] N.-J. Park, D.-Y. Lee, and D.-S. Hyun, A power-control scheme with constant switching frequency in class-d inverter for induction-heating jar application, IEEE Trans. Ind. Electron., vol. 54, no. 3, pp , Jun Fig5.d.Driver output wavefprm [2] S. Faucher, F. Forest, J.-Y. Gaspard, J.-J. Huselstein, C. Joubert, and D. Montloup, Frequency-synchronized resonant converters for the supply of multiwinding coils in induction cooking appliances, IEEE Trans. Ind. Electron., vol. 54, no. 1, pp , Feb [3] P. Y. Chen,M. Jinno, and Y.M. Shie, Research on the reverse conduction of synchronous rectifiers, IEEE Trans. Ind. Electron., vol. 55, no. 4, pp , Apr [4] A. Morozumi, K. Yamada, T. Miyasaka, S. Sumi, and Y. Seki, Reliability of power cycling for IGBT power 91

6 semiconductor modules, IEEE Trans. Ind. Appl., vol. 39, no. 3, pp , May/Jun [5] A. Shenkman, B. Axelrod, and V. Chudnovsky, Assuring continuous input current using a smoothing reactor in a thyristor frequency converter for induction metal melting and heating applications, IEEE Trans. Ind. Electron., vol. 48, no. 6, pp , Dec [6] L. A. Barragán, D. Navarro, J. Acero, I. Urriza, and J. M. Burdío, FPGA implementation of a switching frequency modulation circuit for EMI reduction in resonant inverters for induction heating appliances, IEEE Trans. Ind. Electron., vol. 55, no. 1, pp , Jan [7] J. M. Espí, E. J. Dede, R. García-Gil, and J. Castelló, Design of the L LC resonant inverter for induction heating based on its equivalent SRI, IEEE Trans. Ind. Electron., vol. 54, no. 6, pp ,Dec [8] Z. M. Ye, P. K. Jain, and P. C. Sen, Full-bridge resonant inverter with modified PSM for HFAC power distribution systems, IEEE Trans. Ind. Electron., vol. 54, no. 5, pp , Oct [9] S. Mollov, M. Theodoridis, and A. Forsyth, High frequency voltagefed inverter with phase-shift control for induction heating, in Proc. Inst. Elect. Eng. Electr. Power Appl., Jan. 2004, vol. 151, no. 1, pp [10] V. Esteve, J. Jordan, E. J. Dede, C. Cases, J. M. Magraner, E. Sanchis- Kilders, and E. Maset, Using pulse density modulation to improve the efficiency of IGBT inverters in induction heating applications, in Proc. IEEE 38th PESC, Jun. 2007, pp [14] E. J. Dede, J. Jordán, V. Esteve, and C. Cases, Reliability of current-fed inverters for induction tube welding in short-circuit conditions, in Proc. 28th IEEE IECON, Nov. 2002, pp [15] S.Arumugam, and Dr.S.Ramareddy, Simulation and analysis of class D inverter fed Induction Heating Application, International conference on smart technologies on communication and computing, January [16] S.Arumugam and Dr.S.Ramareddy, Simulation of class E Inverter based Induction Heated using simulink, International Journal of Computer and Electrical Engineering, Vol2, No6, Dec2010. About Authors S.Arumugam has obtained his B.E degree from Bangalore University, Bangalore in the year He obtained his M.E degree from Sathyabama University, Chennai in the year He received his PhD degree in the area of power electronics in Bharath University, Chennai. in the year He has published more than 15 international journals and 35 in National and International conferences. Presently he is working as a Professor and Academic Dean in Ganadipathy Tulsis Jain Engineering College, Vellore, and Tamilnadu. He is a life member of Indian Society for Technical Education. He has published books on Basic Electrical Engg and Electrical machines. E.L. Karthikeyan has obtained his B.E degree from Anna university Chennai in the year 2009.Presently he doing his research in power electronics area. [11] H. Fujita and H. Akagi, Pulse-density-modulated power control of a 4 kw450 khz voltage-source inverter for induction melting applications, IEEE Trans. Ind. Appl., vol. 32, no. 2, pp , Mar./Apr [12] V. Esteve, J. Jordán, E. J. Dede, E. Sanchis-Kilders, and E. Maset, High energy efficiency test system for induction heating generators, in Proc. ICREPQ, Apr. 2006, pp [13] E. Dede, J. Jordán, V. Esteve, J. Espí, and A. Ferreres, Switching modes and short-circuit considerations in a very high frequency, very high power resonant inverters for induction heating applications, in Proc. PCC, Aug. 1997, vol. 2, pp