IRACST - International Journal of Advanced Computing, Engineering and Application (IJACEA), ISSN: 2319-281X,
Vol. 1, No.3, December 2012
Experimental Studies of Series-Resonant Inverters
Using PDM for Induction Hardening Applications
S.Arumugam1 E.L.Karthikeyan2
1. Professor, Ganadipathy Tulsis Jain Engineering College, Vellore, India
s_arumugam@rediffmail.com
2. 2. 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
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IRACST - International Journal of Advanced Computing, Engineering and Application (IJACEA), ISSN: 2319-281X,
Vol. 1, No.3, December 2012
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.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
Fig.3. Switching modes (a) Mode I. (b) Mode II. (c) Mode III.
(d) Mode IV.
B.0utput power
The dotted shape in Fig. 3.1. 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
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
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IRACST - International Journal of Advanced Computing, Engineering and Application (IJACEA), ISSN: 2319-281X,
Vol. 1, No.3, December 2012
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. 3.1. 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
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IRACST - International Journal of Advanced Computing, Engineering and Application (IJACEA), ISSN: 2319-281X,
Vol. 1, No.3, December 2012
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
Fig.4.4. (g)-Output current
Fig4.4. (b)-Gate pulse Q1&Q3
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.
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IRACST - International Journal of Advanced Computing, Engineering and Application (IJACEA), ISSN: 2319-281X,
Vol. 1, No.3, December 2012
Fig.5.e.Output voltage
Fig.5.a. Top view of hardware kit
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 2012.
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About Authors
S.Arumugam has obtained his B.E degree from
Bangalore University, Bangalore in the year
1999. He obtained his M.E degree from
Sathyabama University, Chennai in the year
2005. He received his PhD degree in the area of
power electronics in Bharath University, Chennai.
in the year 2011. 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.
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