Digital Load Adaptive Control for Resonant Inverter for Surface Metal Heating
B Meziane*, H Zeroug
†Electrical Engineering Departmùent, Laboratory of Electrical and Industrial Systems, University of Sciences and Technology Houari Boumediene, Algiers, Algeria.
E-mail*: [email protected], E-mail†: [email protected]
Keywords: Series resonant inverter, Load adaptive control.
Abstract
Induction heating is a well-known technique to produce very high temperature at reduced time for well specific applications such as in melting steel, brazing, and hardening.
Series resonant converter proves to suit better to surface metals treatment and hardening. Efficiency reaches its highest level in the resonant mode, where switching losses are minimized by operating the IGBT at a resonant frequency for switching at the zero crossing point of current or voltage.
However, as heat rises during hardening metal process, the metal exhibits parameter variations, which ultimately affects the overall system performances. Therefore, the inverter system with an effective load-adaptive control circuit with short response time is highly sought to let the system operate efficiently regardless of any variation of the load parameters.
This will bring back the system under resonant mode to allow the system to operate at its optimal performances.
In this paper, an analysis of a series resonant inverter performance is conducted, in which the control frequency is monitored and continuously adapted using phase-locked loop aimed to metal hardening. The control technique is characterized by its flexibility, low cost hardware, and short response time. The system comprises analogue and digital circuits, microcontroller based. The experimental bench is built for the purpose in the range of 1 to 5 kW power, with 01 to 50 kHz operating frequency. First, the process principle and the design procedure of the inverter system with the proposed control scheme are described through modelling and simulation. Then, experimental results are presented to show the validity of this technique, and to evaluate the system performances.
1 Introduction
Induction heating is increasingly used in industry, as this technology has known major enhancements in these last years. The features such as higher efficiency, very short heating times and local heating capabilities have made them superior to other conventional heating systems [1].
This technology is particularly exploited in metal heat treatment applications, such as surface hardening, forging, annealing, brazing, tempering, and soldering.
Further, this technology has been a driving force for a wider use of power electronics and control systems, as the power need grows. In this respect, much attention has been focused on the improvement of high frequency resonant inverters capable of supplying high power to induction heating loads [2,3]. In this regards, resonant inverters with reduced power device switching losses by means of soft switching technique are attractive. They allow higher frequency operation, with higher efficiency, and overall system simplicity in terms of inverter control, protection and maintainability [15]. In addition, the series and parallel inverter system has revealed that voltage source series resonant inverter offers better overall performance than the parallel resonant counterpart with respect to converter utilization [1]. Owing to its advantages, series resonant inverter topology has been retained and exploited in this project for metal hardening process.
Usually, to supply maximum power to the metal, induction heating converters commutate close to resonant frequency.
However, load (heating coil and work-piece) parameters inherently vary owing to the shape of the work-piece, spacing between the work-piece and the coil, working frequency, and work-piece temperature, its electrical conductivity and magnetic permeability [4]. This will result in deterioration of the power factor, oscillations are observed during commutation instant and increase in device losses. To overcome these drawbacks, various control schemes are implemented, in particular phase shift and PLL (phase-locked loop) control strategies. In the latter, the operating frequency is automatically tracked to maintain a zero phase shift between voltage and current signals. For controlling this phase shift, analogue circuits are frequently used [5]. The selection of the components and their replacement can be challenging and very often complicated to ensure an accurate control of the resonant mode, and to achieve a swift response during transient load parameters changes, when various surface and hardening metal heating cycles are being considered.
In this paper, series resonant inverter for metal treatment applications is presented. This inverter is based on full-bridge topology using IGBT devices operating at the resonant frequency switching. The power processed by this type of converter is varied by adjusting the DC supply voltage to the inverter through a chopper placed at the inverter input.
Varying DC link voltage allows full control of the heating power, without affecting the adjustment of the working frequency [4]. The latter is automatically tracked in order to maintain the resonance mode regardless load variations using a PLL scheme that combines hardware and software in order to accomplish high performances in terms of efficiency and speed responses. In order to design an effective system with optimum performances and ultimately assess its performances, a model simulation Matlab/Simulink based was conducted. In addition, an experimental test bed has been built for the purpose for 1 to 5kW power range and within the 1 to 50 kHz operating frequency. The validity of the proposed load-adaptive control circuit is primarily evaluated through simulation at the later by experiments.
2 The principle of induction heating process
The principle of the induction heating process is the induction of the load in fast-changing electromagnetic field of eddy currents [6]. The induced latter make it easy to generate Joule’s heat directly in the work-piece on the basis of the heating principle. The eddy currents in the work-piece are concentrated in a peripheral layer of thickness δ, according to the following formula [3]:
rf
0
(1) Where, µr and ρ are relative magnetic permeability and electrical resistivity of the material, respectively and f is operating frequency. The penetration depth is defined as the thickness of the surface layer wherein 87% of the generated power flows. High energy efficiency of the induction heating process, the ability to generate heat directly in the worked element, no pollution, the possibility of easy automation while ensuring accurate, controlled, and repetitive parameters, as well as the latest developments in power electronics led in recent years to the intensive development of power supplies for induction heating, allowing to meet the diverse requirements of the heat treatment technology in general, in terms of both power output and frequency [6].
3 System modelling
3.1 System description
The general layout of the high frequency converter for induction heating system is shown in Figure 1. The three AC
converter. High-speed IGBTs with fast anti-parallel diodes are used in the inverter. Snubbers are used to reduce the dv/dt stresses on the IGBTs. The inverter is of the voltage-fed load resonant type, i.e. it consists of an equivalent inductance representing the heating coil and work-piece in series with high-frequency compensating capacitor [2]. The inverter output voltage and the inductor current are measured by means of high-frequency voltage and current transducers to provide the necessary inputs information for the PLL control.
Figure 1: General layout of series resonant inverter 3.2 Load Modelling
The heating coil and the load are modelled as a transformer with a single turn secondary winding. The equivalent model of the transformer can be in a simplified form by an equivalent inductor and resistor [7,8]. The heating coil and the load can be represented by an equivalent series inductor Leq and resistor Req, as represented in Figure 2.
Figure 2: Equivalent circuit of the load 3.3 Load variation effect
Nature of the materiel of heating load, relative position between inductor and work-piece, excitation frequency and temperature affect the equivalent load parameters [3]. This can be described by its quality factor Q, defined by [2]:
r res eq eq
r eq
C R R
Q L
1
(2)
res eq
res L C
f 2
1 (3) The system impedance is given by expression (4). At the resonant frequency, the equivalent impedance is equal to a simple resistor.
eq res
eq
eq R
L C R
Z 2 1 )2
( (4)
Figure 3 represents an inductance change with temperature.
The increase of the temperature produces a similar effect on the value of inductance, until the Curie temperature is reached. Then, the value of the inductance decreases abruptly [9,10]. Figure 4 represents inductor Leq and resistor Req
variations according to different working frequency [11,12,13]. The equivalent inductance decreases gradually when the working frequency increases, contrary to the resistor behaviour.
Figure 3: Inductance variation according to the temperature
Figure 4: Leq and Req variations according to the frequency 3.4 Control scheme modelling
The load parameters depend on several variables as mentioned above. Therefore, it becomes necessary to vary operating frequency of the inverter in order to maintain its constant output power irrespective of load variations. This implies that the inverter switching frequency must vary during operation, depending on the resonant frequency of the inverter circuit [5]. The phase-locked loop device for load adaptive resonant frequency tracking is introduced for resonant inverter. The phase-locked loop control system is shown in Figure 5.
Figure 5: Bloc diagram of phase-locked loop control system
4 Simulation results
In order to study this type of structure, and obtain the characteristic for its applications, a simulation has been carried out. Parameters of the simulation are as follows:
41 . 2
; 20
11
; 6
; 35
;
1 1
Q V Vdc
kHz f
F C
H L
Req eq res res
The IGBTs are driven at variable frequency with a 50% duty cycle. The inverter is selected to operate at the resonant frequency for switching at the zero crossing points of current or voltage (soft switching) which reduces loss at IGBTs switches and radio interference. Gate signals, output voltage and current waveforms, coil voltage, and the power variation are respectively shown in Figures 6, 7, 8 and 9.
Figure 6: Driving pulses
Figure 7: Voltage and current at load IGBTs 1 - 4
Pulses 1 - 4
Time (s)
Output voltage (V) and current (A)
Time (s) Time (s)
Pulses 2 - 3
IGBTs 2 - 3
Figure 8: Coil voltage
Figure 9: Power variation
We note that the resonance is well established, for an input DC voltage of 20Volts, the maximum current reaches 25A and the voltage across the coil Vcoil Q.Vdc47.93V. We also note that the maximum power occurs at the resonant frequency, and diminishes as the operating frequency gets away from the resonance mode.
In this simulation, and to examine the performance of the system with load variation effect, we have tuned the equivalent load inductance up to four times its value. This variation appears to correspond approximately to same range encountered in practice, when hardening the metal samples considered in this paper. Further, two control schemes are presented, with and without PLL in view to highlight the advantages of the scheme for load adaptation. Figure 10 represents the variation of the resonant frequency according to the equivalent inductance of the system.
Figure 10: Resonant frequency variation according to the equivalent inductance of the system
Control signal, output voltage and current without PLL are
results in large switching losses. Figure 12 represents the same graphs using PLL control. In this case, the oscillator has come out with a new signal with an adequate frequency of 5.89 kHz that brings back the system to resonance mode for optimal efficiency η = 99.82%.
Figure 11: Control signal, output voltage and current without PLL
Figure 12: Control signal, output voltage and current with PLL
5 System control description
An experimental kit of a full-bridge resonant inverter system has been set up using MITSUBISHI IGBT 2MBI75UA-120 as main switching devices (75A-1200V), with short-circuit and overcurrent protection logic circuits as illustrated in Figure 13.
Since the heating system is tailored toward surface metal treatment and hardening in the low power range, various control have been designed and implemented, to cater for the DC inverter input parameters control and at its high frequency AC output. The coil was designed according the specifications provided. The work piece consists of metallic bars. The spacing between the work-piece and the heating coil was set to 5mm using thermal insulator. The load system parameters are as follows: Req=1Ω, Leq=35µH, Cres=6µF and dead-band was adjusted to 1µs. Also, the system has been operated at an input DC voltage of 20V, with a maximum input current set to 25A and an operating frequency of 11 kHz. These operating conditions are found in the Time (s)
Coil voltage (V)
Time (s) Time (s)
Control signal f = 11 kHz
η = 46.98%
Output voltage (V) and current (A)
Time (s) Time (s)
Control signal f = 5.89 kHz
η = 99.82%
Output voltage (V) and current (A)
5.1 Design principles
The PLL scheme was implemented with a combined hardware and software to perform resonant operation over a range of frequency from 1 to 21 kHz. A block diagram of the PLL system is shown in Figure 14.
Drivers
SKHI 23/12 Power Resonant
Inverter
Oscillator’s Frequency f'OSC Reading Zero-Crossing
Detector
Zero-Crossing Detector Current Measurement
Voltage Measurement
Phase Comparator
Low-Pass Filter
Voltage Controlled Oscillator
PWM Frequency Adaptation to f'OSC
Microcontroller TMS320F28027 CAP1 Phase-Locked Loop
Capture and Detection
Figure 14: Block diagram of proposed
The output voltage and the inductor current are measured with high frequency voltage and current transducers. Zero crossing of these signals are detected by an analog circuit and compared in an XOR gate. The signal generated from the latter is filtered by RC low pass filter to get an average value of voltage. The average voltage conforms to different of phase between voltage and current at load [14]. The DC voltage input to the oscillator, which is proportional to the phase difference between the inverter output voltage and inductor current, is compared with a value corresponding to 90° degrees and the switching frequency is adjusted so that this difference is made zero, when parameters of load are varied. When this condition is achieved, the inverter voltage and current become in phase [1]. In other words, VCO generates an AC signal whose frequency is proportional to its DC input voltage, and adjusts the frequency until the output signal is matched to the input signal [5].
Figure 13: Components of the entire Experimental system 5.2 Control algorithm
The oscillator output represents the new adequate frequency of the system. It is injected right into the TMS320F28027 microcontroller Capture’s input. Through this operation, two TIMERs have been used.
The first is intended for measurement of the input’s signal frequency. The latter is compared with the one previously PWM generated in order to highlight the shift
between them. An adaptation of the control frequency takes place, by using a second Timer.
Microcontroller clock speed equals 60 MHz, allowing for swift information processing, and therefore, a real time adaptation. Flowchart of the PLL control is represented in Figure 15.
6 Experimental results
In order to assess the system and the effectiveness of real time frequency tracking control, two approaches were conducted with and without PLL frequency tracking. In the first, the system performs control search for the resonance frequency signal and adjusts in real time the inverter frequency in Full-bridge inverter circuit Drivers and protection circuits with microcontroller
Various working coil used for metal induction hardening (below), cooling system (above)
Error Phase Err = 0 Voltage and Current
Capture
Phase-Shift Evaluation
New Oscillator’s
Frequency f'OSC Oscillator’s Frequency Equal PWM’s frequency
fOSC = fPWM
Transfer Control Signals to Drivers PWM’s Frequency Adaptation fPWM = f'OSC
Reading the f'OSC by Microcontroller
Yes
No
Power and control circuits
sympathy with any load changes. In the other approach, the PLL output was monitored and fed back manually to system control. As for the load changes, the first concerns the tracking due to inductance variation as a consequence of metal shape affecting the coupling between the work-piece and the coil. In this respect, the work-piece was partially introduced or increased in diameter. The other inductance variation was performed through temperature rising effect.
6.1 Test under resonance mode without load changes Initial tests were conducted under resonant mode for the following conditions: Vdc=10v, Imax=10A, Req=1Ω, work- piece1 Leq1=35µH, and observing the resonant frequency fres=11 kHz, as shown in Figure 16. The oscillator generates a signal with a similar frequency to the one of the control signals as illustrated in Figure 17, and the microcontroller generates the same signal frequency.
6.2 Test under load changes due to work-piece temperature rising effect
This time, the system is tested with same parameters, but with an increase in input power to highlight the effect of the heat using input voltage Vdc=25v, Imax=25A, temperature has risen to 700° in120seconds. Figure 18 shows a slight drift from the resonance, and phase shift is introduced due of parameters variations of the system. Figure 19 shows a slight shift from 90° initially set. In this case, the oscillator has come out with a new signal with an adequate frequency of10.87 kHz, that brings back the system to resonance.
6.3 Test under load changes due to work-piece shape effect
observe the influence of the equivalent inductance. Figure 20 shows clearly that resonance is lost, and phase shift is introduced due of parameters variations of the system. From Figure 21, it is illustrated that the phase-shift between voltage and current is significantly higher. The oscillator has generated a new signal with a frequency of 5.81 kHz.
In last stage, similar tests were performed in a closed loop, with the PLL circuit combined with the microcontroller through software programming. It was found that the system performs well, even for other load changes conditions. A real time signal tracking was possible and responds well to any sudden load changes.
7 Conclusion
In this paper, an effective control strategy of full-bridge series resonant inverter for induction metal surface treatment and hardening has been proposed and described, in which ZCS is ensured with a PLL scheme over wide operation. The PLL control scheme operates satisfactory as predicted, ensuring full inverter resonance mode over load variation, due to heating effect or work piece shape, thus providing maximum power transfer to the load throughout the heating cycle. This was achieved using hardware associated to software, microcontroller based at minimum cost. In addition, the system shows that it can be tailored easily to other heating requirements by software only. The system design presented shows flexibility, high efficiency and very short response time. The experimental results were validated through simulation and confirm the effectiveness of the control system presented in this application.
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Figure 16: Load current and control signal at the resonant
frequency
Figure 17: Phase-shift (upper trace) and oscillator output at the
resonant frequency (lower trace)
Figure 18: Load current and control signal outside of the
resonant frequency
Figure 19: Phase-shift (upper trace) and oscillator output (lower trace)
at 700°C
Figure 20: Load current and control signal outside of the
resonant frequency
Figure 21: Phase-shift (upper trace) and oscillator output with work-piece having large diameter
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Control signal
Control signal Current : 3.5 A/V
Control signal Current : 2.5 A/V
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