variable frequency induction heating using magnetic energy recovery by Ryan Lian

Variable Frequency Induction Heating Using Magnetic Energy Recovery Switch (MERS) Takanori Isobe∗ , Kazuhiro Usuki∗ , Nobuyuki Arai∗ , Tadayuki Kitahara∗ , Kazuhiko Fukutani† and Ryuichi Shimada∗

∗ Solution

Research Organization, Tokyo Institute of Technology, Tokyo 152-8550, Japan Development Bureau, Nippon Steel Corporation, Chiba 293-8511, Japan

† Technical

Abstract— This paper proposes a power converter for induction heating which can control the output frequency. This power converter is using a configuration named MERS (magnetic energy recovery switch). A 90 kVA 150-1000Hz controllable frequency power supply for steel strip induction heating was developed. The power supply used optimally designed IGBTs for the MERS configuration. Loss reduction due to soft-switching and use of the IGBTs makes induction heating possible to operate with variable frequency. Demonstration using the proposed power supply confirms advantages of variable frequency operation.

I. I NTRODUCTION Induction heating is widely used for industrial heating especially in the steel industry. Many high power induction-heaters, up to several mega watts, are installed to hot strip mills [1]. This heating method is efﬁcient compared to other heating methods like gas furnace, because the substance is heated directly by electric power. Moreover, induction heating is a promising heating method to produce value added products because it has high potential for high performance heat control. In general, induction heating uses a high frequency ac power supply and a capacitor connected in shunt or series with the load to compensate reactive power because of low load power factor as shown in Fig 1(a). Therefore this type of converter can reduce ratings of power electronics components; however, frequency cannot be controlled. Moreover, high power induction heating often uses natural commutated current source inverter using thyristors and parallel connected capacitors to turn off thyristors. In this case, it is challenging to operate the inverter when the load condition is changed dynamically or under no load condition. The purpose of this paper is to propose a power converter for induction heating, which can control the output frequency. This power converter is using a switch module named MERS (magnetic energy recovery switch) [2] and consists of a single phase full-bridge with selfcommutated devices and a capacitor. A main purpose of proposed power converter is to reduce losses and power electronics ratings due to advantages of soft switching operation. The losses and ratings reduction makes semiconductors possible to handle full apparent power required for induction heating; therefore, variable frequency operation and robustness to dynamic changing of load condition can be realized. These new feature adds another controllable property to induction heating, making this

(a)

(b) Fig. 1. (a)Conventional circuit configuration for high power induction heating. (b)Circuit configuration of MERS inverter with diode rectifier for controllable frequency induction heating.

heating method more attractive for various industrial fields. In the first part of this paper, circuit configuration and operation principles are described. The second half of this paper describes a 90 kVA 150-1000Hz power supply for induction heating of steel strip and its experimental results. II. O PERATION P RINCIPLES The basic configuration of the proposed converter is shown in Fig. 1(b). A diode rectifier and a dc capacitor of the MERS configuration are connected through a dc inductor. The configuration of the MERS is the same as the full-bridge single-phase inverter; however, the dc capacitor size is comparatively small and this results in the capacitor voltage being changed dynamically from zero to peak voltage by utilizing the resonance between the capacitor and an inductive load.

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(a)

(b)

Fig. 3. Schematic waveforms of load voltage and load current. Applied voltage and ﬂowing current of V-arm device are also shown.

(c) Fig. 2. Operational states of MERS inverter. (a)Capacitor charging. (b)Capacitor discharging. (c)Current free-wheeling.

The MERS configuration has been proposed as a series connected full-bridge configuration with a capacitor and used as an reactive power compensator [3][4][5][6]. This paper proposes the MERS configuration for a dc-ac converter. In this MERS inverter, dc power is supplied from the diode rectifier through the dc inductor into the capacitor of the MERS configuration. Expected main advantage of this configuration is soft switching operation in a wide range of frequencies. Fig. 2 shows operating modes of the MERS inverter and Fig. 3 shows schematic waveforms. When the load current, iload , is flowing with negative polarity and U and Y are turned off, the load current charges the capacitor and the current decreases with the current path as shown in Fig. 2(a). X and V should be turned on before the current becomes zero. Then load current naturally starts to flow in positive polarity and the capacitor discharges as shown in Fig. 2(b). When the capacitor voltage becomes zero, the reverse diodes of U and Y are also turned on since the voltage across the diodes becomes zero, and current flows in two parallel paths as shown in Fig. 2(c). Operation for the other current direction is performed by symmetrical switching.

Capacitor voltage is applied to the load with alternating polarity, and the voltage increases and decreases the load current in periods of (a) and (b). These phenomena are part of a LC resonance between inductance of the load and the capacitor. On the other hand, in period (c), no voltage is applied to the load and current is free-wheeling. Fig. 3 also shows the applied voltage and current ﬂowing in a switch. The current starts to ﬂow in the reverse diode when the applied voltage is zero. The current changes polarity with zero voltage naturally, and the current is shut down immediately with zero voltage. Therefore every switching is performed under zero voltage and/or zero current condition. This means the switching losses and EMI can be reduced. Moreover, this soft-switching realizes the following advantages: 1) Surge voltage caused by turning off appears with almost zero static voltage, while voltage source type converter needs to carry the surge voltage on top of a full rated dc-link voltage. This means the device voltage rating of the MERS inverter can be reduced. 2) Series connection of devices can be achieved with comparatively small snubber circuit and/or not complicated gate control technique because of low dv/dt characteristics. 3) Optimized IGBT for soft-switching can be used. This device can reduce conduction losses due to low on-state voltage [7]. To use this converter in the soft-switching condition, zero voltage period of the capacitor is needed. This is realized

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Fig. 4.

System conﬁguration of 90 kVA prototype power supply for controllable frequency induction heating. TABLE I R ATINGS OF 90 K VA INDUCTION HEATING POWER SUPPLY. Capacity Voltage Current Peak voltage Frequency

90 kVA 900 V 100 A 3000 V 150 - 1000 Hz

TABLE II PARAMETERS OF IGBT OPTIMIZED FOR MERS

Collector-Emitter voltage (max. rating) Collector current (max. rating) C-E saturation voltage (chip, 125◦ C, 150 A)

Fig. 5. Overview of the power supply cabinet installed in a test facility.

under the following condition, 1 √ <f (1) 2π LC , where L is load inductance, C is capacitance of the MERS type inverter and f is switching frequency. For variable frequency operation, the maximum frequency of control range must satisfy this equation. III. D EVELOPMENT OF 90 K VA 150-1000H Z I NDUCTION H EATING P OWER S UPPLY A. System Configuration To demonstrate and investigate the proposed method, a 90-kVA 150-1000 Hz controllable frequency power supply for steel strip induction heating was developed. System configuration and overview of this power supply are shown in Fig. 4 and Fig. 5 respectively. Main specifications of the power supply are listed in Table I. In the line side, a thyristor voltage regulator is installed to control input power and its voltage set-point is given to maintain the output current at a current set-point. This configuration enables controlling output current amplitude. A high impedance transformer in the line side is used instead of using dc inductor. Three series connected IGBTs are used to carry high voltage up to 3 kV. Capacitor

CONFIGURATION .

1200 V 150 A 1.54 V

consists of several capacitor units and total capacitance can be changed from 7.5 μF to 165 μF by wiring modification. B. MERS Inverter The optimum designed IGBTs for the MERS configuration [7] were used. Parameters of the IGBTs used for the power supply are listed in Table. II. For the MERS configuration, high speed switching characteristics are not required because of soft-switching operation. Moreover, large short circuit capability is not required. Consequently, saturation voltage can be prioritized, as well as the forward voltage of FWD (free-wheeling diode). The low saturation voltage of 1.54 V at rated current was achieved, where the conventional IGBT with same rating has 2.10 V saturation voltage. The FWD voltage was also improved. Performance of series connection of IGBTs was evaluated. Fig. 7(a) shows time-trends for the case of 1000 Hz and 100 A. Voltage sharing of three connected IGBTs as shown in Fig. 7(b) was measured. For canceling the effect of different stray capacitance, small capacitors (0.47 μF) were connected in parallel to each IGBT. Voltages across three IGBTs are also shown in Fig. 7(a). Good voltage sharing can be seen even though no special technique is applied to the gate drivers. This indicates the potential for simple series connection of IGBTs in the MERS con-

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(a)

(b) Fig. 6. High impedance transformer. (a)Schematic diagram of manufactured transformer. (b)Overview of the transformer. TABLE III PARAMETERS OF THE HIGH IMPEDANCE TRANSFORMER . Capacity Phase Primary voltage Secondary voltage Secondary current Impedance

40 kVA (200% overload 2 min) 3 400 - 200 V 800 V 28.9 A 61.65%

(b) Fig. 7. Series connection of IGBTs. (a)Time-trends of voltage sharing of three IGBTs for the case of 1000 Hz and 100 A. (b)Conﬁguration.

IV. H EATING E XPERIMENTS ﬁguration, enabling construction of large scale variable frequency induction heating. C. High Impedance Transformer In this power supply, dc reactor is replaced with ac side reactor since dc reactors have a comparatively large dimension, weight and cost. A high impedance transformer which has 61.5% leakage inductance was manufactured to use as ac reactors. Fig. 6(a) shows schematic diagram of the transformer and Fig. 6(b) shows overview of the transformer. The transformer has an auxiliary core, which has linkage with only secondary winding. This causes high leakage inductance and has advantage in copper loss to the combination of an ordinary transformer and ac inductors. Parameters of the high impedance transformer are listed in Table. III.

A. Experimental Setup Experiments using test facilities for induction heating of steel strip were conducted. Fig. 8(a) and Fig. 8(b) show experimental facilities including transverse flux type induction coils. A pair of coils was installed on one edge of the steel strip and the edge was heated by transverse magnetic flux generated by the coils. Typical heating characteristics are shown in Fig. 8(c), which was taken by a thermo view camera. A steel strip was conveyed between the coils. The experiments used two types of steel, carbon steel and stainless steel. This configuration demonstrates edge heating in hot rolling mills of steel industry. Converter operation was started before the strip entered the coil area. One experimental sequence was around 10 s to 20 s period, and frequency and current set-point were changed by steps in the experimental sequence. Electrical parameters depend on target material and position, and

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TABLE IV M EASURED

TYPICAL ELECTRICAL PARAMETERS OF LOAD .

Resistance Inductance Power factor

R L cos φ

4.05 Ω 1.40 mH 0.42

(at 1000 Hz) (at 1000 Hz) (at 1000 Hz)

(a)

(b) (b)

Fig. 9. (a)Current in rms and coil input power active power and apparent power with 1000 Hz and 100 A set-point. (b)Load current and load voltage waveforms at 10 s.

(c)

Fig. 8. Experimental facilities for steel strip induction heating. (a)Layout of induction coil and steel strip. (b)Overview. (c)Thermo view of the surface while in operation.

typical parameters are shown in Table IV. Capacitor of the MERS inverter was ﬁxed at 7.5 μF based on the load inductance shown in Table. IV, and a maximum frequency of 1000 Hz, to satisfy equation (1). B. Full Rating Operation Fig. 9(a) shows current, active power and apparent power with full rated operation. Operation was started at full current set-point, and then, the steel strip entered the coil area at around 2 s. The current was maintained at 100 A while the strip is entering and input active power was increased according to the load increase. These characteristics indicate that this proposed method can operate while the load impedance is changing dynamically such as changing from no load to full load. Waveforms at 10 s are shown in Fig. 9(b). The voltage waveform has zero voltage periods and this conﬁrms IGBTs are turned off with zero voltage. C. Variable Frequency Operation Output frequency can be changed while in operation since series or shunt capacitor to reduce power electronics ratings are not connected in load side. Fig. 10 shows current, active power and apparent power while the switching frequency was changed sequentially. Current set-point

Fig. 10. Current in rms, active power and apparent power while the switching frequency was changed sequentially.

was ﬁxed at 75 A. The time-trend conﬁrms that current was maintained at the set-point by feedback control and frequency changing results in active power changing. Detail waveforms are shown in left part of Fig 11. These waveforms were measured with 7.5 μF capacitor, which is optimized at 1000 Hz, the maximum frequency; therefore, current waveform at 1000 Hz is most similar to pure sinusoidal wave. For lower frequency cases, zero voltage period with free-wheeling load current is extended, therefore, the current waveform becomes near the rectangle waveform. Right part of Fig. 11 shows their analysis into fre-

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(a)

(b)

(c)

(d)

(e)

(f)

Fig. 11. Waveforms at each frequency in the operation shown in Fig. 10. Current set-point was 75 A. Voltage and current waveforms in time domain are shown in (a), (c) and (e). Current waveforms in frequency domain are shown in (b), (d) and (f). (a) and (b) are measured at 5 s, which belong to 175 Hz period. (c) and (d) are at 9 s, 475 Hz. (e)(f) are at 14 s, 1000 Hz.

quency components. These frequency analyses also indicate that the current waveform with 1000 Hz switching frequency includes less other frequency components, and lower switching frequency results in increase of other frequency components. For 475 Hz case and 175 Hz case, major components except switching frequency are odd order harmonic components. However, experiments confirmed that there was no obvious problem related to heating characteristics by the high order harmonics. Current beat was found in the waveforms. The beat frequency was 150 Hz, which is three times of 50 Hz, the line frequency. The beat is caused by input power fluctuation. The thyristor voltage regulator uses diodes for reverse current flow instead of thyristors, therefore, input power from the line side fluctuates at 150 Hz, especially

for low power case. For the case of full rated operation as shown in Fig. 9(b), the beat was not remarkable because of low phase angle of the thyristor voltage regulator. Frequency analysis indicates that the influence of the beat appears as f ± 150 Hz components, which is relatively near to the fundamental frequency, f . Experiments also confirmed that the beat did not cause problem in heating characteristics. D. Heating Characteristics Enabling variable frequency in induction heating field results in heating distribution control. Fig. 12 is a temperature rise distribution with several current frequencies. Surface temperature of heated steel strip was measured by thermo view camera and analyzed into temperature

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Fig. 12. Temperature rise distribution along width direction of the steel strip with several frequencies. Current in rms was ﬁxed at 40 A. The temperature is normalized to the variation between ambient and peak temperature.

[3] T. Takaku, G. Homma, T. Isobe, S. Igarashi, Y. Uchida, and R. Shimada, “Improved wind power conversion system using magnetic energy recovery switch (mers),” Conference Record of the 2005 IEEE Industry Applications Conference, 40th IAS Annual Meeting, vol. 3, pp. 2007–2012, 2005. [4] J. A. Wiik, F. D. Widjaya, T. Isobe, T. Kitahara, and R. Shimada, “Series connected power ﬂow control using magnetic energy recovery switch (mers),” Conference Proceedings of Fourth Power Conversion Conference-NAGOYA, PCC-NAGOYA 2007, pp. 919– 924, 2007. [5] T. Isobe, T. Takaku, T. Munakata, H. Tsutsui, S. Tsuji-Iio, and R. Shimada, “Voltage rating reduction of magnet power supplies using a magnetic energy recovery switch,” IEEE Transactions on Applied Superconductivity, vol. 2, no. 16, pp. 1646–1649, 2006. [6] T. Isobe, J. A. Wiik, T. Kitahara, S. Kato, K. Inoue, N. Arai, K. Usuki, and R. Shimada, “Control of series compensated induction motor using magnetic energy recovery switch,” 12th European Conference on Power Electronics and Applications, EPE 2007, 2007. [7] T. Takaku, N. Iwamuro, Y. Uchida, and R. Shimada, “Experimental demonstration of 1200V IGBT for a magnetic energy recovery switch application,” IEEJ Transaction on Electronics, Information and Systems, vol. 128, no. 4, 2008.

rise distribution along width direction of the steel strip. The center of coils was located on one edge as shown in Fig. 8(a), and the edge is indicated as 0 mm in Fig. 12. In induction heating, alternating magnetic flux induces current in the steel plate, and the current heats the steel. Magnetic flux concentrate in fringe part of the steel plate for high frequencies according to electromagnetic theory, consequently, heating distribution depends on the frequency. Experimental results show that low frequency current has a flatter heating distribution while high frequency current heats the edge part intensively. This result indicates that the variable frequency adds possibility of temperature distribution control to induction heating. V. C ONCLUSION This paper proposed a high frequency ac power supply which operates under soft switching condition. Induction heating has some advantages in many industrial fields because of its efficiency and controllability. The proposed power supply is suitable for variable frequency operation which can add other attractive properties to induction heating. A 90 kVA variable frequency power supply for induction heating using proposed configuration was fabricated. This power supply takes advantage of some features of soft switching, and indicates possibility of loss and rating reduction and construction of large scale system. Heating experiments using the power supply was conducted and it confirmed the variable frequency operation. Heating experiments also demonstrated edge heating, which is used in hot strip mills of steel industry, and the result indicates possibility of heat distribution control. R EFERENCES [1] K. Itoh, Y. Moriura, T. Satoh, K. Arimatsu, N. Nakayama, K. Kimoto, T. Doizaki, and K. Dojoh, “9000kw-1500hz frequency converter for hot bar heater,” in Fourth Power Conversion Conference - NAGOYA, PCC-NAGOYA 2007, 2007, pp. 904–910. [2] T. Takaku, T. Isobe, J. Narushima, H. Tsutsui, and R. Shimada, “Power factor correction using magnetic energy recovery current switches,” IEEJ Transactions on Industry Applications, vol. 125, no. 4, pp. 372–377, 2005.

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