The Study of Two Novel Induction Heating Technology

Youhua Wang, Junhua Wang

Province-Ministry Joint Key Laboratory of Electromagnetic Field and Electrical Apparatus Reliability, Hebei University of Technology, Tianjin, 300130, China

Abstract- With the development of metal metallurgical industry,

there is fast growing interest in induction heating. But there are many disadvantages of the traditional induction heating method. In order to solve that problem, the research of Transverse Flux Heating and Travelling Wave Induction Heating is applied more and more in metallurgical engineering area now. This paper presents the new study and development of these two technologies.

I. INTRODUCTION Longitudinal induction heating was the most often used

method in the past and it is still used today. However, there are many disadvantages of the traditional induction heating method. It is well known that we can increase the operational frequency to decrease the skin depth in order to increase the heating efficiency. For the metal strip with certain depth, 10 kHz or above frequency has to be used if using longitudinal induction heating method. The traditional induction heating method can only applied to metal strips with large thickness.

In this paper two different types of inductors- Transverse Flux inductors and Travelling Wave inductors are considered. While Transverse Flux Induction Heating (TFIH) systems have been studied for many years, Travelling Wave Heating (TWIH) systems are not yet fully appreciated with respect to their main advantages and possible industrial applications [1, 2].

Some standard and specially developed FEM codes based on numerical methods were successfully used for the study of TFIH and TWIH heaters. The main advantages and the particular characteristics of TFIH and TWIH systems such as the effect of slots on induced power distributions were studied. The FEM codes and the results of numerical simulation may be therefore used easily and reliably, for the design of these two systems.

II. THEORETICAL ANALYSIS Maxwell equations are the theoretical basis of all macro-

electromagnetic phenomena, and its differential equations are as following:

DH Jt

∂∇× = +∂

(1)

0B∇ ⋅ = (2) BEt

∂∇ × = −∂

(3)

D ρ∇ ⋅ = (4)

This project overlooks the functions of the accumulative charges, that is 0ρ = . From (1) to (4), H -magnetic field

intensity, J -current density, E -electric field intensity, B -magnetic field intensity, D -electric displacement vector. According to the relationship betweenH、 J 、 E、 B andD , we can get

( )DH J E Et t

σ ε∂ ∂∇× = + = +∂ ∂

(5)

The corresponding vector form ( )H j Eσ εω∇× = + (6)

When σ >> εω , Dt

∂∂

can be neglected. And when the

frequency is up to thousands of hertz, it satisfies σ >> εω .

Therefore, in this project we can neglect Dt

∂∂

. Equation (1) is

simplified as H J∇ × = (7)

For the eddy current field, basic equations can be established with A φ− method. Then we introduce magnetic vector potential A , define it as B = A∇ × , and eventually attain the current density formula

( ) sAJ Jt t

φσ ∂ ∂= − + ∇ +∂ ∂

(8)

TFIH and TWIH comply with the Maxwell equations strictly in theory. From the equations above we can see that the current density in the strip is decided by the magnetic flux and the field intensity, so magnetic field inducted by TFIH and TWIH can create different vortex effect in the strip.

III. TRANSVERSE FLUX INDUCTION HEATING

The TFIH inductor configuration has been selected to be as close as possible to an existing system for assuring the possibility of checking the results by tests.

It consists of two inductors with current carrying conductors located inside the slots of two magnetic yokes and perpendicular to the length of the strip and to the direction of its movement. The inductors are situated on both sides of the strip.

The aluminum work-piece has the following dimensions: thickness 2.5 mm, width from 1200 to 1600 mm, while the air

572

Authorized licensed use limited to: National Taiwan Univ of Science and Technology. Downloaded on May 17, 2009 at 08:57 from IEEE Xplore. Restrictions apply.

gap between magnetic yokes is 60 mm. On each inductor there are three coils and six conductors in every coil (Fig.1).

Fig.1 Coil connections for transverse flux inductor supplied by a three phase

system Compared with Longitudinal Flux Induction Heating (LFIH),

TFIH has many advantages. It has been studied for a few years. Now, researchers in China pay much attention for its study and have attained some notable achievements

a) The relationship between eddy current distribution and coil geometry in TFIH equipment was analyzed. The non-linear coupled electromagnetic-thermal problem was solved through finite element method (FEM). Based on that, new coil geometry was designed to get a homogeneous temperature distribution on the surface of the strip, shown in Fig.2 [2].

Fig.2 New structure of the TFIH equipment

b) A new coupling calculation method “Smooth movement of thermal sources” was proposed [3]. The step can be “copied” from the former place to the latter. Eddy current field can avoid double counting, substantially reducing coupling numerical simulation time. That approach can nearly get the same results, but greatly reduce the computation time. Discrete continuously moving thin strip is shown in Fig.3.

Fig.3 Discrete continuously moving thin strip

c) Neural Networks with FEM was introduced to predict eddy current and temperature distribution on the continuously moving thin conducting strips in TFIH equipments. With the temperature prediction results, a good guessed value of the

temperature dependent parameters for each finite element and initial values for temperature field solution are provided [4].

Previous studies revealed that the eddy current density attains its maximum value around the coil projection, and the farther from the central lines, the less density of the eddy current. Also, the width ratio of the coil and the strip has a considerable influence on the distribution of eddy current. If the ratio is larger, the edge eddy current distribution will be overheating. Instead, smaller ratio may result in eddy current decrease.

The predicted results are presented in Fig.4. Network prediction results speed up the iteratively solution process for the non-linear coupled electromagnetic-thermal problems.

Fig.4 The predicted result of temperature field distribution on the surface of

one half of the thin metal work piece

IV. TRAVALLING WAVE INDUCTIONHEATING Travelling Wave Induction Heating, as one of the multiphase

induction heating systems, has particular features which make them attractive for application to some heating and melting processes in industry. Among the advantages we can mention the possibility to heat quite uniformly thin strips or regions of a body without moving the inductor above its surface, to reduce the vibrations of inductor and load due to the electro-dynamic forces and also the noise provoked by them, to obtain nearly balanced distributions of power and temperature [5]. Fig.5 is the coil connections of TWIH. The fruits of its research are as following.

Fig.5 Coil connections for travelling wave inductor supplied by a three phase

system A double-side TWIH system for the continuous heating of

thin metal sheets has been analyzed by comparing its performance with TFIH system, which is shown in Fig.6. We also deeply studied the distributions of magnetic force, eddy

573

Authorized licensed use limited to: National Taiwan Univ of Science and Technology. Downloaded on May 17, 2009 at 08:57 from IEEE Xplore. Restrictions apply.

current and power density along the position of the strip and improved the uniform of temperature distribution by applying different dispositions of the yokes. Fig.7 displays the temperature distribution of TWIH.

Fig.6 Axisymmetric Slice of the Travelling Wave Induction Heating System

Fig.7 Temperature distribution of TWIH along the strip’s surface

As TWIH using three-phase induction heaters, upper and lower induction heaters can be installed properly to make up better for the low-fields, so as to achieve uniform heating results. And it applies the typical three-phase windings and parametric analysis to assess the key parameters (transmission of electricity to the work-piece, efficiency, power factor, et al) and the distribution of electricity. Compared with the TFIH system, three-phase induction heating heater not only has the same advantages, but owns the ability to produce more uniform temperature distributions, and reduces industrial noise with low vibration.

V. CONCLUSION The simulations have shown that a relative good power and

eddy currents distribution across the strip can be obtained for both systems using the method of spatial control of the electromagnetic field. The TWIH system, however, looks to be more promising for further study because it might solve some problems related to metal deformation and noise from the system, which play important roles to the uniform temperature distribution. Moreover, the values of power density in the load

are able to produce good heating of the strip. Also, it is demonstrated that a convenient choice of the

systems and their parameters (inductor geometry, poly-phase winding arrangement and supply, air gap values and control of the relative position of the inductors) gives the opportunity of obtaining very good energetic performances and a certain degree of control of the value and distribution of the power transferred to the load.

The main advantages and the particular characteristics of TFIH and TWIH such as the effect of slots on induced power distributions were studied. The FEM codes and the results of numerical simulation may be therefore used easily and reliably, for the design of different systems.

ACKNOWLEDGMENT

This work was supported in part by the National Natural Science Foundation of China (50477016) and the Provincial Natural Science Foundation of Hebei, China (E2005000019).

REFERENCES [1] Yang Xiaoguang, Wang Youhua. The Effect of Coil Geometry on the

Distributions of Eddy Current and Temperature in Transverse Flux Induction Heating Equipment. Heat Treatment of Metals. 2003, vol.28, no.7, pp.49-54.

[2] Yang Xiaoguang, Wang Youhua. New Method for Coupled Field Analysis in Transverse Flux Induction Heating of Continuously Moving Sheet. Heat Treatment of Metals, 2004, vol.29, no.4, pp.53-57.

[3] Yang Xiaoguang. Transverse Flux Induction Heating and Its Optimization Problem, The doctoral dissertation submitted to Hebei University of Technology, 2004.

[4] Xiaoguang Yang, Youhua Wang, Fugui Liu, Weili Yan. 3D Finite Element Computation Combined with Neural Networks Prediction of Eddy Current and Temperature Distribution in TFIH Equipments. Electrical Machines and Systems, 2003. ICEMS 2003. Sixth International Conference, 9-11 Nov. 2003, vol.2, pp.724 – 727.

[5] Wang Junhua, Wang Youhua. Comparison of Eddy Current Density between Travelling Wave Inductor and Transverse Flux Inductor. Conference on Theory and Advanced Technology of Electrical Engineering (CTATEE’07), 11-16 August 2007, Zhangjiajie, China, pp.150-153

[6] F.Dughiero, S.Lupi, P.Siega, “Analytical Calculation of Traveling Wave Induction Heating Systems,” International Symposium on Electromagnetic Fields in Electrical Engineering 1993, 16-18 September 1993, Warsaw-Poland, 207-210.

[7] S.Lupi, M.Forzan, F.Dughiero, et al. “Comparison of edge-effects of transverse flux and travelling wave induction heating inductors,” IEEE Transactions on Magnetics, 1999, vol.35, no5, pp.3556 -3558.

[8] Zanming Wang, Xiaoguang Yang, Youhua Wang, et al. “Eddy Current and Temperature field Computation in Transverse Flux Induction Heating Equipment,” IEEE Transactions on Magnetics. 2001, vol.37, no.5, pp.3437-3439.

[9] F.Dughiero, S.Lupi, V.Nemkov, et al. “Travelling wave inductors for the continuous induction heating of metal strips,” Proceedings of the Mediterranean Electrotechnical Conference-MELECON.1994, vol.3, no.3, pp.1154-1157.

x

Strip

y Air Coil(240°) Coil(120°) Coil(0°)

574

Authorized licensed use limited to: National Taiwan Univ of Science and Technology. Downloaded on May 17, 2009 at 08:57 from IEEE Xplore. Restrictions apply.