Closed and quasi-closed yoke con fi gurations for travelling wave induction heaters

The salient features of the travelling wave induction heating (TWIH) make them very important technique in the field of heating flat metals. This study proposed two novel configurations, closed and quasi-closed, of the heater yoke. The proposed yokes designed to reduce the problems of slot effects and reduce the leakage flux, in order to focus the magnetic field within the heating region, which lead to improve the performance of the TWIH system. A finite-element simulation and analysis is represented in this study by using ANSYS programme code. A comparison analysis between the proposed configurations and the traditional one shows a superior performance of the proposed types for workpiece thickness >2 mm. The proposed methods give progress in produced power, efficiency and temperature about; 7–23, 15–40 and 10–25%, respectively, for thickness >2 mm. Moreover, wider and uniformed heat distributions are achieved with conjunction of typical yoke.


Introduction
Travelling wave induction heating (TWIH) system is a very important device in the field of heating rectangular flat metal workpiece, as it has particular features which enable it to overcome many disadvantages associated with other induction heating devices such as Ross coil, multilayer coil and transverse flux induction heating [1].These TWIH features could make this kind of heating more profitable and attractive for many industrial heating processes.The most important features are: performs quite uniform heat distribution over the surface of the strip without moving the inductor, lower mechanical vibration and noise due to the electromagnetic forces, represents nearly balance load to the utility, the ability of using line frequency, gives sufficient mechanical strength and provides protection for the coils by the ferromagnetic core [2].In spite of those features the TWIH systems are not appreciated yet, because they have not been fully investigated as a way of other traditional methods, where a few researches concern to this area of the word [3].The performance of the TWIH system can be affected by many factors such as the edge-effect, slots-effects, winding arrangement etc.Most of researches focus on the two-dimensional (2D) distribution of the induced eddy current and power densities along the centre line of the workpiece under the heater face.Neither the 3D distribution of the eddy current nor the leakage flux had been concerned in the literature.
This paper proposes two novel configurations of the travelling wave (TW) inductor yoke to reduce both of leakage flux and slot effects.Which constrain the magnetic flux density in the area faces of the heater and hence obtains higher and more uniform eddy current and temperature distribution.The closed and quasi-closed yoke configurations are proposed in this paper to achieve this improvement.The features of the proposed systems are: temperature rising and uniformity, eddy current distribution, efficiency and power density are demonstrated.Moreover, the analysis of input power, power factor and magnetic flux density are examined in conjunction with traditional TW inductor.Moreover, the influence of load thickness has been analysed for different types of inductors in order to realise the features of the proposed methods.
There are two main analytical methods to analyse the performance of the TWIH system: the traditional analytical method and the finite-element numerical method.The first method is not suitable for this analysis because the geometry of the system, magnetic non-linearity and skin effect cannot be described, whereas the 2D and the 3D finite-element analysis (FEAs) gives good solution for both transient performance and steady-state analysis of the induction heater [4].To realise the features of the proposed methods both of harmonic and transient 3D FEA is essential in this paper.

Slot effects
The configuration of the conventional double sides TW heater is shown in Fig. 1.It consists of double inductors on the opposite sides of the load.There are relatively large air-gap between the inductor and the strip due to the thickness of the interposing refractory material [5].It is obvious that the magnetic yoke has a salient pole configuration.Their slots make the efficient air-gap between the coils and load (g1) longer than the air-gap between the tooth and load (g) as in Fig. 1.The magnetic permeability of the metal yoke is much higher than the air, so the slots existence could influence the magnetic field distribution.The induced magnetic field density below the tooth position will be bigger than the air position in the strip.This is the first drawback of the traditional TW inductor.For the same reason, salient poles and non-uniform air-gap, some of the magnetic flux lines leak out the yoke and diverge out the area faces of the heater, which causes unfocused induced eddy current and reduces heat generation; this is the second drawback of the TW inductor.Those could affect the power is to be transferred efficiently from the coil to the load [6].
Many aspects which are affecting the performance of the heater have been considered, and the relationships between the air-gap and pole-pitch, the slot-tooth ratio or slot-depth have been analysed and improved in the literature such as crossed yoke, slot wedges, distributed vernier and slot less travelling wave induction heaters [7,8].However, definite criteria for the choice of the winding arrangement and the optimum design are not yet available [8].In this paper, the possibility of focusing the magnetic flux lines to govern the eddy current density within the area faces of the heater is investigated.Thereby designing a novel yoke configurations: closed and quasi-closed.

Configuration of the heater yoke
The double sides travelling wave inductor types and configuration are summarised briefly in [7].The schematic configuration of typical TW inductor shown in Fig. 1, all systems mentioned in this paper consist of double linear inductors located on the opposite sides of the load and perpendicular to the direction of its movement [5].As can be seen, the current carrying conductors located inside the slots of two magnetic yokes.The winding arrangements could affect the results of 3D simulation and the choice of a suitable power supply and heat distribution on the strip surface.Researches demonstrated that the wide and uniform heat range and higher rise heating time can be achieved with short-pitch coil and up-down mode more than of full-pitch and left-right coil [3,9].Therefore, in this paper the short-pitch and up-down coil arrangement is used and excited by three-phase current of 120°p hase shift apart and 50 Hz frequency.The first proposed configuration is a quasi-closed yoke and is depicted in Fig. 2.
The same winding arrangement, excited current and frequency of the traditional system are used here.The top side of the yoke is expanded with the same number and size of yoke tooth are closed the magnetic path from the front and back.These additional tooth somewhat encloses the path of induced eddy current which flows in the area under yoke face.The flux lines of the magnetic field density passing through the ferromagnetic quasi-closed yoke are depicted in Fig. 3.
The second proposed configuration is designed to be fully closed around the coils from five sides as shown in Fig. 4. The additional walls provide a ferromagnetic box focusing the magnetic field within the heating area under the heater face.This configuration gives more reduction in leakage flux and induces more eddy current density.Fig. 5 illustrates the flow of the magnetic field density by the same winding arrangement and excited current.

System performance and analysis
To realise the features of the proposed closed and quasi-closed yokes, the energy transferred efficiency, input power factor, output power density, eddy current and strip temperature must be investigated for different strip thicknesses.Usually the analytical methods are used for the integral parameters investigation.Moreover, these methods are applied for simple configurations and some assumptions must be made and sometimes reduction of dimension may be essential [2], whereas the numerical FEAs are more useful for determination of the induced eddy currents and power distribution if a high accuracy is demanded such as taking the edge-effects and slots-effects of the induction end workpiece into consideration.Therefore, 2D or 3D FEAs is a universal choice for simulation and analysis multiphase systems.Both methods need a relatively long calculation time, which includes large quantities of work to input data and a previous knowledge of the poly-phase exciting current (amplitude, frequency and phase) [8].In this paper, numerical FEAs are used to investigate the electrical and thermal performances.The influence of the load thickness has also been analysed.A comparison analysis is represented in this paper by using both of transient and harmonic FEA of ANSYS ® programme code.The non-linearity of the B-H curve of the steel magnetic yoke is taken into account.The main parameters of the traditional, quasi-closed and closed travelling wave induction heater systems which are simulated and examined are shown in Table 1.
The salient features of the proposed configurations can be demonstrated by examining the eddy current density and temperature distributions over the strip surface.The one half symmetry simulations eddy current density and temperature distributions, respectively, can be shown in Figs. 6 and 7.The input phase current, power factor, strip time-averaged power density, system efficiency, temperature and the rate of energy transferred (which is a criteria proposed from the authors equal to the percentage ratio of output temperature/input power) of the different types of heaters are investigated as illustrated in Figs.8-13.

Discussion
The aim of this paper can be realised clearly by examining Figs. 6  and 7, in which one can see the difference between the eddy current and temperature distributions.In the traditional case, the eddy current flows out of the heating region as in Fig. 6a, because of the leakage flux and slot effect.This leads regressing the heat value and distribution as in Fig. 7a, whereas in the proposed configurations the heating regions are expanded, more uniformity of heat distribution is obtained and progressive in temperature values.This is because of focusing the magnetic field within heating region, which leads to enclose the eddy current path and forbid it from flowing out the region as in Figs.6b, c and 7b, c.The profit in  temperature is about 9% in the quasi-closed and 11% in the closed yoke (for t = 1 and g = 1 cm).
In addition to this gain in temperature, there are many other benefits achieved from the proposed configurations in conjunction with traditional one for strip thickness >2 mm.Obviously, the input and output analyses show two regions of the variation of strip thickness: the first region is for very thin strip (less or equal to 2 mm), in which the performance of the traditional configuration is better than the proposed types.The second region is for thickness >2 mm, in which the proposed configurations give superior performance for most input and output variables except the line power factor.Moreover, it can be seen clearly that the closed yoke gives better performance than quasi-closed yoke, where Fig. 8 shows a reduction in the line current about 10% in quasi-closed and 12% in closed, also Fig. 10 shows an increase in the strip average power about 7-23% depends on thickness and Fig. 11 depicts a progress in temperature between 10 and 25% for thick strip.Fig. 12 shows a profit in efficiency from 15 to 40% in thick strip in spite of the reduction of power factor.Moreover, finally an increase in the duty of energy transferred from 25 to 45% for a thick strip.

Conclusions
Obviously, for workpiece thickness >2 mm, the finite-element simulation and analysis demonstrate the features of the proposed closed and quasi-closed yoke configuration in conjunction with the traditional configuration, where they show convenient focusing of the magnetic field within the heating region, and hence enclose the flow of the induced eddy current under the face of heater for both of closed and quasi-closed yokes.Moreover, the produced temperature being more uniform and higher than traditional by 10-25% depending on strip thickness.Besides this profit they show reduction in the input phase current and increase in output power and efficiency from 15 to 40% in spite of the reduction in power factor.Generally, the closed yokes give better performance from the quasi-closed yoke, but certainly they have higher weight and cost.

Fig. 5
Fig. 5 Magnetic flux distribution of the closed yoke

Fig. 6
Fig. 6 One half symmetry eddy current distribution a Traditional b Quasi-closed c Closed yoke

Table 1
System parameters