TEMPERATURE PREDICTION AND THERMAL MANAGEMENT FOR COMPOSITE MAGNETIC CONTROLLERS OF INDUCTION COILS V. Nemkov, R. Goldstein, J. Jackowski, N. Vyshinskaya, C. Yakey Fluxtrol, Inc., 1388 Atlantic Blvd, Auburn Hills, Michigan 48326, USA ABSTRACT. Temperature control of magnetic controllers (concentrators, cores, shields, shunts) is an essential part of the induction coil design. Prediction and study of the coil copper have been described in a presentation "Influence of Cooling Conditions on Induction Coil Copper Temperatures" (V. Nemkov, R. Goldstein) [1]. That study was made using Flux 2D computer simulation program. The present study is devoted to temperature prediction and control in both copper and magnetic controller. Computer simulation with programs Flux 2D was used for modeling of the whole coil head operating conditions. Flux 2D has no standard option to account for magnetic losses in the concentrator material and their influence on the concentrator temperature. Special procedure has been developed to solve this problem. A case study illustrates the calculation procedure and influence of the coil head design, frequency, controller material selection and application technology on the controller temperature. New composite magnetic materials and temperature management methods are also described. INTRODUCTION Magnetic flux control, i.e. modification of magnetic field intensity and distribution may be accomplished by variation of the coil turn shape and positioning, by insertion of non-magnetic shields (Faraday rings) and magnetic flux controllers (concentrators, cores, impeders, shields). Magnetic flux controllers are made of laminations, ferrites and magnetic composites. The use of magnetic controllers on induction coils can provide the following advantages: - accurately control heat pattern resulting in better part quality - improve the coil efficiency and power factor resulting in higher production or energy savings - protect machine or part components from unintended heating - better utilize power transferred to the part in local heating processes - reduce current demand to the coil improving performance of the whole system. In many applications controllers give more than one benefit. It is important to mention that the controller design, selection of material and application technique can strongly influence lifetime of heavy loaded induction coils. Soft magnetic composites (SMC) are well established materials for magnetic flux control in multiple induction technologies especially at middle and high frequencies [2] with Fluxtrol and Ferrotron materials dominating on the market. Main advantages of these materials are: - excellent machinability that allows the user to create controllers of different shape - with different material grades it is possible to cover the whole range of frequencies used in induction technique, from line frequency 50/60 Hz to 13.56 MHz - high enough temperature resistance (up to 300 C for short exposure time) - high magnetic permeability (up to 120) that is sufficient for all induction applications. Of course, using controllers requires special knowledge in application technique in order to prevent premature failure of the coil. The main failure modes of the heat treating coils are the copper cracking due to overheating in cyclic processes and the concentrator overheating.

These two effects are coupled and in gnral must be considered together. On one side, characteristics of magnetic controller can influence the temperature of hot spots of the coil copper profile [3]. It was shown that in cyclic operations good thermal contact of copper to the controller can reduce maximum temperature of the copper corners. On the other side, if copper temperature in areas in contact with controller is too high due to incorrect coil profile cooling, the concentrator can be prematurely destroyed by heat transferred from the coil, which locally heat the concentrator instead of cooling....

Il II Figure 2 Magnetic flux density distribution in the System with magnetic field lines (left). Sectioned regions of average magnetic flux density values in the core (right) Smaller flux density regions can be created if better discretization accuracy is desired, such as dividing the cross-section into small rectangular or triangle areas. Experience of calculations shows that even rough discretization provides good results due to strong averaging properties of the Fourier equation operator. The power densities in each section of concentrator may be calculated from Eq. 1, and then inputted into...

coefficient a equals to 2 and b equals to 1. It is correct for low frequencies. For middle and especially for high frequency, losses grow with B and f faster than at low frequency and coefficients a can be as high as 2.2 and coefficient b can be 1.25. Therefore these coefficients must be carefully selected for good accuracy or losses defined directly from the curves and tables, not from the approximation expressions 1 or 2. Simulation Results This model allows us to study an influence of a variety of factors on the concentrator temperature such as its material properties, frequency, coil power,...

Color Shade Results Quantity : Temprature Deg. Celsius Time (s.) : 0.003E6" Phase (Deg): 0 .Scale / Color 20 / 31.25 31.25 / 42.5 42.5 / 53.75 53.75 / 65 65 / 76.25 76.25 / 87.5 87.5 / 98.75 98.75 / 110 110 / 121.25 121.25 / 132.5 132.5 / 143.75 143.75 / 155 155 / 166.25 166.25 / 177.5 177.5 / 188.75 188.75 / 200 III Max T = 90 C Max T = 193 C Max T = 127 C Figure 4. Temprature distribution in the coil head: left - 4 kA, 20 kHz, Fluxtrol A with optimal orientation; center and right - 2.2 kA, 200 kHz, Fluxtrol 50 with non-optimal (center) and optimal orientation Results of Table 1 and Fig.4 correspond...

Design of the coil copper and interface with concentrator. Simulation revealed and practice confirmed that in many cases the magnetic controller starts to deteriorate not because of big magnetic losses but because of a local contact to hot copper mainly in the corners of the coil tubing where copper losses are high. This situation is especially critical when hardening inductors have no permanent cooling and their copper is cooled by quenching media after Figure 5. Magnetic properties of Fluxtrol 75: magnetization curve (left) and permeability vs. flux density (right) 0 20 40 60 80 100 120 140...