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World Academy of Science, Engineering and Technology 18 2008 Work Coil Design used in Induction Hardening Machine Han Phyo Wai, Soe Sandar Aung, Jr., and Thidar Win Keywords—Changing of applied frequencies, work coil type and work piece size, design of helical multi-turn type of work coil and induction hardening heat is caused by the induction of current, the process is called induction heating. And as the induction hardening is a branch of induction heating system, the process of induction hardening is the same as that of induction heating. The heat development in the work piece depends on the rating of the induction hardening machine, the electrical resistivity of the work piece and the configuration of the work coil and its relationship of the work piece. Although the induction hardening system generates the heat energy on the work piece, there is no tough between the work coil and work piece. The source of heat is developed only in the work piece but not on the work coil. Therefore, it is a non-contact method of heating. In induction hardening process, the intense of heat is highest at the surface of the work piece and lowest at the centre of it. Depending upon the kind of hardening, the development of heat must be from the surface of the work piece to interior of the limited zone during heating cycles. After being heated for some times, the work piece must be quenched by suitable quenching medium to be desired hardenability. I. INTRODUCTION II. INDUCTION HARDENING SYSTEM N induction type hardening system, the essential requirements are the coil called work coil and the alternating current supply. The work coil may be wound the material to be hardened called work piece or it may be near by the work piece and the work coil is applied by the high-frequency current. The flow of current in the work coil produces a magnetic field or flux that surrounds each turn of the work coil and this flux passes through air or any metal that is within or near the work coil. The alternating current causes the flux to change or the alternating magnetic field. And the change of flux induces a voltage within the work piece. Due to this induced voltage, the induced current is flown through the work piece, and as the current passes through the resistance of the work piece, the heat is developed in the work piece. Since this There are AC power supply, rectifier circuit, inverter circuit, work coil and quenching system for the whole system. The AC power source may be single-phase or three-phase, and it applies line frequency and line voltage. The control rectifier converts the AC voltage to the DC values and applies the desired DC current to the inverter circuit. The inverter changes the DC signals to the AC signals with desired frequency to apply the work coil. The work coil uses the input values and makes the work piece develop heat. When the work piece has been heated for a time, the quenching system is applied to the work piece. Fig. 1 is the block diagram of induction hardening system. Abstract—The induction hardening machines are utilized in the industries which produce machine parts and tools needed to achieve high wear resistance. If they are constructed in local, the industries can utilize them commonly and easily. As the machines are designed and constructed in accordance with the desired products, the quality of products is higher and the products can be used efficiently. In the study of Design and Construction of Induction Hardening Machine, the design of work coil is presented. A small model induction hardening machine is designed for the output power 5 kW and operation frequency 35 kHz. As the work coil is the heart of the machine, a good coil design reduces power requirement and causes good efficiency. The design of work coil that causes the most magnetic flux density in the work piece, the least power consumption and develops desired heat during limited time is studied. It has been attempted to design the helical multi-turn type of work coil for any size of cylindrical shape of work piece with uniform surface. The design study is covered the changing of work piece size, the changing of work coil turns and the changing the applied frequencies. I Han Phyo Wai is with the Electrical power Engineering Department, , Technology University Mandalay, Myanmar (e-mail: hanphyowai2007@ gmail.com). Soe Sandar Aung is with the Electrical power Engineering Department, Technology University Mandalay, Myanmar (e-mail: soesandarag@ gmail.com). Thidar Win is with the Electrical power Engineering Department, Technology University Mandalay, Myanmar (e-mail: malthida80@ gmail.com). Fig. 1 Block diagram of induction hardening system Typically the other requirements are the protection system, choke coil or filter circuit, load matching system and cooling system. In most of the induction hardening systems, the 352 World Academy of Science, Engineering and Technology 18 2008 6) 7) 8) 9) protection circuits, over voltage protection and over current protection are matched with the rectifying triggering circuit because of the complicated operation functions of inverting triggering circuit. The voltage and current sampling circuit senses the work coil voltage and current. It uses the current transformer to detect the over current and uses the voltage transformer to detect the over voltage. The over voltage and current are mainly caused when the operated work piece is changed. For a constant size and shape of work piece, all settings do not need to change, but according to the various size and shape of work piece it is needed to change the voltage and current setting. The power control of work coil is done by several ways. The first one is varying the DC link voltage. The reducing of supply voltage to the inverter can decrease the power at the work coil. As the inverter is supplied from the rectifier, by controlling the rectifying operation the variable voltage DC supply fulfils the required voltage of inverter. Varying the DC link voltage allows full control of the power. The second one is varying the duty ratio of the devices in the inverter. By reducing the on-time of the switches in the inverter the power processed by the inverter can be decreased. At the time of the devices are switched on, power is only sourced to the work coil. When the devices are tuned off, the load current is left to freewheel through the diodes. So, varying the duty ratio of the switches allows full control of the power. The next one is varying the operating frequency of the inverter. If the operating frequency of the inverter is moved away from the resonant frequency of the load circuit, there is less resonant rise in the load circuit and the current in the coil diminishes. Therefore, less circulating current is induced in the work piece. The last one is varying the value of inductance in the matching system. Altering the inductance of the matching system adjusts the value to which the load impedance is translated. In general, decreasing the inductance of the matching system causes the work coil impedance to be transformed down to a lower impedance. This lower load impedance being presented to the inverter causes more power to be sourced from the inverter. Conversely, increasing the inductance of the matching system causes a higher load impedance to be presented to the inverter and causes less power to be sourced from the inverter. Oxidization is less because of short time heating. Distraction is smaller compared to that of other methods. Easy for automation. Skilled operators will not be required to operate the machine. IV. DEPTH OF HARDNESS For work piece, the depth of penetration is that a distance from the surface to the interior of work piece that to be hardened. And it is also called the depth of hardness. It is expressed as δw = 1 2π 1 µ r fσ (1) . Where, δ = depth of hardness, m f = applied frequency, Hz µr = relative permeability, H/m σ = electric conductivity of work piece, mhos/m V. ROLE OF WORK COIL The work coil is the heart of induction hardening system. It is the main part of the system that transfers the electric energy to the heat energy. So, to be the maximum transfer of heat energy to the work piece the role of work coil is very important. The work coil is generally formed to be the largest possible number of magnetic flux lines inserted in the work piece at the area to be heated. The denser the flux at this point, the higher will be the current generated in the work piece. As the current generated in the work piece causes the heat energy, the heating rate and the depth of heat penetration absolutely depend upon the work coil. The work coil faces various types, sizes and shapes of work pieces for a wide variety of heating operations. So, the formation of work coil is the most important one for the whole system to be a good efficiency. A. Types of Work Coil Work coil for induction hardening is made in a wide variety of styles, shapes and sizes. Depending upon the natures of work piece the styles of work coil are changed. According to the geometry of work piece surface the shape of work coil is varied. And the size of work coil is governed by the length of work piece. The work coils are mainly divided into helical, pancake and internal and also they are shown in Fig. 2. A helical coil is generally used for the part or area to be heated located within the coil and, thus, in the area of greatest magnetic flux. A pancake coil is used for heating flat surfaces and so flux from only one surface intersects the work piece. An internal coil is used for heating inner surfaces of holes. In general, helical coils used to heat round work pieces have the highest values of coil efficiency and internal coils have lowest values. Coil efficiency is that part of the energy delivered to the coil that is transferred to the work piece. This should not be confused with overall system efficiency. Besides coil efficiency, heating pattern, part motion relative III. BENEFITS OF INDUCTION HARDENING The benefits of induction hardening are as follow: 1) Depth of hardness can be minutely controlled. 2) Different degrees of hardness can be obtained within a single work piece. 3) As there is no flame or heating element, there is no smoke, ash or pollution during operation, it is a clean operation process. 4) As the localized heating is developed, the dissipation of heat is small and the efficiency of the heating system is good. 5) Uniform results after a heating cycle has been established. 353 World Academy of Science, Engineering and Technology 18 2008 considerations are as follows: Single-turn coils are preferred when the heated area is narrow or restricted. Single-turn coils are more practical where the height does not exceed the diameter. Multi-turn coils are to be preferred for heating long areas. When the length of a coil exceeds eight times its diameter, uniform heating may be difficult. Long area should be heated by progressive feed through short coils. to the coil, and production rates are also important. Because the heating pattern reflects the coil geometry, inductor shape is probably the most important of these factors. Quite often, the method by which the part is moved into or out of the coil can necessitate large modifications of optimum design. If one part is needed every 30 seconds but a 50 seconds heating time is required, it will be necessary to heat parts in multiples to meet the desired production rate. It is important to look at a wide range of coil techniques to find the most appropriate one. C. Inductance Losses When connecting the leads of a heating coil to a generator, especially those of the quick change type, it is desirable to keep them as close together as possible in order to avoid inductance losses between the leads, as might be represented by the example as shown in Fig. 4. The spacing would result in unwanted inductance, so the maximum heating of the work within the coil could not be attained. By providing leads as shown in Fig. 5, where the space is held to a minimum, there is better assurance of maximum heating of the work. Fig. 2 Types of work coil B. Helical Coil Type The copper conductor is wound or formed either symmetrical in contour or shaped to suit the outline of the part to be heated. This type is suited to surface heating of shafts and bars. The shape of helical coil mainly depends on the shape of work piece. So, the coil may be in the shape of round, rectangular, formed, spiral helical and others. Of all, the round coils are commonly used and suitable for several shapes of work piece. The coil includes single-turn or multi-turn. Depending upon the application, it is a single-turn, singleplace coil or a multi-turn, single-place coil. Both coils are used for heating a single part at a time. And it is a single-turn, multiplace coil or a multi-turn, multi-place coil for heating the multi parts at a time. The following figure shows typical configurations for helical coils. Fig. 4 Large coil lead spacing Fig. 5 Narrow coil lead spacing VI. WORK COIL CHARACTERIZATION To achieve uniform heating along the work piece length, the work coil must be modified to provide better uniformity. Adjusting the work coil turns, coil pitch and coupling distance with the work piece to achieve a uniform heating pattern is known as work coil characterization. In applying multi-turn coils, the pitch of the turn windings has a direct relation to the depth of heat penetration. In multiturn work coils, the spacing between turns should not be more than one half of the diameter of the tubing itself. Beyond this limit, or with loosely wound coils, nonuniform heating is likely to occur. If the work coil is placed closely to the work piece, the heated zone is the exact shape of the coil winding but not uniform throughout the surface of work piece and the large heating zone as shown in Fig. 6. By increasing the coil coupling as shown in Fig. 7, the heating becomes more uniform but the less heated layer. A good relation is that the Fig. 3 Typical configurations for helical coils The use of single-turn coil and the use of multi-turn coil are based on usually the area of the zone to be heated. The basic 354 World Academy of Science, Engineering and Technology 18 2008 VIII. DESIGN RESULTS coupling distance should be equal to the coil pitch and not more than two times coil pitch to be uniform heating on the work piece. The number of work coil turns is mainly based on the length of work piece and the pitch of coil windings. Then it is expressed as follow. (2) lw N = dc + C p Where, N = number of. turns of work coil Lw = length of work piece to be hardened Cp = pitch of coil winding The results for work coil, conductor, work piece and electrical properties of the system are calculated. These results are presented by the following tables. Fig. 6 Nonuniform heat penetration TABLE II RESULTS FOR WORK COIL specification shape Number of turns Inner diameter (m) Outer diameter (m) Length (m) Coil pitch (m) Coupling distance (m) Fig. 7 Uniform heat penetration If the work piece is not straight, coupling must decrease. At high frequencies, work coil currents are lower and coupling must be increased. With low and medium frequencies, work coil currents are considerably higher and coupling must be decreased. specification material shape thickness (m) diameter (m) Length (m) To design the work coil, it is needed some specifications of work piece, conductor and operation. The specifications for operating process are the ambient temperature is assumed 300.15 K, the desired hardened temperature is 1116.48 K, the duration of hardened time is about 10 sec of apply frequency is 35 kHz. Table I is for the specifications of conductor used as work coil and work piece material. specification Material Shape Nature of surface Depth of hardness (m) diameter (m) length (m) Cross sectional area (m2) Surface area (m2) Volume (10-6) SPECIFICATIONS OF CONDUCTOR AND WORK PIECE value copper 1.7 × 10−8(at 293.15 K) 1 1040 carbon steel Resistivity (Ωm) 12.7 × 10−8(at 293.15 K) 115.6 7 × 10−8(at 1253.15 K) Permeability (H/m) Specific heat (J/kg.K) 1 434 (at 300 K) 1169 (at 1000 K) 1749.26 1116.48 – 1172.03 7861.13 copper round 0.000702 0.00635 1.282781 Design value 1040 carbon steel cylindrical uniform 0.0009587 0.067008 0.033504 0.000199 0.007053 6.665071 TABLE V RESULTS FOR ELECTRICAL PROPERTIES OF THE SYSTEM Specification Melting temperature (K) Hardened temperature (K) Density (kg/m3) Design value TABLE IV RESULTS FOR WORK PIECE TABLE I material resistivity (Ωm) Permeability (H/m) material round 4 0.070184 0.082884 0.0381 0.03175 0.01588 TABLE III RESULTS FOR CONDUCTOR VII. REQUIRED SPECIFICATIONS FOR DESIGN CALCULATION specification Design value Resistance of work coil (Ω) Resistance of work piece (Ω) Inductance of work coil (µH) Magnetizing Inductance (µH) Power factor Resonated capacitance (µF) Quality factor Total impedance (Ω) supply current (A) Supply voltage (V) 355 Design value 0.003114 0.121220 1.434858 0.551223 0.273791 10.411355 3.512809 1.658596 71.479484 118.555586 World Academy of Science, Engineering and Technology 18 2008 IX. CONCLUSION The study of work coil design for induction hardening machine is for the work piece of cylindrical shape with the uniform surface and the type of work coil is helical multi-turn coil. And the method of hardening is stationary hardening method. The design study does not match with the work piece with irregular surface and the work piece its length less than its radius. So, the further study should be the design consideration of work coil for the work piece with irregular surface and single-turn work coil because helical type single-turn coils are commonly in used. And it should be for the study of the several shape of work piece. As multi-turn coils have a number of coil pitch, rotating and moving of work piece during operation has advantages. So, the scanning method of hardening should be studied in future. ACKNOWLEDGMENT Firstly the author would like to thank her parents: U Ba Thin and Daw San Yee for their best wishes to join the Ph.D research. Special thanks are due to her supervisor Dr. Ni Ni Win, Lecturer, Electrical Power engineering Department, MTU, Myanmar. The author greatly expresses her thanks to all person whom will concern to support in preparing this paper. REFERENCES [1] [2] [3] Curits, F.W.1944. High-frequency Induction Heating. 1st ed. New York:: McGraw-Hill Book Company, Inc. Zinn, S., and Semiatin, S. L. 1988. Coil Design and Fabrication: Basic Design and Modifications. July 2005 <http://www.ameritherm.com/ techonotes. html > Herbert B. D wight. 1945. Electrical Coils and Conductors. 1st Ed. Mc Graw-Hill Publishing Co.Ltd. Han Phyo Wai received her M.E in Electrical Power Engineering from Yangon Technological University, then following two years teaching in Technological University, Myanmar. Her interests include Power Electronic Devices and its applications. 356