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Power and Frequency Control in a 60 kW Induction Steel Heating Furnaces through PLC

H. M. Ünver, M. T. Aydemir Department of Electrical and Electronics Engineering, Kırıkkale University, Kırıkkale, TURKEY Department of Electrical and Electronics Engineering, Gazi University, Ankara, TURKEY unver@kku.edu.tr, aydemirmt@gazi.edu.tr Phone: +90 (544) 771 07 90 Fax: +90 (0318) 218 95 42

AbstractMagnetic permeability (µ) and resistivity (ρ) of the materials experience large variations during heating in induction steel heating furnaces (ISHF). Power unit operating at mid frequencies is expected to observe these variations and keep the output power constant and in the active region. It is not easy to obtain heat, pressure and position control while controlling other units, by usingmicrocontroller based systems in power units. These require a separate control unit, such as Programmable Logic Controllers (PLCs). New generation PLC systems are equipped to provide all the necessary control functions needed in an ISHF, as well as a PWM signal generator that can drive the switches of a dc/ac inverter. This provides the necessary tools for various control techniques such as classicalcontrol, artificial intelligence, fuzzy logic and adaptive control. In this work, controlling of an ISHF through classic control methods and PLC, and simplification of the system structure has been attempted.

Keywords: Induction heating, Inverter, PLC, PID control, Power and frequency control

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Corresponding author: unver@kku.edu.tr, (Halil Murat Ünver)

Phone: +90 544 771 07 90, Fax: +90318 218 95 42.

1. Introduction Induction furnaces are used to heat metals by using the principles of induction. A nice feature of these furnaces is to create heat on the surface of the material. The furnace is designed by considering which part of the material will be heated and how long it will be heated. Since this is a very clean heating process no deterioration is made in quality. Inductionfurnaces have found a wide range of application area in the last decades including heating, melting, welding, shrink fitting, forming, soldering, hot forming, hardening, cooking and plasma physics [1-3]. They can heat in the frequency range of 10 Hz to 60 MHz. Several different types of power supplies operating at different frequencies have been developed and used depending on the type of theapplication. Switching power losses have been reduced, safety at high frequency operation has been increased and weight has been reduced in the course of development of these supplies. In parallel to the developments in the power semiconductor technology, applications utilizing Insulated Gate Bipolar Transistors (IGBT) have become very common. Low conduction losses and low gate power requirement ofIGBTs have made them the primary switching device. Although ISHF have several advantages in practical applications, their design is not simple, and several inherent problems are met. Two serious problems especially become important when the material starts heating up in induction heating. The first problem is that increasing temperature leads to increased resistivity and thus the total resistance(R) of the material increases. This, in turn, causes a significant drop in the power drawn from the supply. The second problem occurs when the material temperature approaches to Curie point. Magnetic permeability (µ) decreases as this point is being approached and becomes unity at this temperature. This results in reduced inductance (L) value, and the resonance frequency changes. The active powertransfer from the supply, again, decreases and heating time increases. Also, thermal losses occurring during this process extends the time to reach the desired temperature point. Modern PLCs have large memory capacities, special functions such as PWM, data communication facility as well as attractive features like being modular and economically affordable.

Due to the PLC utilization in ISHF,...
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