Improvement of the electrohydraulic power controller of an arc furnace in foundry

Abstract

Arc furnaces, in which a metal is melted and its chemical composition is finished, are now widely used in foundry. According to the type of electric current, arc furnaces are known to be divided into dc and ac furnaces. In contrast to standard ac furnaces, dc fur naces have a number of technological advantages; however, they require a more complex power supply and control system [1,2]. The electrode motion drive in arc furnaces of both types is electromechanical or hydraulic. The hydraulic electrode motion drive has received wide acceptance due to its advantages, such as small sizes, the minimum mass/power ratio as com pared to the drives of other types, the ability to gener ate static forces for a long time, the possibility of speed control and overload protection by limiting the pres sure, and operation smoothness and noiselessness. The disadvantages of the hydraulic drive are working fluid leakage through seal and gaps (especially at high pressure), heating of a working fluid, the necessity of ensuring the purity of a working fluid during operation and the prevention of air penetration into it, and fire risk (when a combustible working fluid is used). These disadvantages can be eliminated or substantially decreased by rational designing of hydraulic units and schemes, which is supported by the results in [3][4][5]. The application of the hydraulic drive can significantly simplify the kinematic scheme of the electrode motion mechanism and substantially increase the operation speed.Although the hydraulic drives of arc furnace elec trode motion are widely used, general approaches to designing control systems based on them have not been formulated because of the variety of electric fur nace operation conditions and the related electric equipment. The absence of such a technique requires long term and expensive investigations for adjusting control systems. When synthesizing and adjusting the power controller of an arc furnace, researchers mainly use a simplified concept of a furnace as a control object, which leads to high errors in choosing equip ment parameters and, hence, to an increase in the cost of the entire set and a decrease in the arc furnace capacity. The accuracy and speed of the power con troller of an arc furnace determine its capacity and energy efficiency.The power controllers (i.e., electrode displacement controllers) in an arc furnace have relatively simple kinematics and are characterized by nonlinear and diverse relations, which hinder the determination of the controller parameter. Their modern scheme ( Fig. 1) differs from the traditional scheme [6]. The electrohydraulic electrode displacement controller of an arc furnace is digital and contains programmable logic controller (PLC) 1 to analyze the input signals from measurement and normalization unit 2, which gathers current and voltage signals from current trans former CT and voltage transformer VT and converts them into a form adequate for introduction into PLC. Moreover, PLC receives technological parameters (e.g., the chemical composition o...