Electrode regulator

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Electrode regulators are used in process engineering systems to maintain the arc length. An electric arc burns between two electrodes or between an electrode and the electrically conductive material to be treated. The most common areas of application are arc furnaces and arc welding machines .

Use in electric arc furnaces

The term arc furnace includes both melting furnaces (EAF = Electric Arc Furnace) and ladle furnaces (LF = Ladle Furnace). In both cases, the arc burns between the tip of a vertically arranged graphite electrode and the electrically conductive charge. The latter is usually solid steel scrap in the EAF and liquid steel in the LF. In more recent procedural routes, the input material in the EAF also consists of a mix of steel scrap, liquid pig iron and sponge iron .

Both furnace variants can be designed as three-phase or direct-current furnaces. The three-phase arc furnace has three adjustable electrodes, one for each electrical phase, which are located above the load. The direct current arc furnace has an adjustable electrode above the load as well as an additional fixed bottom electrode.

The controller has the task of keeping the arc length constant by moving the electrode up and down in the event of control deviations. However, the arc length cannot be determined directly, or can only be determined with great effort. For this reason, the electrical measured variables current and voltage, which are a function of the arc length, are used either directly or converted as controlled variables .

Use in three-phase arc furnaces

The impedance and the current are often used as controlled variables . While the electrode current can be measured directly, the impedance is calculated from the quotient of electrode voltage to electrode current. In addition, the arc voltage is occasionally used, which is calculated in a somewhat more complex manner from an electrical model of the furnace system.

The arc length and the effective arc power are of interest as target values ​​for the process. This can also be converted from the electrical model of the system into an impedance or a current and then made available to the electrode controller.

In three-phase ovens, a separate controller is used for each phase. Exceptions are the now outdated mono-arm ladle furnaces, the three electrodes of which are controlled by a single common electrode adjustment system. The electrical coupling of the three phases makes regulation much more difficult. If the arc length changes in one phase, the electrical parameters in the two neighboring phases are also influenced. While the coupling is very large with current as the controlled variable, the use of impedance as the controlled variable results in a significant weakening of the phase coupling. The impedance mode is therefore particularly important for meltdown furnaces (EAF), as the scrap movement during the meltdown results in strong dynamics in all three phases.

Based on the prevailing control deviation (difference between actual value and setpoint ), the controller calculates a corresponding manipulated variable, depending on the version (usually P or PI controller), which is fed to the actuator. Since the electrode position is usually not returned to the controller, the actuator converts the manipulated variable into an upward or downward speed of the electrode. This gives the controlled system an integrating character. The D component is of no advantage here due to the high stochastic signal components.

Use in direct current arc furnaces

In the case of a direct current arc furnace, the alternating voltage provided by the supply network must be converted into a direct voltage. For three-phase bridges rectifiers on thyristor wherein the arc power is determined by the firing angle of the thyristors (used phase control ).

The control principle is similar to the principle of regulated DC drives. A fast internal controller keeps the electrode current constant by specifying the ignition angle and a superimposed slow controller with the arc voltage as the controlled variable then adjusts the electrode. The independent regulation of current and voltage is advantageous here.

Process optimization

Modern, powerful controllers go beyond their actual control task of maintaining the arc length and support the operator in process control. In melting furnaces in particular, intelligent controllers based on neural networks are increasingly being used, which are able to optimize the melting process largely independently.

The goals of optimization are to reduce production costs and increase effectiveness. This is achieved through different approaches, for example through dynamic adaptation

  • of the setpoint
  • the control parameter
  • the arc voltage

the melting progress.

It also makes sense that the controller complies with process boundary conditions that B. imposed by the energy supply company with regard to energy costs and voltage quality . Temporary power limits should not be exceeded or limit values ​​regarding the voltage quality (active power factor , voltage dips , flicker ) should be observed. Operating results so far have shown that production efficiency has been increased while saving electrical and chemical energy.

Control hardware

In practice, electro-hydraulic actuating systems have proven to be effective, in which the electrodes can be moved with relatively short response times via lifting cylinders by means of pumps and proportional valves . In addition, electromotive control systems are occasionally used, in which the electrodes are moved by an electric motor and cable winches.

The older analog controllers are gradually being replaced by industrial computers or programmable logic controllers (PLC).

Use in arc welding machines

While non-consuming graphite electrodes are used in electric arc furnaces, consuming metal electrodes are used in electric welding. Since the arc liquefies both the workpiece and the electrode, the electrode material must be adjusted relatively quickly to maintain constant welding conditions.