Abstract: Introduce the application of S7-300 programmable controller in electric furnace control system and the principle of electrode lifting control.

The steel-making electric arc furnace is a furnace in which scrap steel or reduced iron is charged into the furnace to generate a three-phase AC arc between it and three artificial graphite electrodes to melt the charge and smelt into steel. The electrode lifting of this system is a process of differential adjustment between the set value of the arc current and the actual value. When the set value is greater than the actual value, the electrode drops, and vice versa, it finally reaches the equilibrium point for stable arc smelting. Our company's 16# electric arc furnace uses this principle to adopt PLC automatic sampling and automatic control of electrode lifting, so as to realize the automation of the smelting process.

1. PLC system hardware configuration

As shown in Figure l. The power module is always installed in the first slot of the rack, and the CPU module is installed in the second slot. S7-300 uses the backplane bus to connect each module except the power module. The backplane bus is integrated on the module, and the modules are connected through a U-shaped bus connector. Each module has a bus connector, which is inserted behind each module. When installing, insert the bus connector on the CPU module first. And fixed on the guide rail, and then load each module in turn. Through the configuration of each module of CPU and DI, DO, AI and AO, the input and output control of each switch value and the collection of arc current and arc voltage are ensured, and the electrode rise and fall are controlled through the model.

2. PLC system programming application

2.1 Main program structure

The main program of the system is completed by Obl,

Organization block Obl: Call FB3, FC1O, FCll, FCl2 when selecting automatic gear to automatically complete functions such as mold-in, mold-out, comparison, and PID adjustment.

Function block FCl: It comes from the three-phase arc current and arc voltage analog-to-digital conversion link, which is converted from analog to digital.

Function blocks FCl0, FCll, FCl2: used to limit whether the maximum value and minimum value of the actual arc current and arc voltage are running between this parameter, and it will be protected in time if there is a deviation.

Function block FB3: It is a function block for running PID closed-loop adjustment.

Data block DB: used to classify and store the values of variables in equipment or production lines. Data blocks are also an important way to realize data exchange, data transfer and data sharing between logic blocks.

2.2 Parameter function table

The electric arc furnace smelting electrode lifting process must adopt a closed-loop control method to make the system work stably, so we introduced the function block FB3 (Figure 3) in the PLC and set up the PI dynamic adjustment, limiting and dead zone adjustment links.

2.3 Working principle of electrode lifting system PI

According to the requirements of the smelting process, set the arc current I0 (required current value), arc voltage UO (five-speed voltage regulation of the transformer), during the arc ignition of the electric furnace, the electrode rises and falls frequently, how to make it not only continuous arc but also work in the best Status, we have introduced a proportional integral adjustment link.

Proportional part

I real-IO/UO×U real×1.732=±I07

I07×IN3=KP

In the formula, I is the actual arc current value; I　 O is the set arc current value; UO is the five-position selected arc voltage value of the transformer; U is the actual arc voltage value; I07 is the calculated current value; IN3 one Adjustable ratio multiple; KP-proportion coefficient (stored in the function block DBl.DBDl2).

Integral part

IN4/IN5×DB1.DBD4=DBl.DBD8

(DBl.DBS×I07)+I08=DBl.DBD20

In the formula, IN4 (DB6.DBD48), IN5 (DB6.DBD4), DB1.DBD4-an adjustable integration time constant: DB1.DBD8-integration time constant; DB1.DBD20-integration accumulation coefficient.

In summary, the function block output OUT can be expressed as

OUT=DB1.DBD12+DB1.DBD20 where DBl.DBDl2 is a proportional coefficient: DBl.DBD20 is an integral accumulation coefficient. When the I07 difference is positive, it proves that the actual arc current is large and the arc voltage is low and the electrode rises; when it is negative, it proves that the arc current is small and the arc voltage is high and the electrode is down. The rising and falling reaction speed of the electrode is set by the PI adjustment constant, and the proportional and integral constants can be adjusted according to different situations. Corrected during debugging. In addition, there is dead zone control (also known as smelting stability zone). Once the output value enters this zone, the system self-checks as the stable arc zone (electrode fluctuation ≈ 0), which is the relationship between the given value, arc current, and arc voltage. To ensure the stability of arc current. In order to prevent the arc from breaking, there must be an ascending and descending speed limiter. Generally, the ascending and descending limit values are set to 0.5:1, that is, the ascending speed is 50% of the descending speed. This value can be adjusted according to actual conditions. During the adjustment, the proportion, integral time constant, and the limit value of the rising and falling speed should be appropriately changed to ensure the smooth ignition of the electric furnace and the smooth smelting current.

The electrode control of this system is hydraulic. The electrical signal is converted into a hydraulic signal by the control valve, and the electrode holder is directly moved by the hydraulic cylinder. Since the inertia of the rotating part is eliminated, the starting characteristic is good. After the input signal is given, the time until the electrode reaches the highest speed is about 40-80ms.

3. Conclusion

This system is now operating normally after debugging. The system runs stably, the arc starts quickly, and the smelting is stable. Compared with the traditional analog system, it has the advantages of high control accuracy, strong reliability, low failure rate and easy maintenance. The quality of electric furnace smelting and the degree of automation have taken a qualitative leap compared with the past.