DC Electric Arc Furnaces

Overview of DC Electric Arc Furnaces

DC EAFs offer superior stability and unidirectionality compared to AC EAFs, significantly benefiting heat transfer in metallurgical processes. Theoretical analysis indicates that replacing alternating current with direct current represents a major technological innovation. Since direct current avoids the zero-crossing issue, arc stability improves and grid interference is reduced. The method of smelting metals using direct current in a DC arc furnace is fundamentally the same as in an AC arc furnace. However, due to the high heat generated by the anode on the charge side, the electrothermal conversion efficiency of the DC arc is higher.
Employing a thyristor-based static converter as the DC arc furnace power supply enables conversion of AC power to DC. By adjusting the thyristor conduction angle, the smelting current can be continuously and smoothly controlled, ensuring high currents do not exceed equipment limits. Utilizing a three-phase full-bridge rectifier circuit allows for simplified winding designs and a robust, reliable structure in the rectifier transformer. Since thyristor conduction angle adjustments only take effect during the next thyristor conduction cycle, introducing a time lag, a DC reactor DCL must be connected in series on the DC side. This prevents the dynamic short-circuit current from instantaneously reaching unacceptable levels during arc voltage drops (short circuits).

DC Electric Arc Furnace Features:

1. PLC control ensures high automation, rapid heating rates, and stable, reliable production cycles.
2. Customizable settings during smelting without shutdown, enabling constant power, constant current, constant impedance, or constant voltage control schemes for convenient operation.
3. Voltage adjustment under load; stepless current adjustment. Voltage levels can be adjusted as needed during smelting without interrupting the process. Current can be precisely adjusted in 1-amp increments.
4. Positive and negative polarities can be freely switched during smelting. Uniform furnace chamber temperature effectively resolves temperature inconsistencies caused by the anode effect. (Patented feature)
5. Arc length can be adjusted without power interruption during smelting to suit requirements. Supports both submerged and open-arc smelting, capable of melting various materials to achieve dual-functionality (EAF + SAF) in one unit.
6. The DC power supply main control board (proprietary intellectual property) features soft-start functionality, preventing high-voltage surge damage to thyristor components upon startup. This board incorporates opto-isolation to effectively shield control circuit stability from strong magnetic field interference in production environments. It also includes overvoltage, overcurrent, phase loss, and high-temperature protection to prevent equipment damage from short circuits.
7. The DC furnace’s high central electrode temperature concentrates heat, facilitating deep electrode burial and preventing furnace bottom uplift, making it ideal for high-melting-point product smelting.
8. Compared to AC arc furnaces, DC arc furnaces save 1–2 sets of electrode assemblies and reduce graphite electrode consumption by 30%–50%. For example, a 20-ton AC arc furnace requires approximately 6 kg of graphite electrodes to smelt 1 ton of molten iron, whereas a DC arc furnace of the same capacity requires only 3 kg.
9. Features electrode breakage detection and automatic balancing functions. When an electrode contacts the furnace bottom and causes a short-circuit arc, the furnace ceases operation to prevent further downward movement of the electrode, thereby avoiding breakage accidents.

10. The arc light in DC electric arc furnaces emits heat vertically from the center, ensuring uniform heat distribution and consistent material melting. The DC arc light exerts a strong electromagnetic stirring effect on molten metal, eliminating dead zones in material melting and achieving high product recovery rates.
11. Noise levels are 10 to 20 decibels lower than in AC electric arc furnaces.

Accident Site of Broken Electrode in AC Electric Arc Furnace

12. Extended service life of furnace wall refractories. The arc in an AC arc furnace forms a 45° angle with the graphite electrodes, frequently striking the furnace walls and damaging the refractory lining. In contrast, the DC arc furnace maintains a 12° angle between the arc and graphite electrodes, preventing wall impacts and hot spots formation, thus preserving the refractory integrity.

DC arc furnaces do not damage the furnace walls.

Why DC Arc Furnaces Being 10%-15% More Energy Efficient Than AC Arc Furnaces ?

we will explain it from Power Factor, Harmonics, Eddy Currents, Skin Effect etc items:

1.DC Arc Furnace Power Factor keep 0.95 to 0.98.

AC short-circuit networks readily induce skin effect and proximity effect, making it difficult to maintain balanced three-phase power. This increases reactive power consumption in AC furnaces. Without high/low-end compensation, the furnace’s power factor remains around 0.7 to 0.8. Even after investing in compensation costs, the power factor only reaches approximately 0.8 to 0.9. In contrast, the DC arc furnaces manufactured by our company require no reactive power compensation devices, achieving a natural power factor of 0.95 to 0.98.

Natural Power Factor 0.95-0.98

2. Current Harmonics

In AC circuits, alternating currents generate fluctuating magnetic fields within short-circuit loops, inducing eddy currents that cause heating and energy loss. DC circuits, however, produce only stable magnetic fields in short-circuit loops. Absent eddy currents, no heating occurs, eliminating eddy current losses and energy waste.

3. Eddy Currents

Analysis of AC waveform characteristics reveals that the 50Hz transition from sine to cosine wave results in 100 voltage zero-crossings per second. This equates to 100 instances per second of arc initiation, extinction, re-initiation, and re-extinction. Severe voltage flicker causes unstable AC arcs, harmonic distortion, and voltage-current phase misalignment. Simultaneously, harmonic pollution occurs, with 2nd and 4th even harmonics coexisting with 3rd, 5th, and 7th odd harmonics. These exceed national standards by 5% to 27%, further complicating voltage distortion. DC voltage, however, possesses a non-zero-crossing characteristic with no polarity change, exhibiting only weak-to-strong flicker. Consequently, DC arc light is more stable than AC arc light. In terms of flicker intensity, it is only 20% that of AC furnaces. Moreover, its impact on the power grid is far less than that of AC furnaces, and it does not introduce odd-order harmonic interference such as 3rd, 5th, 7th, 11th, or 13th harmonics into the grid.

4. Skin Effect

AC inherently exhibits the skin effect. In AC circuits, current distribution within conductors is uneven, concentrating in the “skin” layer—a thin outer region where current density peaks near the surface while interior current is minimal. This increases conductor resistance and power loss. DC current lacks this skin effect, enabling short-circuit loops to carry higher currents per cross-sectional area. Consequently, DC systems achieve greater energy efficiency with reduced consumption.

Compare the High-temperature uniformity of AC & DC arc furnace electrodes