1. Electromagnetic stirring
When current flows within a conductor, a magnetic field is generated. If the direction of the current remains unchanged, the direction of the magnetic field will not change.
For low-frequency furnaces, the frequency is 0.5Hz, which means that the magnetic field direction undergoes a different change every 2 seconds, and the electromagnetic thrust also changes accordingly. Due to frequent changes, in this situation, the direction of the magnetic field changes again when the metal liquid starts to move a short distance, and the direction of the thrust changes. The metal liquid also frequently changes the direction of movement back and forth, and the stirring effect is equivalent to no effect.The following diagram is a stirring diagram of a three electrode furnace. From the graph, it can be seen that due to the constant change in the direction of the current, the direction of the magnetic field will also change accordingly.
Overlapping these three images together (as shown in the figure below), it can be seen that there are two opposing thrust forces on each path of liquid flow, which cannot form the so-called rotating magnetic field. According to the Ampere rule, holding the wire with the right hand so that the direction of the extended thumb is consistent with the direction of the current, then the direction pointed by the bent four fingers is the direction of the generated magnetic induction line, which is the rotational magnetic field. So it is a self deception to say that the rotating magnetic field inside the communication furnace or low-frequency furnace forms electromagnetic stirring.
The following diagram is a stirring diagram of a dual electrode DC furnace. As long as the polarity of the positive and negative poles is not changed in a dual electrode DC furnace, the direction of the current and magnetic field will not change, so the electromagnetic thrust remains unchanged and the metal liquid always moves in a fixed direction.
In terms of stirring effect, it is inevitable that stirring with the same direction for a long time will have a better stirring effect. We can manually set the polarity of the positive and negative poles to change every 20 or 30 minutes, while changing the direction of the magnetic field, in the case of continuous electricity (continuous arc light), to achieve a uniform temperature in the molten pool.
2.Raw material area
The area of the raw material area is related to the design of the melt pool and the properties of the material.
The arc radiation zone around the electrode is limited. When the melting point of the material is low and the thermal conductivity is high, the area of the high-temperature melting zone is larger, and vice versa. When smelting materials such as calcium aluminate with low melting points and high thermal conductivity, the diameter of the melting zone can reach 5 times the diameter of the electrode. When melting materials with high melting points and low thermal conductivity such as zirconia, chromium oxide, yttrium oxide, and metallic silicon, the diameter of the melting zone is only 1.2 times the diameter of the electrode.
For a 3-electrode furnace, no matter how the pole center circle is designed, the high-temperature melting zone cannot coincide with the pole center circle. The high-temperature melting zones of the three electrodes overlap, as shown in the following figure. From the figure, it can be seen that this furnace type has three large raw material areas. The furnace shell is circular, and the polar circle is also circular, but the three high-temperature melting zones overlap to form an approximate equilateral triangle shape. This results in a large area of the raw material area.
For a dual electrode DC furnace, there is no concept of a pole center circle. The high-temperature melting zones of the two electrodes overlap, as shown in the following figure. From the figure, it can be seen that the furnace walls on both sides of the furnace blend perfectly with the high-temperature melting zone, leaving only a small portion of the raw material area above and below. The area of the raw material area is much smaller than that of the three electrode low-frequency furnace.
3.Stability of arc light
A short circuit in a conductor generates an arc, which becomes a part of the energized circuit. When the current disappears, the arc light will also disappear. The presence of frequency in a low-frequency furnace means that the arc will start and stop several times in a short period of time. For example, when the low-frequency furnace is operating at 0.5Hz, the arc will start and stop every 2 seconds (30 times per minute). When the low-frequency furnace is operating at 0.1Hz, the arc will start and stop every 10 seconds (6 times per minute).
When the DC arc furnace is not turned off, the current does not stop and the arc light does not start or stop, making the arc very stable.
4. Heating speed
The smelting state of a low-frequency furnace is simulated in a power frequency AC furnace. As shown in the figure below, during operation, only two electrodes of the low-frequency furnace have arc light each time, and the third electrode is in a non powered state. For example, when the equipment is at 0.1Hz, there is always a power outage of 10 seconds for one electrode, and the material around this electrode is in a waste state of heat loss.
The smelting state of the dual electrode DC furnace is shown in the following figure, with continuous arc light and two electrodes always in a heating state. At the same time, when converting AC current into DC current, the total current will increase by about 1.22 times, and the heat generated by arc light will also increase by about 1.22 times. This is one of the reasons why the heating speed of a DC furnace is faster than that of a low-frequency furnace.
5.Harmonics and power factor
All harmonics are caused by incomplete conduction of the thyristor conduction angle in the rectifier bridge. In order to achieve the smelting state of the AC furnace, the low-frequency furnace cannot fully conduct the conduction angle, which will generate a large number of harmonics and pollute the power grid. Mild cases can accelerate equipment aging, interfere with the operation of other precision instruments, and severe cases can damage equipment and power facilities. To solve this problem, it is necessary to add harmonic suppression devices. In addition, for rectifier circuits, the higher the number of pulses, the fewer harmonics. Low frequency furnaces have no concept of pulse waves and can never eliminate the presence of harmonics by increasing the number of pulses.
The thyristor conduction angle of the low-frequency furnace is not fully conductive, which leads to a natural power factor of only about 0.9 and cannot be higher.
The thyristor of the dual electrode DC furnace is only responsible for rectification, with full conduction angle and negligible harmonic pollution. The dual electrode DC furnace effectively reduces harmonic pollution by increasing the number of pulses. For example, a 12 pulse power supply can eliminate harmonics below 13 times, a 24 pulse power supply can eliminate harmonics below 25 times, and a 48 pulse power supply can eliminate harmonics below 49 times. The natural power factor can reach 0.95-0.98.
6.Graphite electrode consumption
There are three main factors that affect the consumption of graphite electrodes:
① The skin effect causes severe heating on the electrode surface;
② In a high-temperature environment inside the furnace, carbon oxidizes to form carbon monoxide and carbon dioxide, which then disappear;
③ In the high-temperature environment inside the furnace, the electrode binder gradually fails, leading to graphite detachment.
Therefore, the more electrodes the device uses, the greater the electrode consumption and the higher the product cost. A low-frequency furnace uses three electrodes, while a dual electrode DC furnace only has two. The electrode consumption of a dual electrode furnace is 33.33% lower than that of a low-frequency furnace
A low-frequency furnace is essentially a three-phase AC furnace. Reducing the frequency is to simulate all the advantages of a DC furnace, but it cannot conceal the fact that AC current flows on the electrodes.
Low frequency furnaces only reduce various drawbacks of AC furnaces, but they have not fundamentally eliminated them, and still cannot match the advantages of DC furnaces.
The low-frequency furnace retains the drawbacks of the AC furnace, such as low power factor, unstable arc breaking, high consumption of graphite electrodes, negligible electromagnetic stirring function, skin effect, and eddy current phenomenon. At the same time, it also increases the harmonic pollution that the AC furnace originally did not have.
March 02, 2024
March 02, 2024
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