The induction coil of the medium frequency induction furnace is made of a hollow copper tube spirally wound. The sparking of the coil occurs between the 匝 and 匝 of the copper tube or between the copper tube and the yoke. Due to the insulation failure, a short-circuit discharge is formed under the action of the voltage, and the spark is flashed (commonly known as sparking), and the continuation occurs. The arc (commonly known as the arc).
Induction coil ignition will bring harm to the equipment, mainly reflected in the following aspects:
1 | Seriously reduce equipment operating efficiency, increase power consumption and time of melting / heating |
2 | High current caused by arcing is extremely easy to burn thyristors and other important power components |
3 | Pulling the arc causes equipment failure or the furnace brings great safety hazards to the operator |
4 | Increase equipment failure rate, increase operation and maintenance costs, reduce enterprise production efficiency, and cause enterprises to lose high profits |
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When the induction coil is working, the alternating current forms an alternating magnetic field through the induction coil. According to Faraday's law of electromagnetic induction, the alternating magnetic lines cut the metal inside the coil to form an induced current. Since the metal itself has resistance heat, the current generates heat in the process of flowing inside the metal, thereby heating or melting the metal. This is the basic principle of induction heating and induction melting.
The induction coil has the following features when it works:
1. There is a high voltage and strong current in the coil;
2. The coil itself is in a high temperature working environment;
3. The insulation performance must be ensured between the coil and the coil. Once the fire is formed or a short circuit is formed, the working efficiency of the electric furnace will be greatly reduced.
Since the coil itself is a conductor, the passage of the induction coil cannot form any path, so the insulation of the induction coil is very important, which directly affects the use efficiency of the induction furnace and the safety of the worker's operating environment. However, unfortunately, the insulation of the induction coil has not received enough attention in the industry . Most manufacturers only spray a layer of conventional insulating varnish on the surface after the coil is finished. Most of these varnishes are used. It is an organic substance that has good insulating ability at normal temperature. However, as the temperature rises, the performance of such insulating varnish deteriorates rapidly, and it is gradually carbonized. When the temperature exceeds 100 ° C , the insulating varnish is completely carbonized. Blackened, completely lost insulation.
The main reason for the insulation damage of the induction coil surface is that the working environment of the induction furnace is mostly harsh. Although there is a water cooling system, it cannot ensure that the insulation paint is always working in a lower temperature environment. This is mainly due to the following reasons:
1. The induced current through the coil has a skin effect, that is, the current is mainly concentrated on the surface of the copper tube, and the higher the frequency of the induced current, the larger the surface current density. Therefore, the heat generated by the copper coil of the induction coil is concentrated on the surface, and the surface temperature in contact with the insulating varnish is much higher than the temperature in the contact portion of the copper tube with the cooling water. Even under normal circulating water cooling conditions, the outlet water temperature is controlled at 50-60 °C, and the temperature of the copper tube surface exceeds 80 °C.
2. The heat transfer of the molten steel in the furnace. The new furnace lining is thicker, which can effectively prevent the heat of the molten steel in the furnace from being transmitted to the surface of the coil. However, with the rapid erosion of the lining in the later stage, the lining of the lining becomes thinner in the later stage, and the heat transferred from the molten steel to the surface of the coil is much higher than that of the new lining. The actual measurement surface, when a new lining (lining thickness of about 15cm ) the temperature of the coil slurry layer 80 [deg.] Vertically, to post lining (thickness of about 5cm ), the coil temperature has risen paste layer close to 200 is deg.] C, at this time Conventional insulating varnishes have completely failed to carbonize.
3. Cooling water cooling capacity is reduced, which is mainly due to the influence of water quality. Cooling water is easy to scale at high temperature, especially in the north and west areas where the water quality is hard. The cooling water fouling is prominent, the copper pipe is blocked, the water pressure is reduced, the cooling capacity is obviously reduced, and the temperature rise in turn accelerates the scaling. . Once this happens, the temperature of the copper tube surface will rise rapidly and the conventional insulating varnish will be destroyed by carbonization in a short time.
Once the insulation paint on the surface of the coil is destroyed and the insulation capacity is lost, it is easy to cause the coil to catch fire, mainly because:
1. Water vapor causes, especially when the new furnace lining is running, the moisture of the lining material is suddenly vaporized by heat, and it will seep through the capillary pores of the coil slurry, and the coil which is relatively low in temperature will immediately condense into water droplets, and the water is a conductor. , immediately caused a fire.
2. The copper tube runs and leaks. Because the circulating water in the copper tube may be caused by local dripping.
3. The metal dust in the workshop accumulates on the surface of the coil, forming a short circuit and sparking.
Once the ignition occurs, the temperature rises rapidly. First, the original residual insulation coating will be completely destroyed, and the ignition will be accelerated until the arc is formed, the copper tube is broken, serious production accidents and property losses.
In addition, the over-loading of the electric furnace is also an important factor that easily causes the coil to ignite. Because the current and voltage passing through the coil increase when the rated power is exceeded, the heat generated by the coil is increased. At this time, the insulating layer is more susceptible to high temperature damage and loses insulation. Ability to fire at high voltages is more likely to occur.
In view of the importance of the insulation of the induction coil, there is no insulating paint on the market to ensure that the induction coil has good insulation performance under severe working conditions, especially at high.
