The medium frequency induction furnace has the advantage of fast heating, high efficient , less heat loss, and lower temperature in the factory, dust reduction, energy saving , working condition improving ,degrade the working strength , obviously cleaning the workroom environment especially for the iron casting.the blast furnace can't compete with the induction furnace at gaining the low sulphur iron molten ,the foundries should choose the medium frequency induction melting furnace according to the transformer capacity, the producing capacity, the investment ,the below item should be noticed in choosing the equipment.
Medium frequency induction furnace usage conditions
1 The capacity of the transformer
As to the SCR full bridge rectifier parallel inverting medium frequency electric power which was normally adopted in this working field, mathematic relationship between the transformer capacity and the rated power: the transformer capacity=rated power*1.2,as to the IGBT half bridge series circuit inversing medium frequency electric power(the common address: one for dual track ,one furnace for melting ,one for muffling ,both working at the same time ): the transformer capacity=the rated power*1.1,for the purpose of reducing the high order harmonious wave in the input power as much as possible, the independent use should be made effort: one medium frequency electric power furnished with one rectifier transformer.
2 The line voltage
Usually, the medium frequency electric power below 1000KW will be supplied by the 3 phase 5 wires 380V,50HZ utility power, equipped with the 6 pulse single rectifier medium frequency electric power, as for the medium frequency electric power above 1000KW will be supplied by the power of 660V(some foundries will use 575V/750V,but both of this are not the standard power level ,the matching spare part was not convenient to be obtain, not recommended) ,12 pulse double rectified medium frequency electric power, it has 2 reason:
(1)raise the rated voltage by raising the line-in voltage
(2) The high order harmonious wave caused by the high power will disturb the input power, by the double rectifier can get the more smooth DC power, the loading current will be the rectangle wave, the load voltage is near to the sine wave so as to reducing the attack of the electric power disturbance to the other equipment, some user blind pursue the high voltage(900V for 1000KW),low current to reach the purpose of saving energy, but they don't know this is sacrificed by the furnace life, gaining don't deserve the paying, the high voltage is easy to reduce the life of the electric apparatus, the copper bus, cable fatigue so as to shorten the furnace life, and another, for the induction furnace manufacturer ,using the high voltage will reduce the material, save the cost.They will be glad to do so.（High price and low value）so the user of electric furnace afford the ultimate loss.
2. The producing capacity
Commonly, according to the unit weight and the demand of the weight of the steel molten per working day, we can decide the capacity of the medium frequency induction furnace and the medium frequency electric power and the frequency.The induction heating equipment is in the catalogue of the non standard product.Up till now ,there is no national standard. The common equipment list is as below in table 1:
From Table 1, domestic medium frequency induction furnace power density is about 500KW / ton, lower than the theoretical optimal value 600-800KW.Under high power density, electromagnetic will have a strong erosion to the lining. So there is a high requirement for the lining material,furnace building ways, smelting technology, materials, auxiliary materials and other requirements. According to the above configuration, each furnace melting time is 75 minutes (including feeding, refloating impurities, hardening and tempering time). If you need to shorten the melting time of each furnace,you can improve the power density of furnace at the rate of 100KW / t with the same capacity of the furnace.
3. The structure of the furnace body
In accordance with the custom of industry practice, the furnace with aluminum alloy structure and tilting reducer is commonly known as aluminum furnace.The furnace with steel structure and hydraulic cylinder is commonly known as steel furnace.Their differences are shown in Table 2:
Compared with aluminum furnace, the furnace with steel shell has five points:
(1) It has hugged,durable, beautiful appearance, especially the large-capacity and has a strong rigid structure. From a security perspective, it’s better to use steel furnace.
(2) The yoke made of silicon steel shields the magnetic field lines generated by the induction coil, reducing the magnetic flux leakage, improving thermal efficiency, increasing production and saving energy at about 5% -8%.
(3) The Furnace lid reduces heat loss and also improves the safety of the equipment.
(4) It has long working life. The oxidation of aluminum in the high-temperature is serious, resulting in metal fatigue and toughness. In the foundry site, we can often see the old and shabby aluminum shell of the furnace using for a year . But the furnace with steel shell due to the little magnetic flux leakage, has a much longer working life than aluminum shell furnace.
(5) The safety performance of the steel shell furnace is much better than aluminum shell furnace. The aluminum shell furnace is easily deformed and has poor security while melting furnace due to high temperature and pressure. Hydraulic tilting furnace with steel shell is safe and reliable.
4. Prices,Why are the prices of the same type of products different vary widely ?How to select the "IF melting furnace?
The price of the same type of medium frequency induction furnace is very different. For the universal application of one ton furnace, for example, sometimes its price is several times different in the market,which is related to the furnace structure, component selection, technical content, service quality and other aspects .
5. The selection of different materials
(1) Furnace shell and magnetic yoke: Aluminum shell furnace on the selection, the standard one ton furnace is made of cast aluminum alloy with an aluminum shell which weighs 400Kg with a thickness of 40mm.But the weight and thickness of the shell made by some manufacturers are often insufficient.As to steel shell furnace, the most important is to choose a yoke .There is a a large price gap when you choose the same type of furnace with different yoke.The general yoke is made of cold rolled silicon steel with high permeability and thickness of 0.3mm.It uses contoured structure and it can be close to the outside of the induction coil,which greatly reduces the outwardly diverging magnetic field lines.The yoke is welding fixed between stainless steel plate and stainless steel clamp and cooled by water.(Some manufacturers use waste, non-oriented, or even removed from the transformer silicon steel scrap to produce a yoke.)
(2) The coil: It should be made of the T2 copper pipe, insulated by static electricity paint spray of the insulator.The pipe should be wrapped by the mica belt and the non base glass belt per once. There is a gap between every wind turn, when casting the refractory cement, the gap should be filled to increase the strength of the adhesion.The interior surface should be smooth for the convenient of the disassembly of the furnace liner so as to protect the coil, on both side of the coil(top and bottom) several turns the water cooling coil will be added so as to increase the strength of the coil.
(3) SCR : The high quality SCR has good sensing ,fast response and low fault rate.When choosing SCR, the electric furnace manufacturers should specify their factory name and the product certification etc. SCR manufacturers with good quality is like: Xiangfan LMH Semiconductor Co., Ltd., Xi'an Power Electronics Research Institute)
(4) Capacitors: As to compensation capacitor banks, the most important is it must be equipped with sufficient amount which generally is the amount of the compensation capacitor. The capacitance value is 18-0 times of power supply, namely: the amount of the compensation capacitor (Kvar)= (20-18) x power supply. And you choose capacitors of the regular manufacturers .
(5) Reactor: Reactor’s main material is silicon It should be selected from the new products of regular manufacturers,and can not be from recycled steel.
(6) The water pipe clipper should be made of the stainless steel. It’s better in the removal copper joint the convenience of the mantling and dismantling, and suitable for the water cooling cable.
In addition,there are other components which are also very important, such as the inverter non-inductive capacitance,non-inductive resistance, water-cooled cables,connectors, copper, pipes, etc. All of them will affect the quality and price of the equipment. Here we do not elaborate.Hope you can choose to buy with attention.Try to claim furnace manufacturer to provide details of the main components, and not just purely value the price while ignoring the structure and quality of the equipment inside.