First, the scope of the application of medium frequency electric furnace and the significance of energy saving 

The medium frequency electric furnace is a commutation device that converts three-phase 50HZ alternating current into (1-8KHZ) single-phase intermediate frequency current, which can be used for non-contact heating of metal materials, and is used in industries such as smelting, casting, forging, quenching, welding, and bending. Widely used in casting, metallurgy, building materials, mining, machine tools, water conservancy, electric power, national defense and other fields. The effect of medium frequency electric furnaces of different enterprises varies greatly. Some enterprises have high energy consumption, low efficiency, and even high failure rate. The company is very dissatisfied and put forward many requirements for technical transformation. Xiangfan Jinhuju Power Electronics Co., Ltd. found out some energy-saving schemes for medium-frequency electric furnaces through exploration and trial and error in practice, so that the energy-saving effect of medium-frequency electric furnaces has been greatly improved.

Second, the working principle of the intermediate frequency electric furnace

The main circuit working principle of the intermediate frequency electric furnace is that the three-phase 50HZ alternating current input from the power grid is rectified into a voltage-adjustable pulsating direct current through a three-phase full-controlled rectifier bridge, filtered by the reactor LD into a smooth direct current, and sent to the single-phase inverter bridge. The output of the transformer is connected to the load. The load of the device is composed of an induction coil (including heating the workpiece) and an electric heating capacitor to form an LC oscillation circuit. The circuit has strong adaptability to the load and is stable and reliable. Due to the function of the inverter bridge, the input DC power is inverted into the required intermediate frequency AC output to the load. The output frequency of the device depends on the resonant frequency F0 of the LC parallel oscillator. Since the trigger pulse signal of the inverter bridge is taken from the load circuit, when the LC parameter of the load circuit changes, the output frequency of the inverter bridge also changes. Plays the role of automatic frequency modulation. The output power of the device can be adjusted by adjusting the rectification firing angle α to achieve the purpose of adjusting the power.

Third, the existing problems [analysis of the cause of power loss]

Because the energy is conserved, it will not be produced by itself, and it will not disappear suddenly. To melt a certain amount of steel, the energy required is certain. However, there is a lot of energy loss in the process of melting steel. The following is a discussion of the loss of electrical energy from three aspects.

1. Loss of the power conversion process.

From 10KV (or 35KV) high-voltage power through the transformer to low-voltage power 400V (or 800V); through the low-voltage cabinet into the intermediate frequency cabinet, into the 1000HZ intermediate frequency current; and then sent to the capacitor cabinet and the furnace body to heat the raw materials. In this complicated conversion process, the power is greatly consumed, and the loss is I2 × R.

This kind of problem is manifested in the defects of many factory layout designs. A: The transformer is very far from the electric furnace, 30 meters, 50 meters, and more than 100 meters. B: The capacitor compensation cabinet is far away from the furnace body, and the water-cooled cable or the channel copper busbar will be more than ten meters. C: Low-cost purchase of poor quality electric furnace equipment, copper copper tube configuration cross-sectional area is too small.

2. Loss of electro-magnetic and magneto-thermal conversion processes.

The IF current generates a magnetic field through the inductor. Due to a certain air gap between the workpiece and the inductor and a certain magnetic flux leakage in the inductor itself, the magnetic field consumes a part of the energy during the transmission.

This type of problem is manifested in the improper use of some factories. A. Purchase an aluminum shell body with poor quality. The aluminum shell is poorly made or the sensor is too close to the aluminum shell. Other factories use ordinary steel plates to fill damaged aluminum casings. B. There is no bottom water between every two furnaces.

3. Loss between heat and the environment.

The molten steel has a very high temperature from the beginning, and the molten steel (or molten iron) in the furnace is basically above 1500 ° C, when heat is diffused into the air, and a part is taken away by the cooling water. The length of the single furnace smelting determines the amount of loss between heat and the environment.

This type of problem is manifested in the improper configuration of some factories. A, the power density is too small. B. The power supply and load are not properly matched. C. The reliability of the electric furnace equipment is poor, often fails, and often requires repeated heating processes.

Fourth, energy saving measures

1. It can be seen from the formula I2×R of the power loss that the smaller the resistance, the more power is saved, and the smaller the current, the more power is saved. Loss for the first electrical energy conversion process. We have taken the following measures:

A. "Golden Torch" electric furnace should communicate with users as much as possible when designing the layout drawings, and explain the principle of energy saving, so that users can get from the power transformer to the position of the furnace as close as possible without affecting the production layout. , all connected copper bars or cable cross-sectional area is large enough.

B. Use a high voltage machine with a line voltage of 800V or 660V. Under certain conditions of rated power, the higher the voltage, the smaller the current. For example: 1250KVA high voltage machine, 800V incoming line voltage, the actual incoming current is 909A. Compared with the 630KVA low-voltage machine with 400V incoming voltage, the actual incoming current is also 909A. Their absolute values of power loss are essentially the same. Relatively speaking, the water consumption per ton of steel is not the same. According to the measured data, the 800V incoming voltage electric furnace saves more than 100KWH of water consumption per ton of steel compared with the 400V incoming line voltage.

2. Loss for electro-magnetic and magneto-thermal conversion processes. We have taken the following measures:

A. The coil is the heart of the whole inductor. The induction coil is powered by the intermediate frequency power supply. Under the action of the intermediate frequency voltage and current, a strong magnetic field is generated. This magnetic field causes the metal in the furnace to generate eddy current and heat. The coil is the key to the thermal energy conversion. Therefore, the coil design is very important. The coil of the furnace is a better solution determined by the computer analysis and calculation according to the principle of electromagnetic field combined with the actual use of the intermediate frequency furnace at home and abroad.

B. Use a steel shell furnace with magnetic enthalpy.

C. It is recommended that the user use the overall lining or good quality lining material to make the molten steel as close as possible to the sensor.

3. Using a fast medium frequency melting furnace to shorten the melting time of a single furnace, the energy lost can be reduced. For the loss between heat and the environment. We have taken the following measures:

A. Increase power density. For example: according to the previous industry standards, 1T electric furnace is only equipped with 500KW power supply, more than two hours, now the power supply of "Gold Torch" 1T electric furnace is generally equipped with 800KW or 1000KW, and the melting time of single furnace is controlled at about 60 minutes.

B. Extend the full power running time.

C. Appropriately increase the operating frequency of the electric furnace.

D. Improve the reliability of the electric furnace equipment.