1 Introduction The device that converts power frequency AC with fixed voltage and frequency into AC with variable voltage or frequency is called “frequency converter”. In recent years, with the development of the resource industry, the mineral processing industry has been expanded and transformed more and more, and the frequency converter has also been widely used in mineral processing production. Frequency conversion and speed regulation technology is a high-tech technology integrating automatic control, microelectronics, power electronics, communications and other technologies. Since the introduction of universal frequency converter into the Chinese market in the 1980s, it has been widely used in a short period of more than ten years. At present, the universal frequency converter is more and more popular with its advantages of intelligence, digitalization and networking. It has been widely used in all walks of life due to its excellent speed regulation performance and remarkable energy saving.

2. Application advantages of frequency converter Since frequency conversion start adopts soft start, it can reduce equipment and

The mechanical impact of the motor extends the service life of the equipment and motor. It has the characteristics of energy saving, energy saving, reliability and high efficiency, ensures the adjustment accuracy, and reduces the labor intensity of workers. The greatest advantage of frequency converter is energy saving and power saving. For fans and pumps, the principle of using frequency conversion speed regulation method to save electricity is based on the relationship between flow, pressure, speed and torque. These relations are: flow=rotational speed, pressure=torque=square of rotational speed, power=cubic power of rotational speed. Namely, the flow of fan or water pump is proportional to the first power of rotational speed, the pressure is proportional to the second power of rotational speed, and the shaft power is proportional to the third power of rotational speed. Q2=Q1? N2/n1 H2=H1? (n2/n1) 2 P2=P1? (n2/n1) 3 In the above formula, Q1, H1 and P1 respectively represent the operating speed, pressure (head) and power at n1, and Q2, H2 and P2 respectively represent the operating speed, pressure (head) and power at n2. For example, when the flow demand is reduced by half, if the frequency converter is adjusted, in theory, only 12.5% of the rated power can save 87.5% of the energy. When the speed is reduced to 80%, the shaft power is reduced to (80%) 3 ≈ 51% of the rated power. It can be seen that when throttling is achieved by reducing the speed to reduce the flow, the power consumed will be much lower. The application of frequency converter can improve economic benefits, but it also brings some problems. The on-site power supply and electrical equipment will have an impact on the frequency converter, and the higher harmonics generated during the operation of the frequency converter will also interfere with the operation of the surrounding equipment. These aspects deserve attention in our use.

3. The practical application of frequency converter in the field of mineral processing is mainly used in the frequency conversion control of the ball mill feed motor and the pulp pool pump motor. In the aspect of ball mill feeding, the power of the motor is generally less than 20kW, regardless of the use of pendulum feed, disc feed, trough feed or belt hauling feed. The selected power of the feed pump motor in the pulp pool varies from tens of kilowatts to 2300 kilowatts. The following is an example of the application of frequency converter in the automation project of a concentrator in Benxi Iron and Steel Co., Ltd. to introduce the precautions for selecting frequency converter. The pendulum feeding motor used by the ball mill at this site is 380V, 5.5kW six-stage motor. The frequency converter is used to adjust the speed of the pendulum feeding motor to control the feeding of the ball mill. There is also a slurry pump fed by the primary cyclone. The motor is 380V and 75kW. 3.1 Selection of frequency converter (1) Select frequency converter with voltage and power suitable for this site. At present, there are many domestic or imported frequency converter brands in the market. This article does not recommend which brand of frequency converter to choose. ABB ACS800 series frequency converter is selected on this site. The voltage level of the frequency converter is generally divided into 200V level and 400V level. Since the motors are 380V, 50Hz three-phase asynchronous motors, we choose the voltage level of the frequency converter to be 400V. The selection of capacity is a key link in the selection of frequency converter. In actual operation, it is often found that the given frequency is high, but the actual frequency cannot be adjusted, and the frequency converter trips frequently, which is that the capacity of the frequency converter is selected too small, the operating current is greater than the rated current of the frequency converter, and the frequency converter over-current trips. However, excessive capacity selection will cause economic waste. There are two empirical methods in actual selection. First, for small power motors, the power of the frequency converter selected should be a higher power level; The second is to select the rated current of the frequency converter to be 1.1~1.5 times of the rated current of the control motor. According to the above description, 7.5kW frequency converter is selected for 5.5kW motor and 90kW frequency converter is selected for 75kW motor. (2) Select the frequency converter according to the load characteristics of the controlled equipment. Due to the wide variety of production machinery, different performance and process requirements, its torque characteristics are complex, but the frequency converter is generally divided into three types: constant torque load, fan pump load and constant power load. Constant torque load means that the load torque is independent of the speed, and the torque remains constant at any speed. Constant torque load can be divided into friction load and potential energy load. The starting torque of friction load generally requires about 150% of the rated torque, and the braking torque generally requires about 100% of the rated torque, so the frequency converter should select those with constant torque characteristics, large starting and braking torque, long overload time and large overload capacity. In this project, the constant torque type frequency converter is selected for the feed of ball mill because the feed material is solid ore, and for the slurry pool pump motor because the slurry is solid particle liquid, the fan pump type should not be selected when selecting, and the constant torque type frequency converter should also be selected generally, which is a key factor when selecting the frequency converter. 3.2 The installation of frequency converter requires high temperature, humidity and dust content of the installation environment due to its high integration, compact overall structure and large heat dissipation. At this site, the environment is relatively poor, there is much dust, high humidity, and there is often slurry splashing, so unless special circumstances generally require that the frequency converter is not placed on the site, but in the low-voltage distribution room or operation room with relatively good conditions, which should consider the interference and heat dissipation. The interference problem is described in detail below. For the heat dissipation problem, the author adopts the following methods: (1) fan cooling: generally, the built-in fan of the converter can take away the heat dissipation inside the converter. If the fan cannot work normally, the converter should be stopped immediately. In addition, an exhaust fan is reserved on the top of the frequency converter cabinet to ensure that the frequency converter starts to operate and the fan runs at the same time in the control circuit design. (2) Reduce the installation environment temperature: because the frequency converter is an electronic device, including electronic components, electrolytic capacitors, etc., the temperature has a great impact on its life. The ambient operating temperature of general frequency converter is generally required to be – 10 ℃~+40 ℃. If measures can be taken to reduce the operating temperature of frequency converter as much as possible, the service life of frequency converter will be extended and its performance will be relatively stable. If conditions permit, a separate inverter low-voltage room can be built, with air conditioning installed inside to keep the temperature of the low-voltage room between 15 ℃ and 20 ℃. However, the frequency conversion cabinet is generally placed in the low-voltage distribution room without air conditioning on site, so the installation space of the frequency converter should meet the requirements of the operation manual of the frequency converter. It is also seen at some sites that the enclosure of the frequency converter is removed, which is also a way to dissipate heat. However, there are security risks, and users are not recommended to do so.

4. Interference suppression of frequency converter The interference of frequency converter is mainly harmonic interference. The solution of conducted interference is mainly to filter out or isolate the conducted high-frequency current in the circuit. Common methods include: 4.1 Common methods of interference suppression of frequency converter (1) The power supply of frequency converter system is independent from that of other equipment, or an isolation transformer is installed at the input side of frequency converter and other electrical equipment, Cut off the flow path of harmonic current; (2) Connect appropriate reactors in series at the input side and output side of the frequency converter, or install harmonic filters. The components of the filters must be LC type to absorb harmonics and increase the impedance of the power supply or load to achieve the purpose of restraining harmonics; (3) The cables between the motor and the frequency converter shall be laid through steel pipes or armored cables, and laid in different cable trenches with other weak current signals to avoid radiation interference; (4) The signal line shall be shielded wire, and the wiring shall be staggered from the main circuit control line of the frequency converter at a certain distance (at least 20 cm) to cut off the radiation interference; (5) The frequency converter uses a special grounding wire and a thick short wire for grounding. The ground wire of other electrical equipment nearby must be separated from the frequency converter wiring, so as to effectively suppress the radiation interference of current harmonics on the adjacent equipment. 4.2 The grounding of frequency converter and reactor design The grounding of frequency converter can largely suppress interference and improve the anti-interference ability of the system. However, if the grounding is poor, it will interfere with the equipment. The grounding modes of frequency converter include multi-point grounding, one-point grounding and bus grounding. Single-point grounding refers to that in a circuit or device, only one physical point is defined as the grounding point, which has good performance at low frequency; Multi-point grounding refers to that each grounding point in the device is directly connected to the nearest grounding point, which has good performance at high frequency; Hybrid grounding is based on the signal frequency and the length of the grounding wire, and the system adopts the mode of single point grounding and multi-point grounding. The inverter itself has a special grounding terminal PE terminal, which must be grounded for safety and noise reduction. The ground wire can neither be connected to the shell of electrical equipment nor to the zero line. One end of the thicker short wire can be connected to the PE end of the grounding terminal, and the other end can be connected to the grounding electrode. The value of the grounding resistance is<100 ω， The grounding wire shall be as short as possible, and the location of grounding electrode shall be selected reasonably. In addition, in order to improve the output current of the frequency converter and reduce the noise of the motor, the distance between the frequency converter and the controlled motor must be considered. Generally, if the frequency converter requires more than 80m, the output end of the frequency converter must be equipped with a reactor. In our design, we usually design a reactor on the line more than 50m; The reactor is generally selected from domestic products.

5 Conclusion Through the above methods, the selected frequency converter has been used in Benxi Steel for nearly one year. The frequency converter has been running stably. In particular, the hydrocyclone solves the problem that the pump sump is often emptied before, and plays a great role in stabilizing production. With the development of science and technology, the frequency converter will be used more and more widely not only in the mineral processing industry but also in other industries. Users will put forward higher requirements for the application of frequency converter, and the frequency converter will also adapt to the requirements of users and achieve faster development. Application and anti-interference measures