Application of frequency converter in feed industry

Variable frequency speed regulation technology is to change the optimal operating parameters of mechanical equipment through the frequency converter and has obvious energy-saving effects. It is partially used in the feed industry but is not yet popular. Through the introduction of this article, it can arouse the attention of feed companies and feed machinery and equipment manufacturers to the experimental research work of using variable frequency speed regulation technology in various mechanical equipment, and accelerate the promotion and application of variable frequency speed regulation technology.

There are many motors used in feed processing machinery and equipment in large and medium-sized feed mills, mainly three-phase AC induction asynchronous motors. Some machines need to adjust the speed to meet the needs of different working conditions, which is also the main measure for energy saving. In recent years, with the maturity of inverter production technology and the increasing application range of inverters, the use of inverters to carry out technical transformation of motor power supply has become an important means for enterprises to save energy, reduce consumption and improve efficiency.

The power supply frequency of my country's power sector is fixed at 50 Hz, and the rotating magnetic field speed of ordinary three-phase AC induction asynchronous motors is also fixed, so they cannot be changed (regulated). At present, the speed regulation methods of three-phase AC induction asynchronous motors mainly include changing the number of magnetic poles p (pole-changing speed regulation), changing the slip rate s (slip rate-changing speed regulation) and changing the frequency f (frequency conversion speed regulation).

①Dual-speed motor: The number of motor poles can be changed by changing the wiring method of the motor coil to change the speed of the motor, that is, fast and slow, two fixed speeds.

② Electromagnetic speed control motor: cooperate with electromagnetic speed control controller, use manual sliding resistor knob or load control instrument to change the motor speed (120 ~ 1200 r/min).

③ Ordinary three-phase AC induction asynchronous motor or variable frequency motor: The frequency of the power supply is changed by the frequency converter, so that the rotating magnetic field speed of the motor changes, which also changes the speed of the motor, and the speed regulation range is wide.

1 Inverter

A frequency converter is an electric energy control device that uses the on-off function of power semiconductor devices to convert industrial frequency power into another frequency.

1.1 The principle of frequency conversion speed regulation of inverter

Rotating field speed of induction motor:

n1(r/min)=60 f/p, where:

f——power frequency (Hz);

p——Number of motor pole pairs; motor speed n

The rotating magnetic field speed of the motor is proportional to the power supply frequency and inversely proportional to the number of pole pairs of the motor. The number of pole pairs of the motor is relatively fixed, and it is not suitable to change this value to adjust the motor speed. Therefore, the speed of the motor can be changed by changing the power supply frequency with a frequency converter, and the speed regulation range is wide and stepless.

1.2 Main functions and features of inverter

具有矢量控制和V/F控制功能；转矩自动提升功能，低频大转矩输出功能；内置PID闭环控制功能；内置RS-485接口，可与电脑联网控制；具有多种保护功能（过电压、欠电压、过电流、缺相、过载、过热、反转保护等）。可对电动机实行软启动和慢速停机，减少启动电流对电源的冲击，可替代大功率电动机的降压启动设备和保护设备。对电动机进行正反转控制非常方便，还有调速范围宽、传动效率高、节电显著等一系列优点。

2 Electric motor

The differences between ordinary three-phase AC induction asynchronous motors and three-phase variable frequency motors are shown in Tables 1 and 2.

2.1 Ordinary three-phase AC induction asynchronous motor

The base frequency of an ordinary three-phase AC induction asynchronous motor is 50 Hz, the fan is a coaxial fan, and the cooling and heat dissipation is affected by the motor speed.

2.2 Three-phase variable frequency motor

The three-phase variable frequency motor has 4 poles and is equipped with a cooling fan. The cooling and heat dissipation is not affected by the motor speed. Although the three-phase variable frequency motor is more expensive, its comprehensive performance at low speed is better than that of ordinary three-phase AC induction asynchronous motors. Generally, ordinary three-phase AC motors are used when the frequency is greater than 35 Hz. When the frequency is ≤35 Hz and it is often operated at low frequency, three-phase variable frequency motors are used.

3 Application of variable frequency speed regulation technology

In the feed industry, some mechanical equipment has used frequency converters for speed control.

3.1 Frequency conversion speed regulation of feeders before feed mills, pellet mills and extruders

According to the current of the main engine motor, adjust the output frequency of the inverter, change the speed of the feeder, change the feeding amount, avoid burning the motor due to overload, and make the motor work in the best state, which is conducive to saving energy.

3.2 Application in electronic scales

In electronic batching scales and electronic packaging scales, in order to accurately control the batching accuracy, a computer program can be used to control the speed of the feeder (feeder) through a frequency converter, thereby changing the feeding amount, improving the batching accuracy and weighing accuracy, and achieving a static accuracy of ±0.1%FS and a dynamic accuracy of ±0.2%FS.

3.3 Use frequency converter to control the speed of the grinder to meet the requirements of different types of feed crushing particle size

Different types of grinders have different requirements for grinder speeds due to differences in their rotational inertia, different wear conditions of hammers and screens, different types of feed to be crushed, and different required particle sizes. According to the test, without changing the feed amount, the grinder speed is adjusted through the frequency converter to find the speed at the minimum current of the motor as the optimal speed. The application of this method can increase the output by 5% to 15% and reduce the power consumption by 5% to 15%. With the wear of the hammer and the change of the material, the optimal speed point should also be adjusted accordingly.

Other aspects such as micro-crushers, micro-powder grading equipment, liquid adding equipment, negative pressure suction dust removal, etc. can all use variable frequency speed regulation technology to change their optimal operating parameters.

The application of variable frequency speed regulation technology in the feed industry is not yet common. Whether some machines need to be controlled by frequency converters and whether the performance parameters of variable frequency speed regulation technology of some machines are appropriate still need to be determined through a large number of production tests.

4 Key points for using frequency converter

4.1 When the frequency converter changes the frequency, the voltage also changes at the same time. Therefore, the electromagnetic torque of the motor also changes accordingly. When matching with the motor, its working conditions must be considered.

4.2 正确使用变频器，必须认真注意散热问题。变频器的故障率随温度升高而呈指数的上升，使用寿命随温度升高而呈指数的下降，环境温度升高10 ℃，变频器使用寿命显著降低，变频器的发热量一般为50～60 W/kW。

4.3 The installation environment of the inverter must be kept dry, free of corrosive gases and dust, and avoid strong vibration.

4.4 The power supply of the inverter must be stable to avoid damage to the inverter due to power failure.

4.5 When the inverter is working, it will generate high-order harmonics, which will affect other equipment. Some shielding and isolation measures must be taken.

4.6 A certain frequency point of the inverter at low speed may cause resonance of the machine, which can be avoided by setting the jump frequency point and width.

5 Summary

The core technology of frequency converters is mastered by developed countries, and the price of imported frequency converters is very high. Some domestic high-tech electrical equipment manufacturers have been able to develop and produce frequency converters with good performance through introduction and digestion and absorption, and the price is only 1/3 to 1/2 of the imported ones.

At present, the price of frequency converter is still higher than that of ordinary control and protection appliances, and technical transformation requires a lot of funds. Relevant technicians are needed to install and debug and train on-site operators. However, by using frequency conversion speed regulation to adapt to the needs of different working conditions, improve production and processing quality, reduce power consumption, and save costs, the technical transformation investment can be recovered after 1 to 2 years, and the economic benefits of the enterprise can be increased. Frequency converter is a mature technical equipment and an ideal equipment for energy-saving transformation of feed mills. It has a high promotion value. The majority of feed companies should raise their awareness, attach importance to technology introduction, and accelerate the promotion and application of frequency conversion speed regulation technology. In application, it is necessary to select reasonably, install correctly, and do a good job in parameter setting and debugging of the frequency converter to achieve good use results.