Abstract This paper describes the application of variable frequency speed control technology in industry in recent years, focusing on the correct selection and maintenance of frequency converters in sugar factories. Keywords Frequency converter; speed control; selection; maintenance 0 Introduction According to statistics, in 2002, the total installed power of fans and pumps in China reached 3.6 x 10⁸ kW, and the annual power consumption was 8.2 x 10¹² kWh, accounting for about one-third of the country's total power consumption. The application of variable frequency speed controllers in the operation of fans and pumps can generally achieve a power saving rate of 20%-40%, with a payback period of 1-2 years. It not only has a significant power saving effect, but also plays an important role in meeting production process requirements and ensuring product quality, resulting in significant economic benefits. Therefore, vigorously promoting the energy-saving operation of variable frequency speed control for fans and pumps is not only an important technical means for enterprises to save energy and reduce consumption, but also an inevitable requirement for realizing the transformation of economic growth patterns. For sugar mills, many with a daily crushing capacity of 1500 t/d or 2500 t/d were expanded or newly built before the 1980s. Their electrical control equipment is relatively outdated, using traditional speed control for asynchronous motors, and actuators to adjust dampers and valves for fans and pumps. When adjusting the load according to production conditions, dampers and valves must be adjusted simultaneously to adapt to the load changes, without reducing the energy consumption from the grid or changing the motor output power. Using frequency converters (VFDs) can completely avoid these problems. Damperes and valves are fully open, eliminating air and water pressure losses, and the motor speed can be automatically adjusted according to the operating conditions to adapt to the load, thus significantly reducing the electrical load. In the sugar mill process, the large number of traditional speed-regulating motors, fans, and pumps results in high overall energy consumption, often causing power shortages, which greatly restricts production development. Adopting frequency conversion speed control technology to upgrade traditional speed-regulating motor, fan, and pump control equipment has significant potential for energy saving and consumption reduction. Especially after the successful reform of state-owned enterprises, sugar factories in Guangxi, Yunnan, and Zhanjiang, Guangdong, have continuously expanded their production scale, with daily crushing volumes generally reaching 5,000-10,000 tons per day. Now, with the reduction of personnel in each workshop, only by upgrading the traditional and outdated speed control equipment can high-quality management and safe production be ensured, achieving energy conservation, consumption reduction, and lower production costs. 1. Overview of Frequency Converter Development AC drive and control technology is one of the fastest-growing technologies currently. This is closely related to the rapid development of power electronic device manufacturing technology, control technology, microcomputers, and large-scale integrated circuits. In other words, whenever a new generation of power electronic devices emerges, a new type of general-purpose frequency converter with smaller size and higher power is produced. Whenever new microcomputer control technology emerges, a new generation of general-purpose frequency converters with more comprehensive functions, wider adaptability, and easier operation appears. From the perspective of the global general-purpose frequency converter application market, it can be roughly divided into three major parts: Japan, the United States, and Europe. Currently, most of the general-purpose frequency converters imported into China are from these brands. Its products are developing and changing in three directions: First, towards multi-functional and high-performance frequency converters that can achieve optimal operation without adjustment; second, towards small and easy-to-operate frequency converters that can be operated through simple control; and third, towards frequency converters with large capacity, high starting torque, and environmental protection functions. 2. Frequency Converter Selection Methods 2.1 Frequency Converter Capacity Selection The basic principle for selecting frequency converter capacity is that the maximum load current must not exceed the rated current of the frequency converter. Generally, the selection is based on the motor capacity specified in the frequency converter's instruction manual. However, when the motor is not a 4-pole motor (general-purpose frequency converters are usually designed based on a 4-pole motor model), the frequency converter capacity cannot be selected solely based on the motor capacity; it is also necessary to consider whether the rated current of the frequency converter is greater than the rated current of the motor. Therefore, the inverter should usually be selected based on the rated current of the asynchronous motor, or based on the actual operating current (maximum value) of the asynchronous motor. When selecting, it should be noted that the inverter's overload capacity allows for a momentary overload of 150% or 120% of the rated current per minute. This is meaningful for setting the motor's starting and braking processes, but cannot be compared to a short-term overload of over 200% for several minutes. In any situation where the motor may experience a short-term overload during operation, the inverter capacity should be increased by one level. 2.1.1 Selection of Inverter Capacity for Continuous Operation Since the inverter supplies the motor with pulsating current, the pulsation value is larger than that of the current supplied by the mains frequency. Therefore, an appropriate margin must be left in the inverter capacity, as shown in the following formula: If the inverter is selected based on the maximum current of the motor during actual operation, the inverter capacity can be appropriately reduced, as shown in Figure 2-01. 2.1.2 Selection of Inverter Capacity for Acceleration and Deceleration The maximum output torque of an inverter is determined by its maximum output current. Generally, for short-duration acceleration and deceleration, the inverter can reach 130%-150% of its rated output current (depending on the inverter capacity). Therefore, the output torque can be increased during short-duration acceleration and deceleration; conversely, if only a smaller acceleration/deceleration torque is needed, the inverter capacity can be reduced. Due to current pulsation, the maximum output current of the inverter should be reduced by 10% before selection, as shown in Figure 2-02. 2.2 Selection of Inverter Type Based on their control functions, general-purpose inverters are divided into two main categories, each with two types: for V/F control, there are ordinary function type and constant electromagnetic torque control function type; for vector control, there are with and without speed sensors. The basic principle for selecting the inverter type is to choose according to the load requirements. The selection method is as follows: (1) For boiler blowers, induced draft fans, feed water pumps and various pumps and conveyor belts in the refining industry, V/F control frequency converters can be selected. Because the load of blowers and pumps has low requirements in terms of overload capacity, and because the load torque is proportional to the square of the speed, the load is lighter when running at low speed (except for Roots blowers). Also, because these types of loads do not have any requirements for speed accuracy, ordinary functional frequency converters are selected. However, the loads of induced draft fans and conveyor belts have constant torque characteristics, so it is best to use frequency converters with constant torque control function. (2) For press machine motors, 1# and 2# sugarcane cutter motors, vector control frequency converters must be selected. Because these types of loads require relatively hard mechanical characteristics at low speeds in order to meet the dynamic and static index requirements of the production process for the control system. The vector control method decomposes the stator current into a specified magnetic field current and torque current, which are arbitrarily controlled separately, and the stator current after the two are combined is supplied to the motor. Therefore, in principle, the same control performance as a DC motor can be obtained. 3. Inverter Maintenance Although the reliability of new-generation inverters is very high, improper use and maintenance can still lead to malfunctions or poor operation, shortening the equipment's lifespan. Even the latest generation of inverters will experience performance changes due to long-term use and environmental factors such as temperature, humidity, vibration, and dust. Proper use and maintenance can extend the machine's lifespan and reduce production losses caused by sudden failures. Therefore, daily maintenance and inspection are indispensable. 3.1 Inverter Inspection Precautions Operators must be familiar with the basic principles, functional characteristics, and specifications of the inverter and have experience operating it. Before maintenance, the power must be disconnected, and attention should be paid to the charging section of the main circuit capacitors. Work should only proceed after confirming that the capacitors have completely discharged. 3.2 Routine Inspection Items Check for any abnormal phenomena during inverter operation; whether the installation site environment is abnormal; whether the cooling system is normal; whether the inverter, motor, transformer, reactor, etc., are overheating, discolored, or have an unusual odor; whether the inverter and motor have abnormal vibrations or sounds; whether the main circuit voltage and control circuit voltage are normal; whether the filter capacitor has an unusual odor; and whether all displays are normal. 3.3 Main Items and Maintenance Methods for Periodic Inspections General periodic inspections should be performed once a year, and insulation resistance checks can be performed once every three years. The focus of periodic inspections is on parts that cannot be inspected during inverter operation. Key inspection items and maintenance methods are as follows: 3.3.1 Cooling System Inspection Cooling Fan. The cooling fan is fully sealed, and maintenance work does not require cleaning or lubrication. However, it should be noted that the fan blades should be fixed first, and then the radiator should be cleaned with compressed air to protect the bearings. A precursor to cooling fan damage is increased bearing noise or radiator temperature exceeding normal levels. Frequent over-temperature warnings or malfunctions in the inverter indicate abnormal cooling fan operation. Radiator. Under normal operating conditions, the radiator should be cleaned once a year. In environments with heavy pollution, radiator cleaning should be more frequent. Please use a soft brush to clean the radiator, or use compressed air to clean it. 3.3.2 Electrolytic Capacitor Inspection Visually inspect the electrolytic capacitors for leakage and deformation. Generally, the service life of an electrolytic capacitor is 100,000 hours, with a static capacitance value greater than 85% of the nominal value. The actual service life depends on the inverter's operating method and ambient temperature; lowering the ambient temperature can extend its service life. 3.3.3 Contactor and Charging Resistor Inspection Check if the contactor contacts are rough, and check the charging resistor for signs of overheating. 3.3.4 Terminal Block and Control Power Supply Inspection Check if screws, nuts, and other fasteners are loose and tighten as necessary; check conductors, insulation, and transformers for corrosion, overheating, discoloration, or damage; check if the insulation resistance is within the normal range; confirm that the control power supply voltage is correct, and confirm that the protection and display circuits are normal. 4 Conclusion As long as the working characteristics of the frequency converter are mastered and the frequency converter is correctly selected and maintained in different working ranges, the application technology of frequency converter speed control will be increasingly valued by sugar factories, thereby accelerating the technological innovation of traditional equipment by the new generation of electrical equipment. After the automatic speed control equipment gradually enters the sugar factory, the production process is more stable, the production safety rate is continuously improved, and the goal of modern enterprise quality management can be achieved. References [1] Wu Zhongzhi, Wu Jialin. Frequency converter application manual [M]. Beijing: Machinery Industry Press, 2001. [2] Zeng Yi, Wang Xiaoliang, Wu Hao, et al. Design and maintenance of frequency conversion speed control system [M]. Jinan: Shandong Science and Technology Press, 2002. A brief discussion on the selection and maintenance of frequency converters in the technical transformation of sugar factories: PDF