1. Nature of the load
The nature of the load determines the selection and use of the frequency converter. Generally, loads can be divided into the following three categories:
(1) Reduced torque characteristic loads. Reduced torque characteristic loads require torque to be proportional to the square of the speed (such as fan and pump loads). These types of loads are most suitable for frequency converter control. General three-phase AC asynchronous motors operate at frequencies below 15Hz, and it is difficult for the motor to reach its rated output torque. However, when reduced torque characteristic loads operate at low speeds, the required torque is reduced accordingly, which perfectly suits the characteristic of frequency converters driving asynchronous motors to reduce output torque at low speeds.
(2) Constant torque loads. These loads require input torque that is essentially independent of speed (e.g., conveyor belts, feeders, elevators, mixers, and the traveling mechanisms of processing equipment). Using a frequency converter for these loads allows for stepless speed regulation, significantly improving product quality and labor productivity. Based on the characteristics of constant torque loads, several points should be noted when using frequency converter control:
① When selecting the V<sub>F</sub> curve of a frequency converter, choose the characteristic curve with a larger starting torque;
② Appropriately increase the additional current setting during ramp ascent to ensure continuous starting and overcome the viscous effect;
③ When setting the motor current limit, the allowable overload current ratio of the motor should be appropriately relaxed. In addition, for constant torque loads with lifting capabilities (such as elevators and hoists), an external braking unit is required during the descent or when the motor stops.
(3) Constant power load. A constant power load refers to a load in which the output torque increases as the motor speed decreases (the motor output power remains constant), such as lathes, drilling machines, grinding machines, etc. For this type of load, the use of frequency converters must be approached with caution; three points should be noted when using them:
① Appropriately select high-capacity frequency converters and high-capacity motors;
② When the mechanical strength and output torque of the AC asynchronous motor can meet the requirements of high-speed operation, a 6-pole or 8-pole motor should be used;
③ Use a combination of pole-changing motor and frequency converter or a combination of mechanical stepped speed regulation and frequency converter.
2. Selection of frequency converter
Based on a clear understanding of the load characteristics, it's essential to understand the performance characteristics of various frequency converters to make an appropriate selection. Currently, the market offers many imported frequency converters, such as those from Fuji and Yaskawa (Japan), Siemens (Germany), and VACON (Finland). Among them, Fuji frequency converters (available in P-type and G-type) are slightly more expensive but have stronger load-carrying capacity. For loads requiring reduced torque, choose the P-type; for constant power loads, choose the G-type; for constant torque loads, either P-type or G-type is acceptable. For constant torque loads, the P-type frequency converter has lower overload capacity, torque limitation, and price compared to the G-type with the same power. Therefore, when selecting, prioritize the P-type with a higher power rating, followed by the G-type with the same power, and avoid the P-type with the same power whenever possible. For electromechanical equipment in sealed, dust-free, and pollution-free environments, Yaskawa frequency converters, which offer good load capacity, simple operation, reliable operation, and low noise, are recommended.
With the development of production technology, the performance and reliability of domestically produced frequency converters (such as Senlan frequency converters and Hong Kong ZL frequency converters) are almost the same as those of imported frequency converters. They are also cheaper, easier to maintain, and their operation methods and program settings are more suited to Asian users. Therefore, for general speed control equipment, considering cost-effectiveness and maintenance, domestically produced frequency converters should be selected whenever possible.
3. Inverter maintenance
The operating environment of a frequency converter directly affects its normal function and lifespan. Therefore, to extend its service life and improve energy efficiency, regular maintenance and replacement of some components are essential. Frequency converters are used in harsh working environments (such as the building materials and paper industries), making their maintenance a crucial aspect of equipment management. As the saying goes, "There are no motors that last 50 years, but there are frequency converters that last 50 years." Therefore, proactive maintenance and operation measures can fully utilize the frequency converter's capabilities and extend its lifespan.
