What principles should be followed when selecting a frequency converter? Introduction to frequency converter capacity selection method

1. Frequency Converter Selection Principles

　　(01) Combined with the overall framework of the project, starting from the process characteristics and electrical control, the load type, usage environment, communication architecture and interface type must all be considered, such as serial port, DP or PN communication interface.

　　(02) Select the inverter according to the load characteristics. If the load is a constant torque load, you can choose the Siemens G120 inverter. If the load is a fan or pump load, you can choose the Siemens G120XA inverter.

　　(03) When selecting a frequency converter, the actual motor current value should be used as the basis for selecting the frequency converter, and the rated power of the motor can only be used as a reference. In addition, it should be fully considered that the output of the frequency converter contains high-order harmonics, which will cause the power factor and efficiency of the motor to deteriorate. Therefore, compared with using the power grid to power the motor, the current of the motor increases by 10% and the temperature rise increases by about 20%. Therefore, when selecting the motor and the frequency converter, this situation should be taken into consideration and an appropriate margin should be left to prevent excessive temperature rise and affect the service life of the motor.

　　(04) If the inverter is to be operated with a long cable, measures should be taken to suppress the influence of the long cable on the coupling capacitance to the ground to avoid insufficient output of the inverter. Therefore, the inverter should be enlarged by one gear or an output reactor should be installed at the output end of the inverter.

　　(05) For some special application scenarios, such as high ambient temperature, high switching frequency (especially in building automation and other application places with high noise restrictions), high altitude, etc., the inverter capacity will be reduced and the inverter needs to be enlarged by one gear.

　　(06) When the inverter is used to control several motors in parallel, the total length of the cable from the inverter to the motor must be within the allowable range of the inverter. If it exceeds the specified value, the inverter should be selected by one or two gears. In addition, in this case, the control mode of the inverter can only be V/F control mode, and the inverter cannot protect the motor from overcurrent and overload protection. At this time, a fuse must be added to each motor to achieve protection.

　　(07) For some special application scenarios, such as high ambient temperature, high switching frequency, high altitude, etc., the inverter capacity will be reduced and the inverter needs to be enlarged by one gear.

　　2. Selection method of inverter capacity

　　1. Steps for selecting inverter capacity

　　(1) Understand the nature and changing rules of the load, calculate the load current or make a load current diagram I=f(t);

　　(2) Preselect the inverter capacity;

　　(3) Check the pre-selected inverter and, if necessary, check the overload capacity and starting capacity. If both pass, the pre-selected inverter capacity is selected; otherwise, start again from (2) until it passes.

　　On the premise of meeting the requirements of production machinery, the smaller the inverter capacity, the more economical it is.

　　2. How to select the capacity of the inverter

　　There are many misunderstandings in the actual operation of inverter capacity selection. Here are three basic capacity selection methods that complement each other.

　　(1) From the perspective of current

　　Most inverter capacity can be expressed from three perspectives: rated current, available motor power and rated capacity. The latter two are given by inverter manufacturers based on standard motors produced in their own country or company, or are reduced with the inverter output voltage, which makes it difficult to accurately express the inverter's capacity.

　　When selecting a frequency converter, only the rated current of the frequency converter is a key quantity that reflects the load capacity of the semiconductor frequency converter. The load current should not exceed the rated current of the frequency converter, which is the basic principle for selecting the capacity of the frequency converter. It should be emphasized that before determining the capacity of the frequency converter, the process conditions of the equipment and the motor parameters should be carefully understood. For example, the rated current of submersible electric pumps and wound rotor motors should be greater than the rated current of ordinary cage asynchronous motors. The motors commonly used in the metallurgical industry for roller tables not only have much larger rated currents, but also allow them to be in a short-term stalled working state. In addition, most roller table transmissions are multi-motor transmissions. It should be ensured that the total load current does not exceed the rated current of the frequency converter under fault-free conditions.

　　(2) From the perspective of efficiency

　　The system efficiency is equal to the product of the inverter efficiency and the motor efficiency. The system efficiency is high only when both are working at a high efficiency. From the efficiency point of view, when selecting the inverter power, pay attention to the following points:

　　1) The inverter power value is most suitable when it is equivalent to the motor power value, so that the inverter can operate at a high efficiency value;

　　2) When the power classification of the inverter is different from that of the motor, the power of the inverter should be as close to that of the motor as possible, but should be slightly greater than that of the motor;

　　3) When the motor is frequently started, braked, or started under heavy load and works frequently, a higher-level inverter can be selected to facilitate long-term and safe operation of the inverter;

　　4) After testing, the actual power of the motor is indeed sufficient, so you can consider using a frequency converter with a power smaller than the motor power, but pay attention to whether the instantaneous peak current will cause overcurrent protection action;

　　5) When the power of the inverter and the motor are different, the setting of the energy-saving program must be adjusted accordingly to achieve a higher energy-saving effect.