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In a variety of fans, water pumps, oil pumps, along with the rotation of the impeller, the air or liquid in a certain speed range of resistance is roughly proportional to the second power of the speed n. As the rotational speed decreases, the torque decreases by the second power of the rotational speed. The power required for this load is proportional to the third power of speed. When the required air volume and flow are reduced, the frequency converter can be used to adjust the air volume and flow by means of speed regulation, which can greatly save electric energy. Because the power required at high speed increases too fast with the speed and is proportional to the cubic power of the speed, the fan and pump loads should not run over the power frequency.
Users can choose different types of frequency converters according to their actual process requirements and application occasions. The following points should be paid attention to when selecting the frequency converter:
Frequency converter selection should be based on the actual motor current as the basis of frequency converter selection, motor rated power only as a reference. In addition, the output of frequency converter should be fully considered to contain rich high-order harmonics, which will deteriorate the power factor and efficiency of the motor. Therefore, compared with the power supply to the motor by frequency converter and power grid, the current of the motor will increase by 10% and the temperature rise will increase by about 20%. Therefore, when choosing motors and frequency converters, this situation should be taken into account, and the appropriate margin should be left to prevent the temperature rise from being too high and affecting the service life of the motors.
If the frequency converter needs to run with a long cable, measures should be taken to suppress the influence of the long cable on the ground coupling capacitance to avoid insufficient output of the frequency converter. Therefore, the frequency converter should be enlarged by one or two grades or the output reactor should be installed at the output end of the frequency converter.
When using frequency converter to control high speed motor, because the reactance of high speed motor is small, it will produce more high harmonics. These high harmonics will increase the output current value of the converter. Therefore, when choosing the frequency converter for high-speed motors, it should be slightly larger than that for ordinary motors.
When using frequency converter to drive gear reduction motor, the range of use is restricted by the lubrication mode of gear transmission part. When lubricating oil is lubricated, there is no limit in the low speed range; In the high speed range above rated speed, there may be a danger of running out of lubricating oil. Therefore, do not exceed the maximum speed allowable value.
When the inverter drives the winding rotor induction motor, it mostly uses the existing motor. Compared with the general prisoner squirrel cage motor, the winding impedance of the wound motor is small. Therefore, it is easy to overcurrent trip caused by ripple current, so a transducer with a slightly larger capacity than usual should be selected. Generally, the winding motor is mainly used in the situation where the flywheel torque GD2 is large, so more attention should be paid when setting the acceleration and deceleration time.