Constant power load:
The required torque of machine tool spindle and coiler and uncoiler in rolling mill, paper machine and plastic film production line is generally inversely proportional to the speed, which is the so-called constant power load. The constant power property of the load should be in terms of the speed variation range. When the speed is very low, it changes to constant torque at low speed due to the limitation of mechanical strength. The constant power area and constant torque area of the load have a great influence on the selection of the transmission scheme. When the motor is in constant flux speed regulation, the allowable output torque remains unchanged, which belongs to constant torque speed regulation; In the field weakening speed regulation, the allowable output torque is inversely proportional to the speed, which belongs to constant power speed regulation. If the range of constant torque and constant power speed regulation of the motor is consistent with the range of constant torque and constant power of the load, that is, under the so-called "matching" condition, the capacity of the motor and the capacity of the frequency converter are small.
Fan and pump loads:
In various fans, water pumps and oil pumps, with the rotation of the impeller, the resistance of air or liquid in the speed range is roughly proportional to the power of speed n. As the speed decreases, the speed decreases to the power of the speed. The power required by this load is proportional to the third power of speed. When the required air volume and flow are reduced, using the frequency converter to adjust the air volume and flow through speed regulation can greatly save electric energy. As the power required at high speed increases too fast with the speed, which is proportional to the third power of the speed, it is usually not necessary to make the fan and pump load operate at more than power frequency.
Siemens can provide different types of frequency converters. Users can choose different types of frequency converters according to their actual process requirements and application occasions. Pay attention to the following precautions when selecting frequency converter:
1. Select the frequency converter according to the load characteristics. If the load is constant torque load, Siemens mmv/mdv frequency converter should be selected. If the load is fan and pump load, Siemens eco frequency converter should be selected.
2. When selecting the 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 the power supply of power frequency grid, the current of the motor increases by 10% and the temperature rise increases by about 20%. Therefore, when selecting motor and frequency converter, this situation should be taken into account, and appropriate allowance should be left to prevent excessive temperature rise from affecting the service life of the motor.
3. If the frequency converter wants 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 enlarge the first gear selection or install the output reactor at the output end of the frequency converter.
4. When the frequency converter is used to control several motors in parallel, it should be considered that the total length of the cable from the frequency converter to the motor is within the allowable range of the frequency converter. If the specified value is exceeded, the frequency converter shall be selected by amplifying the first gear or the second gear. In addition, in this case, the control mode of the frequency converter can only be v/f control mode, and the frequency converter cannot protect the overcurrent and overload protection of the motor. At this time, it is necessary to add a fuse on each motor to achieve protection.
5. For some applications, such as high ambient temperature, high switching frequency, high altitude, etc., the capacity of the frequency converter will be reduced at this time, and the frequency converter needs to enlarge the first gear selection.
6. When using frequency converter to control high-speed motor, because the reactance of high-speed motor is small, high-order harmonic also increases the output current value. Therefore, when selecting the frequency converter for high-speed motor, it should be slightly larger than that of ordinary motor.
7. When the frequency converter is used for pole changing motor, full attention should be paid to selecting the capacity of the frequency converter so that its large rated current is below the rated output current of the frequency converter. In addition, the motor should be stopped before pole number conversion during operation, otherwise the motor will idle and the frequency converter will be damaged in case of bad conditions.

Previous：MCZ Frequency Next：Jhb600 multifunctional universal frequency converter 0.75-7.5kw