Driving motor by inverter has become an irreversible trend. In the actual use process, due to the unreasonable matching relationship between inverter and motor, it often leads to some problems. The load characteristics of the equipment driven by the frequency converter should be fully understood when selecting the frequency converter. 

We can divide the production machinery into three types: constant power load, constant torque load and fan and water pump load. Different load types have different requirements for frequency converters, so we should make a reasonable match according to the specific situation. 

Constant power load

Machine tool spindle and rolling mill, paper machine, plastic film production line in the coiler, uncoiler and other requirements of the torque, generally inversely proportional to the speed, is a constant power load.  The constant power nature of the load should be in terms of a certain range of velocity variations. When the speed is very low, restricted by mechanical strength, at low speed will change to constant torque load.  When the motor adjusts the speed with constant flux, it adjusts the speed with constant torque. And in weak magnetic speed regulation for constant power speed regulation. 

Fan and pump loads

Fan, water pump, oil pump and other equipment with the rotation of the impeller, as the speed decreases, the torque decreases by the square of the speed, the power required by the load is proportional to the third power of the 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 electricity. Because the power required at high speed increases too fast with the speed, the fan and pump loads should not run over the power frequency. 

Constant torque load

TL is always constant or almost constant at any speed.  When the frequency converter drags the load with constant torque, the torque at low speed should be large enough and have enough overload capacity.  If it is necessary to run steadily at low speed, the heat dissipation performance of the motor should be considered to avoid burning the motor due to high temperature rise.  

Problems to be paid attention to when choosing frequency converter:

— Power frequency motor is driven by frequency converter, the current of the motor will increase by 10-15%, and the temperature rise will increase by 20-25%. 

— When using frequency converter to control high speed motor, more high harmonics will be produced.  These high harmonics will increase the output current value of the converter.  Therefore, the choice of frequency converter, should be larger than the ordinary motor. 

— Wound motor compared with ordinary squirrel cage motor, prone to overcurrent trip problem, should choose a slightly larger capacity than the usual frequency converter. 

— When using frequency converter to drive a gear reduction motor, the range of use is limited by the lubrication mode of the gear rotation part. There is a risk of running out of oil when the rated speed is exceeded. 

— Take the current value of the motor as the basis of frequency converter selection, and the rated power of the motor is for reference only. 

— Inverter output is rich in high harmonics, which will reduce the power factor and efficiency performance of the motor. 

— If the converter needs long cable operation, the influence of cable on performance should be considered, and special cable should be selected if necessary.  In order to make up for this problem, the frequency converter should be amplified by one or two options. 

— High temperature, frequent switch, high altitude and other special occasions will make the capacity of the frequency converter rain, it is recommended that the frequency converter press the magnifying one gear to choose. 

— Compared with the power supply, the output capacity of the synchronous motor driven by the inverter will be reduced by 10~20%. 

— For loads with large torque fluctuations such as compressors and vibrators and peak loads such as hydraulic pumps, the operation of power frequency should be fully understood and a larger frequency converter should be selected.