There are many kinds of production machinery, with different performance and process requirements, and their torque characteristics are also different. Therefore, before the application of frequency converter, we should first make clear the nature of the load carried by the motor, that is, the load characteristics, and then select the frequency converter and motor. There are three types of load: constant torque load, fan pump load and constant power load. Different types of load should choose different types of inverter.

(1) Constant torque load: it can be divided into friction load and potential energy load. The starting torque of friction clothing usually requires about 150% of the rated torque, and the braking torque usually requires about 100% of the rated torque. Therefore, the frequency converter with constant torque characteristics, large starting and braking torque, overload time and overload capacity should be selected. Potential energy load usually requires large starting torque and energy feedback function, which can quickly realize forward and reverse rotation. The frequency converter with four quadrant operation ability should be selected.

(2) Fan and pump load: the wind load of negative pump is a typical square torque load. The load is very small at low speed and is proportional to the square of speed. The combination of general frequency converter and standard motor is suitable. This kind of load does not require high performance of frequency converter, only requires economy and reliability, so it is necessary to select the inverter with U / F = const control mode.

(3) Constant power load: constant power load refers to the load whose torque is inversely proportional to the speed, but the power remains constant, such as coiler, machine tool, etc. For the clothing with constant power characteristics, we should pay attention to the following problems: in the frequency range above power frequency, the output voltage of frequency converter is constant control, so the torque produced by motor and load torque have opposite tendency, the combination of standard motor and general frequency converter is difficult to adapt, so special design is needed.