Different load characteristics determine the dynamic changes of motor performance parameters. Different working conditions may lead to changes in the torque, current or speed of the motor. All these changes must be taken into account when selecting the inverter.

Compressor, vibrator and other equipment belong to the load condition of torque fluctuation, while oil pump equipment has peak load. These are typical variable loads. If the inverter is matched according to the rated current or output value of the motor, unexpected phenomena such as over-current protection action may occur due to peak current. Therefore, it is necessary to select the frequency converter with higher rated output current than its maximum current to ensure the normal operation of the equipment.

Roots blower is a positive displacement blower, which has the characteristics of high output air pressure, and is mostly used in the exhaust slot of sewage treatment plant. During the operation, the output pressure of the pump is basically constant, and the output air volume changes in proportion to the speed. For the motor body, its torque characteristic is basically constant torque. However, when the air volume increases, the pressure loss of piping and the pressure loss of the air blowing out part in the water increases. About 20% of the pressure loss is related to the rotation speed, which is not constant. When selecting the rated capacity of the frequency converter, it should be 20% higher than the rated value of the motor, and the speed should be adjusted above 20% of the rated speed.

Compared with the same general motors, the rated current of motors in deep well pumps is larger. The selected capacity should be within the rated current of the inverter. In addition, the cooling of the motor is carried out by the running water of the pump. Therefore, a certain flow rate is required. For pumps with high head, such as deep well pump, the rotation speed is correspondingly high. The lower limit should be set according to the rotation speed of the pump.

The moment of inertia of the centrifuge is large, so it needs a large acceleration torque. Moreover, it takes a long time to accelerate and takes a few minutes. Therefore, in order to make the overload protection of the frequency converter not act during acceleration, the motor current should be selected below the rated current of the frequency converter when the frequency converter accelerates. In addition, in order to stop quickly, a large deceleration torque is required, so that the braking resistance will not overheat due to the rise of regenerative voltage during deceleration.