A frequency converter is used to drive the asynchronous motor for speed regulation. After the asynchronous motor is determined, the frequency converter should usually be selected according to the rated current of the asynchronous motor, or according to the current value (maximum value) in the actual operation of the asynchronous motor. When the operation mode is different, the calculation method and selection method of the inverter capacity are different, and the conditions that the inverter should meet are also different. When selecting the capacity of the inverter, the rated current of the inverter is a key quantity, and the capacity of the inverter should be selected according to the maximum working current that may occur during operation.