In order to ensure that the functional characteristics of the VFD can reliably realize the process requirements, the following technical parameters should be paid close attention to when selecting the VFD according to the actual working current of the motor.

The actual working current of the motor is the most critical factor in the selection of the VFD. When the VFD is working for a long time, the output current of the inverter must be greater than the actual working current of the motor.

In the project, the motor is usually selected first, and then the VFD is selected according to the motor. The actual working current of the motor is not the rated current which marked on the motor nameplate. When selecting the VFD, you should be familiar with the working conditions and estimate the relationship between the working current of the motor and the time before determining the corresponding inverter model.

 1.1 Generally, the VFD drives the motor with constant torque load, then the VFD is selected based on the rated current of the motor.

1.2 Generally, the inverter drives the motor of fan and pump , then the inverter is selected based on the rated current of the motor.  

1.3 For a motor that is often overloaded for short periods of time, it is necessary to calculate the overload period and overload current.

When the inverter drives the motor with this type of load, the maximum output current Imax of the inverter is required to be greater than the peak current of the motor, and the I2t of the inverter is within its allowable range. When selecting the inverter, it may be enlarged by one or several gears to meet the requirements of the site requirement

Now take a 10kW, 20A rated current motor as an example: If the motor works intermittently, the peak current is 40A (2 times the rated current) during overload operation within 1 second, and then it stops running for 20 seconds.At this case, the inverter overload curve is used for the selection: firstly, the curve of the motor current change with time is drawn, and secondly, whether the output current curve of the inverter can cover the motor current curve (whether the output current of the inverter exceeds the actual operation of the motor current), only the inverter model whose output current curve of the inverter covers the motor current curve is suitable for the motor with heavy load. For the selection of heavy-duty inverters, there are often some empirical data for reference.