Could you tell anyone which duty motor is suitable for VFD application. Whether S1 or S9 duty. Motor to drive the load of axial flow fan in aircooler application in oil and gas industry.
Duty is determined by the load to which the motor is connected and the operating parameters thereof; not by the source of power that the motor is connected to...i.e., the mains, or VFD, or Generator, etc.
So, to answer your question, look to the duty of the load to which the motor is connected.