I am thinking about using an analog output in a VFD to output the KW. The intent is to monitor the KW in the PLC to determine if the fan is running. The system has pressure switches that fail occasionaly. The last time this happend the belt came off the fan and was a couple of hours before anybody noticed. With the motor and fan running the VFD is outputting about 7KW. I can wire the analog output to the PLC and monitor the power. If the power fell below say 3KW (to allow for speed changes because of process needs) then I could set an alarm. The question here is doesnt the power output of the drive equal the amount of air moved by the fan minus the amount of power lost due to friction etc. in the motor, fan, bearing etc.? Does this make sense or should I just go get another #@#% pressure switch?