Vfd Fan Power

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?

Is the fan's drive signal controlled by PID?

It might be easy enough to install a Sail Switch--a microswitch with a "sail" on it that makes when air flows across it. These switches usually have a pretty good lifespan.

You could try a current switch. These guys make one http://www.greystoneenergy.com/English/products/power/list_power.html it works well. Amos

Fan power draw is roughly proportional to air flow for a given discharge pressure and speed. If you are varying the speed the power vs. CFM curve shifts down as the cube of the speed.

For what you want to accomplish you can make it work. Just measure kW at a min and a max speeds, calculate the linear relationship, and use it to set a low load shutdown alarm.

Of course, it might be easier and cheaper to buy a flow switch.

Thanks for all the suggestions. Yeah I know, it would be cheaper to just install a flow switch but the spare I/O is there and it would be a LOT easier to just run a cable from the drive to the analog input card (about six feet in the same panel) than to try to get to the pipe waaaay up in the air above a big machine...

Tom, you confirmed what I thought I read somewhere... maybe here.


Thanks

I was asking if the fan was PID driven because you might set an alarm if your output goes 100% for an extended time. That is provided you're not having to run 100% ALL the time anyway.

Good Luck