VFD Application

[khara]

Hi All,
I have recently installed a VFD unit on a 22Kw vent fan motor. The calculated yearly KWH without the VFD was calculated at 192852kwh. The data obtained from the VFD produced a yearly Kwh saving of 44195kwh; however according to the afinity law (at 80% the multiplier factor for the kwh was calculated to be 0.512). Therefore the obtained kwh at 80% for the year was calculated as 22*8766*.512=98470kwh; consequently abstaining a yearly Kwh saving of 94112Kwh. Can anyone give some suggestions as to the discrepancy between the calculated kwh savings and the VFD savings.
Thanks

 

[jraef]

When we say a motor is "22kW", that is the MECHANICAL power rating of the motor, meaning XX torque at YY speed. The ABSORBED electrical power from the line source for that motor is higher by the losses of efficiency of the motor, drive, everything in the electrical path. In addition, just because your motor is rated 22kW does not mean the LOAD is actually requiring all 22kW.


"Will work for salami"

 

[mikekilroy]

in addition, you say "The data obtained from the VFD produced a yearly Kwh saving of 44195kwh."

What does this mean? Exactly what data?

 

[waross]

Have you read the KW demand from the VFD display at the original speed and at the new speed? Calculate your savings from that data and don't forget to subtract the losses of the VFD from the savings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[khara]

Thanks all,
The data received comprised of the total Kwh counter, the duration and the saved kwh from using the VFD. I will take a look at the losses of the VFD and also obtain data on running the VFD at 100% speed. Then explore the suggestion of Waross.

 

[LionelHutz]

Your measured savings number appear wrong because you don't have the proper measured data. You wouldn't be using a theoretical before VFD kWh number if you had before VFD measured data. You can't measure the savings after installing the VFD when you don't know the measured usage before installing the VFD.

Your simple calculations are wrong because you didn't include the motor efficiency.

Also, using the simple affinity law calculation shown on certain websites and by certain VFD salesmen can produce rather optimistic predictions.