Troubleshooting Fan Speeds

Troubleshooting Fan Speeds

Let’s say you are out in the field doing some troubleshooting on an air system fan, or any other type of motor-driven sheave (pulley) system, and you need to determine fan revolutions per minute or RPMs.  

The easiest way is to measure the RPM with your tachometer.  You reach into your tool bag only to remember that you let a team member borrow it and they have not yet returned it.  Not to worry because you always have a backup-plan – math!

You can use the following formula to determine the fan RPM:

Find These Variables
·                     Record the motor RPM from the motor tag
·                     Measure the motor (driver) sheave diameter in inches
·                     Measure the fan (driven) sheave diameter in inches

Example Problem
Motor RPM from the motor tag - 1750 RPM
Motor (driver) sheave diameter – 10.0” Pitch Diameter (P.D.)
Fan (driven) sheave diameter – 14.0” P.D

Remember, if you know any three variables you can solve the problem. The variable you are trying to find does not have to be the fan RPM.
Note – If you need to take off the drive guard remember ALWAYS lockout/tag out.  However, if the fan cannot be shut down during troubleshooting activities, then do the best you can to measure through the guard. In most cases, this will be sufficient for understanding the fan speed for troubleshooting purposes.


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