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How to Determine the Resonant Frequency of a Cooling Tower Fan

Determining the resonant frequency of a cooling tower fan, or any object, requires finding the frequency at which the fan will respond to vibrations. When an object resonates, any vibrations are amplified by the structure of the material, so the vibrations continue to intensify. Similar to a crystal wine glass that can be shattered by the right note, a cooling fan can be damaged if the motor runs at or near its resonant frequency. The type of material can greatly mitigate the resonance. For example, metal fan blades resonate far more vigorously than do fiberglass.

Things You'll Need

Tuner
Rubber mallet

Instructions

- 1
  Power off the fan. Never work on a fan that is powered on or that may turn on while you're working on it.
- 2
  Strike the hub of the fan firmly with the rubber mallet. Start with the amount of force you would use to swat a fly. Don't hit the fan blades, as they might break or move out of alignment.
- 3
  Move the tuner close to -- or touching -- one of the fan blades. Sound tuners should have the microphone placed as close to a fan blade without touching it as is possible. Record the frequency of the vibration in Hertz or vibrations per second. Vibration sensors may need to touch the fan blade; touch the base of the blade just near the hub so that you allow the rest of the blade to resonate freely.
- 4
  Take several measurements on all of the fan blades to develop an average resonant frequency.
- 5
  Increase the force of the mallet blow if you're unable to take consistent readings. The goal is to use as little force as you can to create the vibration so that you don't do any damage to the fan. Start softly and slowly increase the force.
- 6
  Add all the frequency measurements and divide by the total number of readings to get the average.

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