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I have two Harbor Breeze 52in. Builders Series Ceiling Fans. Under normal operation these fans are smooth and quiet. However, during power outages like we've had recently, I use a Devilbiss 6000 watt emergency generator that is wired into my house circuitry. When using the generator these same two fans hum extremely loud. They are so noisy that we don't use them and use a tower floor fan instead.

Why are these fans so noisy when using the generator? Everything else in the house works normally when powered by the generator and I have it wired and connected according to manufacturers and local code specs.

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The power output from normal generators is actually pretty noisy, it is not a regular sine wave like you get from the power company, and may have high frequency harmonics. According to one source I found, total harmonic distortion (THD) from a generator can be >15%, where normally acceptable THD is 5-6%.

Here's an animation from wikipedia, showing a normal sine wave, and then the effect harmonics have on the overall signal:

enter image description here

In an AC motor (simplified explanation) the sine wave directly runs the magnets in the motor, causing it to spin. The harmonics effectively reverse the polarity of the magnets mid-rotation - it's as if you're flicking back and forth between forward/reverse hundreds of times a second. I believe this is what causes the hum.

As for why your tower fan works, hard to say. The motor design may be less susceptible to the effects of harmonics, it may have electronics built into it, or they may be DC motors.

So the solutions:

  • Don't run your fans while on generator power
  • Use a power conditioner after the generator (which may or may not help, depending on the quality of the incoming power)
  • Get an "inverter generator". They are more expensive, but produce a pure sine wave output, and are more efficient because they are capable of lowering their speed to match the load
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    Greg is probably right, assuming the gen put out a modifided Sine wave. You may also want to check the operating freq of the gen. It is common for gen sets to be way off calibration for 60HZ but still be close to 120VAC.
    – shirlock homes
    Commented May 4, 2011 at 8:55
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    Distortion of the sine wave can also be affected by how heavily-loaded the generator is.
    – myron-semack
    Commented May 4, 2011 at 13:58

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