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Saturday, October 31, 2009

High Freq Hum

I have an older Bounder diesel pusher and I'm on my second battery charger for the coach batteries. Unfortunately I continue to hear a very large humming noise from the charger. Also, the charger is located just under the bed and this creates quite a problem with sleep. Is this humming normal and if so can it be moved away from ear distance for sleeping? Jack, (Toledo, OH)

Jack, what you are hearing is an audible mechanical resonance created by a harmonic distortion in the converter/charger. I know that sounds like double-speak, but the phenomenon is somewhat technical. I'll try to explain the happenings.

Most AC electrical loads are linear, meaning the current is proportional to the AC voltage. Examples include regular AC lighting and simple motors. Linear devices rarely vary in frequency. Non-linear electrical loads, such as your AC/DC converter, can produce electrical waveforms that are multiples of the normal 60 Hertz sine wave associated with the 120-volt AC electrical system in the RV. These multiple waves, called harmonics, are actually superimposed over the original sine wave. The original, fundamental sine wave combined with any of the overlaying harmonics will result in a non-sinusoidal wave form of an entirely different shape.

This is very similar to the harmonics associated with stringed musical instruments. (I could demonstrate this easier on my guitar!) This new waveform then, has the ability to create a mechanical resonance within the transformer section of the converter. Overheating of the transformer, additionally, can lead to increased levels of harmonic distortion which usually manifests itself as an audible hum via the mechanical resonance. (Hang on, we’re almost there!)

The laminated steel plates that make up the transformer literally begin to vibrate at a frequency that is able to be heard. Unfortunately, there's not much you can do to eliminate this resonance short of redesigning the converter to include special notch filters to block out or eliminate the offending frequencies. Sound baffles can be placed between you and the converter, but care must be taken to avoid any build up of heat in the converter area.

The best cure is to simply upgrade to one of the newer, high frequency converter/chargers like the one pictured above. They are engineered with devices that can cancel out the harmonics and allow the waveform to maintain the 60 Hertz necessary. Another benefit of upgrading is the optimum battery charging abilities available with the sophisticated, smart converter/charger. (Whew!)


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