Transformer Humming

[VTer]

Hi All,

This is just a curiously question more than anything.

I often hear much older and experienced engineers refer to the transformer humming as a 120Hz sound (on 60Hz system) or 100Hz sound on a 50Hz system which makes sense but in most cases these transformers are 3-phase units which in my mind would be a 360Hz or 300Hz sound. What am I missing?


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla

 

[burnt2x]

Because transformer noise or sound (hum) is produced by vibration of the core and winding and is dependent on the frequency of the exciting voltage (2 peaks per cycle), the fundamental noise frequency will be 120 Hz for 60Hz and 100 for 50Hz systems. I cannot believe "much older and experienced engineers" say it's 120 or 100 Hz only. There is no such thing as perfect transformers and there are 3rd, 5th, and 7th harmonics noise as well that you will hear from transformers! To prove this, try using a noise analyzer and see for yourself.

 

[Skogsgurra]

burnt_twice. Yes, we much older and experienced engineers say that you have a 100 (or 120) Hz hum from a transformer. Just as we say that a piano or a flute emits 440 Hz when an 'a' is played. There are harmonics in all those cases, but that is understood and it would be very impractical to include a full description of the spectrum.

To answer VTer's question: If you have three sound sources that each emit a 100 (or 120) Hz tone, then all you will hear is the sum of those tones. And that is still a 100/120 Hz tone.

Another situation is when you run, say, a DC motor from a thyristor controlled rectifier that is connected to a three-phase trasformer. Your armature current then contains 300 (or 360) Hz components (because of the six-pulse rectification) that emit the typical "thyristor sound".

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

The (fundamental) frequency of the audible hum is twice the line frequency. The hum is caused by small changes in the size of the core as the magnetic field changes. The rate of change is twice the line frequency. Adding more phases doesn't change the frequency. There will be some audible harmonics, I'm sure, but main tone will be 120 or 100 Hz.

http://en.wikipedia.org/wiki/Magnetostriction

David Castor

http://www.cvoes.com

 

[WoodrowJWeen]

Always thought transformers hummed because they didn't know the words. Sorry, couldn't resist.

 

[rwidup]

Noise is caused by magnetostricition (changes in shape) of the core laminations while the transformer is energized. Transformers emit a low-frequency, tonal noise that people living in their vicinity experience as an irritating “hum” and can hear even against a noisy background. The electrical power industry produce a range of solutions to abate humming, which originates in the transformer’s core and, when it is loaded, in the coil windings. Core noise is generated by the magnetostriction of the core’s laminations when a magnetic field passes through them. It is also known as “no-load noise,” as it is independent of the load passing through the transformer. As the phenomenon occurs it causes air columns to be formed in the spaces between the transformer core and the low-voltage windings of the core and other adjacent parts of the transformer, and these air columns cause audible noise as it moves between the various parts of the transformer. Magnetostriction takes place at twice the frequency of the supply load: for a 60 Hz supply frequency, a lamination vibrates at 120 cycles per second. And, the higher the density of the magnetic flux, the higher the frequency of the even number harmonics.

Also, the audible sound produced by fan-assist cooled (FAC) transformers is partially due to the energizing of the core by the alternating voltage applied to the windings and also by the fans forcing air through the coils. The noise generated by the core, and whose fundamental frequency is twice the frequency of the applied voltage, will create audible sound will be present even under no load conditions. The vibrations producing audible sound can occur in the core, coil, mounting, housing, and in the conduit. The transmission of sound from the transformer can be by various media such as air, metal, concrete, wood or any combination. Amplification of audible sound can occur in a given area due to the presence of reflecting surfaces.

Transformer hum also arises through the vibration caused when the load current passes through the windings, interacting with the leakage flux it generates. This “load noise” level is determined by the size of the load current.

In IEEE C57.94, Recommended Practice for Installation, Application, Operation, and Maintenance of Dry-Type General Purpose Distribution and Power Transformers, they tell us that the audible sound produced by transformers is due to energizing of the core by the alternating voltage applied to the windings. This creates vibrations whose fundamental frequency is twice the frequency of the applied voltage. The vibrations producing audible sound can occur in the core, coil, mounting and in the housing. The transmission of sound from the transformer can be by various media such as air, metal, concrete, wood or any combination. Amplification of audible sound can occur in a given area due to the presence of reflecting surfaces.

 

[Skogsgurra]

The thought that transformers will some day know the words is frightening.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...