Why do transformers hum?

Discussion in 'General Electronics Chat' started by Sparky49, Oct 27, 2011.

  1. audio76

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    Aug 30, 2014
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    I think I learned more in two or three posting here than my last year of college! Thanks guys for the insight. I'll be able to sleep tonight....
     
  2. subtech

    AAC Fanatic!

    Nov 21, 2006
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    In the majority of North America, the fundamental power system frequency is 60 Hz.
    In many other locations around the world, the power system frequency is 50 Hz.
    Almost all large power transformers (let's say 5 MVA +) are three phase units,
    so the "hum" you'll hear when you are near them is the third harmonic of their
    fundamental operating frequency. Here in the US, 180 Hz.
    What you hear, you can also feel. (don't touch anything in a substation or generating plant
    unless you know what you are doing...)
    There are incredibly strong forces at work inside these units, and the sound you hear
    is just as others have said, it is physical materials vibrating due to electrical forces acting on them.
    Windings, core steel, and the all the mounting/bracing hardware are moving.
    It may help to think of those three windings inside of that metal tub as incredibly strong
    electro-magnets interacting with each other, and being held in place with a good deal of
    bracing components such as I-beams, angle stock, and pipe or tubing.
    Designing the bracing and clamping components in large transformers is an art unto itself,
    and is usually left to those precious few with the guts and smarts to stick out the many years
    of training in order to gain the necessary experience.
    Most likely what you have heard is the hum that results from normal operation.
    During fault conditions, if you are within a few meters, you will feel the ground vibrate.
    The foundations these units set on are designed to take into account all of that mass
    shaking/vibrating significantly.
    It's an interesting field. (no pun intended)
     
  3. inwo

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    Nov 7, 2013
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    audio76 likes this.
  4. DerStrom8

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  5. ian field

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    Oct 27, 2012
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    The core is made of stacked laminations, in the concentrated magnetic field they act as loads of armatures and radiate sound waves.

    Some of the small transformers in equipment are vacuum impregnated with varnish and baked hard - if done properly, the transformer can be pretty much silent.

    There is also some stray magnetic field around the transformer that deflects the steel housing - substation transformers are usually oil filled which provides a fair bit of damping - but I've yet to see one that's silent.

    The effect is a bit pronounced in microwave ovens, the mains transformer is rarely more than an inch from the steel case, the transformers are designed for cheapest production possible - supressing stray magnetic field wasn't high on the list of priorities.
     
  6. wayneh

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    Sep 9, 2010
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    No one has mentioned the hum or crackling buzz of high tension wires. Whatever causes that sound is also in play at the substation, and it obviously has nothing to do with the transformer laminations. Transformers hum, but it's not the only thing.
     
  7. DerStrom8

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    Feb 20, 2011
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    In those cases it's the wires or towers that actually vibrate. Same idea as the transformer laminations.
     
  8. wayneh

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    I suspect there is some discharge sound as well, to produce the crackle. Not really sure what is going on. It's not a steady, pure tone hum.
     
  9. ian field

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    Oct 27, 2012
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    That's the high voltage verging on corona discharge - The 400kV National Grid cables near my flat are pretty loud in damp or foggy weather.

    Just up the road, the overhead cables pass over a road - you can't hear anything at all directly underneath them in dry weather.
     
  10. mbohuntr

    Senior Member

    Apr 6, 2009
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    Correct, The corona effect is caused by sharp edges on connectors that create opposing electromagnetic fields. If they are severe enough, they can be observed with the naked eye at night and resemble a blue gas flame. The funny looking rings and heart shaped devices are corona rings designed to prevent the metal erosion caused by corona.
     
  11. ian field

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    When I hear the cables buzzing *REALLY* loud - I wonder how much energy is expended over the whole grid!

    Mind you - the energy expended heating up the eddie's in the town's main substation transformer is more than I could use without some help.
     
  12. mbohuntr

    Senior Member

    Apr 6, 2009
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    We do eddy current testing, but I don't really understand it..... All those "noises" are pretty cool. The shop got into the subject of ferro-resonance recently, really interesting stuff.
     
  13. ian field

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    If you had a solid core transformer; all the energy would be eddy currents, and next to nothing out of the secondary - basically one *BIG* fat shorted turn.

    Laminations break up the conductivity of the core so there is no shorted turn.

    There's still eddy currents, but hopefully so small as only a small percentage of the total power.
     
  14. BadBadger

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    Feb 1, 2014
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    And yet a speaker's voice coil pulled in one direction from zero, then pushed in the other from zero, produces a 60Hz tone when fed a 60Hz current. If an object's mechanical motion exactly follows an EM field's waveform, it will produce the exact waveform in the form of sound.

    I'm thinking the difference is due to the materials in (core) or around (other stuff) a transformer being magnetized by the flux with varying, but the same, polarities. For example, at any point in the cycle, two adjacent ferrous objects in the EM field will acquire the same magnetic polarities, therefor repelling each other, regardless of the polarity being plus or minus. That would mean they are repelling each other twice during a single cycle.

    As opposed to the flux produced by a voice coil acting against a speaker's permanent magnet, which has a fixed polarity. In which case the voice coil attracts or repels the magnet in synchrony with the applied current.

    Does this sound right?
     
  15. BadBadger

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    Feb 1, 2014
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    And then, as mentioned previously, there is magnetostriction, which is similar, but on a molecular scale. And no doubt the stronger of the effects. Molecules will attract each other twice during a cycle causing deformation (vibration) in an affected material.

    220px-Magnetostriction_by_Zureks.gif
     
  16. kubeek

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    Yes, basically the sheet metal is attracted by any polarity of the magnetic field, so you get two peaks in attraction in each period, hence the frequency doubling. Voice coil is attracted by one polarity and repelled by the other, so it moves at the same frequency as the field.
     
  17. BadBadger

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    Feb 1, 2014
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    Of course, I forgot about the simple fact that the transformer's windings would attract things, too. :rolleyes: Thanks for condensing my ramblings! I wonder if anyone can get it down to 10 words or less. :D

    Seems to me that with these multiple effects, there can be an awful lot of shaking going on.
     
  18. wayneh

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    Sep 9, 2010
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    Powerful electromagnetic fields induce mechanical motion in nearby conductors. (9 words)
     
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  19. BadBadger

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    Feb 1, 2014
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    I'll accept that. lol
     
  20. ian field

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    Oct 27, 2012
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    Even the "soft" magnetic material used in laminations has *SOME* remanance, so it will be very slightly magnetised in alternating directions - this increases the deflection a small amount.
     
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