humming transformers


by PanCerowany
Tags: humming, transformers
PanCerowany
PanCerowany is offline
#1
Feb24-10, 06:02 AM
P: 5
Why transformers hum?

I have heard two versions. Oxygen has unpaired electrons and it reacts to changing magnetic field. Material that transformer is made of vibrates in changing magnetic field.

Both explanations tell that something vibrates in a magnetic field, but what it is?
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uart
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#2
Feb24-10, 06:32 AM
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It is caused by magnetostriction.

See : http://en.wikipedia.org/wiki/Magnetostriction
torquil
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#3
Feb24-10, 06:32 AM
P: 640
Quote Quote by PanCerowany View Post
Why transformers hum?

I have heard two versions. Oxygen has unpaired electrons and it reacts to changing magnetic field. Material that transformer is made of vibrates in changing magnetic field.

Both explanations tell that something vibrates in a magnetic field, but what it is?
The second one is correct. It is called magnetostriction. The varying magnetic field inside the transformer influences the metal:

http://www.federalpacific.com/univer.../chapter2.html
http://www.federalpacific.com/univer...ransnoise.html

It is also possible that the windings themselves move around a bit from the forces acting on them.

Torquil

sophiecentaur
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#4
Feb24-10, 07:11 AM
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humming transformers


Why transformers hum?

Maybe because they don't know the words?
Boom Boom
Phrak
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#5
Feb24-10, 07:27 AM
P: 4,513
Magneto restiction?
sumit sawai
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#6
Feb25-10, 07:10 AM
P: 5
2nd one is correct it is called as magnetostriction
core material in transfprmer is feromagnetic in changing magnetic field of transformer there is slight change in shape of material wich causes vibration
which produce huming noice
PanCerowany
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#7
Feb26-10, 07:59 AM
P: 5
OK, thanks.
m.s.j
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#8
Mar2-10, 07:25 AM
P: 219
In power transformers:

1-The most striking point is the strength of the component at 100 Hz or twice the normal operating frequency of the transformer. Consideration of magnetostrictive strain in the transformer core reveals that magnetostriction can be expected to produce a longitudinal vibration in the laminations at just this measured frequency. Deviation from a ‘square-law’ magnetostrictive characteristic would result
in even harmonics (at 200, 400, 600 Hz, etc.), while the different values of magnetostrictive strain for increasing and decreasing flux densities a pseudohysteresis effect lead to the introduction of odd harmonics (at 300, 500, 700 Hz, etc.).
The sensitivity of the ear to noise increases rapidly at frequencies above 100 Hz. On the 40 phon contour, it requires an increase of 12 dB in intensity to make a sound at 100 Hz appear as loud as one at 1000 Hz. The harmonics in a transformer noise may thus have a substantial effect on an observer even though their level is 10 dB or more lower than that of the 100 Hz fundamental. Although longitudinal vibration is the natural consequence of magnetostriction, the need to restrain the laminations by clamping also leads to transverse vibration. As already pointed out, two similar sources sound about 25% louder than one. By the same token, complete elimination of the transverse vibration would reduce the loudness of the transformer noise by only about a fifth. Although valuable, this reduction, even if technically and economically possible, is insignificant compared with the halving of the loudness which can be achieved by a reduction of 10 dB in the noise level of both longitudinal and transverse vibrations.

2-The other main source of noise from the transformer core is due to alternating attractive and repulsive forces between the laminations caused by flux transfer across the air gaps at the leg to yoke and inter-yoke joints. These forces can be reduced by special building and design techniques of which the best known and most widely used is the step-lap form of construction.
Even variations of 10% in flux density have been shown to produce changes of noise level of the order of only about 2 dB, although the character of the noise may vary appreciably. From this it will be apparent that it is most uneconomic to obtain a reduction in noise level by the employment of low flux densities. This is perhaps demonstrated best by reference to experience with cold-rolled steel.
To make the optimum use of these newer materials, it is necessary to operate them at flux densities of 1.65 1.85 tesla. While this higher flux density tends to lead to a higher noise level for a given size of core, current results suggest that the difference is quite small for a given transformer rating, due to the smaller core made possible by the use of the higher flux density material.
Considerable work is being undertaken to obtain even quieter operation by suitable treatment of the raw material and by particularly careful assembly of the finished core laminations.
Turning to other possible sources of noise emitted by a transformer, the forces present between the individual conductors in the winding when the transformer is loaded must be considered. These forces are, however, of a sinusoidal nature so that any vibration consists of a fundamental at 100 Hz with negligible harmonics. The fundamental is thus effectively dwarfed by the much greater 100 Hz fundamental generated by the core, while there are no harmonics to add to the annoyance value. Acoustic measurements confirm this conclusion by showing that the noise level on most transformers increases by not more than 2 dB (15% rise in loudness) from no load to full load. Any variation is in fact attributable more to changes in flux density than to variations in the forces in the windings.

3- The other major source of noise is the transformer cooler. Fans produce noise in the frequency range 500- 2000 Hz, a band to which the ear is more sensitive than it is to the 100 Hz fundamental produced by the core. The predominant frequencies are dependent on many factors including speed, number of blades and blade profile. Sound power level is dependent upon number and size of fans as well as speed and, for many forced-cooled transformers, the cooler can prove to be a significantly greater source of noise than the transformer itself. These comments on transformer noise assume the absence of resonance in any part of the unit. Normally the minimum natural frequency of the core and windings lies in the region of 1000 Hz.
The natural frequency of the tank or fittings being lower, resonance of these is much more likely to occur, since the vibrations of the core can be transmitted by the oil to the tank. If any part of the structure has a natural frequency at or near 100, 200, 300, 400 Hz, etc., the result will be an amplification of noise at that particular frequency.

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PanCerowany
PanCerowany is offline
#9
Mar12-10, 08:19 AM
P: 5
lol, www.electrical-riddles.com vs www.electric-gibberish.info - obviosuly more sense in riddles than in gibberish


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