Transformers in service cause sound, which in the long run may seriously discomfort people in the environment.
Sound may be defined as any pressure variation in air (or other elastic media) that the human ear can detect. The pressure variations travel through the medium from the sound source to the listener’s ears. The number of cyclic pressure variations per second is called the frequency of the sound, and is expressed in Herz (Hz). The frequency of a sound produces its own distinctive tone. A transformer ‘hum’ is low frequency, fundamentally 100 Hz or 200 Hz, while a whistle is high frequency, typically above 3 kHz. The normal range of hearing for a healthy young person extends from approximately 20 Hz to 20 kHz.
A further characteristic used to describe a sound is the amplitude of the pressure variations, which is expressed in Pascals (Pa). The weakest sound that a healthy human ear can detect is strongly dependent on the frequency; at 1 kHz it has amplitude of 20 µPa. The threshold of pain corresponds to a sound pressure of more than a million times higher. Therefore, to avoid the use of large numbers, the decibel scale (dB) is used.
The dB-scale is logarithmic and uses 20 µPa as the reference level, p0, which then corresponds to 0 dB. Sound pressure level Lp expressed in dB is defined in the following equation:
2 0 2 p p p lg 10 L = ⋅ dB (48)
where p is the sound pressure measured by a microphone. Sound pressure is a scalar quantity, which means it has magnitude only.
To provide a feeling of how a few well-known types of sound are situated on the dB-scale some values are listed below.
Source of sound Sound pressure level in dB
Quiet living area 45
Normal conversation at 1 m distance 60
Medium factory noise 75
Factory maximum limit 85
City street with heavy traffic 95
Circle saw at 1 m distance 105
Comprehensive investigations are made to correlate human perception of ‘loudness’ at various frequencies and sound pressures. The curves in Figure 11-17 are results of such investigations. These curves will vary somewhat from one person to another, but they can be regarded as average curves for how young persons with healthy ears respond.
Each curve represents sound that is perceived as ‘equally loud’ across the whole frequency range. The lowest curve that goes through zero dB sound pressure at1000 Hz represents the hearing threshold. The human ear is not able to hear anything below this curve. It appears that the ear is most sensitive in the frequency range between 3 and 4 kHz, where a sound pressure even below 20 µPa is audible.
For frequencies below 700 Hz the threshold curve ascends, which means that for the lower frequencies the sound pressure has to be increased to make the sound audible. The curve also rises at frequencies above 4 kHz.
The three upper curves go through 40, 70 and 100 dB at 1 kHz. A microphone responds quite differently to sound pressure. In order to imitate the response curves of the human ear filters are inserted in the measuring equipment. Three different filters are standardized, named A, B and C filters. They imitate the curves going through 40, 70 and 100 dB at 1 kHz respectively.
Measurement results made with one of the three filters inserted are denoted dB(A), dB(B) or dB(C).
Ear sensivity curves
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Sound frequency (Hz)
Sound pressure (dB)
Hearing thresholdC-weighted filter B-weighted filter
A-weighted filter
Transformer handbook. Draft. Rev. 02Q Page 142 of 197
When measuring the sound pressure around a transformer, the A-filter is used because this corresponds to the sound pressure level that normally prevails.
The sound pressure caused by a transformer is measured in several points around the transformer at a distance of 0,3 meters from the outer vertical surface of the transformer. This measurement result is in itself of limited interest, because nobody normally is standing 0,3 meters from the transformer. More interesting is the sound pressure level at larger distance from the transformer where people may stay.
To be able to estimate the sound pressure at larger distance, the sound power level of the transformer must be determined. The term ’sound power’ needs to be explained.
A sound source radiates power into the surrounding air resulting in a sound pressure field. Sound power is the cause. Sound pressure is the effect. The sound pressure, which is heard or measured with a microphone, is dependent on the distance from the source and the acoustic environment. So, the sound pressure at a certain distance from the source alone cannot quantify the strength of the source. It is necessary to determine the sound power of the source, which is independent of the environment and a unique descriptor of the strength of the sound source.
Sound power is the rate at which energy per unit time is radiated from the source. Its dimension is Watt. However, sound power Lw is normally also expressed in dB and logarithmic scale according to the following formula:
0 w W W lg 10 L = ⋅ db (49)
In this formula W0 is an arbitrarily chosen reference value equal to 10-13 Watt, which corresponds to a quite weak sound source. This reference value is chosen without regard to the previously mentioned reference value for sound pressure.
The sound power of one of the strongest sound power sources of concern, a large jet motor, has a sound power of about 100 000 Watts. The total power range to deal with is then 1018. Instead of working with such extremely high figures in acoustical subjects, the whole power range is covered by 180 dB.
Based on the measured sound pressure the sound power of the transformer in dB can be calculated according to formulas given in IEC 60076-10 Ed. 1.0.
In a large free field the sound pressure at larger distances from the transformer can be calculated. In practice there are often one or more walls or other items in the surroundings of the transformer that will reflect sound from the transformer and make a prediction of the sound pressure at various places in the neighbourhood difficult.
Sources of sound generation
The dominant generating source of transformer sound is magnetostriction. Magnetostriction is the change in dimensions which takes place in certain materials when they are subjected to a change in magnetic flux. In magnetic core steel the dimensional change is in the range of 10-7 to 10-5 meters per meter length at typical induction levels.
The effect does not depend on the sign of the flux, only on its magnitude and orientation relative to certain crystallographic axes of the material. Therefore, when excited by a sinusoidal flux, the fundamental frequency of the dimensional change will be twice the exciting frequency. The effect is highly non-linear, especially at high, near saturation, induction levels. The non-linearity results in a significant harmonic content in the vibration spectrum of the core.
In three-phase cores the change in dimension in each core limb does not occur simultaneously, which means that the whole core will be subject to pulsating distortions that also generate sound. A DC bias in addition to the AC magnetization of the core may significantly increase the vibration amplitudes of the core and consequently the sound level. The DC bias may also cause a considerable difference in the positive and negative peaks of the flux density, which in turn makes the fundamental frequency of the sound equal to the frequency of the service voltage instead of twice this frequency.
There are a few means available to the transformer designer to reduce the sound generated from the core:
• Reduce the flux density in the core from the usual 1.7 – 1,85 Tesla down to 1,2 Tesla. This can be done either by increasing the core cross section or by increasing the number of turns in the windings. There is not much to gain in reduced sound level by going below 1,2 Tesla. Reduced flux density gives larger geometric dimensions, increased load losses and higher weight and manufacturing costs. The no load losses will decrease,
• Avoid combinations of core cross section and limb height that make natural frequencies of the core coincide with frequencies of the magnetic field,
• Making the framework that holds the core together heavier and stiffer,
• Inserting a pad of damping material between the active part of the transformer and the tank bottom.
Another source of sound from transformers is vibrations in the windings due to pulsating mechanical forces acting on the winding conductors. The windings are situated in the magnetic leakage field. The force acting on a winding conductor is proportional to the product of the current floating in the conductor and the local flux density of the magnetic field where the conductor is situated.
At normal flux densities in the core, 1,7 T and above, the sound due to the core will overshadow the sound from the windings. But in cases where especially low sound levels are specified and the flux density in the core is low (down to 1,2 T) in order to fulfil the required sound level, the sound produced by the windings may give a noticeable contribution to the total sound level of the transformer.
Plane metal plates between stiffeners in the transformer tank may act like a membrane of a loudspeaker if any natural frequency of the plate coincides with a dominant frequency of the vibrations from the active part. The total sound level of the transformer may in such situations be considerably increased.
The measurement of sound from transformers may sometimes be disturbed by high background sound or sound from one or several other strong sound sources in the surroundings. To determine the sound power of the transformer in such situations sound intensity measurements are made. Sound intensity is the time-averaged product of the pressure and particle velocity of the medium in which the sound waves are propagating. Sound intensity is a vector quantity describing the magnitude and direction of the net flow of sound energy at a given position.
It is outside the scope of this chapter to explain the physics that form the basis for determining the sound power of a source by means of sound intensity measurements. We will just mention that two closely spaced microphones are used and that the measurement procedure and the determination of the results are described in IEC 60076-10.
Cooling fans create turbulent flows of air resulting in pressure fluctuations with a wide range of frequencies. The sound level is highly dependent on the speed of the fan wheel periphery.
The transmission of sound from the transformer to the surroundings can be reduced by installing sound-damping panels around the transformer or place the transformer inside a separate building. Within large buildings transformer sound may propagate widely through structural parts of the building, like floors and walls. Inserting plates of suitable damping material between the transformer and its fundament can effectively reduce this.
The IEC transformer standards do not prescribe any permissible limits for transformer sound. They just describe how sound characteristics of transformers can be determined. Permissible limits are entirely subject to agreement between purchaser and supplier in each case.
The CENELEC transformer standard HD 428.1 S1:1992 has standardized limits for the sound level for the smallest distribution transformers.
Transformer handbook. Draft. Rev. 02Q Page 144 of 197