5.6 TRANSFORMER NOISE

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Although transformer noise does not cause hearing damage, in general, the

continuous hum emitted by large transformers can be quite annoying. The

metal cores of transformers are usually laminated to reduce hysteresis losses.

One source of transformer noise depends on the magnetic field variations in

the laminations. As a result of magnetostriction effects, the laminations

change in length as the magnetic field changes. The total change in length

is usually on the order of micrometers, but this small dimensional change is

sufficient to produce the transformer hum at frequencies of 120 Hz, 240 Hz,

and 360 Hz, which are harmonics of the 60 Hz excitation current for the

transformer. The harmonic frequencies fall in the 125 Hz and 250 Hz octave

bands, so the hum noise tends to appear largest in these octave bands.

The level of noise generated by the transformer increases with increase

of the magnetic flux density or with the kVA rating of the transformer. The

flux density is determined by the design of the transformer and does not vary

significantly with the load on the transformer. This means that the noise

generated is practically independent of the transformer load. The hum noise

is present at the no-load condition for the transformer.

One approach to reducing the noise generated by a transformer is to

reduce the transmission of sound between the core, where the noise is generated,

and the transformer housing, from where the sound is radiated into

the surroundings. This may be accomplished by using spray cooling, evaporative

cooling, or gas cooling, instead of conventional oil cooling. A

resilient acoustic barrier in the oil between the core and housing could

also be used to reduce the transformer hum noise.

There are two general types of transformers, depending on the method

of dissipating energy from the transformer. The self-cooled transformer is

actually cooled by natural convection. The forced air-cooled transformer is

Noise Sources 177

TABLE 5-6 Solution for Example 5-3

Octave band center frequency, Hz

63 125 250 500 1,000 2,000 4,000 8,000

LW, dB 142:1 142:1 142:1 142:1 142:1 142:1 142:1 142:1

CF3, dB 25 18 12 7 4 8 14 21

LW(octave band), dB 117:1 124:1 130:1 135:1 138:1 134:1 128:1 121:1

Equation (5-6) _24:6 _24:6 _24:6 _24:6 _24:6 _24:6 _24:6 _24:6

Lp(octave band), dB 92:5 99:5 105:5 110:5 113:5 109:5 103:5 96:5

Copyright © 2003 Marcel Dekker, Inc.

cooled by forced convection heat transfer assisted by a fan. The fan provides

an additional source of noise for this type of transformer.

Based on tests conducted by the National Electric Manufacturers

Association (NEMA), the following correlations may be used to estimate

the overall sound power level for transformers. For a self-cooled transformer,

the sound power level is related to the kVA rating of the transformer

by the following expression:

LW ј 45 ю 12:5 log10рkVAЮ (5-26)

For a forced air cooled transformer, the following expression applies:

LW ј 48 ю 12:5 log10рkVAЮ (5-27)

The octave band sound power level spectrum may be determined for either

type of transformer from the following expression:

LWрoctave bandЮ ј LW _ CF4 (5-28)

Values of the conversion factor CF4 are given in Table 5-7.

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