Inside a transformer, the core which is made in the form of laminated sheets also undergoes expansion and contraction due to the changing magnetic flux. This expansion and contraction occurs twice in an ac cycle. The fundamental frequency of the noise or vibration is double that of the frequency of the power supply. Thus a supply with a frequency of 50 Hz will cause noise or vibration whose fundamental frequency is 100 Hz.
In addition to the fundamental frequency, there are also harmonics whose frequencies are odd multiples of the fundamentals such as the 3rd harmonic, 5th harmonic, etc. A proper study of the noise and vibration spectrum is necessary to devise methods of reducing them.
Since, the core of the transformer is made of laminated steel sheets; these sheets experience unequal expansion and contraction when exposed to the magnetic flux. Hence, they rub against each other causing the distinct hum. The constant cyclic forces generated in the transformer core cause vibration which is carried to the different parts of the transformer body. In addition, they also cause noise. Thus when trying to reduce the hum of the transformer, both noise and vibration needs to be addressed. The noise of the transformer is measured in decibels (dB).
People can find the noise of a transformer disturbing and may oppose locating a transformer near their residence. In such circumstances, measures for reducing the impact of the sound may be explored.
Vibrations can be addressed by the fitment of supports or dampers. Noise can be reduced by mounting baffles and planning the location of the transformer.
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