A. Cooling fan
B. magnetostriction in an iron core
C. Mechanical vibration
D. All of the above
Answer: B. magnetostriction in an iron core
Explanation:
The transformers using ferromagnetic core produces noise.
Such a noise exists in the form of an electric hum around transformers. Such noise may be annoying to the nearby residential area.
We can think of a transformer core, therefore, as behaving like a giant loudspeaker producing a continuous humming or buzzing noise, which is well within a human’s audible frequency range, and extremely irritating.
The noise is produced due to the vibration of the enclosure and accessories. The vibrations are produced mainly due to stray magnetic fields and due to magnetostriction.
Magnetostriction is a phenomenon due to which the length of the ferromagnetic core increases when magnetized and gets back to the original when demagnetized.
Due to this, there is an increase and decrease in the cross-section of the core. As the laminations change their dimensions, the core vibrates to produce the noise. The core sound is dominant in no-load conditions.
In load conditions, the vibrations in the tank and winding cause noise. The load noise is caused due to electromagnetic forces resulting from leakage fields produced by load currents.
These forces are proportional to the square of the load currents. Similarly, there is noise due to sound generated by cooling equipment such as pumps and cooling fans.
Thus the various factors responsible for producing transformer noise are,
Magnetic forces producing vibrations due to stray magnetic fields.
The vibrations are produced due to magnetostriction.
The vibrations are due to electromagnetic forces generated due to the load currents.
Mechanical vibrations due to the degree of tightness of clamping the core by nuts and bolts.
The joints in the core.
The sound is produced by cooling equipment like fans and pumps.
The damping
The noise productions in transformers can be reduced by
The no-load sound generated due to the magnetostriction can be reduced by lowering the flux density in the core.
By using proper core material with low loss, high permeability, and low noise generation.
By properly tightening the core by clamps and bolts and properly fixing laminations and frames.
By providing sound insulation to the tank from the ground and surrounding air.
By using cushion padding and oil barriers which help in insulating the sound of vibration.
By using proper innovative designs and materials like stiffeners while designing the tank walls.
The noise cannot be completely eliminated but taking the frequency of vibration of the transformer outside the audio frequency range is most helpful.