Magnetostriction and Its ill-effects:The property of ferromagnetic materials, due to which their dimensions are changed due to change of magnetic field, is called magnetostriction effect.
Magnetostriction
In fact, the applied magnetic field changes the magnetostrictive strain until reaching its saturation value (O). This causes change in the dimensions of ferromagnetic materials.
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The effect was first identified in 1842 by James Joule while observing a sample of iron.
In simple terms, as per this phenomenon, if a piece of magnetic sheet steel is magnetised it
will extend itself. When the magnetisation is taken away, it goes back to its original condition.
A transformer is magnetically excited by an alternating voltage and current so that it becomes extended
and contracted twice during a full cycle of magnetisation.
Hence, when the core becomes magnetised, it expands then it contracts and expands again as
the field is reversed. A 50 Hz transformer, therefore, expands and contracts at 100 Hz, producing a
buzzing sound at that frequency.
Transformer buzzing noise (or hum) is caused by a phenomenon called magnetostriction.
Although, these extensions are only small dimensionally, and therefore cannot usually be seen by
the naked eye. They are, however, sufficient to cause vibration resulting in a noise.
This effect also causes losses which produce heat in ferromagnetic cores.
Thus, the first cause of humming noise is magnetostriction, the other cause may be due to details
of core construction, size and gauge of laminations or joints in the core .
The other reasons for humming sound in a transformer may be Loosening of laminations, mounting defects, unbalancing or insulation deterioration etc.
Methods to control transformer noise: The transformer noise can be controlled by
- using low value of flux density core,
- by proper tightening of the core by clamps, bolts etc.
- by sound insulating the transformer core from tank walls
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