Why Do Transformers Hum?


Quick Answer

Transformers hum when magnetized sheet steel expands or extends during operation. When the steel loses its magnetization, the steel returns to its original size, and the humming stops. The technical term for the process is magnetostriction.

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Full Answer

The alternating voltage and current working inside a transformer create a cycle of magnetization that expands and contracts the transformer twice. The precise point of magnetization on the sheet is not always the same, which means that the expansion and contraction do not take place regularly. The sheet starts to show erratic behavior when compared to nearby sheets, and so the movement continues to spread during the expansion part of the cycle.

The expansion that takes place throughout the sheets does not produce movement that is visible without a microscope, but it is sufficient to cause vibrations that produce a hum. Applying voltage to transformers creates magnetic force lines inside the core, and the degree to which this flux takes place influences the level of magnetostriction. The greater the magnetostriction, the greater the noise. Because these processes take place within a fixed cycle that is part of factory specifications, the humming sound does not signal anything wrong with the transformer; rather, if a transformer does not hum periodically, it is not working properly.

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