Methods for controlling transformer core noise

The primary source of transformer noise is the core. Core noise is caused by core vibration resulting from the magnetostriction of silicon steel sheets and magnetic attractive forces. The magnitude of magnetostriction is related to the material of the silicon steel sheets and the tightness of the core; the higher the magnetic flux density of the core, the greater the magnetostriction of the silicon steel sheets. The condition of the stacking joints in the transformer core affects the flow of magnetic flux, causing the core to generate noise of varying degrees and magnitudes. If the AC power supply contains harmonic components, it will lead to increased noise. The fundamental frequency of core noise is twice the power supply frequency, along with noise at various integer multiples of that frequency. Additionally, electromagnetic forces between winding conductors or between coils can also cause vibration and lead to noise. The following are several countermeasures for suppressing noise:

Reduce the magnetic flux density of the core: Although reducing the magnetic flux density of the core can lower transformer noise, it requires increasing the cross-sectional area of the core. This leads to an increase in the transformer's weight, which offsets the noise reduction effect, and also increases the cost of the transformer. Therefore, the reduction of the core's magnetic flux density should be limited to a maximum of 80%.

Use silicon steel sheets with low magnetostriction: Selecting high-permeability grain-oriented silicon steel sheets can reduce noise by approximately 2–4 dB; if laser-irradiated high-permeability grain-oriented silicon steel sheets are used, the noise can be reduced even more significantly.

Uniformize the core binding force: During the core assembly process, ensure that the core is not subjected to localized stress, which can cause performance degradation. When using binding straps to tighten the core, it is advisable to ensure that the binding force is uniform across all parts.

Improve core joints and assembly methods: Minimize the number of core joints and reduce the gaps between them. Changing the core stacking method to step-lap joints has a significant effect on improving transformer noise.

Varnish impregnation under vacuum: A highly effective noise reduction method for small transformers is to place the fully assembled transformer in a varnish tank for vacuum impregnation.