In recent years, large transformers have been increasingly used in residential and public places, which has put forward requirements for the noise of large transformers and small and medium-sized transformers. The noise problem of transformers is receiving increasing attention from people. How to reduce transformer noise has become an urgent problem to be solved in the transformer manufacturing industry.

The audible noise emitted by transformers is caused by the magnetostrictive deformation of the iron core and the electromagnetic force inside the winding, oil tank, and magnetic shield. In the past, it was believed that the noise generated by the longitudinal vibration of iron core laminations induced by magnetic fields was the main component of transformer noise. The amplitude of this vibration is related to the magnetic flux density in the iron core laminations and the magnetic properties of the iron core material, but not to the load current.

Specifically, there are three sources of transformer noise: the iron core, the winding, and the cooler, which is the sum of noise caused by no-load, load, and cooling systems. The reason for the noise generated by the iron core is that the silicon steel sheets that make up the iron core undergo small changes, namely magnetostriction, under the action of an alternating magnetic field. Magnetostriction causes the iron core to vibrate periodically with changes in excitation frequency. The reason for the vibration of the winding is that the current generates electromagnetic force in the winding, and the leakage magnetic field can also cause structural components to vibrate. The electromagnetic force (and vibration amplitude) is proportional to the square of the current, while the emitted sound power is proportional to the square of the vibration amplitude. Therefore, there is a clear relationship between the emitted sound power and the load current.

Factors affecting transformer noise

1、 Factors affecting no-load noise

The main reason for the noise generated by the iron core is that the size of cold-rolled silicon steel sheets will undergo slight changes under the action of alternating magnetic fields. Due to the fact that the variation period of magnetostriction is half the frequency of the power supply, the vibration of the transformer body caused by magnetostriction is based on twice the frequency of the power supply. Therefore, the vibration of silicon steel sheets is mainly caused by the magnetostriction characteristics of ferromagnetic materials. The magnitude of magnetostriction is related to the material of silicon steel sheets. The greater the magnetostriction, the greater the noise. When the magnetic field strength is the same, high-quality silicon steel sheets also have less magnetostriction, resulting in less noise. The magnetostriction rate is also related to the magnetic field strength, and the stronger the magnetic field, the larger the ε. The magnetostriction rate is also related to whether the surface of the silicon steel sheet is painted and annealed, because the coating has adhesion to the silicon steel sheet, which can prevent deformation of the silicon steel sheet. The magnetostriction of annealed silicon steel sheets is much smaller than that of unannealed silicon steel sheets under the same magnetic field strength. This is because choosing a good annealing process can significantly reduce magnetostriction. The no-load noise of transformers is not only related to their material, but also to the condition of the joints.

2、 Other factors affecting no-load noise

(1) The influence of iron core structure. The noise is related to the diameter of the core column and iron yoke, the height and width of the iron core window, and the quality of the iron core. For every 1t decrease in iron core quality, the noise can be reduced by 1/3dB (A) value. For every 0.1 decrease in the ratio of the height of the iron core window to the diameter of the iron core, the transformer noise needs to be reduced by 2-3dB.

(2) The frequency of no-load noise is based on twice the frequency of the power supply as its fundamental frequency. In addition to the fundamental magnetic flux, there are also high-frequency magnetic fluxes in the iron core, so the frequency of no-load noise also has secondary or higher high-frequency waves. When the natural frequency of the iron core oil tank is close to the noise frequency, noise resonance phenomenon will occur, resulting in an increase in noise. Therefore, the natural frequencies of the transformer iron core and oil tank should avoid some high-frequency waves.

(3) In order to prevent iron core resonance, the natural frequency of the iron core should also be considered when designing low-noise transformers. When the rated frequency of the transformer is 50Hz, the natural frequency of the iron core should avoid the following frequency bands: 75-125Hz, 165-235Hz, 275-325Hz, 375-425Hz. If the natural frequency avoids the resonance band, the iron core will not resonate.

Article source:Transformer silicon steel sheethttp://www.cheungshun.com