How to solve the influence of silicon steel core on transformer noise (1)

- Jul 29, 2019-

In recent years, large transformers have been used more and more in residential and public places, which puts certain demands on the noise of large transformers and small and medium-sized transformers. The noise problem of transformers is getting more and more attention. How to reduce transformer noise has become an urgent problem for transformer manufacturers.

2 Causes of transformer noise

The audible noise emitted by the transformer is caused by the magnetostrictive deformation of the core and the electromagnetic forces in the windings, the fuel tank and the magnetic shield. In the past, it has been thought that the noise generated by the magnetic field induced core lamination vibration in the longitudinal direction is the main component of transformer noise. The amplitude of the vibration is related to the magnetic flux density in the core lamination and the magnetic properties of the core material, but has little to do with the load current.

Specifically, the transformer noise has three sound sources, one is the core, the other is the winding, and the third is the cooler, which is the sum of the noise caused by the no-load, load and cooling systems. The reason why the iron core generates noise is that under the action of the alternating magnetic field, the silicon steel sheet constituting the iron core undergoes a slight change, that is, magnetostriction, and the magnetostriction causes the iron core to periodically vibrate with the change of the excitation frequency. The reason why the winding generates vibration is that the current generates an electromagnetic force in the winding, and the leakage magnetic field can also cause the structural member to vibrate. The electromagnetic force (and vibration amplitude) is proportional to the square of the current, and the emitted sound power is proportional to the square of the vibration amplitude. Therefore, the transmitted sound power has a significant relationship with the load current.

3 Factors affecting transformer noise

3.1 Factors affecting no-load noise

The reason why the iron core generates noise is mainly that the size of the silicon steel sheet changes slightly under the action of the alternating magnetic field. Since the variation period of magnetostriction is half a cycle of the power supply frequency, the vibration of the transformer body caused by magnetostriction is based on twice the power frequency, so the vibration of the silicon steel sheet is mainly the magnetic material of the ferromagnetic material. Caused by the telescopic properties.

The magnetostriction rate is related to the material of the silicon steel sheet. The larger the magnetostriction rate, the greater the noise. When the magnetic field strength is the same, the silicon steel sheet of good material has a small magnetostriction, so the noise is also small.

The magnetostriction rate is also related to the strength of the magnetic field. 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 or annealed because the coating has adhesion to the silicon steel sheet to prevent deformation of the silicon steel sheet. A silicon steel sheet annealed at the same magnetic field strength is much smaller than a magnetostrictive sheet of a non-annealed silicon steel sheet, which is due to the selection of an optimum annealing process, which can double the magnetostriction.

The no-load noise of the transformer is related to the condition of the seam, in addition to its own material.

3.2 Other factors affecting no-load noise

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

(2) The frequency of no-load noise is the fundamental frequency of twice the power supply frequency. In addition to the fundamental frequency flux, there are high-frequency magnetic fluxes in the core, so there are more than two high-frequency waves in the no-load noise frequency. When the natural frequency of the iron core tank is close to the noise frequency, a noise resonance phenomenon will occur to increase the noise. Therefore, the natural frequency of the transformer core and the fuel tank should avoid some high frequency waves.

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