Magnetostrictively induced mechanical resonance of electrical steel strips

Extensive research has been carried out over the years to reduce the acoustic noise resulting from vibration of electromagnetic cores mainly caused by magnetostriction. This project presents the results of a basic experimental study of magnetostriction in strips of magnetic materials commonly used in electromagnetic cores which gives an important new understanding of the phenomenon. The presence of mechanical resonance in the laminations is highlighted here for the first time. A standard magnetising system was built and a new method of measuring magnetostriction was used. A single point laser vibrometer was used to measure magnetostrictive vibration of the samples. The magnetostriction of grain-oriented materials cut at various angles to the rolling direction, non-oriented samples with different silicon content and nickel iron strips was measured over a wide range of magnetising frequencies and at peak flux densities up to 1.O Tesla. Magnetostriction measurement results were used to identify magnetisation induced mechanical resonance of the samples. The magnetising frequency at resonance was derived from the relationship of velocity, frequency and wavelength of an electromagnetically excited strip. Theoretical value of the fundamental resonant frequency and its harmonics were calculated and compared with measured values. The variation of the acoustic noise in a three- phase transformer core under no-load condition with various switching frequencies and different modulation indices was measured under pulse-width modulation and sinusoidal voltage excitation. All measurements were repeated at least five times to assess experimental accuracy and uncertainties. Results suggest that under resonance, transformer cores can produce excessive noise and potentially long term deterioration of lamination coating and possible core failure. Extrapolation of the results to larger cores infer that the phenomenon can possibly occur in cores with different length laminations leading to variability of noise output according to how close the magnetising frequency or predominant harmonics are to the resonant frequency. These findings demonstrate the importance of the interaction between basic magnetostriction and geometrical factors that are contributing towards the total noise output and care that must be taken when characterising the basic magnetostriction of samples of different sizes magnetised at different frequencies