Effects of transformer core design on noise level

The advantages of the so-called step-lap core design were investigated on a single-phase dry-type model. A-weighted noise level and noise spectrum were measured for different core variants at core inductions from 1.4 to 1.7 or 1.8 tesla. The number of steps in the over-lap area and the way of stacking (1, 2 or 3 laminations per stagger layer) was varied. The models were assembled from conventional grain-oriented laminations (grade M4/0.27 mm). The weight of the core was about 105 kg. It has been found that the models with multistep-lap joints, in comparison to the conventional single step-lap one, show significant noise level reduction only at low and moderate core inductions. The noise level reduction is up to 7.5 dB at inductions 1.4-1.5 tesla, while at 1.7 tesla it is practically negligible, To obtain a significant improvement of noise level the multistep-lap has to consist of 3-4 steps. An increase of number of steps over 3-4 is not justified, because that can further reduce the noise level for 0.5 dB only. The number of laminations per stagger layer has a small influence on the noise level.