Magnetic properties of grain-oriented silicon iron. Part 3: Magnetostrictive stress sensitivity of simple assemblies of strips

During the construction of large power transformers, the magnetostriction of the material is increased, so that the noise output of the core is higher than that predicted from laboratory measurements. One way of producing improved transformers has been studied in this work which has investigated the magnetostriction/stress behaviour (in the plane of the sheet and rolling direction) of groups of similar samples held together by both taping and bonding, and the way in which the properties of individual strips combine to give the properties of an assembly. Under tension, bonded groups had a similar magnetostriction/strees curve to that of a single lamination; but under compression the magnetostriction remained reasonably constant until a compressive strees of about 400 lbf/in2 had been applied. Beyond this point, the asembly buckled. Further associated experiments were carried out on the vibration patterns or modes of vibration of the 100, 200 and 300 Hz harmonics for each assembly of strips held vertically between two fixed end points in such a way as to similate a transformer limb. These showed that the amplitude of vibration of the bonded assembly was less, by about a factor of 10, than that of a taped assembly. During bonding, it seems that stresses are produced which counterbalance the important detrimental compressive stresses arising in the plane of the sheet during core construction.