Magnetic forces and magnetostriction both contribute to the vibrations of magnetic cores and thus to the noise of electrical machines and transformers. In this article, a method is described to calculate vibrations of magnetic cores due to these magnetic forces and magnetostriction. Magnetic forces can be calculated on the basis of an analytical expression. Magnetostriction, however, has to be measured and a magnetostriction model has to be provided to calculate the contribution of the magnetostriction to the vibrations. In this article, the numerical method is used to compare the contributions of the magnetic forces and the magnetostriction to the vibrations of a two-pole and a four-pole three-phase induction machine.
Analysis of the results reveals that in the case of the two pole machine, the contributions of the magnetic forces and the magnetostriction to the vibrations of the most important eigenmodes (modes 2 and 2′) are in phase and thus add up, while in the case of the four-pole machine, the contributions of the magnetic forces and the magnetostriction to the vibrations of the most important eigenmodes (modes 4 and 4′) are in counter phase and thus subtract.
Although this is an interesting result, it is only proven here for the described machines with no load and only for the main eigenmodes. We can conclude that it is not justified to assume that the magnetostriction in all cases will increase the vibrations of an electromechanical device.
Read the full article Noise in Electrical Machines