Experimental investigations on the stators of electrical machines in relation to vibration and noise problems

Accurate assessment of excitation forces and vibration behaviour is essential to arrive at a suitable design for quiet operation of an electrical machine. From the vibration point of view, the stator of an electrical machine can be modelled as a system consisting of a number of masses interconnected by springs and damping elements to facilitate the analytical solution of the dynamic behaviour of the structure. The distribution of the mass and stiffness, which are essential to the determination of the resonances, are well documented in the literature. However, not much information is available on the nature and amount of damping present in electrical machine stators. Since vibration damping is not amenable to mathematical formulation, often one has to resort to experimental information for its determination. The authors present a fundamental study on the vibration behaviour of electrical machine stators using an experimental modal analysis. Modal testing is a process of forcing a structure to vibrate predominantly at a selected resonance. To achieve this, distributed electromagnetic forces were used. Detailed investigations were conducted on the stator models of a 120 hp induction motor to study their vibration behaviour, and to critically examine the damping present in them. Physical interpretations of the results are given to enable a better understanding of the vibration behaviour of electrical machine structures.