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An innovative approach for the evaluation of the axial component of the magnetic field in the terminal parts of the alternators

Fabrizio D'Agostino, Roberto Biondi - Ansaldo Energia

Techniques and methodologies for electromagnetic design calculations and generators operation are becoming more and more key factors to success in today's industrial software market. Specific electromagnetic issues and needs from the field have to be investigated by using 3D FEM analyses, and in the majority of the cases, specific design practices have to be developed ad hoc.
Objective of this article is to show how to solve complex problems like the evaluation of the axial component of the magnetic field, predominantly acting in the terminal parts of the turbo-alternators. Such a component, as known, is highly detrimental since it causes the circulation of eddy currents tangent to the rolling direction of the lamination packets and the generation of an electromagnetic force applied to the laminations packets themselves. Therefore, a proper evaluation of the axial component of the magnetic field allows to estimate the magnitude of the eddy current losses, as well as their spatial distribution on the tooth surface. In correspondence with the terminal parts of the turbo-alternators, the last laminations packets are built with a small radial slot used to reduce the path of the eddy currents and consequently the heat generation due to the joule effect.
On the other hand, this radial slot weakens the mechanical structure of the stator tooth and the electromagnetic force generated by the axial component of the magnetic field can cause vibrations resulting in a friction on the bar insulation. Furthermore, to the stator's conductor winding are also applied the electrodynamics forces generated by the Lorentz's effect which increase the friction effect between the bar insulation and the stator tooth. This article will illustrate the design practice used to carry out a 3D FEM Analysis with Ansys Maxwell for the calculation of the electromagnetic fields and a 3D FEM Analysis coupled with Ansys Structural Mechanical for the optimum design of the radial slot.