Electromagnetic Forces On Coils And Bars Inside The Slot of Hydro-Generator

The coils and bars installed in the stator core are subjected to transversal electromagnetic (EM) field inside the stator core slot that in turn, develops electromagnetic forces between the top and bottom layers. This subject matter and the theoretical background have been discussed in various academic papers in the past and is well known among equipment manufacturers. It is not, however, well known among plant engineers. There is a misconception about the amplitude and direction of those forces and how they are applied to hydro-generators stator winding.An electromagnetic finite element analysis (FEA) was performed during a recent project to study the effect of electromagnetic forces on stator and rotor. A detailed model of the stator and rotor was prepared and the electromagnetic forces on the top and bottom bars were calculated and analyzed. This analysis demonstrated how saturation of the stator teeth, actual magnetic field distribution inside the slot, eddy current in the damper bars and the shape of the salient poles impacted the electromagnetic forces in the slot.This paper summarizes the theory and basics of EM forces on conductors inside the core slot, provides calculation of the electromagnetic forces at rated operating conditions and fault conditions as function of the phases on the top and bottom bar, and provides a comparison of the forces calculated using an electromagnetic FEA. Furthermore, the forces inside the slot are discussed, which is an important factor for securing the winding inside the slot.