Effect of ventilating holes on radial flux and losses in stator slots of turbogenerators

An accepted practice among some manufacturers of turbogenerators is to provide for the ventilation of stator laminations through axial holes in the teeth in addition to the core. This is especially the case for machines without radial ducts separating packs of laminations. These holes are located in areas of the teeth subject to high degrees of saturation. Under these conditions, flux is diverted into adjacent stator slots. The main component of the diverted flux is radial and is in the area of the strands. Furthermore, this radial flux component is time varying and hence establishes eddy currents in the strands. These eddy currents create additional losses. Therefore, the size and location of such ventilation holes is critical if the added component of radial flux losses is not to produce spots of excessive heating in the winding. This phenomenon is presented through results of a comprehensive magnetic field study on two 4 pole, 659 MVA asymmetrically slotted rotor and 2 pole, 733 MVA standard rotor turbogenerators. From flux density and loss distribution curves, recommendations as to the size and location of these holes, to significantly reduce these losses are made.