Generators and fatigue problems

Generators consitst of a rotating rotor placed inside a not moving stator. This can be seen on page 2 of this pdf. A 2 pole generator will turn at 25 revolutions per second. Due to the weight of the rotoraxis and its big span. fatigue due to changing bending stresses is a problem. Above all big sharp changes in the thickness of the rotor axis can cause fatigue problems. By using a bigger radius where the axis changes in thickness and using a groove to redirect tension concentrations make it possible to lower fatigue problems.  

Fatigue problems were also present in cooling fans needed to arrange a cooling airflow through the generator. The cooling fans are connected with the rotating rotor axis. These fans are made by pouring liquid aluminum into a mold. Due to a big difference in thickness of this product problems occur. During cooling down of the aluminum the thin blade of the fan can allready be solid. The thicker stubshaft wil become solid later. Due to this an after suction of aluminum occurs leaving voids and air inclusions near the transition from blade to stubshaft. This happens because all liquid metals shrink when they get solid.

Holllow areas inside this fan are the starting point for fatigue cracks. By lowering the change in thickness this problem can be overcome. This can be done by placing an insert into the stubshaft so the thickness of the stubshaft lowers. Take into account that these hollow areas are posioned at that position where the stresses in the fan are highest.

Also the position of the stubshaft of the fan is wrong.  The total air force on the wing shaped blade will be at 1/3 of the length of the blade. Due to the fact the stubshaft is positioned at 1/2 the blade length you also introduce torsional forces the fan has to deal with. These torsional forces will not be present if we position the stubshaft at 1/3 of the length of the blade.

Due to the breaking of one cooling fan a generator costing more than 1 milion euro could be thrown away. By reducing the thickness jump also the caster profits. It will lower its quantity of rejected fans reducing its costs.

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