The magnetic rotor field is generated by a field winding F on the rotor which is fed with an adjustable direct current. In addition, the rotor has a short circuited damper winding D at the surface. This winding serves to dampen electrical and mechanical oscillations and to shield the field winding from inverse rotating fields in case of asymmetries or harmonics in the stator currents. (In rotors without an explicitly realized damper winding, eddy currents in the rotor iron can have a similar effect.) Depending on the application of the generator, two different types of rotors are used that are shown in Figure.
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Figure: Cross-sections through different rotor types.
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ROUND ROTOR
Round rotors are used with high-speed turbines such as steam or gas turbines. For this reason, generators with round rotors are also called turbo generators. They can have ratings as high as 1800 MVA per unit. Due to the large centrifugal forces, the rotor consists of a long, narrow, solid steel cylinder.The field windings are mounted in slots that are mill-cut into about 2/3 of the perimeter. Because of the discrete distribution of the windings on the rotor surface, the magnetic flux density in the air gap always has a stair-step form. Through proper distribution of the windings these stair-steps can be made approximately sinusoidal!
SALIENT POLE ROTOR
Salient pole rotors are used with low-speed hydro turbines with rated powers of up to 800 MVA per unit. In order to obtain the appropriate electrical power frequency in spite of the low rotor speed, salient pole rotors typically have multiple pole pairs. For run-of-river power stations the number of poles can be as high as p = 200! Such rotors have very large diameters (several meters) and short lengths.The field windings are mounted on the individual poles. By properly designing the geometric form of the poles, the magnetic flux density in the air gap at the stator surface can also be made approximately sinusoidal!