A synchronous machine consists of a stator and a rotor. The stator is the stationary part of the machine with the three-phase windings to produce the rotating field as described in the previous section. The casing is made of
stacked sheets containing the three-phase windings of the stator, usually located inside open slots along the inner borehole (distributed winding).
As described before, the rotating field would cause a magnetic or compass needle inside the stator to rotate with the frequency of the rotating field. However, in the borehole of the stator of a synchronous generator, it is not a magnetic needle but a rotor that can be driven by the rotor blades of a wind turbine. Such a rotor must be magnetic so that it can follow the frequency of the rotating field. Permanent magnets or rotor windings with a DC current produce the magnetic field of a synchronous machine rotor. The rotor windings are also called excitation windings. The DC current that flows in these windings is fed from outside through slip rings.
There are two types of synchronous machine rotors, which are shown in Figure 5.23. They are cylindrical and salient-pole rotors. The cylindrical rotor, or turbo rotor, has a solid drum. It has slots in the longitudinal direction that contain the excitation windings. A cylindrical rotor can resist centrifugal forces better due to its massive construction. However, the material requirements of cylindrical rotors are also higher.
A salient-pole rotor has two or more salient poles. These rotors can have two poles, four poles (see Figure 5.23) or even more. The theoretical description of a salient-pole rotor is much more complicated than that of a cylindrical rotor since its construction causes asymmetries. This section describes cylindrical rotors only. For a description of salient-pole rotors and further details of electrical machines see the specialized literature (e.g. Hindmarsh, 1995; Fitzgerald et al, 2002).
The rotational speed nS = f1 / p of the stator field depends on the frequency f1 of the three-phase current and the pole pair number p of the stator as described before. The rotational speed for a frequency of 50 Hz and two poles (p = 1) is ns = 3000 min-1. For 60 Hz it becomes 3600 min-1.
The rotor of a synchronous machine has the same speed as the stator field. The north pole of the rotor always follows the south pole of the stator. If a synchronous machine operates as a motor, the north pole of the rotor and the south pole of the stator are not directly on top of each other; the load of the motor causes a shift between the rotor and stator poles. The load angle & that rises with the load describes this shift.
If a synchronous machine operates as a generator, for example in a wind turbine, the synchronous speed of the rotating field in the stator also defines the rotor speed. However, there is also a shift between the poles of the rotor and stator. Now, the rotor pole moves ahead of the stator pole. The load angle changes its sign from negative to positive. The load angle & increases with the force that drives the rotor. The rotational speed always remains constant, i.e. the rotor runs synchronously with the stator frequency. The rotational speed ns of the rotor only changes with the frequency f of the rotating field or the pole pair number p.
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