Environmental Engineering Reference
In-Depth Information
Figure 5.23
Cross-section through a Synchronous Machine; Left: Cylindrical
Rotor, Right: Salient-pole Rotor
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
n
S
=
f
1
/
p
of the stator field depends on the
frequency
f
1
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
n
S
= 3000 min
-1
. For 60 Hz it becomes 3600 min
-1
.
