Analysis
of the Synchronous Generator
Over
the years, synchronous generators, also called alternators, have been used as a
reliable ac electric power source. Therefore, it is of great importance to have
a basic
knowledge of
this kind of ac machinery, its construction. principles of operation and performance
characteristics.
Physical
Construction
A typical
synchronous generator consists of the following essential elements:
The
Stator
The stator of a
synchronous generator is identical to that for an induction machine. It is built of thin
iron laminations of highly permeable steel core. The laminated core is to reduce
the magnetizing losses such as the eddy current and hysteresis losses. The
stator accommodates the three-phase armature windings which are the distributed
stator windings. The distributed windings are embedded in the slots inside the
stator core 120 electrical degrees apart in the space to minimize the space
harmonics in the resultant air gap flux waveform. They are also made
short-pitch in order to produce a smooth sinusoidal voltage waveform at the
stator terminals. The reason behind having the armature windings fixed on the
stator is that they need to be well-insulated due to the high voltages and
transient currents they may experience during different operation modes. A cross-sectional
sketch of a stator of a synchronous machine is shown below in Figure.
Basic
Stator Scheme for a 2-pole 3-phase synchronous generator
The
Rotor
The rotor of the
synchronous generator hosts windings which carry the dc field current and are
connected to the excitation source of the generator via brushes and the slip
rings assembly. The synchronous generator has two different rotor
configurations based on its speed. Turbo- generators used for high speed
operation have a round rotor structure.
These generators
are commonly referred to as non-salient pole or cylindrical-rotor synchronous
generators. They have a uniform air-gap and normally have either two or four
field poles, depending on the required speed of operation. Hydro-generators
used for low speed operation have rotors with salient poles structure. These
generators are known as salient pole synchronous generators. Because of the
rotor saliency, a non-uniform air gap is formed between the rotor and stator
inner surface. The salient-pole rotor has a comparatively larger number of
poles. A common practice is to have damper or amortisseur windings
equipped to the rotor of this type in order 10 damp out the speed oscillation
in the rotor. Damper windings are copper or brass bars embedded in the salient 32
pole faces with both their ends shorted-circuited by means of shorting rings to
form a cage structure similar to that for a squirrel cage rotor of an induction
machine. These windings play a major role in retaining the generator synchronism
during the dynamic transient or hunting. The cross-sectional view
of two rotor structures of the synchronous generator are shown in Figure.
Elementary rotor
structure of2-pole alternator: (a) cylindrical rotor, (b) salient-pole rotor
