Synchronous Generator

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