In this article, I am describing the construction of synchronous generator and hoping this article will boost your knowledge and imaginations.
AC generators always run at a constant speed (synchronous speed). That is why they are also known as synchronous generators. Practically in all medium and large synchronous generators, the armature is housed in the stator while the DC field system is placed on the rotor.
Construction of Synchronous Generator
There are mainly two synchronous generator parts:
- Stator (Armature )
- Rotor (DC Field System)
The armature is an iron ring, formed of laminations of special magnetic iron or steel alloy (silicon steel) having slots on its inner periphery to accommodate armature winding and is known as the stator. The whole structure is held in a frame which may be of cast iron or welded steel plates.
The field rotates in between the stator, so that flux of the rotating field cuts the core of stator continuously and therefore, causes eddy current loss in the stator core. The stator core is laminated to minimize the eddy current loss,
The rotor is of two types, namely:
- Salient pole type
- Smooth cylindrical type
Salient Pole Rotor
It is like a flywheel which has a large number of alternate North and South bolted on it. The magnetic wheel is made of cast iron or steel of good magnetic quality. These magnetic fields are energized or excited by a DC source.
The salient pole rotors are used only by low and medium (120 – 500 rpm) speed synchronous generators such as those driven by water turbines. Because of their low speeds, they require a large number of poles.
Such rotors have large diameters and short axial lengths. The salient pole structure is simpler and cheaper to manufacture than a cylindrical rotor.
Smooth Cylindrical Rotor
It consists of a smooth solid forged-steel cylinder having a number of slots milled out along its outer surface for housing field magnetizing field coils. Two or four regions are left un-slotted for creating non-projecting poles.
Such rotors are used in steam turbine driven alternators which run at very high speeds (up to 3600 rpm). Such rotors have small diameters and very long axial lengths.
Excitation System of Synchronous Generator
The rotor poles of a synchronous generator are electromagnets. They require excitation for their operation. The field winding needs a DC supply to produce the required flux. There are several excitation systems which are used to provide DC excitation current for the generator.
In one system, power is taken from the AC generator terminals, is rectified and then supplied to the rotor field system through slip-rings and brushes.
In some systems, excitation supply is obtained from a small DC shunt generator called an exciter. It is mounted on the same shaft as that of the synchronous generator.
One another system, which is known as brushless excitation system of synchronous generator, a small 3-phase generator mounted on the shaft of the main generator itself, is used as an exciter. The output of the exciter is rectified and fed directly to the rotating field poles of the synchronous generator.
The brushless excitation system has no commutator, slip-rings or brushes which make the system simple and reliable.
There are two methods of ventilation:
- Natural Ventilation
- Closed Circuit Ventilation
In natural ventilation method, a fan is attached to one end of the machine. Air is the medium by which ventilation takes place, and the heat of machine parts is carried away.
In the closed-circuit ventilation method, the medium used for ventilation is hydrogen. Hydrogen is circulated with the help of water-cooled heat exchangers. In the modern large capacity alternators, this method is preferred.
Working of Synchronous Generator
The three-phase EMFs are generated by three-phase synchronous generators (or alternators). A three-phase synchronous generator has three identical windings. Every phase winding is displaced at 120o electrical apart.
When a magnet (rotor) is rotated using some prime-mover in these windings, the stator conductors are cut by the magnetic field of the rotor. Hence an EMF is induced in each winding.
These EMFs are of same magnitude and frequency but are displaced from one another by 120 electrical degrees.
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