The synchronous generator is a primary source of electrical energy.They operate on the fundamental principle of electromagnetic induction. A synchronous machine is an AC machine in which the rotor moves at a speed which bears a constant relationship to the frequency of currents, in the armature winding. The synchronous machine is AC rotating machine which can be used as either generator or motor. In the case of a generator, the machine has to be driven at a constant speed equal to the synchronous speed. Whereas synchronous motor automatically rotates at a constant speed equal to the synchronous speed.
synchronous generators are also called the alternators for the supply of electrical energy. Synchronous machines are generally constructed in larger sizes. Small size alternators are not economical. The modern trend is to build alternators of a very large size capable of generating 500 MVA or more. The synchronous motor is rarely built in small sizes owing to superior performance characteristics and economical construction of induction motors.The shaft of the machine is driven by the prime mover at a constant speed equal to the synchronous speed (i.e. Ns = 120f/P).
Synchronous generator consist of mainly 3 parts:
It consists of uniformly distributed three-phase armature winding. The armature is an iron ring formed of laminations of special magnetic iron having slots on its inner periphery to accommodate armature conductors and is known as a stator. This structure is held in a frame which may be of cast iron or welded steel plates. The field rotates in between the stator; the flux of the rotating field cuts the core of the stator continuously and therefore causes eddy current loss in the stator core. To minimize the eddy current loss, the stator core is laminated.
The excitation is usually provided from a small dc shunt or compound generator, known as an exciter, mounted on the shaft of the alternator itself. The field system of the alternator is rotated within the armature ring and is known as the rotor. The exciting current is supplied to the rotor through two slip rings and brushes. The rotor is the rotating part of the machine with a number of magnetic poles excited by the dc source from the exciter.
There are two types of rotor:
cylindrical type rotor:
This type of rotor has got smooth magnetic poles in the form of a closed cylinder. These types of rotors are used in very high-speed prime movers like steam turbine,gas turbine etc. To reduce the peripheral velocity, the diameter of the rotor is reduced and axial length is increased. It is also known as non-salient pole type rotor.Construction of these type of rotors is more compact & robust.The operation is less noisy as compared to salient pole rotor structure.
Salient pole rotor:
This type of rotor is used for low and moderate speed alternators such as Water turbine .Salient poles cannot be used in high-speed prime movers on account of a very high peripheral.The salient poles are made of thick steel laminations to reduce eddy current loss. The salient pole field structure has large diameter and short axial length.
It is a self-excited dc generator mounted on the shaft of the main machine to produce a magnetic field in the rotor. The function of the exciter is to supply dc current to the field winding of the rotor. The dc current generated by the exciter is fed to the field winding of the alternator through a slip ring and carbon brushes.