Synchronous Generator [Alternator] Introduction
Synchronous machines are principally used as alternating current generators. They supply the electric power used by all sectors of modern society. Synchronous machine is an important electromechanical energy converter. Synchronous generators usually operate in parallel forming a large power system supplying electrical power to consumers or loads. For these applications the synchronous generators are built in large units, their rating ranging form tens to hundreds of Megawatts. These synchronous machines can also be run as synchronous motors.
Synchronous machines are AC machines that have a field circuit supplied by an external DC source. Synchronous machines are having two major parts namely stationary part stator and a rotating field system called rotor.In a synchronous generator, a DC current is applied to the rotor winding producing a rotor magnetic field. The rotor is then driven by external means producing a rotating magnetic field, which induces a 3-phase voltage within the stator winding.Field windings are the windings producing the main magnetic field (rotor windings for synchronous machines); armature windings are the windings where the main voltage is induced (stator windings for synchronous machines).
Types of synchronous machines:-
According to the arrangement of armature and field winding, the synchronous machines are classified as rotating armature type or rotating field type.
In rotating armature type the armature winding is on the rotor and the field winding is on the stator.
The generated emf or current is brought to the load via the slip rings. These type of generators are builtnonly in small units.
In case of rotating field type generators field windings are on the rotor and the armature windings are on the stator. Here the field current is supplied through a pair of slip rings and the induced emf or current is supplied to the load via the stationary terminals.
Based on the type of the prime movers employed the synchronous generators are classified as
1. Hydrogenerators :
The generators which are driven by hydraulic turbines are called hydrogenerators. These are run at lower speeds less than 1000 rpm.
2. Turbogenerators:
These are the generators driven by steam turbines. These generators are run at very high speed of 1500rpm or above.
3. Engine driven Generators:
These are driven by IC engines. These are run at aspeed less than 1500 rpm.
Hence the prime movers for the synchronous generators are Hydraulic turbines, Steam turbines or IC engines.
Hydraulic Turbines: Pelton wheel Turbines: Water head 400 m and above
Francis turbines: Water heads up to 380 m
Keplan Turbines: Water heads up to 50 m
Steam turbines: The synchronous generators run by steam turbines are called turbogenerators or turbo alternators. Steam turbines are to be run at very high speed to get higher efficiency and hence these types of generators are run at higher speeds.
Diesel Engines: IC engines are used as prime movers for very small rated generators.
This is a small introduction to Synchronous Machines.
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