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Synchronous Generator – Construction and Working Principle
A synchronous generator is a synchronous machine which converts mechanical power into AC electric power through the process of electromagnetic induction.
Synchronous generators are also referred to as alternators or AC generators. The term "alternator" is used since it produces AC power. It is called synchronous generator because it must be driven at synchronous speed to produce AC power of the desired frequency.
A synchronous generator can be either single-phase or poly-phase (generally 3phase).
Construction of Synchronous Generator or Alternator
As alternator consists of two main parts viz.
Stator – The stator is the stationary part of the alternator. It carries the armature winding in which the voltage is generated. The output of the alternator is taken form the stator.
Rotor – The rotor is the rotating part of the alternator. The rotor produces the main field flux.
Stator Construction of Alternator
The stator of the alternator includes several parts, viz. the frame, stator core, stator or armature windings, and cooling arrangement.
The stator frame may be made up of cast iron for small-size machines and of welded steel for large-size machines.
The stator core is assembled with high-grade silicon content steel laminations. These silicon steel laminations reduce the hysteresis and eddy-current losses in the stator core.
The slots are cut on the inner periphery of the stator core. A 3-phase armature winding is put in these slots.
The armature winding of the alternator is star connected. The winding of each phase is distributed over several slots. When current flows through the distributed armature winding, it produces an essential sinusoidal space distribution of EMF.
Rotor Construction of Alternator
The rotor of the alternator carries the field winding which is supplied with direct current through two slip rings by a separate DC source (also called exciter). The exciter is generally a small DC shunt generator mounted on the shaft of the alternator.
For the alternator, there are two types of rotor constructions are used viz. the salient-pole type and the cylindrical rotor type.
Salient Pole Rotor
The term salient means projecting. Hence, a salient pole rotor consists of poles projecting out from the surface of the rotor core. This whole arrangement is fixed to the shaft of the alternator as shown in the figure. The individual field pole windings are connected in series such that when the field winding is energised by the DC exciter, the adjacent poles have opposite polarities.
The salient pole type rotor is used in the low and medium speed (from 120 to 400 RPM) alternators such as those driven by the diesel engines or water turbines because of the following reasons −
The construction of salient pole type rotor cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speed.
If the salient field pole type rotor is driven at high speed, then it would cause windage loss and would tend to produce noise.
Low speed rotors of the alternators possess a large diameter to provide the necessary space for the poles. As a result, the salient pole type rotors have large diameter and short axial length.
Cylindrical Rotor
The cylindrical rotors are made from solid forgings of high-grade nickel-chrome-molybdenum steel.
The construction of the cylindrical rotor is such that there are no-physical poles to be seen as in the salient pole rotor.
In about two-third of the outer periphery of the cylindrical rotor, slots are cut at regular intervals and parallel to the rotor shaft.
The field windings are placed in these slots and is excited by DC supply. The field winding is of distributed type.
The unslotted portion of the rotor forms the pole faces.
It is clear from the figure of the cylindrical rotor that the poles formed are non-salient, i.e., they do not project out from the rotor surface.
The cylindrical type rotor construction is used in the high-speed (1500 to 3000 RPM) alternators such as those driven by steam turbines because of the following reasons −
The cylindrical type rotor construction provides a greater mechanical strength and permits more accurate dynamic balancing.
It gives noiseless operation at high speeds because of the uniform air gap.
The flux distribution around the periphery of the rotor is nearly a sine wave and hence a better EMF waveform is obtained.
A cylindrical rotor alternator has a comparatively small diameter and long axial length. The cylindrical rotor alternators are called turbo-alternators or turbo-generators. The alternator with cylindrical rotor have always horizontal configuration installation.
Working Principle and Operation of Alternator
An alternator or synchronous generator works on the principle of electromagnetic induction, i.e., when the flux linking a conductor changes, an EMF is induced in the conductor. When the armature winding of alternator subjected to the rotating magnetic field, the voltage will be generated in the armature winding.
When the rotor field winding of the alternator is energised from the DC exciter, the alternate N and S poles are developed on the rotor. When the rotor is rotated in the anticlockwise direction by a prime mover, the armature conductors placed on the stator are cut by the magnetic field of the rotor poles. As a result, the EMF is induced in the armature conductors due to electromagnetic induction. This induced EMF is alternating one because the N and S poles of the rotor pass the armature conductors alternatively.
The direction of the generated EMF can be determined by the Fleming’s right rule and the frequency of it is given by,
$$\mathrm{𝑓 =\frac{𝑁_{𝑠}𝑃}{120}}$$
Where,
$N_{s}$ is the synchronous speed in RP
P is the number of rotor poles.
The magnitude of the generated voltage depends upon the speed of rotation of the rotor and the DC field excitation current. For the balanced condition, the generated voltage in each phase of the winding is the same but differ in phase by 120° electrical.
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