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Types of Alternators used in Different Power Stations
Alternator:
Alternator, also called synchronous generator, is an electromechanical device which converts mechanical energy into alternating emf (Electromotive force or voltage) of constant frequency. In U.S.A. and a few other countries the frequency is 60 Hz. The AC voltages generated may be single phase or 3-phase depending on the power supplied.
Generation of emf:
In 1831 Faraday discovered that an emf can be induced (or generated) due to relative motion between a magnetic field and a conductor of electricity. This voltage was termed as the induced emf since the emf is produced only due to motion between the conductor and the magnetic field without actual physical contact between them. The principle of electromagnetic induction is best understood by referring to Fig.1 below.
Figure 1
The magnetic field is produced by the two fixed poles one being the North Pole from which the magnetic flux lines emerge and enter into the other pole known as the South Pole. It was found that the
magnitude of the voltage induced in the conductor is proportional to the rate of change of flux lines linking the conductor.
Mathematically it is given as:
Where
φ = flux in Webers
t = time in seconds
e = average induced emf in volts
The above equation however holds good when the conductor is stationary. In practical rotating machinery, for the conductor of active length l moving with a velocity of v in a magnetic field of flux density B, as shown in Fig.1, the instantaneous induced emf is expressed as:
Where
B= flux density in Tesla (Wb/m2)
l = active conductor length (m)
v = relative linear velocity between the conductor and the field (m/s)
Thus the instantaneous voltage e and the average value E of the induced emf are:
Figure 2
Direction of induced e.m.f
The direction of emf (and current flow) is given by Fleming’s Right Hand Rule as shown below:
Figure 3
Elementary Synchronous Generators (Alternators)
On the basis of rotor winding, there are two types of constructions of Alternators.
cylindrical-rotor (non-salient pole)
Figure 4
salient-pole rotor
Figure 5
Hydro-Electric Power Station
Brush less type of Alternator is used in electrical power generation plant as main source of power.
As the speed of operation is low for hydro turbines, used in Hydro-electric plants, salient pole type Synchronous generators are used.
As N (Speed of Rotor) is inversely proportional to P (number of Poles), more Poles can be attached to Rotor for required frequency.
These allow better ventilation and also have other advantages over smooth cylindrical type rotor.
Such generators are characterized by their large diameters and short axial lengths. These generators look like a big wheel.
Steam Power Station
These include coal fired and furnace oil based power stations.
Brush less type of Alternator is used in electrical power generation plant as main source of power.
Steam turbines are found to operate at fairly good efficiency only at high speeds. The high speed operation of rotors tends to increase mechanical losses and so the rotors should have a smooth external surface.
Hence, smooth cylindrical type rotors with less diameter and large axial length are used for
Synchronous generators driven by steam turbines with either 2 or 4 poles.Poles are subjected to less mechanical stress.
Nuclear Power Station
Brush less type of Alternator is used in electrical power generation plant as main source of power.
As nuclear Power Plant also produces steam to turn turbine (and hence generator), the same Alternator used for steam power plant.
Smooth cylindrical type rotors with less diameter and large axial length are used for
Synchronous generators driven by steam turbines with either 2 or 4 poles.
Diesel Power Station
Brush less type of Alternator is used in electrical power generation plant as main source of power.
Speed of Diesel engine is approximately 1800 rpm, with this speed 4 pole Salient type rotor can be used.
Sources:
Electrical Machines - Prof. Krishna Vasudevan, Prof. G. Sridhara Rao, Prof. P. Sasidhara Rao
Chapman Electric Machinery Fundamentals 5th edition
Principles of Power Systems by V.K Mehta
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