In this topic, you study Alternator or Synchronous Generator – Working Principle, Construction, Classification, Advantages & Diagram.
Alternating current generators are normally known as alternators or synchronous generators (being always run at synchronous speed). Presently, they are the most commonly used machines for generating electrical power on large scale for commercial purposes. All modern-day power stations are equipped with alternators for the generation of electrical power. Since the three-phase system has become the standard system throughout the world for generation, transmission and distribution of electrical energy, in this topic, we shall confine our discussion on alternators to mainly three-phase alternators.
PRINCIPLE OF WORKING OF AN ALTERNATOR
The operation of all electrical generators, whether dc or ac is based on the fact that when a conductor is moved in magnetic field or a magnetic field moved with respect to the conductor, according to Faraday’s law of electromagnetic induction, an electromotive force is set up in the conductor. Thus, as long as there is relative motion between a conductor and magnetic field, a voltage will always be generated in the conductor.
Three Phase Alternator or Three Phase Synchronous Generator
Three-phase alternator has three windings spaced 120o (electrical) apart around the armature. Such an alternator produces three independent alternating voltages. These voltages in the individual windings have the same magnitude and frequency but have a phase difference of 120o. To understand this more clearly, let us again consider an elementary two-pole, three-phase alternator shown in Fig. 3.4.
Fig. 3.4 : (a) Elementary three-phase alternator with two poles, (b) Waveforms of three phase e.m.f.s., (c) Phasor diagram
Construction of an Alternator
The armature of the alternator consists of three single-turn rectangular coils R1 R2 , Y1 Y2 and B1 B2 fixed to one another at angles of 120o. This means that the corresponding ternlinals (i.e. starting terminals R1, Y1 and B1 or finishing terminals R2, Y2 and B2) of the three coils are 120o apart. The ends of each coil are connected to a pair of slip-rings carried on the shaft. The coils are placed in the uniform magnetic field provided by the North and South poles Of the magnet. The carbon brushes are pressed against the slip-rings to collect the induced currents in the coils (some of these details are omitted in the figure for simplicity).
Operation of an Alternator
Suppose the three coils are rotating in an anti-clockwise direction at uniform speed. Then as seen previously in the case of an elementary single-phase alternator (with only one armature coil), each coil will have its own generated e.m.f. and current which will be alternating in nature. In the position shown in Fig. 3.4 (a), the plane of the coil RI R2 is perpendicular to the magnetic field and its conductors move parallel to the field. Since there is no flux cutting, e.m.f. generated in the coil is zero. After exactly 120o the coil Y1 Y2 will occupy this position and e.m.f. induced in it will be zero. Same thing will be true regarding the maximum values achieved by the e.m.f.s in the respective coils when in their horizontal positions. Thus, it will be observed that e.m.f. generated in the coil Y1 Y2 will be attaining its zero or maximum value 120o latter than the e.m.f. generated in the coil R1 R2. In other words, the e.m.f. generated in the coil Y1 Y2 will be lagging that in the coil R1 R2 by 120o. Similarly, the e.m.f. generated in the coil BIB2 will lag that in the coil Y1 Y2 by 120o. The waveforms of the respective electromotive forces even though identical will, therefore, be displaced through 1200 as shown in Fig. 3.4 (b). It means that these three e.m.f.s with the same amplitude will have a phase difference of 120o. The phasor diagram shown in Fig. 3.4 (c) also shows their phase relationships (ER, EY and EB being the r.m.s. values Of e.m.f.s in the coils R1 R2, Y1 Y2 and B1 B2 respectively). If the instantaneous value of the e.m.f. generated in the coil R1 R2 is represented by
eR = Em sin ωt
then the instantaneous values of the e.m.f.s generated in the coils Y1 Y2 and B1 B2 will be
given by
ey = Em sin (ωt – 120o)
eB = Em sin (ωt – 240o)
Thus, we get three independent alternating voltages from the three-phase alternator having three windings spaced 120o apart around the armature. These voltages have the same magnitude and frequency but they have a phase difference of 120o.
Classification of an Alternator
Apart from classifying the alternators as single-phase and three-phase as seen above, they can also be classified in the following manner based upon their speed of operation, capacity, generation voltage and construction.
Classification Based on Speed of Operation
Various types of prime movers with different speed ranges are used for driving the alternators. Depending upon their speed of operation, the
alternators may be roughly divided into three classes:
(i) Low speed alternators,
(ii) Medium speed alternators,
(iii) High speed alternators.
Table below gives the speed range and the types of prime movers used for the alternators coming under the above mentioned classes.
| Class
|
Speed Range
|
Prime Movers Used
|
| (i) Low speed alternators | 60 to 500 r.p.m. | Water turbines, Steam or Internal combustion engines. |
| (ii) Medium speed alternators | 600 to 1000 r.p.m. | High head water turbines, Internal
combustion engines, Electric motors. |
| (iii) High speed alternators | 1500 to 3000 r.p.m. | Steam turbines, Electric motors. |
Classification Based on Capacity and Generation Voltage
Depending upon their capacity and generation voltage, alternators may be broadly classified as follows :
(i) Small capacity alternators: 250, 420, 1100, 3300 V
(ii) Medium capacity alternators: 3300, 6600, 11000 V
(iii) Large capacity alternators: 6600, 11000, 33000 V
Classification Based on Construction
Similar to a dc generator, an alternator consists essentially of the field magnet system and the armature (part of the machine in which e.m.f. is induced). Only difference is that the commutator is replaced by slip-rings. The generation of an e.m.f. in the armature conductors depends on the relative motion between the conductors and the field flux. In a dc machine, the armature is always a rotating member to make the action Of the commutator possible. But in the alternator, as there is no commutator, either the armature or the field may be the rotating member. Accordingly, there are
two types of alternators :
(i) Revolving armature type.
(ii) Revolving field type.
The alternators which we have studied schematically represent the revolving armature type three-phase alternators respectively. In this type of alternator, the armature rotates through a stationary magnetic field.
The revolving field type alternator has a stationary armature winding and a rotating field winding. Fig. 3.5 illustrates the elementary single-phase and three-phase alternators of this type.
Fig. 3.5 : Rotating field type elementary 2-pole alternators (a) Single-phase, (b) Three-phase
Advantages
The advantages of the rotating field construction for alternators are as follows :
(i) The high voltage generated in the armature winding need not be brought to the external circuit through slip-rings and brushes, but the load can be directly connected to the terminals of the alternator.
(ii) It permits better insulation for the armature coils than would be possible on a rotating armature. This is because with steady armature, more space can be easily provided for insulation. The increased space may be made available by either increasing the size of the Stator or by deepening the slots without mechanically weakening the armature. Moreover, the centrifugal force and vibrations are also absent. Hence, it is possible to achieve generation voltage as high as 33000 V.
(iii) Since the voltage applied to the rotating field winding is low dc voltage (110 or 250 V), the problem of sparking at the slip-rings is not encountered. Moreover, only two slip-rings are required and they can be easily insulated because of low voltage.
(iv) Overall construction is considerably simplified. With the simpler and more robust mechanical construction of the rotor, a higher speed is possible, so that a greater output can be obtained from a machine of given size.
(v) In the case of large machines, it is easy to make the necessary arrangements for forced air-cooling or hydrogen cooling on a stationary armature by increasing the size of the stator core and providing radial air ducts and ventilation holes.
