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It is clear that for the fixed number of poles, the alternator has to be rotated at a particular speed to keep the frequency of the generated emf constant at the required value. Such a speed is called Synchronous Speed of the alternator denoted as Ns.
So Ns=120f⁄P Where f= Required Frequency
In our nation, the frequency of an alternating emf is standard equal to 50 Hz. to get 50 Hz frequency, for the different number of poles, the alternator must be driven at different speeds called Synchronous Speeds. Following table gives the values of the synchronous speeds for the alternators having a different number of poles.
From the table, it can be seen that the minimum number of poles for an alternator can be two hence maximum value of synchronous speed possible in our nation i.e. for the frequency of 50 Hz is 3000 rpm.
We have seen that for 2 pole alternator, one mechanical revolution corresponding to one electrical cycle of an induced emf Now consider 4 pole alternator i.e. the field winding is designed to produce 4 poles. Due to 4 poles, the magnetic axis exists diagonally as shown in figure-
Now in position 1 of the conductor, the velocity component is parallel to the flux lines while in position 2, there is the gathering of flux lines and entire velocity component is perpendicular to the flux lines.
figure-
So at position 1, the induced emf in the conductor is zero while at position 2, it is maximum. Similarly, as conductor rotates, the induced emf will be maximum at positions 4, 6 and 8 and will be minimum at positions 3, 5 and 7. So during one complete revolution of the conductor, inducted emf will experience four times maxima, twice in either direction and four times zero. This is because of the distribution of flux lines due to the existence of four poles.
So if we plot the nature of the induced emf, for one revolution of the conductor, we get the two electrical cycles of the induced emf, as shown in figure-
So for a four-pole alternator, we can write,
3600 electrical = 7200 mechanical
From this, we can establish the general relation between degrees mechanical and degrees electrical as.
3600 electrical = 3600xP/2 mechanical
P= No. of Poles
i.e.
10 electrical = (P/2)0 mechanical
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The Alternator works on the principle of Electromagnetic induction. When there is a relative motion between the conductors and the flux, emf gets induced in the conductors. The dc generators also work on the same principle. The only difference in the practical alternator and a dc generator is that in an alternator the conductors are stationary and field is rotating. But for the understanding purpose, we can always consider the relative motion of conductors concerning the flux produced by the field winding.
Consider a relative motion of a single conductor under the magnetic field produced by two stationary poles. The magnetic axis of the two poles produced by field is vertical, shown by dotted lines.
Let the conductor starts rotating from Position 1. At this instant, the entire velocity component is parallel to the flux lines. Hence there is no cutting of flux lines by the conductor. So d∅/dt at this instant is zero and hence induced emf in the conductor is also zero.
As the conductor moves from position 1 towards position 2, the art of the velocity component becomes perpendicular to the flux lines and proportional to that, emf gets induced in the conductor. The magnitude of such an induced emf increases as the conductor moves from position 1 towards 2. At position 2, the entire velocity component is perpendicular to the flux lines. Hence there exists maximum cutting of the flux lines. And at this instant, the induced emf in the conductor is at its maximum. As the position of conductor changes from 2 towards 3, The velocity component perpendicular to the flux starts decreasing and hence induced emf magnitude also starts decreasing. At position 3, again the entire velocity component is parallel to the flux lines and hence at this instant induced emf in the conductor is zero. As the conductor moves from position 3 towards 4, the velocity component perpendicular to the flux lines again starts increasing. But the direction of the velocity component now is opposite to the direction of velocity component existing during the movement of the conductor from position 1 to 2. Hence and induced emf in the conductor increases but in the opposite direction.
At position 4, it achieves maxima in the opposite direction, as the entire velocity component becomes perpendicular to the flux lines. Again from position 4 to 1, induced emf decreases and finally at position 1, again becomes zero. This cycle continues as conductor rotates at a certain speed. So if we plot the magnitudes of the induced emf against the time, we get an alternating nature of the induced emf as shown in the figure. This is the working principle of an alternator.
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Natural Ventilation- A fan is attached to either end of the machine. The ventilating medium is nothing but an atmospheric air which is forced over the machine parts, carrying away the heat. This circulation is possible with or without ventilating ducts. The ventilating ducts if provided may be either axial or radial.
Closed Circuit Ventilating System- An atmospheric air may contain injurious elements like dust, moisture, acidic fumes etc. which are harmful for the insulation of the winding. Hence for large capacity machines, closed-circuit system is preferred for ventilation. The ventilating medium used is generally Hydrogen. The hydrogen circulated over the machine parts is cooled with the help of water-cooled heat exchangers. Hydrogen provides very effective cooling than air which increases the rating of the machine up to 30 to 40 % for the same size. All modern alternators use closed-circuit ventilation with the help of hydrogen as a ventilating medium.
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The synchronous machines whether alternator or motor are necessarily separately exciting machines. Such machines always require the flux for the operation which is provided by the field winding. Giving dc supply to the field winding, for the production of the necessary flux is called excitation.
For the small machines, the required dc supply is obtained from a dc generator called exciter. It is mounted on the main shaft of the alternator. The dc output of the exciter is given to the field winding of the alternator through slip ring and brush assembly as generally, the field winding is on the rotor. In some machines, a current is supplied to the exciter by another dc generator called pilot exciter. But this arrangement is not very sensitive, quick and effective if it is required to change the excitation of the alternator.
For the medium size machines instead of dc generators, ac generators are used called ac exciters. The output of such ac exciters is rectified and then given to the rotor of the main alternator using slip ring and brush assembly. The excitation can be varied and controlled as per requirement in such a method.
Still, these two methods are not very much suitable for the large alternators hence a brush-less excitation system is used for the large alternators.
Brush-less Excitation System With the increase in rating of an alternator, the supply of necessary magnetic field becomes difficult as the current values may reach up to 4000 A. If we use conventional excitation systems such as a dc generator whose output is supplied to the alternator field through brushes and slip rings then problem invariably associated with slip rings commutators and brushes regarding cooling and maintenance.Thus modern excitation systems are developed which minimizes these problems by avoiding the use of brushes. Such excitation system is called Brush-less excitation system which is shown in figure-
It consists of silicon diode rectifiers which are mounted on the same shaft of the alternator and will directly provide necessary excitation to the field. The power required for rectifiers is provided by an ac exciter which is having stationary field but rotating armature.
The field of an exciter is supplied through a magnetic amplifier which will control and regulate the output voltage of the alternator since the feedback of output voltage of the alternator is taken and given to the magnetic amplifier. the system can be made self-contained if the excitation power for the magnetic amplifier is obtained from a small permanent magnet alternator having stationary armature having stationary armature which is driven from the main shaft. The performance and design of the overall system can be optimized by selecting proper frequency and voltage for ac exciter. the additional advantage that can be obtained with this system is that it is not necessary to make arrangement for spare exciters, generator field circuit breakers and field rheostats.
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Most of the alternators prefer rotating field type of construction. In the case of alternators, the winding terminology is slightly different than in the case of dc generators. In generators, the stationary winding is called 'Stator' while the rotating winding is called 'Rotor'. So most of the alternator stator as armature and rotor as the field.
Stator- The stator is a stationary armature. This consists of a core and the slots to hold the armature winding similar to the armature of a dc generator. The stator core uses a laminated construction. It is built up of special steel stamping insulated from each other with varnish or paper. The laminated construction is basically to keep down eddy current losses. Generally, the choice of material is steel to keep down hysteresis losses. The entire core is fabricated in a frame made of steel plates. The core has slots on its periphery for housing the armature conductors. the frame does not carry any flux and serves as the support to the core. Ventilation is maintained with the help of holes cast in the frame.
Rotor- There are two types of rotors used in alternators-
Salient pole type
Smooth cylindrical type
Salient pole type- This is also called Projected pole type as all the poles are projected out from the surface of the rotor.
The poles are built up of thick steel lamination. the poles are bolted to the rotor. The pole face has been given a specific shape. The field winding is provided on the pole shoe. These rotors have large diameters and small axial lengths. The limiting factor for the size of the rotor is the centrifugal force acting on the rotating member of the machine. As mechanical strength of salient pole type is less, this is preferred for low-speed alternators ranging from 125 rpm to 500 rpm. The prime movers used to drive such rotor are generally water turbines and IC engines.
2. Smooth cylindrical type- This is also called non-salient type or non projected type of rotor.
The rotor consists of a smooth solid steel cylinder, having the number of slots to accommodate the field coil. the slots are covered at the top with the help of steel or manganese wedges. The unslotted portions of the cylinder itself act as the poles. The poles are not projecting out and the surface of the rotor is smooth which maintains a uniform air gap between the stator and the rotor. These rotors have small diameters and large axial lengths. this is to keep peripheral speed within limits. The main advantage of this type is that these are mechanically very strong and thus preferred for high-speed alternators ranging between 1500 to 3000 rpm. Such high-speed alternators are called turbo-alternator. the prime movers used to drive such type of rotors are generally steam turbines, electric motors.
Difference between salient pole type rotor and smooth cylindrical type rotor-
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The various advantages of the rotating field can be stated as-
As everywhere ac is used, the generation level of ac voltage may be higher as 11 kV to 33 kV. This gets induced in the armature. For the stationary armature, large space can be provided to accommodate a large number of conductors and the insulation.
It is always better to protect high voltage winding from the centrifugal forces caused due to the rotation. So high voltage armature is generally kept stationary. The avoids the interaction of mechanical and electrical stresses.
It is easier to collect larger currents at very high voltages from a stationary member than from the slip ring and brush assembly. The voltage required to be supplied to the field is very low (110 V to 220 V) and hence can be easily supplied with the help of slip ring and brush assembly by keeping it rotating.
The problem of sparking at the slip rings can be avoided by keeping field rotating which is low voltage circuit and high voltage armature is stationary.
Due to the low voltage level on the field side, the insulation required is less and hence the field system has very low inertia. It is always better to rotate low inertia system than high inertia, as efforts required to rotate low inertia system are always less.
Rotating field makes the overall construction very simple. With simple, robust mechanical construction and low inertia of the rotor, It can be driven at high speeds. So greater output can be obtained from an alternator of a given size.
If the field is rotating, to excite it by an external dc supply two slip rings are enough. One each for positive and negative terminals. As against this, in the three-phase rotating armature, the minimum number of slip rings required is three and can not be easily insulated due to high voltage levels.
The ventilation arrangement for high voltage side can be improved it is kept stationary.
I hope that this article will be informative for you all.
It is known that the electric supply used, nowadays for commercial as well as domestic purposes, is of alternating type. Similar to d.c. machines, the a.c. machines associated with alternating voltages are also classified as generators and motors. The machines generating a.c. e.m.f. is called alternators or synchronous generators. While the machines accepting input from a.c. supply to produce mechanical output are called Synchronous motors. Both these machines work at a specific constant speed called synchronous speed and hence in general called synchronous machines. All the modern power stations consist of large capacity three-phase alternators. In this topic, the construction, working principle and the emf equation of three-phase alternator are discussed.
Difference between D.C. generator and Alternator It is seen that in case of a dc generator, basically, the nature of the induced emf in the armature conductors is of alternating type. By using commutator and brush assembly it is converted to dc and made available to the external circuit. if commutator is dropped from a dc generator and induced emf is tapped from an armature directly outside, the nature of such emf will be alternating. Such a machine without a commutator, providing an ac emf to the external circuit is called an alternator. The obvious question is how is it possible to collect and emf from the rotating armature without a commutator.
So The arrangement which is used to collect an induced emf from the rotating armature and make it available to the stationary circuit is called slip ring and brushes assembly. Concept of Slip Rings and Brush Assembly- Whenever there is a need for developing contact between the rotating element and the stationary circuit without conversion of an emf from ac to dc, the slip rings and brush assembly can be used. In the case of three-phase alternators, the armature consists of three-phase winding and an ac emf gets induced in these windings. After connecting windings in star or delta, the three ends of the windings are brought out. Across these terminals three-phase supply is available. But the armature i.e. these terminals are rotating and hence stationary load can not be connected directly to them. Hence slip rings, made up of conducting material are mounted on the shaft. Each terminal of winding is connected to an individual slip ring permanently. Hence three-phase supply is now available across the rotating slip rings. The brushes are resting on the slip rings, just making contact.
The brushes are stationary. Hence as brushes make contact with the slip rings, the three-phase supply is now available across the stationary brushes.
Hence any stationary load can then be connected across these stationary terminals available from the brushes. The schematic arrangement is shown in the figure-
Not only the induced emf can be taken out from the rotating winding outside but an induced emf can be injected to the rotating winding from outside with the help of slip ring and brush assembly. The external voltage can be applied across the brushes, which gets applied across the rotating due to the springs.
Now the induced emf is basically the effect of the relative motion present between an armature and the field. such a relative motion is achieved by rotating armature with the help of prime mover, in case of a dc generator. as the armature is connected to commutator in a dc generator, armature must be a rotating member while fielding as a stationary, but in case of alternators it is possible to have,
The rotating armature and stationary field
The rotating field and stationary armature
I hope that this article will be informative for you all.
