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Describe the principle, construction.working of a single phase a.c. generator with their diagram ?
A.C. Generator

Principle
An a.c generator is based on the principle that an induced emf is produced in a coil whenever there is change in the magnetic flux linked with it.  Because of this, an induced current is set up in the coil the direction of which is given by Fleming's  right hand rule.

Construction.
An a.c. Generator consists of following four main parts.

1. Armature: A rectangular coil ABCD consisting of a large number of  turns of insulated copper wire wound over a soft iron core is called the armature.  It is placed between the pole pieces of  a strong magnet NS. The coil rotates about a horizontal axis perpendicular to the magnetic lines of force.

2. Field magnet: Two strong magnets N and S between which the coil rotates is called the field magnet. In case of small dynamo, it is a permanent magnet while in the case of big dynamo, an electromagnet is used.

3. Slip rings: R1 and R2 are two  hollow metal rings. The ends of the armature coil are connected to slip rings  which rotate about the same axis as the armature  rotates. The rings are insulated from each other.

4. Brushes: B1 and B2 are two flexible metal plates or carbon rods, called the brushes. These brushes  touch the metal rings R1 and R2. The brushes are stationary in their positions  and rings rub against them as the armature coil rotates. The brushes B1 and B2 are connected to a load in the external circuit.

Working

Suppose the armature  coil is in the position as shown in fig 1(a). Rotate the coil in the clockwise direction. Then the arm CD moves downwards and AB moves upwards. According to Fleming's right hand rule, the induced current flows from C to D and from A to B within the armature coil. Therefore, during the first- half rotation, the current flows in the external circuit from Y to X. During  the second half rotation, the arm CD moves upwards and AB moves downwards.  According to Fleming's right hand rule, the induced current flows from D to C and from B to A within the armature coil. Therefore, the current flows in the  external load from Y to X as shown in fig 1(b) so, the direction of the induced current across the load in the external circuit reverses after every half rotation of the armature coil. The current, thus, produced, is alternating  in nature.

EMF in A.C. Dynamo

The emf in the rotating armature (fig 2) can be calculated as follows:
Let         N= total number of turns in the armature coil
        A= Area of the armature
        B= magnetic field in which the armature is present
           = flux linked with the armature at any instant
         = angular frequency of rotation of armature.

and    
   
The magnetic flux  through each turn of the armature is given by the dot product of face area A and magnetic field B

i.e

The emf E induced in each turn of the armature, according to Faraday's law,


Since the armature has N turns the instantaneous emf E induced across the  armature is
                                                                                                                     …......(1)

The induced emf E is maximum when sin     Let maximum value be Eo
    …......(2)
Equation (1) reduces to

Where Eo= NAB  is called  the peak value of amplitude of the  induced emf. Thus the peak value of the induced emf depends upon.

(i)  Number of turns N of the coil
(ii) The face area A of the coil
(iii) The magnetic field B
(iv) The angular velocity   of the coil.

Further E is zero when    or when case when B    is perpendicular to the place of the armature and time rate of change of flux is zero.  If we plot a graph between E and   we get a  sine curve as shown in Fig.3. Thus, the emf varies sinusoidally  with time,.  The current so produced is known as single phase a.c and the generator is called the single phase a.c. generator.

Fig:-1(a)  and Fig:-1(b)



        Fig:-(2)

     Fig:-(3)


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