An electric generator : It is an electrical device which is used to convert mechanical energy into electrical energy.
But the word generator is a misnomer. Generator generates nothing but only converts one form of energy into another.
Principle : Its working is based upon the principle of mutual induction i.e.,
..An induced emf is produced in a coil whenever there is change in magnetic flux in the neighbouring coils.
Construction : It consists of following parts :
(i) Armature : Armature coil (ABCD) consists of large number of turns of insulated copper wire wound over a soft iron core. It can rotate about central axis.
(ii) Field magnet : The field magnet NS is an electromagnetic or permanent magnet in the field of which the armature rotates.
(iii) Slip rings : The two ends of coil are connected to two slip rings `R_(1)" and "R_(2)`. These rings rotate along with the armature coil.
(iv) Brushes : There are two carbon brushes `B_1" and "B_2` which are kept fixed and touch `R_1" and "R_2` lightly. These are used to pass the current from armature coil to external load R.
Working : As the armature coil is rotated in the magnetic field, angle `theta` between the field and normal to coil changes continuously. So magnetic flux linked with the coil changes. An emf is induced in the coil.
Suppose the plane of coil is perpendicular to plane of paper in which magnetic field is applied with AB at front and CD at the back. The amount of magnetic flux linked with the coil in this position is maximum. As the coil is rotated anticlockwise, AB moves inwards and CD moves outwards. The amount of magnetic flux linked with the coil changes. According to Fleming.s right hand rule, current induced in AB is from A to B and in CD, it is from C to D. In the external circuit, current flows from `B_2 " to "B_1`, as shown in Fig. (a).
After half rotation of coil, AB is at back and CD is at front. Therefore, on rotation further, AB moves outwards and CD moves inwards. The current induced in AB is from B to A and in CD, it is from D to C. Through external circuit, current flows from `B_1" to "B_2` as shown in Fig. (b). When the coil is rotated continuously, the above process is repeated and induced current in the external circuit changes direction after every half rotation of the coil. Hence, the current induced in the coil external circuit will be alternating in nature as shown in Fig.
Suppose
n= Number of turns in the coil.
A= Face area enclosed by each turn of the coil.
`vec(B)=` Strength of magnetic field.
`omega=` Uniform angular velocity of coil in clockwise direction.
Starting from vertical position, let the armature be in the position shown in fig. any instant t. Normal to armature at this instant makes an angle `omega t" with "vecB`. The magnetic flux `phi_(B)(t)` at any instant t through each turn of the armature is given by the dot product of the face area `vecA` and the magetic field `vecB`, i.e.,
`phi_(B)= n vec(B).vec(A)= n BA cos theta = n BA cos omega t`
As the coil is rotated, magnetic flux linked with the coil changes and hence an e.m.f. is induced in the coil. At this instant t, if E is the e.m.f. induced in the coil, then
`E= (d phi_B)/(dt)= -(d)/(dt)(n BA cos omega t)`
`= -n BA (d)/(dt) (cos omega t)`
`= -n BA (-sin omega t) omega`
`E= n BA omega sin omega t`
The induced e.m.f. will be maximum, when
`sin " "omega t` = maximum = 1
`E_("max")= E_(0)= n BA omega xx 1`
Therefore, eqn. (2) becomes `E= E_(0) sin omega t`
