Question

Explain in detail the principle, construction and working of a single phase AC generator.

Answer 1

AC Generator (Dynamo) - Single Phase: The ac generator is a device used for converting mechanical energy into electrical energy.

Principle: It is based on the principle of electromagnetic induction according to which an emf is induced in a coil when it is rotated in a uniform magnetic field.

Essential parts of an AC generator

i) Armature: Armature is a rectangular coil consisting of a large number of loops or turns of insulated copper wire wound over a laminated soft iron core or ring. The soft iron core not only increases the magnetic flux but also serves as a support for the coil.

ii) Field magnets: The necessary magnetic field is provided by permanent magnets in the case of low power dynamos. For high power dynamos, field is provided by electron magnet. Armature rotates between the magnetic poles such that the axis of rotation is perpendicular to the magnetic field.

iii) Slip rings: The ends of the armature coil are connected to two hollow metallic rings $R_1$ and $R_2$ called slip rings. These rings are fixed to a shaft, to which the armature is also fixed. When the shaft rotates, the slip rings along with the armature also rotate.

iv) Brushes: $B_1$ and $B_2$ are two flexible metallic plates or carbon brushes. They provide contact with the slip rings by keeping themselves pressed against the rig. They are used to pass on the current from the armature to the external power line through the slip rings.

Working: Whenever, there is a change in orientation of the coil, the magnetic flux linked with the coil changes, producing and induced emf in the coil. The direction of the induced current is given by Fleming's right hand rule.

Suppose the armature $ABCD$ is initially in the vertical position. It is rotated in the anticlockwise direction. The side $AB$ of the coil moves downwards and the side $DC$ moves upwards Figure (a). Then according to Flemings right hand rule the current induced in arm $AB$ flows from $B$ to $A$ and in $CD$ it flows from $D$ to $C$. Thus the current flows along $DCBA$ in the coil. In the external circuit the current flows from $B_1$ to $B_2$.

On further rotation, the arm $AB$ of the coil moves upwards and $DC$ moves downwards. Now the current in the coil flows along $ABCD$. In the external circuit the current flows from $B_2$ to $B_1$. As the rotation of the coil continues, the induced current in the external circuit keeps changing its direction for every half a rotation of the coil. Hence the induced current is alternating in nature figure (b).

As the armature completes $v$-rotations in one second, alternating current of frequency $v$ cycles per second is produced. The induced emf at any instant is given by $e=E_0\sin \omega t$.

The peak value of the emf, $E_0=NBA\omega$ where $N$ is the number of turns of the coil, $A$ is the area enclosed by the coil, $B$ is the magnetic field and $\omega$ is the angular velocity of the coil.