With the help of a labelled diagram, explain the principle, construction and working of an a.c. generator.
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Slip rings are used in AC generators to maintain electrical connectivity while reversing current direction every half cycle.
Split rings (commutators) are used instead in DC generators to convert internal AC into unidirectional DC.
Concept: • An AC generator (or alternator) is an electrical device that converts mechanical energy into alternating electrical energy.
• It works on the principle of Faraday's Law of Electromagnetic Induction. Step 1: Principle
When a closed armature coil is rotated rapidly in a uniform magnetic field, the magnetic flux linked with the coil continuously changes with time.
According to Faraday's law of electromagnetic induction, an electromotive force (emf) is induced in the coil, generating an alternating current in the external circuit. Step 2: Construction
An AC generator consists of four main parts:
1. Armature (Coil): A rectangular coil $ABCD$ consisting of a large number of turns of insulated copper wire wound over a soft iron core to increase magnetic field intensity.
2. Field Magnet: A strong electromagnet or permanent horse-shoe magnet providing a strong uniform magnetic field perpendicular to the axis of rotation.
3. Slip Rings: The two ends of the armature coil are connected to two metallic hollow rings $R_1$ and $R_2$, which rotate along with the coil.
4. Carbon Brushes: Two stationary flexible carbon blocks $B_1$ and $B_2$ remain in light sliding contact with slip rings $R_1$ and $R_2$ to conduct current to the external load circuit. Step 3: Working
When the armature coil $ABCD$ is rotated mechanically in the magnetic field with angular velocity $\omega$:
During the first half-rotation, side $AB$ moves upwards and side $CD$ moves downwards.
By Fleming's Right-Hand Rule, induced current flows along $ABCD$ through brush $B_1$ to $B_2$ in the outer circuit.
In the second half-rotation, side $AB$ moves downwards and $CD$ moves upwards.
The direction of induced current reverses, flowing along $DCBA$ from $B_2$ to $B_1$ in the outer circuit.
Hence, the direction of current in the load resistor reverses periodically after every half rotation, generating alternating current. Step 4: Conclusion
The continuous rotation of the armature coil in a magnetic field induces an alternating voltage across the slip rings, providing AC current to the connected external load.
