Question

With the help of a labelled diagram, explain the principle, construction and working of an a.c. generator.

Hint:

Remember the differences between generators:
• A.C. Generator uses slip rings.
• D.C. Generator uses split-ring commutators. The principle of both generators is Faraday's law of electromagnetic induction.

Answer 1

Concept: An a.c. generator is a device that converts mechanical energy into electrical energy. It works on the principle of electromagnetic induction discovered by Michael Faraday. Whenever the magnetic flux linked with a coil changes, an emf is induced in the coil. If the coil rotates continuously in a magnetic field, the induced emf changes periodically in magnitude and direction, producing alternating current.

Step 1: Principle of an A.C. Generator The working of an a.c. generator is based on Faraday's law of electromagnetic induction. According to Faraday's law: Whenever the magnetic flux linked with a closed circuit changes, an emf is induced in the circuit. The magnitude of the induced emf is equal to the rate of change of magnetic flux linked with the circuit. Mathematically, \[ e=-\frac{d\Phi_B}{dt} \] where
• \(e\) = induced emf,
• \(\Phi_B\) = magnetic flux linked with the coil. The negative sign represents Lenz's law.

Step 2: Labelled Diagram of an A.C. Generator

Where:
• \(N\) and \(S\) are the pole pieces of a strong magnet.
• \(ABCD\) is a rectangular armature coil.
• \(R_1\) and \(R_2\) are slip rings.
• \(B_1\) and \(B_2\) are carbon brushes.
• The external circuit is connected through the brushes.

Step 3: Construction of an A.C. Generator The main parts of an a.c. generator are:
• Armature Coil: A rectangular coil \(ABCD\) consisting of a large number of turns of insulated copper wire wound over a soft iron core.
• Strong Magnetic Field: The coil is placed between the pole pieces \(N\) and \(S\) of a strong magnet.
• Slip Rings: The ends of the coil are connected to two metallic slip rings \(R_1\) and \(R_2\).
• Carbon Brushes: Two stationary carbon brushes \(B_1\) and \(B_2\) press against the slip rings and provide electrical contact with the external circuit.
• Mechanical Arrangement: A shaft rotates the armature coil with a constant angular velocity.

Step 4: Working of an A.C. Generator When the armature coil is rotated in the magnetic field:
• The angle between the magnetic field and the normal to the coil changes continuously.
• Therefore, the magnetic flux linked with the coil changes continuously.
• Due to electromagnetic induction, an emf is induced in the coil. During the first half rotation:
• Current flows in one direction through the external circuit. During the next half rotation:
• The direction of induced current reverses.
• Current flows in the opposite direction through the external circuit. Thus, the direction of current changes after every half revolution. Hence, the output current is alternating in nature.

Step 5: Nature of the Output Since the direction of induced current changes periodically, \[ \text{Output Current} = \text{Alternating Current (AC)} \] The induced emf varies sinusoidally with time and is represented by \[ e=e_0\sin\omega t \] where
• \(e_0\) = maximum emf,
• \(\omega\) = angular velocity of rotation. Result: An a.c. generator works on the principle of electromagnetic induction. A rotating coil placed in a magnetic field experiences a continuous change in magnetic flux, producing an alternating emf and hence alternating current in the external circuit.