JEE Main & Advanced Physics Electro Magnetic Induction ac Generator/Alternator/Dynamo

An electrical machine used to convert mechanical energy into electrical energy is known as ac generator/alternator.

(1) Principle : It works on the principle of electromagnetic induction i.e., when a coil is rotated in uniform magnetic field, an induced emf is produced in it.

(2) Construction : The main components of ac generator are

(i) Armature : Armature coil (ABCD) consists of large number of turns of insulated copper wire wound over a soft iron core.

(ii) Strong field magnet : A strong permanent magnet or an electromagnet whose poles (N and S) are cylindrical in shape in a field magnet. The armature coil rotates between the pole pieces of the field magnet. The uniform magnetic field provided by the field magnet is perpendicular to the axis of rotation of the coil. 

(iii) Slip rings : The two ends of the armature coil are connected to two brass slip rings R₁ and R₂ . These rings rotate along with the armature coil.

(iv) Brushes : Two carbon brushes (\[{{B}_{1}}\] and \[{{B}_{2}}\]), are pressed against the slip rings. The brushes are fixed while slip rings rotate along with the armature. These brushes are connected to the load through which the output is obtained.

(3) Working : When the armature coil ABCD rotates in the magnetic field provided by the strong field magnet, it cuts the magnetic lines of force. Thus the magnetic flux linked with the coil changes and hence induced emf is set up in the coil. The direction of the induced emf or the current in the coil is determined by the Fleming's right hand rule.

The current flows out through the brush \[{{B}_{1}}\] in one direction of half of the revolution and through the brush \[{{B}_{2}}\] in the next half revolution in the reverse direction. This process is repeated. Therefore, emf produced is of alternating nature.

\[e=-\frac{Nd\varphi }{dt}=NBA\omega \sin \omega t={{e}_{0}}\sin \omega t\] where \[{{e}_{0}}=NBA\omega \]

\[i=\frac{e}{R}=\frac{{{e}_{0}}}{R}\sin \omega t={{i}_{0}}\sin \omega t\]\[R\to R\] Resistance of the circuit