question_answer 16)

Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Answer:

An AC generator converts mechanical energy into electric energy. Principal : Whenever in a closed circuit (i.e., a coil), the magnetic field lines changes, an induced current is produced. Construction : It consists of the following four parts. 1. Armature : Armature (abcd), also called the coil, consists of a large number of turns of insulated copper wire would over a soft iron core. It revolves around an axle between the two poles of a strong magnet. 2. Field Magnet : The magnetic field (B) is supplied by a permanent magnet in a small  dynamo (also called a magneto) and by an electromagnet in case of a big commercial dynamo (usually called a generator). The poles of the magnet are shown as N-S in figure (a). 3. Slip Rings : R1 and R2  are two hollow metal rings held at different heights. The end d of the armature coil is connected to ring R1. The end c of the coil is passed through R1 without touching it and is connected to R2. These rings rotate with the rotation of the armature. 4. Brushes or Sliding Contact: B1 and B2 are flexible metal plates or carbon rods. These are called brushes or sliding contacts. B1 is in constant touch with and R1 and B2 is in constant touch with R2. It is with the help of these brushes that the induced current is passed on from the armature and the rings to the external circuit containing a resistance, R and a galvanometer, G. Brushes are stationary i.e. these do not rotate with the rotation of the armature. Working : The working of an AC generator is clear from figure (a) and (b). As the armature is rotated about an axis (shown dotted), the magnetic flux linked with the armature changes. Therefore, an induced current is produced in the armature. (a) Let us suppose that the armature abcd is rotating anticlockwise so that the arm ad moves inwards and bc moves outwards. Applying Fleming's right-hand rule, we find that the induced current in the armature and in the circuit is as shown in Fig. (a) due to which G shows deflection towards the right. (b) After the armature has turned through 180°, it occupies the position shown in Fig. (b). with the armature rotating in the same direction (i.e., anticlockwise), bc moves inwards and ad moves outwards. Thus, again applying Fleming's right-hand rule, we find the induced current in the external circuit (R and G) flows in the opposite direction due to which the direction of deflection in the galvanometer is towards left. Thus, we see that the direction of induced current changes in external circuit after every half revolution of the armature, i.e., after the armature has turned through an angle of 180° from its initial position. Hence, the induced current is alternating in nature.