Electric generator

Electric generator

An alternating current (AC) generator, as shown in Figure 3.24, consists of a rotating rectangular coil ABCD called armature placed between the two poles of a permanent magnet. The two ends of this coil are connected to the two slip rings S₁ and S₂. The inner sides of these rings are insulated. Two conducting stationary brushes B₁ and B₂ are kept separately on the rings S₁ and S₂ respectively. The two rings S₁ and S₂ are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. Outer ends of the two brushes are connected to the external circuit.

When the coil is rotated, the magnetic ux linked with the coil changes. is change in magnetic ux will lead to generation of induced current. The direction of the induced current, as given by Fleming’s Right Hand Rule, is along ABCD in the coil and in the outer circuit it ows from B₂ to B₁. During the second half of rotation, the direction of current is along DCBA in the coil and in the outer circuit it ows from B1 to B2. As the rotation of the coil continues, the induced current in the external circuit is changing its direction for every half a rotation of the coil.

To get a direct current (DC), a split ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field while the other is in contact with the arm moving down. Thus a unidirectional current is produced. The generator is thus called a DC generator (Figure 3.25).