GENERATION OF ALTERNATING VOLTAGE AND CURRENT

An alternating voltage can be generated either by rotating a coil in a uniform magnetic field at constant speed as shown in Figure 6.3 or by rotating a uniform magnetic field within a stationary coil at a constant speed as shown in Figure 6.4.

Fig. 6.3  Production of ac voltage (rotating coil, field stationary)

Fig. 6.4  Production of ac voltage (rotating field, coil stationary)

The first method is generally applied in small AC generators, whereas second method is applied in large AC generators due to economical considerations. In both cases, magnetic field is cut by the conductors (or coil sides) and an emf is induced in them. The direction and magnitude of the induced emf in the conductors depend upon the position of the conductors explained as follows:

For simplicity, consider a coil placed in a uniform magnetic field to which a load (LM) is connected through brushes and slip rings as shown in Figure 6.5. When it is rotated in anticlockwise direction at a constant angular velocity of ω radians per second, an emf is induced in the coil sides. The cross-sectional view of the coil and its different positions at different instants are shown in Figure 6.5.

Fig. 6.5  Position of coil at various instants. Wave shape of generated voltage

The magnitude of induced emf depends upon the rate at which the flux is cut by the conductors. At (i), (iii), and (v) instants, induced emf in the conductors A and B is zero as they are moving parallel to the magnetic lines of force and the rate of flux cut is zero, whereas the magnitude of emf induced in the conductors A and B is maximum at instant (ii) and (iv) as the conductors are moving perpendicular to the magnetic lines of force and the rate of flux cut is maximum.

The direction of emf induced in the conductors is determined by applying Fleming’s right-hand rule. At instant (ii), the direction of emf induced in conductor A is outward, whereas at instant (iv), the direction of induced emf in the conductor A is inward (i.e., the direction of induced emf at this instant is opposite to that of the direction of induced emf at instant (ii).

The wave shape of the emf induced in the coil is also shown in Figure 6.5.