Generation of Alternating Voltage

An alternating voltage can be generated either by rotating a coil in a uniform magnetic field at a constant speed or by rotating a uniform magnetic field within a stationary coil at a constant speed. In small AC generators, the coil rotates between the magnetic field, whereas in large AC generator magnetic field is rotated around the coil due to economical considerations.

In both the cases magnetic field is cut by the conductors and an emf is induced in them. The direction and magnitude of the induced emf depend on the position of the conductors.

Process of Generating Alternating Voltage

Consider a coil placed in a uniform magnetic field to which load is connected through brushes and slip rings. When the coil is rotated in an anticlockwise direction at a constant angular velocity of ω radians per second an electromagnetic force is induced in the coil sides. The cross-sectional view of the coil and its different position at different instants are shown below in the figure.


The magnitude of induced emf depends on the rate at which flux is cut by the conductors. At (1), (3) and (5) instants, induced emf in the conductors is zero as they are moving parallel to the magnetic line of force and the rate of flux cut is zero. Whereas the magnitude of emf induced in the conductors A and B is the maximum at instants (2) and (4) as the conductors are moving perpendicular to the magnetic line of force and the rate of flux cut is maximum.

The direction of emf induced in the conductor is determined by Fleming’s right-hand rule. At instant (2), the direction of the emf induced in conductor A is outward whereas, at the instant (4), the direction of the 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 an instant.

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