Principle: The working of an AC. generator is based on the principle of electromagnetic induction. When a closed coil is rotated in a uniform magnetic field with its axis perpendicular to the magnetic field, the magnetic flux linked with the coil changes and an induced emf and hence a current is set up in it.
Working: As the armature coil rotates, the magnetic flux linked with it changes and so an induced current flows through it. Suppose, initially the coil PQRS be in the vertical position and it is rotated in the clockwise direction. The side PQ moves downward and SR moves upward. According to Fleming’s right-hand rule, the induced current flows from Q to P and from S to R.
So, during the first half rotation of the coil, the induced current flows in the direction SRQP, with brush B1 acting as positive terminal and brush B2 as negative terminals. During the second half-rotation, the side PQ moves upwards and SR moves downward. The direction of induced current is reversed, i.e. it flows along PQRS so that the brush B1 act as the negative terminal.
Thus, the direction of current in the external circuit is reversed after every half cycle. Hence, alternating current is produced by the generator. Such a generator which generates alternating current is called an A.C. generator or an alternator.
