AC. Generator: It is a device which converts mechanical energy into alternating form of electrical energy.
Principle: It works 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 field lines passing through the coil change and an induced emf and hence a current is set-up in it.
Construction: It consists of the following main parts:
1. Field magnet: It is a strong horse shoe-type permanent magnet with concave poles.
2. Armature: ABCD is a rectangular armature coil. It consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core. It can be rotated about an axis perpendicular to the magnetic field of the field magnet.
3. Slip rings: These are two brass rings S₁ and S₂ rigidly connected to the two ends of the armature coil. As the coil rotates, slip rings also rotate about the same axis of rotation.
4. Brushes: These are two graphite rods B₁ and B₂ which are kept pressed against the slip rings S₁ and S₂. Through these brushes, the current induced in the armature coil is sent to the external circuit.

Fig. A.C. generator

Working: As shown in Fig, suppose the armature coil ABCD is in the horizontal position. Now the coil is rotated clockwise. The coil cuts the magnetic lines of force. The arm AB moves upwards while the arm CD moves downwards. According to Fleming’s right hand rule, the induced current flows from A to B in arm AB and C to D in arm CD i.e., the induced current flows along ABCD. The induced current flows in the circuit through brush B₂ to B₁.
After half the rotation of the armature, the arm CD moves upwards and AB moves downwards. The induced current now flows in the reverse direction i.e., along DCBA. The current flows from B₁ to B₂. Thus the direction of current in the external circuit changes after every half rotation. Such a current which changes its direction after equal intervals of time is called alternating current. This device is called A.C. generator.