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explain the working of ac and dc

A.C. 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.
                            
Working: 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 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 that is, 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.

D.C.generator: A d.c generator is a device which converts mechanical energy into electrical energy of direct form.
The principle and construction of d.c. generator are similar to that of a.c.generator except that a d.c. generator has split ring commutator instead of slip rings. Split ring commutator: It consists of two semi cylindrical brass rings S1 and S2 rigidly attached to the ends of the armature coil. As the armature coil rotates, the two split rings also rotate about the same axis of rotation.
Working: Suppose initially the plane of the coil is horizontal. The split ring S1 touches the brush B1 and S2 touches B2. When coil ABCD is rotated clockwise, magnetic field lines passing through it change. The arm AB moves upwards and arm CD moves downwards. According to Fleming's right hand rule, the induced current in the armature coil flows in the direction ABCD. In the external circuit, current flows from B2 to B1. As the coil crosses its vertical position, the split ring S1 comes in contact with B2 and S2 with B1. The arm AB now moves downwards and arm CD moves upwards. The induced current in the armature flows along DCBA. But in the external circuit, the current still flows from B2 to B1. Hence, a unidirectional current flows in the external circuit.
                           


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