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Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
An electric generator converts mechanical energy into electrical energy. The following figure shows an electric generator. It includes a rotating rectangular coil ABCD placed between the two poles of a permanent magnet. The ends of this coil are connected to the two rings R1 and R2 whose inner side are made insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2. These rings are internally attached to an axle. The axle may be mechanically rotated from outside to the coil inside the magnetic field. Outer ends of these brushes are connected to the galvanometer to show the flow of current in the external circuit. When the axle connected to the two rings is rotated in such a way that the arm AB moves up in the magnetic field produced by the permanent magnet. Suppose the coil ABCD is rotated clockwise. By applying Fleming’s right-hand rule, the induced currents are generated in these arms along the directions AB and CD. Hence, an induced current flows in the direction ABCD. If there are large numbers of turns in the coil, the current produced in each turn adds up to give a large current through the coil which means the current flows from B2 to B1. After the half rotation, arm CD moves up and AB moves down. Due to this, the directions of the induced currents in both the arms change, which increases the net induced current in the direction DCBA. Now, the current in the external circuit flows from B1 to B2. Therefore, after each rotation the polarity of the current in the respective arm changes. Such a current that changes direction after equal intervals of time is called an alternating current. So, this device is called an AC generator. In order to get a direct current, a split-ring type commutator should be used. With this arrangement, one brush is at all times in contact with the arm moves up, while the other moves down. Therefore, a unidirectional current is generated. Hence, the generator is called a DC generator.
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