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Electric Generator

Electric generator is a device for producing large currents for use in homes and industry. it converts mechanical energy into electrical energy. 

A generator is based on the principle that an induced current is produced in a conductor (in form of a closed circuit) whenever there is a change in the number of lines of force passing through it.


The essentials parts of A.C.Generator

  1. Field Magnet
    It is a powerful horse-shoe magnet.
  2. Armature
    It is consists of a soft iron core on which a large number of turns of insulated copper wire are wound. The armature is rotated rapidly in the field of the magnet. In the diagram only one turn of wire ABCD is shown, for simplicity. The coil rotates about a horizontal axis perpendicular to the lines of force.
  3. Slip Rings and Brushes
    The current generated is taken out to the external circuit by means of an arrangement called slip rings and brushes. The ends of the coil are connected to the slip rings, which rotate about the same axis as the coil. The two rings are insulated from each other. The brushes are made of graphite (or carbon). One end of each brush touches the slip ring and the other end is connected to the external circuit. The brushes do not rotate with the coil.


The coil (i.e. armature) is rotated rapidly say in the clockwise direction. Suppose, at the start, the plane of the coil is perpendicular to the magnetic field with the arm AB at the top is shown in Figure. At this position, the number of lines of force passing through the coil is maximum.
From Fleming's right-hand rule, the current induced in the upper arm is along BA and in the lower arm along DC. By the time the coil rotates through 90o, the induced current has increased to its maximum value because the rate of change of the number of lines of force passing through the coil has been changing. At this position, the plane of the coil is parallel to the field and the number of lines of force passing through the coil becomes zero. Since the rate of change of lines of force is maximum, the induced current is maximum when the coil is horizontal. 
If the rotation of the coil is continued, the induced current decreases and becomes zero when the coil has rotated through 180o. During this time the direction of the induced current is the same as in the previous quarter cycle. On rotating the coil further through 90o it is seen that the arm AB, which was descending in the previous half cycle, now begins to ascend and the arm CD starts descending. The induced current is therefore, reversed. The current in the coil is now along directions AB and CD. The induced current becomes more and more negative becoming negative maximum when the coil has rotated through 270o to a position shown in figure. In the last quarter cycle, the current becomes zero when the coil has rotated through 360o to a position shown in Figure. On continuing the rotation of the coil, the sequence of events described above is repeated.
The variation of induced current is represented by the curve shown in Figure, where the letter V stands for the vertical position of the coil and the letter H stands for the horizontal position of the coil.
Notice that the induced current is positive during the first half cycle and negative during the second half and so on. Such a current is called alternating current (ac).


Note: In India, power plants produce ac which changes direction after every 1/100 second, i.e. the frequency of ac is 50 Hz with a peak voltage of 220 V.

D.C. Generator
An alternating current (ac) generator becomes a direct current (dc) generator if the two split rings are replaced by a single split ring commutator as shown in Figure. The brushes are arranged in such a way that when the coil goes through the vertical position, the two halves S1 and S2 of the split rings commutator change contact from one brush to the other. This reverses the direction of the current, which, therefore, never becomes negative, i.e. it remains unidirectional (or direct). This particular arrangement of the split rings is called a 'current reverser' or 'commutator'. DC generators are used in bicycles and cars. AC generators are used in industry and electric power plants.

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