A.C. Generator – If a coil is rotated in a magnetic field, the magnetic flux passing through it would be maximum when its position is perpendicular to the lines of force (fig. 17. 15 a) and the flux would be zero (fig 17. 15b) when its plane is parallel to the lines of force because in this position, the lines do not cut through the surface of the coil. Thus, when a coil rotates in a magnetic field, the flux passing through it continuously changes. This change of flux induces an emf in the coil.
This is the principle on which an A.C generator works. A simpler generator consists of a rectangular coil, which is rotated between the poles of a permanent magnet. Both the ends of the coil are soldered to the two slip rings fixed on the arm of the coil (fig 17.16)
Two carbon brushes are kept in contact with these slip rings with the help of two springs. Current is drawn from the coil through these brushes. When the coil is rotated between poles of the magnet, a current is induced in accordance with the principle described above. This current after passing through the slip rings and the carbon brushes flows into the circuit.
Fig 17. 17 shows the emf that is generated during one complete rotation of the col. Suppose in the beginning the coil is vertical with its side a upward and side B downward as shown in fig. 17. 18 (a). In this figure the sides A and B have been shown by their vertical sections. In this position the sides A and B are moving parallel to the field due to which the rate of change of flux through the coil is zero. So, according to Faraday’s law, the emf induced in the coil is zero (fig. 17. 17 point 0).
After completing one quarter of the rotation, the position of the coil is shown in fig. 17. 18-(b) at this instant both the sides A and B are moving tat right angles to the field and the rate of change of flux through the coil is maximum. Hence the emf induced at this instant is maximum (fig. 17. 17point ¼).
After one half rotation of the coil, the position of sides A and B is shown in fig 17. 18©. Now again the sides are moving parallel to the field, so the emf induced is zero (fig 17.17, point 1/2 ). In this position, the side A is located downward and side B upward. As the coil further rotates, the direction of induced emf changes. After completing three quarters of its rotation, the coil assumes the position as shown in Fig. 17. 18(d). at this instant, the emf is at its maximum value but it direction is opposite to that of Gig. 17. 17 (point ¼).
The position of the coil after one complete rotation is shown in Fig. 17. 18(e). now the emf is again zero because this position is the same as shown in fig. 17. 18(a). Henceforth the coil starts its next rotation and with it. The next cycle of induced emf also starts. In this way the rotation of the coil in a magnetic field generates an alternating induced emf.
for your information
this robot after sensing the magnetic field of the under-ground current cable can indicate the passage of cable.