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How the alternating voltage induced in the conductor can be made unidirectional?


The voltage induced in a conductor rotating in a magnetic field is alternating in nature


Imagine the coil to be rotating in clock wise direction. We know an e. m. f. is induced in it which is proportional to the rate of change of flux linkages. When the plane of the coil is at right angles to lines of flux i.e. when it is in position, 1, then flux linked with the coil is maximum but rate of change of flux linkage is minimum.

As the coil continues rotating further, the rate of change of flux linkage increases, till position 3 is reached where   Ѳ=90. But at position 1, where  Ѳ=0 the flux linked with the coil is minimum.

In the next quarter revolution i.e. from 90 to 180, the flux linked with the coil gradually increase but the rate of flux linkages decreases. Hence, the induced e. m. f. decreases gradually till position 5 of the coil; it is reduced to zero value.  

So, we find that in the first half revolution of the coil , no (or minimum) e. m. f. is induced in it when in position 1, maximum when in position 3 and no e. m. f. when in position 5. The direction of this induced e. m. f. can be found by applying Fleming’s Right-hand rule which gives its direction from A to B and C to D. Hence, the direction of current flow is ABMLCD . The current through the load resistance R flows from M to L during the first half revolution of the coil.









 In the next half revolution i.e. from 180 to 360, the variations in the magnitude of e. m. f. are similar to those in the first half revolution. Its value is maximum when coil is in position 7 and minimum when in position 1. But it will be found that the direction of the induced current is from D to C and B to A as shown in figure . Hence, the path of current flow is along DCLMBA which is just the reverse of the previous direction of flow.  
Therefore, we find that current which we obtain from such a simple generator reverses its direction after every half revolution. Such a current undergoing periodic reversals is known as alternating current.
It is seen that in the first half revolution current flows along (ABMNLCD) i.e. the brush no.1 in contact with segment ‘a’ acts as the positive end of the supply and ‘b’ as the negative end. In the next half revolution , the direction of the induced current in the coil has reversed. But at the same time, the position of segment ‘a’ and ‘b’ have also reversed with the result that brush no.1 comes in touch with the segment which is positive i.e. segment ‘b’ in this case. Hence current in the load resistance again flows from M to L. The waveform of the current through the external circuit as shown in figure.     .
This current is unidirectional but not continuous like pure direct current. It is only due to the rectifying action of the spilt ring (also called commutator ) that it becomes unidirectional in the external circuit. Hence it should be clearly understood that even in the armature of a d. c. generator the induced voltage is alternating
 

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