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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