Consider a single turn loop ABCD rotating clockwise in a uniform magnetic field with a constant speed .As the loop rotates, the flux linking the coil sides AB and CD changescontinuously.
Hence the e.m.f. induced in these coil side also changes but the e.m.f. induced in one coil side adds to that induced in the other.
(i) When the loop is in position no. 1, the generated e.m.f. is zero because the coil sides (AB and CD) are cutting no flux but are moving parallel to it
When the loop is in position no. 2, the coil sides are moving at an angle to the fluxand, therefore, a low e.m.f. is generated as indicated by point 2.
(iii) When the loop is in position no. 3, the coil sides (AB and CD) are at right angle to
the flux and are, therefore, cutting the flux at a maximum rate. Hence at this instant,
the generated e.m.f. is maximum as indicated by point 3.
(iv) At position 4, the generated e.m.f. is less because the coil sides are cutting the flux
at an angle.
(v) At position 5, no magnetic lines are cut and hence induced e.m.f. is zero as
indicated by point 5
(vi) At position 6, the coil sides move under a pole of opposite polarity and hence the
direction of generated e.m.f. is reversed. The maximum e.m.f. in this direction (i.e.,
reverse direction, will be when the loop is at position 7 and zero when at position 1.
This cycle repeats with
each revolution of the coil.
Note that e.m.f. generated in the loop is alternating one. It is because any coil side, say
AB has e.m.f. in one direction when under the influence of N-pole and in the other
direction when under the influence of S-pole. If a load is connected across the ends of
the loop, then alternating current will flow through the load.
The alternating voltage generated in the loop can be converted into direct voltage by
a device called commutator. We then have the d.c. generator. In fact, a commutator is
a mechanical rectifier.
