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Direct Current (D.C.) Dynamo or Generator

Principle. An induced emf is produced in a conductor whenever there is a change in the magnetic flux linked with it. It is a device for converting mechanical energy into direct electrical energy.

ABCD is a rectangular coil and it is called armature. N, S are poles of magnet. The ends of armature are connected to commutator segments. The split ring commutator consists of a hollow cylinder R1R2 which is divided into two segments R1 and R2 well insulated from each other. Brushes B1 and B2 lightly touch the two segments. The brushes and the split rings are mounted in such a way that the contact of one brush with the first segment changes over to that with the second segment, when the plane of the coil is at right angles to the magnetic lines of forces. At this instant, the induced emf is zero.

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As the coil rotates in the magnetic files, emf is induced in the coil due to electromagnetic induction. The direction of emf can be found from Fleming’s right hand rule. The direction of emf changes at the end of half rotation. The positions of half-rings connected to armature interchange.
Consequently, one brush is at positive potential and the other brush is at negative potential. Current flows in the external circuit in a single direction. The current is unidirectional and pulsating. The emf is not constant but varies sinsuoidally with time.

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The D.C. dynamo described above has only one coil and hence is called a single phase dynamo. The current is unidirectional but not steady. Usually, a steady emf and current are required by the consumers. This is accomplished by mounting a number of coils at equal angles on the axle and using a split ring with double the number of segments.  The ends of any coil are connected to two segments diametrically opposite each other.

A two phase D.C. dynamo has two coils with their planes at right angles to each other. The ring is divided into four segments. The ends of each coil are connected to the two segments diametrically opposite each other. As the armature rotates, both the coil develop induced emf varying according to the since curve. But, when the induced emf in one coil is maximum, in the other it is zero. Further, the brushes remain in contract with the coils only for one quarter of a cycle. The ring and the brushed are so adjusted that during the time the brushes remain in contact with the segments, the coil passes through a position of maximum induced emf.
The resultant emf and hence current is much less fluctuating.

By increasing the number of conductors round the circumferences of the armature, the emf and current curves may be made much smoother. But with a large number of coils, two problems are created:
1.    Each coil contributes current for a very short period during each revolution.
2.    The split parts of ring become many and hence very thin. When more than one of them touches the brush simultaneously, sparking takes place due to shortening of two coils. Sparking damages brushes and dynamo does not supply current. Above problems are solved by using a continious winding.

Continuous winding. In this, same wire is wrapped on the core. Core is rotated by 60o after each turn. Thus winding has three coils equally inclined to each other and all in series, Being in series, all coils contribute current for whole time.
As coil has only two free ends, split ring has only two parts, which are broad enough. This avoids sparking. Brushes do not damage and supply of current from dynamo is continious.

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