It is a device that converts mechanical energy into electrical energy and produces direct current and direct emf.
Its principle is based on Faraday’s law. When the coil rotates by any means in the magnetic field, the flux changes and produces directly induced emf.
Its Construction is the same as that of an a.c generator with the difference that the slip ring is replaced by a split(commutator).
Two halves of ring A, A (split rings) are called commutator B, and B is the carbon brushes that slide over the split ring and connect the coil to the external circuit. The coil wound on an iron cylinder(core). The (coil + core) is called the armature.
When a coil is rotated by a means in the magnetic field, an a.c emf produces in it which is converted into direct emf by means of split rings. The position of carbon brushes is so adjusted as the position in which the emf reverses the connection of to the external circuit is interchanged. As a result, brush B always remains in contact with a half which is + ve and brush B always remains in contact with a half which is ve.
In this way, the lower halves inverted and ve halves are obtained at the output, but fluctuate in magnitude by wounding large no of coils around the cylindrical core and split rings we can obtain constant emf.