How commutator are convert AC current into DC current and equally Dc to AC current ?

To understand how commutator converts from AC to DC, first you need to understand how the commutator work and what is commutator ?

Introduction :

Commutator

The commutator is of cylindrical shape and is made up of wedge - sheet of mica. The segments are held together by means of two 'V' shape rings that fit into the V– grooves cut into the segments. Each armature coil is connected to the commutator segment through riser.

Commutator Construction

It is an important part of the DC machine and the major purpose is

It connects the rotating armature conductors to the stationary external circuit through carbon brushes. Carbon are use for brushes because carbon are good conductor material and carbon provide lubrication effect on the commutator.
It convert the alternative current induced in the armature conductors into unidirectional current in the external load circuit in generator action where as it convert the alternating torque into unidirectional (continuous) torque produced in the armature in motor action.

How commutator convert AC to DC current ?

Consider the two ends of the coil are connected to only one slip-ring dvide into two parts (segments) 'A' and 'B'. Both are insulated by the mica layer.

Two brushes rest on these parts of the slip-ring.

In this case when the coil is rotated is counter clockwise direction by an external force (in DC generator), the coil situated in the armature also rotate.

The magmatic flux is cut by the coil and an EMF is induced in it. The coil are shorted with the brush segment with are rest on the slip-ring. However the flow of current in the external resistor or circuit will become unidirectional current.

At second instant the flow of current in the external resistance is from M to L as well as the flow of current in the external resistor is from M to L in the fourth instant

It's wave shape.

Hence a alternating current is converted into unidirectional current in the external circuit with the help of a splitring (commutator).

In an actual machine, there are number of coil connected to the number of segments of the ring called Commutator.

Commutator

The emf or current delivered by these coils to the external load. The actual flow of current flowing in the external load which fluctuate slightly. The number of coil placed on the armature is even much more than this and a pure direct current is obtained at the output.

Thus, in actual machine working as a generator, the function of commutator is to convert the alternating current produced in the armature into direct in the external circuit.

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