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Commutation in DC machines is the process by which the reversal of current takes place. In DC generator this process is used to convert the induced AC in the conductors to a DC output. In DC motors commutation is used to reverse the directions of DC current before being applied to the coils of the motor.
What is Commutation : Working Principle, Effects on DC Machines
In our daily life, the usage of DC machines for our day to day needs has become a common thing. DC machine is an energy conversion device that makes electro-mechanical conversions. There are two types of DC machines- the DC motors and the DC generators. DC motors convert DC electrical power into mechanical motion whereas the DC generators convert the mechanical motion into DC power. But the catch is, the current generated in a DC generator is an AC but the output of the generator is DC!! In the same way, the principle of the motor is applicable when the current in the coil alternates, but the power applied to a DC motor is DC!! Then how are these machines running? The answer to this wonder is the small device named “Commutator”.

upwards or downwards depending upon the direction of current in the conductor. This gives rise to motor effect.

Motor Effect on Coil
Motor Effect on Coil


When an Electromagnetic coil is placed in between two magnetic with north facing south of another magnet, the magnetic lines moves the coil upwards when current is in one direction and downwards when the current in the coil is in reverse direction. This creates the rotatory motion of the coil. To change the direction of current in the coil, two half-moon shaped metals are attached to each end of the coil called Commutator. Metal brushes are placed with one end attached to the battery and the other end connected to the commutators.
Commutation in DC Machine
Each Armature coil contains two commutators attached at its end. For the transformation of current, the Commutator segments and brushes should maintain a continuously moving contact. To get larger output values more than one coil is used in DC machines. So, instead of one pair, we have a number of pairs of Commutator segments.The coil is short-circuited for a very short period of time with the help of brushes. This period is known as commutation period. Let us consider a DC motor in which the width of the Commutator bars is equal to the width of the brushes. Let the current flowing through the conductor be Ia. Let a, b, c be the Commutator segments of the motor. The current reversal in the coil .i.e. commutation process can be understood by the below steps.

Position-1

position 1

Let the Armature starts rotating, then the brush moves over the commutator segments. Let the first position of the brush commutator contact be at segment b as shown above. As the width of the commutator is equal to the width of the brush, in the above position the total areas of commutator and brush are in contact with each other. The total current conducted by the commutator segment into the brush at this position will be 2Ia.

Position-2

Now the armature rotates towards the right and the brush comes in contact with the bar a. At this position, the total conducted current will be 2Ia, but the current in the coil changes. Here the current flows through two paths A and B. 3/4th of the 2Ia comes from the coil B and remaining 1/4th comes from coil A.When KCL is applied at the segment a and b, the current through the coil B is reduced to Ia/2 and the current drawn through segment a is Ia/2

Position-3

At this position half of the brush, a surface is in contact with segment a and the other half is with segment b. As the total current drawn trough brush is 2Ia, current Ia is drawn through coil A and Ia is drawn through coil B. Using KCL we can observe that the current in coil B will be zero.

position 3

Position-4

In this position, one-fourth of the brush surface will be in contact with segment b and three fourth with segment a. Here the current drawn through coil B is – Ia/2. Here we can observe that the current in coil B is reversed.

position 4

Position-5

At this position, the brush is in full contact with segment a and the current from coil B is Ia but is reverse direction to the current direction of position 1.Thus commutation process is completed for segment b.