Commutation in DC Machine or Commutation in DC Generator or Motor

The voltage generated in the armature winding of a DC generator is alternating because it is placed in a rotating magnetic field. Commutation in a DC machine, especially in a DC generator, is the process that converts this alternating current into direct current using the commutator and stationary brushes.
In a DC Motor, the input direct current (DC) is converted to an alternating current (AC) in the armature through commutation.

This transformation of current from the rotating armature of a DC machine to the stationary brushes needs to maintain continuously moving contact between the commutator segments and the brushes. As the armature rotates, the coils under one pole (e.g., the N pole) move between a positive brush and a negative brush. The current flows inward to the commutator segments through these coils.
Then the coil is short circuited with the help of a brush for a very short fraction of time (1⁄500 sec). It is called commutation period. After this short-circuit time the armature coils rotates under S pole and rotates between a negative brush and its succeeding positive brush. Then the direction is reversed which is in the away from the commutator segments. This phenomena of the reversal of current is termed as commutation process. We get direct current from the brush terminal.
The commutation is called ideal if the commutation process or the reversal of current is completed by the end of the short circuit time or the commutation period.

If the current reversal isn’t completed during the short circuit time, sparking occurs at the brush contacts, causing overheating and damage to the commutator surface. This situation is known as poor commutation.

Physical Concept of Commutation in DC Machine

For the explanation of commutation process, let us consider a DC machine having an armature wound with ring winding. Let us also consider that the width of the commutator bar is equal to the width of the brush and current flowing through the conductor is I_C.
Let the commutator is moving from left to right. Then the brush will move from right to left.
At the first position, the brush is connected the commutator bar b (as shown in fig 1). Then the total current conducted by the commutator bar b into the brush is 2I_C.

When the armature starts to move right, then the brush comes to contact of bar a. Then the armature current flows through two paths and through the bars a and b (as shown in fig 2). The total current (2I_C) collected by the brush remain same.
As the contact area of the bar a with the brush increases and the contact area of the bar b decreases, the current flow through the bars increases and decreases simultaneously. When the contact area become same for both the commutator bar then same current flows through both the bars (as shown in fig 3).
When the brush contact area with the bar b decreases further, then the current flowing through the coil B changes its direction and starts to flow counter-clockwise (as shown in fig 4).
When the brush fully contacts bar a and disconnects from bar b, the current I_C flows counter-clockwise through coil B, ending the short circuit.
In this process the reversal of current or the process of commutation is done.

Methods of Improving Commutation

There are three methods of sparkles commutation:

1. Resistance Commutation
2. Voltage Commutation
3. Compensating Windings