Commutation in DC Machines | Generator

what is a commutation in DC Generator or commutation definition

In the DC generator the currents induced in the armature conductors is alternating. To make these currents unidirectional (i.e. DC) in the external circuit commutators are used. The commutators convert the alternating currents in armature of the generator into DC.

Consider the diagram of 2 pole lap wound generator as shown in figure. There are two parallel paths between the brushes. The currents enter in the armature gets divided into two parts, half currents moves in the “ABC” and another half moves in the direction “ADC.

After the rotation of armature, now the armature coils get interchanged. The armature coils which was initially in contact with negative brush, now comes in contact with positive brush and armature coils which was initially in contact with positive brush comes in contact with negative brush.

Now the direction of current in the armature coils gets reversed. Now the direction of currents in armature coils is “CDA” and “CBA” as shown in above figure. This reversal of current in the armature coils is called commutation. This reversal of current takes place along the magnetic neutral axis or brush axis. This process by which currents in the short circuited coil is reversed when its passes the brush axis is called commutation. The period for which the coil remains short circuited is called commutation period.

If the reversal of currents is completed by the end of the commutation period, it is called ideal commutation. If the current reversal is not completed by the end of the commutation period, sparking occurs between the brush and commutator segment, causing damage to both.

Let us discuss the process of commutation in detail. Consider the figure 2 below where winding is shown straight for simplicity. Consider the brush width equal to the one Commutator segment and one mica insulation.

1. Initially the brush is in contact with commutator segment “b” . The current in the brush will flow coil “B” and coil “C”. The Total current flowing in the brush is 40A , 20 A from coil “B” and 20A from coil “C”. The direction of current in coil “B” is “1 to 2”. As shown in figure “2(a)”
2. With the rotation of armature, the brush makes contact with commutator segment “a”. Thus short circuits the coil “B”. Now there are two paths to brush – one though “2b” and another “1a”. The brush is 1/4 th on commutator segment “a”, thus current will 10A through the path “1a” and 3/4th on commutator segment “b”, thus current will be 30A. The direction of current in coil “B” is still “1 to 2”. As shown in figure “2(b)”
3. With the further rotation of the armature, the brush is 1/2th on commutator segment “a” and 1/2th on commutator segment “b”. The resistance of two parallel paths will be equal. Thus no current will flow through coil “B”. The brush still takes 40A current, 20A from path “1a” and 20A from path “2b” from coil “C”. As shown in figure “2(c)”
4. With further rotation of the armature, the brush is now 3/4th on commutator segment “a” and 1/4th on commutator segment “b”. The brush still take 40A current, 30A through path “1a” and 10A through path “2b”. The current in the coil “b” is 10A. The direction of current coil “B” is now reversed i.e. 2 to 1. As shown in figure “2(d)”
5. With further rotation of the armature. The brush comes fully in contact with commutator segment “a”. The brush still takes 40A current, 20A from coil “A” and 20A from coil “B”. but the direction of current in coil “B” is reversed. Thus coil “B” has undergone the commutation. As shown in figure “2(e)”

During commutation the coil under consideration remains short circuited by the brush. For ideal commutation the current in the coil undergoing commutation should be reversed completely. But ideal commutation cannot be attained in practice. This is due to presence of inductance in the armature coils. When the current in the coil undergoing commutation changes the self induced emf is produced in the coil, which is called reactance voltage. This reactance voltage opposes the change of current in the coil undergoing commutation.

The current and Time graph for the coil undergoing commutation is shown below.

The horizontal line “AB” represents a constant current of 20A upto to beginning of the commutation. From the finish of the commutation, it is represented by another horizontal line “CD” . The way in which current changes from “B” to “C” depends upon the condition under which the coil undergoes commutation. If the current changes at uniform rate, then it is called ideal commutation, The reversal current will be completed at the of the commutation. Under Ideal commutation, no sparking will take place between brushes and commutator. when the self inductance of coil is taken into consideration, the change in current in the coil is represented by curve “BK”, the reversal of current in coil is not completed at the end of commutation. The current “CK” (= 5A) is flowing from commutator segment “b” to the brush through air. This results in sparking on commutator. The sparking causes overheating of commutator and brush contact and causing damage to both.

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