Commutation in DC Machines
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Commutation is the process by which the current in a short circuited coil is reversed as it crosses the MNA. During commutation, the coil is briefly short-circuited. If current reversal from positive to zero to negative is completed by the end of the short circuit period, commutation is ideal. If not completed, sparking can occur in the brushes, making commutation non-ideal. Commutation is illustrated through figures showing the current in a coil decreasing to zero and then reversing as it transitions from one side of the brush to the other during the short circuit period.
Commutation in DC Machines
- 2. COMMUTATION??? The process by which the current in the short circuited coil is reversed while it crosses the MNA is called ‘Commutation’ The brief period during which the coil remains short- circuited is known as ‘Commutation Period’
- 3. If the current reversal ie. The change from ‘+I’ to zero and then to ‘-I’ is completed by the end of short circuit or commutation period, then the commutation is ‘ideal commutation’. If current reversal is not complete by that time, then it will result in sparking in the brushes resulting in a ‘non-ideal commutation.’ Let us discuss the process of commutation or current reversal in more detail with the help of the figures.
- 4. Consider the fig shown below- Coil B is about to be short circuited because brush is about to come in contact with commutator segment ‘a’. It is assumed that each coil carries 20A,so that brush current is 40A. Prior to the beginning of short-circuit, coil B belongs to the group of coils lying to the left of brush & carries 20A from left to right.
- 5. In the fig shown here coil B has entered its period of short circuit and approximately at one-third of this period. The through coil B has reduced down from 20A to 10A because the other 10A flows via segment ‘a’. As the area of contact of brush is more with segment ‘b’ than with segment ‘a’, it receives 30A from the former, the total again being 40A.
- 6. Again consider the fig shown- Now the coil B is in the middle of the short-circuite period. The current through it has decreased to zero. The two currents of 20A each, pass to the brush directly from coil A & coil C as shown. The brush contact areas with the two segment ‘b’ & ‘a’ are equal.
- 7. Consider the shown below:-In this fig coil B has became the part of the group of coils lying to the right of the brush. It is seen that brush contact area with segment ‘b’ is decreasing rapidly whereas that with segment ‘a’ is increasing. Coil B now carries 10A in the reverse direction which combine with 20A supplied by coil A to make up 30A that passes from segment ‘a’ to the brush, the other 10A is supplied by coil C to the brush.
- 8. From the fig show now depicts the moment when coil B is almost at the end of commutation period. For ideal commutation, current through it should have reversed by now but, as shown it is carrying 15A only (instead of 20 A). The difference of current between coils C & B ie. 20-15=5A, jumps directly from segment ‘b’ to the brush through air producing spark.
- 9. If the change of current through coil B are plotted on a time base it will be represented by a horizontal line AB upto the beginning of commutation period. From the finish of commutation the current will be represented by another horizontal line CD. The way in which current changes from its positive value to zero and then to negative value depends on how coil B undergoes commutation.
- 10. If the change of current through coil B are plotted on a time base it will be represented by a horizontal line AB upto the beginning of commutation period. From the finish of commutation the current will be represented by another horizontal line CD. The way in which current changes from its positive value to zero and then to negative value depends on how coil B undergoes commutation.