Commutation in DC machine

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Commutation in DC machine

commutation in DC machine is the process in which generated alternating current in the armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes.

it will takes place, the coil undergoing commutation is short circuited by the brush. The brief period during which the coil remains short circuited is known as Tc.

If the current reversal is completed by the end of Tc, it is called ideal commutation.

If the current reversal is not completed by that time, then sparking occurs between the brush and the commutator which results in progressive damage to both.

Causes of Poor Commutation

1. Poor mechanical conditions like uneven surface of commutators, vibrations of brushes etc.
2. Poor electrical conditions such as increased voltage between the bars of commutators, increase in the current density of brushes etc.
  • Practically Ideal Commutation is not possible.
  • This is mainly due to the fact that the armature coils have appreciable inductance. When the current in the coil undergoing commutation changes, self-induced e.m.f. is produced in the coil. This is generally called reactance voltage.
  • This reactance voltage opposes the change of current in the coil undergoing commutation. The result is that the change of current in the coil undergoing commutation occurs more slowly than it would be under ideal commutation.

how sparking placed between the commutator segment and brush.

  • At the end of commutation or short-circuit period, the current in coil A is reversed to a value of 12A (instead of 20 A shown above fig.) due to inductance of the coil.
  • When the brush breaks contact with segment, the remaining 8 A current jumps from segment to the brush through air causing sparking between segment and the brush.

Calculation of Reactance Voltage

Reactance voltage = (Coefficient of self-inductance) x (Rate of change of current)

  • The commutation period is very small,
  • say of the order of 1/500 second.

Methods of Improving Commutation

There are three methods:

  1. Resistance Commutation
  2. EMF commutation
  3. Compensating winding

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