Methods of Improving Commutation

Satisfactory commutation requires the coil current to reach its required reversed value before the trailing commutator segment leaves the brush.
Three design measures can form the methods of improving commutation:

1. Resistance commutation
2. E.M.F. commutation
3. Compensating windings

Resistance Commutation

Resistance commutation uses carbon brushes whose higher contact electrical resistance helps transfer current between adjacent commutator segments during brush overlap. It reduces sparking when the brush grade, pressure and contact area suit the machine.

During the commutation period, current I_C from coil C can reach the brush through two routes. The direct route passes through segment b. The 2^nd route passes through short-circuited coil B and segment a. With a low-resistance metallic brush, current I_C may remain concentrated in the 1st path because it has less resistance than the 2^nd path.
As a carbon brush moves from segment b to segment a, its contact area with b decreases while its area with a increases. In a simplified contact model, the electrical resistance R_b rises and R_a falls. This changing resistance transfers current towards segment a and assists reversal through the 2^nd path.

Higher brush resistance shortens the electrical time constant of the commutating path and encourages current transfer, but it also adds brush voltage drop and heat.

ρ is the bulk resistivity of the brush or conductor material.
l is the current-path length.
A is its cross-sectional area. Real brush contact also depends on contact pressure, surface condition and current density, so R = ρl/A is only a simplified guide to the sliding contact.

E.M.F. Commutation

Coil inductance opposes the rapid change of current during the short commutation interval. E.M.F. commutation applies a reversing emf to counter the reactance voltage and drive current towards its required reversed value.

Reactance Voltage:
Reactance voltage is the inductive voltage in the short-circuited coil that opposes current reversal during commutation. Its magnitude depends on coil inductance and the rate of current change.
A reversing emf can be produced in two ways:

1. By brush shifting.
2. By using inter-poles or commutating poles.

Brush Shifting Method of Commutation

Brushes may be shifted from the geometric neutral axis to the magnetic neutral axis. A DC generator normally uses a shift in the direction of rotation. A motor normally uses a shift opposite the direction of rotation. The commutating coil then cuts main-pole fringe flux that induces a reversing emf. Because armature reaction moves the magnetic neutral axis as load changes, a fixed brush shift is correct for only one load condition. Brush shifting can also add demagnetizing armature ampere-turns, so interpoles are preferred where load varies.

Method of Using Inter-Pole

Narrow auxiliary interpoles sit between the main poles in the neutral regions. A series connection with the armature makes their flux change with armature current. Their polarity is selected to oppose armature-reaction flux in the commutation zone and to induce the reversing emf required in the short-circuited coil. Correctly designed interpoles improve commutation over a range of loads, but no design can promise spark-free operation under every fault, overload or maintenance condition.

Compensating Windings

Compensating conductors sit in slots in the main-pole faces and carry armature current in the direction needed to oppose armature mmf under the pole arc. They reduce flux distortion, local saturation and the risk of flashover during rapid load changes.
Compensating windings add copper, slots and manufacturing cost. They are therefore used mainly in large or highly loaded machines with rapid current changes, reversal or plugging. Interpoles still provide the direct reversing emf in the commutation zone.