Commutation in DC Machines: Types and Windings

In DC machines, there are mainly three methods to obtain good commutation. They are

• Resistance Commutation
• Voltage Commutation
• Compensating Windings

Resistance Commutation

In the resistance commutation method of improving commutation, the high resistance carbon brushes are used. This increases the resistance between the contact of commutator segments and brushes. This high contact resistance has the tendency to force the current in the short-circuited coils to reverse according to commutation requirements and then increase in the reverse direction.

Voltage Commutation

In voltage commutation method, the arrangements are made to induce a voltage in the coil undergoing commutation, which will neutralise the reactance voltage. This induced voltage is in opposite direction to the reactance voltage. If the value of induced voltage becomes equal to the reactance voltage, then the quick reversal of current in the short circuited coil will take place and there will be sparkless commutation.

There are two methods of inducing voltage opposite to the reactance voltage

• Brush Shift
• Interpoles or Commutating Poles

Brush Shift

The magnetic neutral axis (MNA) is shifted in the direction of rotation (for the generator) and against the direction of rotation (for the motor) due to the effect of armature reaction. The armature reaction creates a flux in the neutral zone and due to this flux, a small voltage is induced in the commutating coil. Therefore, by shifting the brushes to the new MNA, the sparkless commutation can be obtained.

Interpoles or Commutating Poles

Some small field poles are also attached to the yoke of the machine and placed midway between the main field poles, are known as interpoles or commutating poles. The windings of the interpoles are connected in series with the armature, because the interpoles must produce fluxes that are directly proportional to the armature current.

The MMF produced by the armature and interpoles being acted simultaneously by the same armature current. Due to this, the armature flux in the neutral zone which tends to shift the MNA, is neutralised by the interpole flux. Therefore, the neutral axis is adjusted in position regardless of the load.

In DC machines, the following rules are used for deciding the polarity of the interpoles

• For a Generator, the polarity of interpole must be the same as that of the next main field pole in the direction of rotation.
• For a Motor, the polarity of interpole must be opposite to that of the next main field pole in the direction of rotation.

The interpoles only provide the flux which is sufficient to assure good commutation. They do not overcome the distortion due to the cross-magnetisation effect.

Compensating Windings

To improve the commutation in DC machines, the commutating windings are used, which eliminates the effect of armature reaction and flashover by balancing the armature flux.

Commutating windings are the auxiliary windings placed in the slots provided in the pole faces parallel to the armature conductors and are connected in series with the armature. The direction of current in the compensating winding is opposite to the armature winding, hence the commutating windings produce a flux which is opposite to the armature flux. Therefore, they neutralise the armature flux and the flux per pole being undisturbed by the armature flux regardless of the load conditions.