INTERPOLES OR COMMUTATOR POLES

The purpose of commentators and brushes in a dc machine is to reverse the current in a conductor when it goes from one pole to the next. This is illustrated in Fig. 1.42a. When the conductor x is under the north pole, it carries a dot current, but after passing through the brush it comes under the south pole (conductor y) and thus carries the cross current. In the developed diagram shown in Fig. the position of a coil (or turn) undergoing commutation is shown. When the coil passes the brush, its current changes direction. Figure shows a linear change of current in the coil. This is an ideal situation, providing a smooth transfer of current. However, current commutation in a dc machine is not linear for two reasons.

Fig.: Current commutation in a dc machine.

Coil inductance. The coil  undergoing commutation has inductance, which will delay current change.

Reactance voltage. The coil undergoing commutation is in the interpolar region, as can be seen in Fig. The armature winding mmf acts along the q-axis and therefore produces flux in the interpolar region. Consequently, when the coil moves in this region, a voltage, called a reactance voltage, is induced in the coil. This reactance voltage delays current change in the coil.

The actual current through a coil undergoing commutation is shown in Fig. When the coil is about to leave the brushes, the current reversal is not complete. Therefore, the current has to jump to its full value almost instantaneously and thus will cause sparking.

To improve commutation, a small pole, called an interpole or commutation pole, is created. Its winding carries the armature current in such a direction that its flux opposes the q-axis flux produced by armature current flowing in the armature winding. As a result, the net flux in the interpolar region is almost zero. If current in the armature winding reverses, the current in the interpole also reverses and hence these fluxes always oppose, as shown in Fig.  

Recall that the compensating winding on the pole face also provides flux in the q-axis. However, it cannot completely remove fluxes from the interpolar region. Similarly, interpoles cannot completely overcome the demagnetizing effects of armature reaction on the main poles. Consequently, both compensating and interpole windings are essential for improved performance of a dc machine. In almost all modern dc machines of large size, both interpoles and compensating windings are used. Figure shows the smaller interpoles (in between the larger main poles) and the pole face compensating windings in a large dc machine.