Armature Reaction

You will remember that the subject of armature reaction was covered in the section on DC generators. The reasons for armature reaction and the methods of compensating for its effects are basically the same for DC motors as for DC generators.

There is a distorting effect that the armature field has on the flux between the pole pieces. However, the effect has shifted the neutral plane backward against the direction of rotation. This is different from a DC generator, where the neutral plane shifted forward in the direction of rotation.

FIGURE

As before, the brushes must be shifted to the new neutral plane. As shown in the figure, the shift is anticlockwise. Again, the proper location is reached when there is no sparking from the brushes.

Compensating windings and interpoles cancel armature reaction in DC motors. Shifting brushes reduces sparking, but it also makes the field less effective. Cancelling armature reaction eliminates the need to shift brushes in the first place. Compensating windings and interpoles are as important in motors as they are in generators. These are relatively expensive; therefore, most large DC motors depend on interpoles to correct armature reaction. Compensating windings are the same in motors as they are in generators. However, interpoles are slightly different. In a generator, the interpole has the same polarity as the main pole ahead of it in the direction of rotation. In a motor, the interpole has the same polarity as the main pole behind it.

The interpole coil in a motor is connected to carry the armature current the same as in a generator. As the load varies, the interpole flux varies, and commutation is automatically corrected as the load changes. It is not necessary to shift the brushes when there is an increase or decrease in load. The brushes are located on the no-load neutral plane. They remain in that position for all conditions of load.

The DC motor is reversed by reversing the direction of the current in the armature. When the armature current is reversed, the current through the interpole is also reversed. Therefore, the interpole still has the proper polarity to provide automatic commutation.