3. Slotting the pole pieces;
Both longitudinal gaps and oblique slots have been tried. The reduction of cross section of the pole piece causes it to become highly saturated and to offer large reluctance to the cross field.
4. The use of auxiliary poles.
These are small poles placed between the main poles and so wound and connected that their action opposes that of the cross field.
Normal Neutral Plane.—This may be defined as a plane passing through the axis of the armature perpendicular to the magnetic field of the machine when there is no flow of current in the armature, as shown in [fig. 288]. It is the plane in which the brushes would be placed to prevent sparking when the machine is in operation were the field not distorted by armature reaction, and there were no self-induction in the coils.
Commutating Plane; Lead of the Brushes.—It has been found that in order to reduce sparking to a minimum, the brushes must be placed in certain positions found by trial and designated as being located in the neutral plane.
When the brushes are in the neutral plane, they are in contact with commutator segments connecting with coils that are cutting the lines of force at the minimum rate.
Ques. Define the term "commutating plane."
Ans. This is a plane passing through the axis of the armature and through the center of contact of the brushes as shown in [figs. 289] and [300].
Ques. What is the angle of lead?