Ans. An imaginary plane passing through the axis of the armature and the center of contact of the brush.
Ques. What two planes are referred to in stating the position of the brushes?
Ans. The normal neutral plane and the commutating plane.
The angle intercepted by these two planes represents the lead, thus in stating that the brushes have a lead of 6°, means that the angle intercepted by the normal neutral plane and the commutating plane is 6°.
Fig. 301.—The proper position of the brushes, if there were no field distortion and self-induction in the armature coils, would be in the normal neutral plane. In the actual dynamo these two disturbing effects are present which makes it necessary to advance the brushes as shown in [figs. 302] and [303] to secure sparkless commutation.
Ques. What is the difference between the normal neutral plane and the neutral plane?
Ans. This is illustrated in [figs. 301] and [302]. The normal neutral plane is the position of zero induction assuming no distortion of the field as in [fig. 301]. The neutral plane is the position of zero induction with distorted field as in [fig. 302] and as is found in the actual machine; the distortion is exaggerated in the figure for clearness.
Fig. 302.—Brush adjustment for field distortion. The effect of the latter is to twist the lines of force around in the direction of rotation, thus maximum induction takes place in an inclined plane. The brushes then must be advanced to the neutral plane which is at right angles to the plane of maximum induction. This gives the proper position of the brushes neglecting self-induction.