Ques. What is the normal plane of maximum induction?
Ans. A plane, 90° in advance of the normal neutral plane, being the position of maximum induction with no distortion of field, as in [fig. 301].
Ques. What is the plane of maximum induction?
Ans. A plane 90° in advance of the neutral plane, being the position of maximum induction in a distorted field as in [fig. 302].
Fig. 303.—Brush adjustment for self-induction. For convenience an electric current is regarded as having weight and hence possessing the property of inertia. The current then during commutation cannot be instantly brought to rest and started in the reverse direction but these changes must be brought about gradually by an opposing force. Hence by advancing the brushes beyond the neutral plane as illustrated, commutation takes place with the short circuited coil cutting the lines of force so as to induce a current in the opposite direction; this opposes the motion of the current in the short circuited coil, brings it to rest and starts it in the opposite direction, thus preventing sparks. Figs. 301 to 303 should be carefully compared and thoroughly understood.
Ques. What should be noted with respect to the different planes?
Ans. The commutating plane should be carefully distinguished from the normal neutral plane and from the neutral plane, as shown in [fig. 303].
Commutation.—In order to understand just what happens during commutation, a section of a ring armature may be used for illustration, such as shown in [fig. 304]. Here the coils A, B, C, D, E, are connected to commutator segments 1, 2, 3, 4, and the positive brush is shown in contact with two segments 2 and 3, the brush being in the neutral position. Currents in the coils on each side of the neutral line flow to the brush through segments 2 and 3; the brush then is positive.
