Hawkins Electrical Guide v. 02 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

Fig. 304.—Commutation. This takes place during the brief interval in which any two segments of the commutator are bridged by the brush. The coil connecting with the two segments under the brush is thus short circuited. During commutation the current in the short circuited coil is brought to rest and started again in the reverse direction against the opposition offered by its so called inertia, or effect produced by self-induction.

Now, as the armature turns, the commutator segments come successively into contact with the brush. In the figure, segment 3 is just leaving the brush and 2 is beginning to pass under it, hence, for an instant the coil C is short circuited.

Ques. In [fig. 304], what are the current conditions?

Ans. Previous to contact with segment 2, current flowed in coil C in the same direction as in coil B.

Ques. What occurs while the brush is in contact with segments 2 and 3?

Ans. During this brief interval, the current in C is stopped and started again in the opposite direction.

Similarly each coil of the armature as it passes the brush will be short circuited and have its current reversed. This is known as commutation.

Ques. What is the effect of field distortion with respect to commutation?

Ans. The neutral plane no longer coincides with the normal neutral plane but is advanced in the direction of rotation of the armature as shown in [fig. 302].

The reaction of the poles N' and S' of the armature field on the poles S and N of the main magnetic field tends to crowd the lines of force into the upper pole face of the south pole of the magnet, and into the lower pole face of the north pole. This effect is due to the strong magnetic attraction between the opposite poles S and N' and N and S', and the equally strong repulsion between like poles N and N' and S and S'. Hence, the plane of maximum induction no longer coincides with the normal plane of maximum induction, but is advanced in the direction of rotation, depending upon the strength of the armature current, being shifted forward for an increase of current, and backward for a decrease of current. This distortion of the field and the consequent shifting of the plane of maximum induction naturally results in the shifting of the neutral plane from the vertical position to the inclined position as shown.