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

Ans. The paths may be in parallel or series parallel according as the winding is of the lap or wave type.

Fig. 284.—Morday's method of measuring the variation of voltage around the commutator by use of a single exploring brush and volt meter. It consists in connecting one terminal of the volt meter (preferably an electrostatic one) to one brush of the machine, and the other terminal to the exploring brush, which can be moved from point to point, readings being taken at each point.

Variation of Voltage Around the Commutator.—There are numerous ways of determining the value of the induced voltage in an armature at various points around the commutator. In the method suggested by Morday, it can be measured by the use of a single exploring brush and a volt meter as shown in [fig. 284].

In this method, one terminal of the volt meter is connected to one of the brushes of the dynamo, and the other terminal is joined by a wire to a small pilot brush which can be pressed against the commutator at any desired part of its circumference. With the machine running at its rated speed, the exploring brush is placed in successive positions between the two brushes of the machine. In each position a reading of the volt meter is taken and the angular position of the exploring brush noted.

Fig. 285.—Cross magnetization. This is defined as lines of magnetic force set up in the windings of a dynamo armature which oppose at right angles the lines of force created between the poles of the field magnet. The figure shows this cross flux which is due to the armature current alone.

Ques. How does the voltage vary between successive pairs of commutator segments?

Ans. The variation is not constant.

Cross Magnetization; Field Distortion.—In the operation of a dynamo with load, the induced current flowing in the armature winding, converts the armature into an electromagnet setting up a field across or at right angles to the field of the machine. This cross magnetization of the armature tends to distort the field produced by the field magnets, the effect being known as armature reaction. To understand the nature of this reaction it is best to first consider the effect of the field current and the armature current separately.