Hawkins Electrical Guide v. 01 (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

Ques. Describe the winding of a series dynamo.

Ans. In this machine, the field magnets are wound with a few turns of thick wire joined in series with the armature brushes as shown in fig. 190.

Ques. What is the effect of this arrangement?

Ans. All of the current generated by the machine passes through the coils of the field magnets to the external circuit. The current in passing through the field magnets, energizes them and strengthens the weak field due to the residual magnetism of the magnet cores, resulting in the gradual building up of the field.

Ques. For what service is the series dynamo adapted?

Ans. It may be used for series arc lighting, series incandescent lighting, and as a booster for increasing the pressure on a feeder carrying current furnished by some other generator.

Ques. What is the effect of the series winding in the operation of the machine?

Ans. Its characteristic is to furnish current at an increased voltage as the load increases. If sufficient current be drawn to overload the machine, the voltage will drop.

Since the armature coils, field magnets and external circuits are in series, any increase in the resistance of the external circuit lessens the power of the machine to supply current, because it diminishes the current in the coils of the field magnets and therefore diminishes the effective magnetism. Again, a decrease in the resistance of the external circuit will increase the voltage because more current will flow through the field magnets. Accordingly, when the external circuit has lamps in series (as is common in an arc light circuit) the switching on of an additional lamp both adds to the resistance of the circuit and diminishes the power of the machine to supply current. When the lamps are in parallel, the switching on of additional lamps not only diminishes the resistance of the circuit, but causes the field magnets to be further excited by the increased current, so that the greater the number of lamps put on, the greater becomes the risk of inducing too much current.

The series dynamo has also the disadvantage of not starting action until a certain speed has been attained, or unless the resistance of the external circuit be below a certain limit.