Ques. What may be said of the field due to this residual magnetism?
Ans. It presents a very weak field, and the voltage that could be generated by the armature revolving in such a field would be only about two to ten volts.
Ques. How then can commercial voltages such as 100 or more volts be obtained with a self-exciting dynamo?
Ans. Part or all of the current induced in the armature is passed through the windings of the field magnets, thus strengthening the field. The voltage, therefore, will “build up,” increasing until the maximum has been reached.
The maximum voltage will depend upon the capacity of the field magnets as determined by the construction, and upon the strength of current used to excite them.
Ques. How long does the process of “building up” require?
Ans. The time required to fully excite the field magnets is from ten to twenty seconds, the rise in field strength being indicated on the voltmeter or by the gradual increase in the brilliancy of the pilot lamp.
Ques. Name three important classes of dynamo.
Ans. Series wound, shunt wound, and compound wound.