Ques. State some of the features of the multi-polar dynamo.
Ans. In this class of machine, the armature and field magnets are surrounded by a circular frame, or ring yoke to which the field magnets are attached. This ring arrangement has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of the magnetic flux is shorter.
Ques. What important advantage is gained by the use of multi-pole field magnets?
Ans. Commercial voltages are obtained at moderate armature speed.
The difficulty experienced with bipolar machines is that, with a dynamo of large output, the speed at which its armature would have to rotate to generate commercial voltages would be excessive.
It is evident that with two or more magnetic fields, secured by increasing the number of poles, the armature inductors revolving between them cut more magnetic lines in one revolution than with a single field, hence, a given voltage is obtained with less speed of the armature than in the bipolar machine.
For instance, if a bipolar dynamo be required to run at say 900 revolutions per minute to generate 125 volts, a four pole machine of equal output will require only 450 revolutions, and one of eight poles only 225 revolutions per minute.
Ques. What is a self-exciting dynamo?
Ans. A machine in which the initial excitation of the field is due to the residual magnetism retained by the cores.