Ans. In [fig. 286], the resultant north pole n, n, n, where the lines emerge from the ring, attracts the south pole, s, s, s, where the lines enter the field magnet, hence a load is brought upon the engine, which drives the dynamo, in dragging the armature around against these attractions. The stronger the current induced in the armature, the greater will be the power necessary to turn it.
Ques. Why does this reaction in the armature require more power to drive the machine?
Ans. The effect produced by the armature reaction is in accordance with Lenz's law which states that: In electromagnetic induction, the direction of the induced current is such as to oppose the motion producing it.
Fig. 287.—Actual distortion of field resulting from cross magnetization, as shown by iron filings.
Remedies for Field Distortion.—Since the distortion of the magnetic field of a dynamo causes unsatisfactory operation, numerous attempts have been made to overcome this defect, as for instance, by:
1. Experimenting with different forms of pole piece;
The reluctance of the pole piece should be increased in the region where the magnetic flux tends to become most dense. The trailing horn of the pole piece may be made longer than the advancing horn and cut farther from the surface of the armature, so as to equalize the distribution of the magnetic flux.
2. Lengthening the air gap;
This increases the reluctance, and also necessitates more ampere turns in the field winding. The field distortion, however, will not be so great, as it would be if the magnetic field of the machine were weaker.