Fig. 295.—Illustrating drag on armature inductors. In moving a wire carrying a current through a magnetic field, the lines of force are distorted, and the effect on the wire is the same as though the magnetic lines were elastic cords tending to shorten themselves. They, therefore, oppose the motion of the wire; hence, in dynamo operation, more or less power is absorbed in overcoming this drag on the numerous inductors. In the figure the inductor is being moved upward against the "drag" due to the magnetic field.
It is evident by inspection of the lines in [fig. 295], that there is a drag upon the conductor in the direction shown by the arrow.
Smooth and Slotted Armatures.—The inductors of an armature may be placed on a smooth drum or in slots cut in the surface parallel to the axis.
In the first instance, the magnetic drag comes on the inductors and in the case of slots, upon the teeth.
The effect of embedding the armature inductors in slots is to distort the magnetic field as shown in [fig. 296]. Most of the lines of force pass through the teeth, thus, not only are the inductors better placed for driving purposes, but, being screened magnetically by the teeth, the forces acting on them are reduced, the greater part of the magnetic drag being taken up by the core.
It should be noted that, although screened from the field, the inductors in a slotted armature cut magnetic lines precisely as if they were not protected. The effect is as though the magnetic lines flashed across the slots from tooth to tooth, instead of passing across the intermediate slot at the ordinary angular velocity.
Comparison of Smooth and Slotted Armatures.—The slotted armature has the following advantages over the smooth type:
1. Reduced reluctance of the air gap;
2. Better protection for the winding;
3. Inductors held firmly in place preventing slippage;
4. No magnetic drag on inductors;
5. No eddy currents in inductors;
6. Better ventilation;
7. Opposition to armature reaction.
Due to increased density of flux through the teeth.
The disadvantages of slotted armatures may be stated as follows: