Ques. What is the effect of lead?
Ans. It produces a demagnetizing effect which tends to weaken the field magnets.
Ques. Describe this demagnetizing effect in detail.
Ans. Tracing the armature currents, in [fig. 289] according to Fleming's rule, it will be seen that current in inductors 1 to 18 flow from the observer indicated by crosses representing the tails of retreating arrows and in inductors 19 to 36, toward the observer from the back of armature, indicated by dots representing the points of approaching arrows. In determining these current directions the inductors to the right of the neutral line are considered as moving downward, and those to the left as moving upward. The current in inductors 1 to 15 and 19 to 33, tends to cross magnetize the magnetic field of the machine, but the current in inductors 34 to 36 and 16 to 18 tends to produce north and south poles as indicated. These poles are in opposition to the field poles and tend to demagnetize them. Hence, the inductors lying outside the two upright lines are known as cross magnetizing turns, and those lying inside, as demagnetizing turns.
The breadth of the belt of demagnetizing turns included between the two upright lines is clearly proportional to the angle of lead; therefore, the demagnetizing effect increases with the lead.
Eddy Currents; Lamination.—Induced electric currents, known as eddy currents, occur when a solid metallic mass is rotated in a magnetic field. They consume considerable energy and often occasion harmful rise in temperature. Armature cores, pole pieces, and field magnet cores are specially subject to these currents.
Fig. 290.—Arago's experiment illustrating eddy currents. Arago found that if a copper disc be rotated in its own plane underneath a compass needle, the needle was dragged around as by some invisible friction. The explanation of this phenomenon, known as Arago's rotations, is due to Faraday, who discovered that it was caused by induction. That is, a magnet moved near a solid mass of metal, induces in it currents, which, in flowing from one point to another, have their energy converted into heat, and which, while they last, produce (in accordance with Lenz's law) electromotive forces tending to stop the motion. Thus, in the figure, there are a pair of eddies in the part passing between the poles, and these currents oppose the motion of the disc. Foucault showed by experiment the heating effect of eddy currents, but such currents were known years before Foucault's experiments, hence they are incorrectly called Foucault currents.
Ques. Describe the formation of eddy currents.
Ans. In [fig. 291], a bar inductor is seen just passing from under the tip of the pole piece N of the field magnet. Noting the distribution of the lines of force, it will be seen that the edge c d is in a weaker field than the edge a b, hence, since the two edges move with the same velocity, the electromotive force induced along c d will be less than that induced along a b. This gives rise to whirls or current eddies in the copper bar as shown.