|Home | Articles | Forum | Glossary | Books|
.The experimental setup illustrated in FIG. 16, although of primitive nature, involves important ramifications for motors. In this discussion, suppose that material “X” is:
(a) An insulator.
(b) A perfect conductor.
(c) A very good conductor.
(d) Iron or other ferromagnetic substance.
In the first instance (a), when the material is an insulator, such as a sheet of glass, Lucite, mica, etc., nothing at all happens—at least physically. The purist might postulate transient phenomena with regard to charges, but such disturbances are of no practical importance to motors. Indeed, sensitive and complex instrumentation would have to be employed to detect any such cause-and-effect relationship. In this discussion, the interest is in gross physical movement. That is why the sheet of material, X, is placed on rollers. For all practical purposes, the sweeping magnetic field produced by the movement of the magnet produces no electrical or magnetic effects capable of setting material X in motion.
In situation (b), a perfect conductor is used for material X. Suppose that this is superconducting material without regard to the awkward implementation of such an experiment. You have, of course, complete liberty to imagine such an investigation. It happens again that the sheet is subjected to no physical displacement. This is because a perfect conductor does not allow penetration of currents induced by a time- varying magnetic field. This self-shielding property is not entirely foreign—a similar occurrence was encountered under the name “skin effect.” In principle, an ideal conductor that is also not ferromagnetic will experience virtually no mechanical force in this experiment.
If the material is a very good conductor (c), say copper or aluminum, it will move in the direction of the moving magnet. Such material will have induced in it so-called eddy currents. By Lenz’s law, the magnetic fields of these eddy currents will oppose the inducing field and will thereby carry the material away from the advancing mag net. This is the very same principle involved in the eddy-current disk associated with the watt-hour meter in FIG. 15. In this situation, the magnet moves and the conductive material is initially at a standstill. But the relative direction of the forces produced in the two cases are the same. Knowing in advance that the eddy-current disk generates a retarding torque, imagine the magnets to be in motion and that the disk is initially at a standstill. A little contemplation reveals what must happen in such a case.
The disk would actually rotate in the direction of the “orbiting” magnets. With a little additional imagination, envisage the use of stationary ac-energized electro magnets iii place of the moving permanent magnets. Then, the sweeping magnetic field could be produced electrically, rather than by actual physical motion. And, thereby brought into existence is the induction motor!
The situation indicated in (d) is easy to visualize. Seemingly, everybody is able to deduce the effect produced with ferromagnetic material. As in situation (c), the sheet would move with the magnet. However, the action is not quite the same. In (c) a quick movement must be imparted to the magnet to get a somewhat delayed motion of the sheet material. In (d) the sheet follows the magnet right down to zero velocity. Although this would be detected in this simple setup, the situation in (c) also tends to produce heat in the material, because you are essentially inducing a short- circuited current path by the same induction process used hi generators. Finally, the situation of (d) results in a downward force on the material, as well as the translatory motion.
The phenomenon demonstrated in (d) is useful for certain clutches, brakes, and coupling devices. However, the all-important implications with regard to motor action are those derived from situation (c).
Another experiment that shows the force associated with induced eddy-cur rents is shown in FIG. 17. The copper disk is, in essence, the physically free but shorted secondary of a transformer. It goes without saying that actual transformers are also subject to such disruptive force. Indeed, in large transformers, the designer must give as careful attention to the physical integrity of the windings as to purely electrical or magnetic considerations. This experiment could be performed with a dc source, in which the repulsive force developed would be of a transitory rather than a sustained nature. The use of ac relates the experiment more closely to the induction motor.
With regard to both motor action and generator action, technical literature continues to perpetuate confusion about that old bugaboo—the direction of current. This is particularly noticeable when you compare the left-hand and right-hand rules in different books. These “rules” enable you to determine the directions of magnetic fields, mechanical force, and current by assigning these parameters to different fingers of a specified hand. Usually, a dot signifies that current flows out of the page, and a cross indicates that current flows into the page. There have been two opposite conventions based on the way the electrical current is supposed to flow. It is customary for texts to state that it makes no difference which convention is adopted as long as one is consistent in using the selected one. This assertion is true, but confusion nonetheless continues because of a persevering sloppiness in semantics that continues unabated in motor books.
Specifically, if you choose to deal with the older concept that electric current flows from the positive terminal of a battery or other active sources, through the cir cult, and thence back to the negative terminal, the terminology current or current flow should be used. (Actually, current flow is a grammatical redundancy because “current” already suggests the process of flowing. However, current flow has gained such popular usage that it is considered acceptable by most editors.) Although the concept of current flowing from the positive terminal of the active device and re turning to the negative terminal is an obsolete one as far as electrical theory is concerned, it has gained such a strong foothold in technology over the years that it is often referred to as the “conventional” direction of current.
The direction of the electron flow is actually the opposite of conventional cur rent flow. The moving electrons, which constitute the electrical current in metallic conductors, leave the negative terminal of the source and return to the positive terminal. To describe such a situation, the terms electron flow or electron current flow are used. In this guide, I use the notion of electron flow when I am considering the directional property of current. However, wherever feasible, a technique will be used that neatly circumvents the confusions discussed earlier.
|PREV:||NEXT:||The simplest dynamo||Guide Index||HOME|