Volume I Part 11 (1/2)
[Ill.u.s.tration: Fig. 68. Principles of Magneto Generator]
In order to concentrate the magnetic field within the s.p.a.ce in which the armature revolves, pole pieces of iron are so arranged in connection with the poles of the permanent magnet as to afford a substantially cylindrical s.p.a.ce in which the armature conductors may revolve and through which practically all the magnetic lines of force set up by the permanent magnets will pa.s.s. In Fig. 68 there is shown, diagrammatically, a horseshoe magnet with such a pair of pole pieces, between which a loop of wire is adapted to rotate. The magnet _1_ is of hardened steel and permanently magnetized. The pole pieces are shown at _2_ and _3_, each being of soft iron adapted to make good magnetic contact on its flat side with the inner flat surface of the bar magnet, and being bored out so as to form a cylindrical recess between them as indicated. The direction of the magnetic lines of force set up by the bar magnet through the interpolar s.p.a.ce is indicated by the long horizontal arrows, this flow being from the north pole (N) to the south pole (S) of the magnet. At _4_ there is shown a loop of wire supposed to revolve in the magnetic field of force on the axis _5-5_.
Theory. In order to understand how currents will be generated in this loop of wire _4_, it is only necessary to remember that if a conductor is so moved as to cut across magnetic lines of force, an electromotive force will be set up in the conductor which will tend to make the current flow through it. The magnitude of the electromotive force will depend on the rate at which the conductor cuts through the lines of force, or, in other words, on the number of lines of force that are cut through by the conductor in a given unit of time. Again, the direction of the electromotive force depends on the direction of the cutting, so that if the conductor be moved in one direction across the lines of force, the electromotive force and the current will be in one direction; while if it moves in the opposite direction across the lines of force, the electromotive force and the current will be in the reverse direction.
It is, evident that as the loop of wire _4_ revolves in the field of force about the axis _5-5_, the portions of the conductor parallel to the axis will cut through the lines of force, first in one direction and then in the other, thus producing electromotive forces therein, first in one direction and then in the other.
Referring now to Fig. 68, and supposing that the loop _4_ is revolving in the direction of the curved arrow shown between the upper edges of the pole pieces, it will be evident that just as the loop stands in the vertical position, its horizontal members will be moving in a horizontal direction, parallel with the lines of force and, therefore, not cutting them at all. The electromotive force and the current will, therefore, be zero at this time.
As the loop advances toward the position shown in dotted lines, the upper portion of the loop that is parallel with the axis will begin to cut downwardly through the lines of force, and likewise the lower portion of the loop that is parallel with the axis will begin to cut upwardly through the lines of force. This will cause electromotive forces in opposite directions to be generated in these portions of the loop, and these will tend to aid each other in causing a current to circulate in the loop in the direction shown by the arrows a.s.sociated with the dotted representation of the loop. It is evident that as the motion of the loop progresses, the rate of cutting the lines of force will increase and will be a maximum when the loop reaches a horizontal position, or at that time the two portions of the loop that are parallel with the axis will be traveling at right angles to the lines of force. At this point, therefore, the electromotive force and the current will be a maximum.
From this point until the loop again a.s.sumes a vertical position, the cutting of the lines of force will still be in the same direction, but at a constantly decreasing rate, until, finally, when the loop is vertical the movement of the parts of the loop that are parallel with the axis will be in the direction of the lines of force and, therefore, no cutting will take place. At this point, therefore, the electromotive force and the current in the loop again will be zero. We have seen, therefore, that in this half revolution of the loop from the time when it was in a vertical position to a time when it was again in a vertical position but upside down, the electromotive force varied from zero to a maximum and back to zero, and the current did the same.
It is easy to see that, as the loop moves through the next half revolution, an exactly similar rise and fall of electromotive force and current will take place; but this will be in the opposite direction, since that portion of the loop which was going down through the lines of force is now going up, and the portion which was previously going up is now going down.
The law concerning the generation of electromotive force and current in a conductor that is cutting through lines of magnetic force, may be stated in another way, when the conductor is bent into the form of a loop, as in the case under consideration: Thus, _if the number of lines of force which pa.s.s through a conducting loop be varied, electromotive forces will be generated in the loop_. This will be true whether the number of lines pa.s.sing through the loop be varied by moving the loop within the field of force or by varying the field of force itself. In any case, _if the number of lines of force be increased, the current will flow in one way, and if it be diminished the current will flow in the other way_. The amount of the current will depend, other things being equal, on the rate at which the lines of force through the loop are being varied, regardless of the method by which the variation is made to take place. One revolution of the loop, therefore, results in a complete cycle of alternating current consisting of one positive followed by one negative impulse.
The diagram of Fig. 68 is merely intended to ill.u.s.trate the principle involved. In the practical construction of magneto generators more than one bar magnet is used, and, in addition, the conductors in the armature are so arranged as to include a great many loops of wire.
Furthermore, the conductors in the armature are wound around an iron core so that the path through the armature loops or turns, may present such low reluctance to the pa.s.sage of lines of force as to greatly increase the number of such lines and also to cause practically all of them to go through the loops in the armature conductor.
Armature. The iron upon which the armature conductors are wound is called the _core_. The core of an ordinary armature is shown in Fig.
69. This is usually made of soft gray cast iron, turned so as to form bearing surfaces at _1_ and _2_, upon which the entire armature may rotate, and also turned so that the surfaces _3_ will be truly cylindrical with respect to the axis through the center of the shaft.
The armature conductors are put on by winding the s.p.a.ce between the two parallel faces _4_ as full of insulated wire as s.p.a.ce will admit.
One end of the armature winding is soldered to the pin _5_ and, therefore, makes contact with the frame of the generator, while the other end of the winding is soldered to the pin _6_, which engages the stud _7_, carried in an insulating bus.h.i.+ng in a longitudinal hole in the end of the armature shaft. It is thus seen that the frame of the machine will form one terminal of the armature winding, while the insulated stud _7_ will form the other terminal.
[Ill.u.s.tration: Fig. 69. Generator Armature]
Another form of armature largely employed in recent magneto generators is ill.u.s.trated in Fig. 70. In this the shaft on which the armature revolves does not form an integral part of the armature core but consists of two cylindrical studs _2_ and _3_ projecting from the centers of disks _4_ and _5_, which are screwed to the ends of the core _1_. This =H= type of armature core, as it is called, while containing somewhat more parts than the simpler type shown in Fig. 69, possesses distinct advantages in the matter of winding. By virtue of its simpler form of winding s.p.a.ce, it is easier to insulate and easier to wind, and furthermore, since the shaft does not run through the winding s.p.a.ce, it is capable of holding a considerably greater number of turns of wire. The ends of the armature winding are connected, one directly to the frame and the other to an insulated pin, as is shown in the ill.u.s.tration.
[Ill.u.s.tration: Fig. 70. Generator Armature]
[Ill.u.s.tration: Fig. 71. Generator Field and Armature]
The method commonly employed of a.s.sociating the pole pieces with each other and with the permanent magnets is shown in Fig. 71. It is very important that the s.p.a.ce in which the armature revolves shall be truly cylindrical, and that the bearings for the armature shall be so aligned as to make the axis of rotation of the armature coincide with the axis of the cylindrical surface of the pole pieces. A rigid structure is, therefore, required and this is frequently secured, as shown in Fig. 71, by joining the two pole pieces _1_ and _2_ together by means of heavy bra.s.s rods _3_ and _4_, the rods being shouldered and their reduced ends pa.s.sed through holes in f.l.a.n.g.es extending from the pole pieces, and riveted. The bearing plates in which the armature is journaled are then secured to the ends of these pole pieces, as will be shown in subsequent ill.u.s.trations. This a.s.sures proper rigidity between the pole pieces and also between the pole pieces and the armature bearings.
The reason why this degree of rigidity is required is that it is necessary to work with very small air gaps between the armature core and its pole pieces and unless these generators are mechanically well made they are likely to alter their adjustment and thus allow the armature faces to sc.r.a.pe or rub against the pole pieces. In Fig. 71 one of the permanent horseshoe magnets is shown, its ends resting in grooves on the outer faces of the pole pieces and usually clamped thereto by means of heavy iron machine screws.
With this structure in mind, the theory of the magneto generator developed in connection with Fig. 68 may be carried a little further.
When the armature lies in the position shown at the left of Fig. 71, so that the center position of the core is horizontal, a good path is afforded for the lines of force pa.s.sing from one pole to the other.
Practically all of these lines will pa.s.s through the iron of the core rather than through the air, and, therefore, practically all of them will pa.s.s through the convolutions of the armature winding.
When the armature has advanced, say 45 degrees, in its rotation in the direction of the curved arrow, the lower right-hand portion of the armature f.l.a.n.g.e will still lie opposite the lower face of the right-hand pole piece and the upper left-hand portion of the armature f.l.a.n.g.e will still lie opposite the upper face of the left-hand pole piece. As a result there will still be a good path for the lines of force through the iron of the core and comparatively little change in the number of lines pa.s.sing through the armature winding. As the corners of the armature f.l.a.n.g.e pa.s.s away from the corners of the pole pieces, however, there is a sudden change in condition which may be best understood by reference to the right-hand portion of Fig. 71. The lines of force now no longer find path through the center portion of the armature core--that lying at right angles to their direction of flow. Two other paths are at this time provided through the now horizontal armature f.l.a.n.g.es which serve almost to connect the two pole pieces. The lines of force are thus shunted out of the path through the armature coils and there is a sudden decrease from a large number of lines through the turns of the winding to almost none. As the armature continues in its rotation the two paths through the f.l.a.n.g.es are broken, and the path through the center of the armature core and, therefore, through the coils themselves, is reestablished.
As a result of this consideration it will be seen that in actual practice the change in the number of lines pa.s.sing through the armature winding is not of the gradual nature that would be indicated by a consideration of Fig. 68 alone, but rather, is abrupt, as the corners of the armature f.l.a.n.g.es leave the corners of the pole pieces.
This abrupt change produces a sudden rise in electromotive force just at these points in the rotation, and, therefore, the electromotive force and the current curves of these magneto generators is not usually of the smooth sine-wave type but rather of a form resembling the sine wave with distinct humps added to each half cycle.
[Ill.u.s.tration: Fig. 72. Generator with Magnets Removed]
As is to be expected from any two-pole alternating generator, there is one cycle of current for each revolution of the armature. Under ordinary conditions a person is able to turn the generator handle at the rate of about two hundred revolutions a minute, and as the ratio of gearing is about five to one, this results in about one thousand revolutions per minute of the generator, and, therefore, in a current of about one thousand cycles per minute, this varying widely according to the person who is doing the turning.