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 space between the two parallel faces 4 as full of insulated wire as space 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 bushing 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. IMG:384091999325875926_fig069_t.gif.png:Illustration_ Fig. 69. Generator Armature Fig. 69. Generator Armature View full size illustration. Another form of armature largely employed in recent magneto generators is illustrated 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, po e es distinct advantages in the matter of winding. By virtue of its simpler form of winding space, it is easier to insulate and easier to wind, and furthermore, since the shaft does not run through the winding space, 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 illustration. IMG:384091999325875926_fig070_t.gif.png:Illustration_ Fig. 70. Generator Armature Fig. 70. Generator Armature View full size illustration. IMG:384091999325875926_fig071_t.gif.png:Illustration_ Fig. 71. Generator Field and Armature Fig. 71. Generator Field and Armature View full size illustration. The method commonly employed of a ociating the pole pieces with each other and with the permanent magnets is shown in Fig. 71. It is very important that the space 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 rods 3 and 4 , the rods being shouldered and their reduced ends pa ed through holes in flanges 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 illustrations. This a ures 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 nece ary to work with very small air gaps between the armature core and its pole pieces and unle these generators are mechanically well made they are likely to alter their adjustment and thus allow the armature faces to scrape 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 ing from one pole to the other. Practically all of these lines will pa through the iron of the core rather than through the air, and, therefore, practically all of them will pa 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 flange will still lie opposite the lower face of the right-hand pole piece and the upper left-hand portion of the armature flange 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 ing through the armature winding. As the corners of the armature flange pa 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 flanges 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 flanges are broken, and the path through the center of the armature core and, therefore, through the coils themselves, is reëstablished. As a result of this consideration it will be seen that in actual practice the change in the number of lines pa ing 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 flanges 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. IMG:384091999325875926_fig072_t.gif.png:Illustration_ Fig. 72. Generator with Magnets Removed Fig. 72. Generator with Magnets Removed View full size illustration. 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 you sand revolutions per minute of the generator, and, therefore, in a current of about one you sand cycles per minute, this varying widely according to the person who is doing the turning. The end plates which support the bearings for the armature are usually extended upwardly, as shown in Fig. 72, so as to afford bearings for the crank shaft. The crank shaft carries a large spur gear which meshes with a pinion in the end of the armature shaft, so that the user may cause the armature to revolve rapidly. The construction shown in Fig. 72 is typical of that of a modern magneto generator, it being understood that the permanent magnets are removed for clearne of illustration. Fig. 73 is a view of a completely a embled generator such as is used for service requiring a comparatively heavy output. Other types of generators having two, three, or four permanent magnets instead of five, as shown in this figure, are also standard. IMG:384091999325875926_fig073_t.gif.png:Illustration_ Fig. 73. Five-Bar Generator Fig. 73. Five-Bar Generator View full size illustration. Referring again to Fig. 69, it will be remembered that one end of the armature winding shown diagrammatically in that figure, is terminated in the pin 5 , while the other terminates in the pin 7 . When the armature is a embled in the frame of the generator it is evident that the frame itself is in metallic connection with one end of the armature winding, since the pin 5 is in metallic contact with the armature casting and this is in contact with the frame of the generator through the bearings. The frame of the machine is, therefore, one terminal of the generator. When the generator is a embled a spring of one form or another always rests against the terminal pin 7 of the armature so as to form a terminal for the armature winding of such a nature as to permit the armature to rotate freely. Such spring, therefore, forms the other terminal of the generator.