Зображення сторінки
PDF
ePub

THE ARMATURE.

Fig. 66 shows the drum form of armature (ThompsonHouston).

Fig. 65 shows the drum armature (Edison).

Fig. 67 represents the uncompleted form of the drum armature and Fig. 68 the same in order for use.

Fig. 69 represents the toothed core-disk of which drum armatures are largely built up. These have two advantages over smooth armatures. (1.) The teeth present an excellent means of driving the copper conductors which lie between them. (2). The teeth may be brought very close to the polar surfaces of the field-magnet.

To set against these real advantages are the disadvantages of somewhat greater labor required in milling out the channels between the teeth of the assembled core; the extra difficulty of insulating the core from the conductors; and the liability of the teeth to set up eddy-currents in the polar faces. The latter can be cured by making the teeth numerous and narrow, also by laminating the polar faces with grooves, and by enlarging the clearance. Or, best of all, by finally serving the entire armature outside the copper conductors with a layer of iron wire. Armatures built up of toothed core-disks have been much used in recent years, particularly for motors. The advantages offered by toothed core-disks are possessed to a still higher degree by core-disks pierced with apertures, for the purpose of ventilation, just within the periphery.

Fig. 70 represents the ring or gramme armature. The arrangement of the lines of force in the magnetic field

THE ARMATURE.

between the two poles N and S when the ring is inserted therein, is shown by dotted lines in the figure. If the armature core be rotated in the direction of the arrow, the whole of the conductors, being immovably fixed to the armature,

FIG. 70.

So

ww

DIAGRAM OF RING ARMATURE.

must necessarily partake of the movement. Owing to this peculiar arrangement a very intense magnetic field is created between the outer surface of the armature core and the polefaces, while the interior space within the core remains almost entirely free from lines of force.

THE ARMATURE.

A broad distinction may be set up between wire wound armatures and those with built up coils consisting of bars and

connectors, or of specially constructed together instead of being wound on.

I. Wire wound armatures.

2. Bar armatures.

portions that are put These are classified as:

Wire-wound armatures are usual for outputs below 100 amperes, including all arc-lighting machines. For armatures having outputs exceeding 200 amperes bar-armatures are more frequent, owing to the inflexible nature of wires that are thick enough to carry these currents. The two classes comprise several varieties as under :

[blocks in formation]

The armatures of alternators may be of ring, drum, pole, or disk type; but in all cases the grouping of the windings is different from that which would be adopted for a direct-current dynamo.

The ring armature is considered the most desirable, because it is not so liable to give out; and if it does, the cost of repairing is less than for a drum armature, and the time lost on account of the damage is less, as a ring armature may

FIG. 76.

[ocr errors]

THE ARMATURE.

be put in service as soon as the repairs are made; and again when an armature short-circuits, it seldom burns out more than two or three coils. If it is of the ring type the coils can be readily removed and new ones wound in their places, and if the workman is sufficiently skilled he will not have to disconnect the commutator to make the repairs; therefore, the cost of replacing the damaged coils will be small. With a drum armature the case is quite different.

[graphic]

KEY WAY.

Armature Cores.-The cores of all practical armatures are now invariably laminated or constructed of iron wire ribbons or disks and are frequently constructed with deep channels or

[merged small][graphic][merged small]

grooves in the outer periphery, as shown in Fig. 72, in which the conductors are wound. The projections or teeth in this method of construction present an excellent means of driving

THE ARMATURE.

and protecting the conductors, but the difficulty of insulating the latter from the core is increased by their use, and they also have a tendency to produce eddy currents in the pole pieces of the field magnets, causing a heating of the latter. The latter disadvantage can, however, be obviated to a great extent by making the teeth very narrow and numerous.

In most cases notches or keyways are stamped on the inner periphery on the disks as in Fig. 71.

[merged small][graphic][merged small][ocr errors]

This core is built up of thin plates of sheet iron of the shape shown at B, Fig. 73. Three of them form a perfect ring with the teeth projecting from the circumference. Upon the

inner surface are two hooks which enclose the bolts of the spider. In order to break joints the segments are placed over alternate spaces. This method of building up the core and securing the segments binds the whole firmly together. The

« НазадПродовжити »