266. The Commutator is in two parts, which must be insulated from each other. The 2 sections are made out of thin tin or copper in the shape of an inverted T, as shown at 10, Fig. 121. The arms of the T are about ⅜ in. wide, the horizontal ones reaching about half around the spool, E. The vertical arm reaches over the top of E, and is held down by a small screw, J. The sections, 10, must not touch the shaft. The outside wires ([§ 123]) of 12 and 16 are fastened under these screws, J, and they must not touch the shaft. Bend the tin sections so that they will be as nearly round as possible. The spool, E, has been sawed off so that it will go between the field-magnets. Wind paper around the shaft to make it fit solidly into E. S is a small screw that holds E in place, if the paper does not hold it tight enough.
| Fig. 121. | Fig. 122. |
Fig. 122 shows a section of the spool and tin sections with the brushes pressing against them. The sections do not touch each other, and the brushes touch opposite sections. It is evident, then, that the current must pass through the coils 12 and 16 in order to get from one section of the commutator to the other, provided you have no short circuits through the shaft or elsewhere. The slots in the commutator must be directly under the center line of the yoke, 14, as seen in Fig. 121.
267. The brushes, 9 and 19, Fig. 120, are made of very thin tin or copper. They are cut to the shape shown, the narrow part being about ⅛ in. wide, and long enough to reach at least to the center-line of the apparatus. The foot, or bottom part of the brushes, should be about 1¼ × ¾ in. These are used to fasten them to the base and to make connections. If you have no thin metal for brushes, use copper wires, and arrange them so that they will press gently against the commutator.
268. Connections. The inside ends ([§ 123]) of the field-magnets are held at 4. The outside end of coil 2 is joined to X, and that of coil 6 to 8, the foot of the brush which presses against 10. The section, 10, of the commutator is joined to 11, the outside end of coil 12, its inside end being fastened to the inside end of coil, 16, either by twisting them together, or by fastening them under the top nuts of the armature yoke, 14. The outside end of coil 16 is joined to the other commutator section, 18. The brush, 19, completes the circuit. In the foot of 19 is the binding-post, Y.
If the current enters at X, it will pass through 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and out at Y, provided 10 and 18 are in contact with 9 and 19. Be careful not to have any short circuits. If, for example, the wire 7 touches 4, or if 3 touches 8, or if the wires 11 and 17 touch the shaft, your current will not pass where you expect, and you will have trouble.
269. Adjustment. The armature cores should just clear the poles of the field-magnets as they turn. This must be regulated by the thickness of A and the position of the nuts on the shaft, S B. The slots in the commutator must be under the center of the yoke, 14. The brushes, 9 and 19, must touch 10 and 18, but not so hard that they will stop the motor. Wire brushes are more easily adjusted than tin or sheet-copper ones. The tin arm, T, must hold the shaft properly. The point of the shaft must allow it to turn easily. The motor will turn clockwise if the attachments are made as shown. Use 1 or 2 good bichromate cells, like [App. 3] or [4].
270. Operation. The current will pass through the field-coils in the same direction, as long as the battery wires are not changed. The current is reversed in the armature-coils every time the brushes change from one section to the other of the commutator; that is, it flows in one direction during one-half of a revolution, and in the opposite direction during the other half. This reverses the poles of the armature-magnets every ½ revolution. (See text-book for full explanations and for simple experiments with electric motors.)
271. Attachment for Winder. In winding small electro-magnets for armature, etc., in which cores are used that are not 5⁄16 in. in diameter, your winder will have to be slightly changed. Its 5⁄16 stove-bolt will have to be removed, and a ¼ in. one put in instead. This may be done by making a handle for the ¼ in. bolt. To keep this from wobbling in the 5⁄16 hole, wind stiff paper around the bolt until it fits quite tightly. The whole winder is explained as [App. 93].