ELECTRIC MOTORS.
_249. An Electric Motor_ is really a machine. If it be supplied with a
proper current of electricity, its armature will revolve; and, if a
pulley or wheel be fastened to the revolving shaft, a belt can be
attached, and the motor made to do work. There are many kinds of motors,
and many simple experiments which aid in understanding them. All that
can be done here, however, is to show how to make simple motors. (See
text-book for experiments.)
APPARATUS 144.
_250. Electric Motor._ Fig. 116, 117. Fig. 116 shows a plan or top view,
and Fig. 117 shows a side view, with a part of the apparatus removed,
for clearness.
The base, B, is 5 x 4 x 7/8 in. The upright, U, is 3-1/2 x 1-1/2 x 1/2
in., and is nailed or screwed to B. The binding-posts, X and Y are like
App. 46. 4 is a screw binding-post.
251. The Field-Magnets, as the large electro-magnets on a motor are
called, are made of 5/16 machine-bolts, 2-1/2 in. long. The washers are
1-1/2 in. apart inside. (See App. 88 for full directions.) The bolt
cores are 2 in. apart, center to center. (See App. 89.) The tin yoke, D,
is made like App. 71, and it is fastened to the base, like App. 90. The
hole for the screw, however, is made a little to one side of the center,
so that a dent can be made at the center for the bottom of the shaft, 8,
to turn in. Make the dent with a center punch. The yoke is fastened to
B, so that one edge of it is 1-1/2 in. from the back edge of B. (Fig.
116).
252. The Armature, A, is made of 6 or 8 thicknesses of tin, 2-1/2 in.
long and 3/4 wide. (See App. 71.) In its center is punched or drilled a
1/4 in. hole, so that it can be slipped onto the 1/4 in. "sink-bolt," 8.
If you have taps you can make the hole a little smaller than 1/4 in.,
and thread it so that it will screw onto 8. A must be heavy enough to
revolve a few times when once started. It is pinched between two nuts, 9
and 11, so that it just clears the poles when it turns. (See App. 145
for another form of armature.)
[Illustration: Fig. 116.]
253. The shaft or axle, 8, is made of a "sink-bolt" that is 3 in. long
and 1/4 in. in diameter. These sink-bolts are threaded over their entire
length, and are furnished with two nuts, 9 and 11, Fig. 117. File or
grind the end of 8 to a point, so that it will turn easily in a dent
made for it in the yoke, D, or in a dent made in another piece of tin
fastened over the yoke. The shaft is held in a vertical position by the
arm, C.
254. The Arm, C, is made of 2 or 3 thicknesses of tin. It is 3 x 3/4
in.; it has in one end a hole for the shaft to revolve in easily, and in
its other end a slot is cut. A screw-eye and bur are used to hold C to
the upright, U. By this means the shaft can be moved and regulated as to
position.
[Illustration: Fig. 117.]
255. The Commutator, 9, (Fig. 117), is made of one of the nuts furnished
with the shaft. Two of its corners are filed or ground off, so that it
has the shape shown at the right, in Fig. 117. The copper wire, 10, rubs
against 9, as the pointed part of it comes around. 10 is really a
"brush," and carries the current into 9 at the right time.
256. Connections. Join the two inside ends (Sec. 123) of the coils to 4.
The outside end of 2 is joined to X; the outside end, 7, of the other
coil, 6, is carried up under or around the screw-eye, S I, and then its
bare end reaches out and gently scrapes against the top of the shaft, 8.
The wire, 10, leads from Y to the back of the base, where it is carried
up to a screw, 12, which holds it to U. Its bare end reaches out to
gently scrape against the commutator, 9, when it swings around. This
wire, 10, should not press against 9 during the entire revolution.
257. Adjustment. Suppose the current enters at X. When the "brush," 10,
presses against the commutator, 9, the current passes through X, 1, 2,
3, 4, 5, 6, 7, down 8 to 9, and out through 10 to Y. (The current, of
course, goes down into D and into the bolt-cores also; but it can go no
farther, if the coils are properly insulated, and A is not allowed to
touch the cores. It is better to have the end of the shaft rest upon a
piece of glass, having a slight depression made with a file, or in a
dent made in tin which rests upon wood, the tin having no connection
with D.) If 10 should continue to press against 9, the current would
continue to pass, and A would be held firmly in place, directly over 2
and 6, and, of course, the shaft could not revolve. If, however, the
brush leaves 9 (See plan of 9 at side of Fig. 117), just as A gets over
the coils, or an instant before it gets there, the weight of A will
carry it beyond the coils. No current should pass again, until A is at
least at right angles to a line drawn through the center of the coils.
If the current again passes, the ends of A will be attracted by the
bolt-cores.
In other words, the current should pass a little less than one-half of
the time, and this is divided into two parts. Suppose you start A with
your finger; the current should be shut off automatically just before
the center of A gets over the center of the bolt-cores. A makes 1/4 of a
revolution without current, and just after it gets beyond this, the
current passes for nearly 1/4 of a revolution, which brings the ends
over the poles again. The next 1/4 of a turn it has no current, because
the flat side of 9 is opposite the brush, 10, as during the first 1/4.
The last 1/4 the current passes again. The exact position of the
commutator will depend upon the way you arrange the brush. The positions
of 9 and 10 can be found by trial, so that the circuit will be promptly
opened and closed at the proper moment. Start the motor by turning the
armature.
258. Batteries. The amount of power needed will depend upon how well you
make the motor. One cell of App. 3 or 4 will run a well made one, but it
is better to use 2 cells. Join the wires to X and Y.
[Illustration: Fig. 118.]
APPARATUS 145.
_259. Armature for Motors._ Fig. 118 shows another form of armature that
may be used for small motors like App. 144; in fact, you may find that
this form is easier to make than that of App. 144. M is a 5/16 machine
screw, 1-1/2 in. long, 9 being the nut furnished with it. 9 is filed as
explained in Sec. 255, and forms the commutator. C is the arm (Sec.
254). A is the armature (Sec. 252). A is held firmly in place between
the spool, E, and 9. S is a set-screw which passes through E, and holds
the piece of 1/4 in. dowel, F, in place. N is a needle-point fastened
in the end of F. N revolves in a dent made in a piece of tin, H, which
rests upon a wooden strip, G. G is cut away on its underside, so that
it will straddle the yoke, D, Fig. 117; it is nailed to the base. This
is given as a suggestion. By making F a little longer, N can turn in a
dent made in the yoke, below G.
260. Adjustments. M, being 5/16 in. in diameter, will screw solidly into
the hole in E. Place 9 upon it first, then A, and screw it about 1/2 way
into E. 9 will serve as a lock-nut by turning it so that it will pinch A
and hold it firmly against the top of E. F should reach half way into E.
Put N in place after you have H and G arranged. You can then cut the
upper end of F at such a place that it will bring A about 1/8 in. from
the top of the magnet-cores. Paper wrapped around F will make a good fit
in E. The current should enter M and leave 9, as fully explained in App.
144. (See Sec. 257).
APPARATUS 146.
_261. Electric Motor._ Fig. 119, 120, 121, 122. Fig. 119 shows a front
view, and Fig. 120 a side view of the whole motor. Fig. 121 shows the
part that revolves, and includes the shaft, armature and commutator.
Fig. 122 shows a section of the commutator. All the dimensions are taken
from a model. You can modify the size to suit.
262. Wood-work. The base is 7 x 5 x 7/8 in. The uprights, U, are 3-1/2 x
1 x 3/4 in. They are screwed or nailed to the base from below, their
1-in. sides being towards you in Fig. 119. They are 4-1/4 in. apart,
inside, in this model. The piece, A, is 2-1/2 x 7/8 x 5/8 in., and is
cut away on the underside to straddle the yoke. Fig. 118 is a
suggestion as to its shape. A is screwed or nailed to B.
263. Tin-work. The horizontal arm, T, is made of 3 thicknesses, and
holds the shaft in a vertical position. T is 6-1/4 x 3/4. In its ends
are slots, and in its center is a hole so that the 1/4 in. shaft can
revolve easily, but not too loosely. The slots allow an adjustment, the
screws, S, holding T to U. The shaft rests in a dent made in a piece of
tin which is tacked to A. The yokes are elsewhere described.
[Illustration: Fig. 119.]
264. Field-Magnets. In this model they were made of 5/16 bolts, 2 in.
long, placed 2 in. apart center to center. The washers are 1-1/8 in.
apart inside. (See App. 88 for full directions.) App. 89 and 71 should
be studied. Except in size, they are made as in App. 144. They have 8
layers of No. 24 or 25 wire.
265. The Armature, Fig. 121, on this style of motor consists of a
regular horseshoe electro-magnet, made in the same general way as the
field-magnets. The electro-magnets, 12 and 16, are smaller, however,
than the field-magnets. The cores are 1/4 in. stove-bolts, 1-1/4 in.
long under the head. They are placed 2 in. apart, center to center. They
are insulated and wound as fully explained in App. 88. These 1/4 in.
bolts require a change in your winder. (See App. 147 for this.) If you
wish to use 5/16 bolts, you may use the same axle for your winder as
before. The washers are 5/8 in. apart, inside. The cores are wound with
4 or 6 layers of No. 24 or 25 wire. This makes them about 3/4 in. in
diameter. They are held in a tin yoke, 14, made of 5 or 6 thicknesses of
tin. 14 is 3 x 3/4 in., and has 3 holes punched in it. The two outside
holes are 2 in. apart. Through these pass the bolts, which are held
firmly by the 2 nuts. The shaft, S B, is a sink-bolt, 3 in. long, and
1/4 in. in diameter. (See Sec. 253.) The inside ends (Sec. 123) of the
coils should be firmly twisted together or held under the top nuts to
make a good connection between them.
[Illustration: Fig. 120.]
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
3/8 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 (Sec. 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.
[Illustration: Fig. 121.]
[Illustration: 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 1/8
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
1-1/4 x 3/4 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 (Sec. 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 1/2 revolution. (See text-book for full
explanations and for simple experiments with electric motors.)
APPARATUS 147.
_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 1/4 in. one put in instead.
This may be done by making a handle for the 1/4 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.