AN electric motor has two essential parts. These are a field magnet for
furnishing a strong magnetic field and a revolving armature.
It is an easy matter to make a strong electric motor suitable to operate
on batteries by the exercise of a little careful workmanship.
The field frame and armature of the motor shown in Figure 133 are
laminated, that is, built up of separate sheets of iron. They may be
made out of sheet tin or ordinary stove pipe iron. The cheapest and
simplest method of securing good flat material is to get some old scrap
from a tinner’s or plumbing shop.
*The Details of the Field* are shown in Figure 134. The exact shape and
dimensions can be understood by reference to the illustration. Lay out
one lamination very carefully as a pattern. Cut it out and smooth up the
edges, making certain that it is perfectly true to size and shape. Then
use it as a template to lay out the other laminations by placing it on
the metal and scribing a line around the edges with a sharp pointed
needle. Enough laminations should be cut out to make a pile five-eighths
of an inch high when tightly pressed together.
[Illustration: FIG. 133.—The completed Electric Motor.]
*The Armature* is made in exactly the same manner as the field frame,
that is, by cutting out a pattern according to the shape and dimensions
shown in Figure 136 and using it as a template to lay out the other
laminations. Enough should be cut to make a pile five-eighths of an inch
high when tightly squeezed together.
[Illustration: FIG. 134.—Details of the Field Frame.]
[Illustration: FIG. 135.—The Assembled Field ready for Winding.]
The armature is one and three-sixteenths inches in diameter. The hole in
the field frame which accommodates the armature is one inch and
one-quarter in diameter so that there is a space in between for the
armature to revolve in.
The hole through the center for the shaft should be of such diameter
that the laminations will force very tightly on a shaft one-eighth of an
inch in diameter. The laminations should be very carefully flattened and
then forced over the steel shaft which is two and one-eighth inches
long. Clean up all the rough edges with a file and smooth the outside so
that it will revolve properly in the field without scraping.
[Illustration: FIG. 136.—Details of the Armature Lamination.]
[Illustration: FIG. 137.—The Armature assembled on the Shaft ready to
Wind.]
Figure 137 illustrates the armature assembled on the shaft and ready to
be wound.
*The Armature Windings* consist of four layers of No. 22 B. & S. Gauge
double cotton covered magnet wire wound around each leg. The iron should
be very carefully insulated with shellaced paper before the wire is put
in position so that there will not be any danger of short circuit due to
the sharp edges of the metal cutting through the insulation. Each leg
should contain the same number of turns of wire and all should be wound
in the same direction.
The Commutator is illustrated in Figure 138. It consists of a piece of
brass tubing seven-sixteenths of an inch long, five-sixteenths inside
and three-eighths of an inch outside. It should be forced onto a piece
of fibre five-sixteenths of an inch in diameter and seven-sixteenths of
an inch long. Split the tube into three equal parts by dividing it
longitudinally with a hack-saw. Make a fibre ring which will force onto
the tube very tightly when it is in position on the fibre core and so
hold the three commutator sections firmly in position. The sections
should be so arranged that there is a small space between each two and
they are perfectly insulated from each other. The fibre core should have
a one-eighth inch hole through the centre so that it may be forced
tightly onto the shaft and up against the armature after the windings
are in position. The commutator should be in such a position that the
split between each two sections come directly opposite the centre of
each winding. Suppose that the windings are lettered A, B, and C, the
commutator section between A and B is numbered 1, that between A and C
is No. 2, and the one between C and B is No. 3. Then the inside terminal
of B is connected to the outside terminal of A and soldered to the end
of commutator section No. 1 close to the winding. The inside end of B is
connected to the outside terminal of C and to commutator section No. 2.
The inside end of winding C is connected to the outside of B and to
commutator section No. 3. The connection of the armature windings to the
commutator are represented by the diagram in Figure 139.
[Illustration: FIG. 138.—The Commutator.]
[Illustration: FIG. 139.—Diagram showing how the Armature Coils are
connected to the Commutator Sections.]
[Illustration: FIG. 140.—The Bearings.]
*The Field Winding* consists of five layers of No. 18 B. & S. double
cotton covered wire. A much neater job may be made of this part of the
work if two fibre heads are cut to slip over the field and support the
ends of the winding as shown in the illustration in Figure 133.
*The Bearings* are illustrated in Figure 140. They are made out of
three-eighths inch brass strip one-sixteenth of an inch thick by bending
and drilling as shown in the illustration. The location of the holes is
best understood from the drawing. The larger bearing is assembled on the
field at the side towards the commutator.
Assembling the motor is a comparatively easy matter if it is done
properly and carefully. The bearings are mounted on the field frame by
screws passing through the holes B and B into a nut on the outside of
the bearing at the opposite side of the field.
The armature should revolve freely without binding and without any
danger of scraping against the field. Slip some small fibre washers over
the ends of the shaft between the armature and the bearings so as to
take up all end play.
*The Brushes* are made of spring copper according to the shape and
dimensions shown in Figure 141. They can be cut out with a pair of
snips.
Each brush is mounted on a small fibre block supported on the large
motor bearing. The holes marked A and C in the illustration should be
threaded with a 4-36 tap. The hole B should be made one-eighth of an
inch in diameter and drilled all the way through the block.
The holes A and C are used to fasten the blocks to the bearing. The
brushes are fastened to the blocks by means of a 6-32 screw with a nut
on the lower end.
*The Base* is a rectangular block, three inches wide, three and one-half
inches long and three-eighths of an inch thick. The motor is fastened to
the base by four small right angled brackets bent out of strip brass and
secured to the field frame by two machine screws passing through the
holes H and H into a nut at the opposite end.
[Illustration: FIG. 141.—The Brushes.]
One terminal of the field winding is connected to a binding post mounted
on the base. The other terminal of the field is connected to the right
hand brush. The end of the wire should be placed under the head of the
screw which holds the brush to the fibre block. The brush should be on
the under side of the block so that it bears against the under side of
the commutator.
The left hand brush bears against the upper side of the commutator and
is connected to a second binding post on the base of the motor. This
makes it a "series" motor, that is, the armature and the field are
connected in series.
[Illustration: FIG. 142.—The Fibre Block for supporting each Brush.]
The motor is now ready to run. Put a drop of oil on each bearing and
make certain that the curved portion of the brushes bear firmly against
the centre of the commutator on opposite sides. The armature having
three poles, should start without assistance and run at high speed as
soon as the current is applied. Two cells of dry or other battery should
be sufficient. The motor may be fitted with a small pulley so that its
power may be utilized for driving small models.