It is very difficult to construct a first class dynamo without resort to
the materials and methods employed in the manufacture of commercial
machines. The necessity for careful workmanship in every detail, can
hardly be overestimated. Poor workmanship and poor materials will always
result in an inefficient machine. Telegraph instruments, toy motors,
etc., may be constructed from all sorts of odds and ends of materials,
and they will work fairly well, but in order to make a real dynamo it
will be necessary to use certain materials for which nothing can be
substituted.
[Illustration: FIG. 123—Front view of the Field Casting.]
The field casting must be soft gray cast iron and the magnet wire should
be of good quality.
Both the field and the armature of the small dynamo described in the
following pages are cast iron and patterns will be necessary in order to
form the moulds for the castings. It may be possible for several
experimenters to club together and make the patterns, or have them made,
for building this dynamo.
The dynamo described has an output of about ten watts. It may also be
used as a motor and as such will develop considerable power. The
castings for this machine are already on the market and may be obtained
from the publishers of this book.
[Illustration: FIG. 124.—Side elevation of the Field Casting.]
*The Field* is shown in Figures 123 and 124. The details in both
illustrations are fully dimensioned and probably no comment in that
direction will be necessary.
If the experimenter decides to make his own patterns he should use every
care to make certain that they are carefully and accurately made. They
should be made of wood and finished by rubbing with fine sandpaper until
perfectly smooth and then given a coat of shellac. The parts should also
be given a slight "draft" or taper toward one side so that the pattern
may be easily withdrawn from the mould.
[Illustration: FIG. 125.—Details of the Armature.]
The easiest way to bore out the "tunnel" of the field is to perform the
work on a lathe. If no lathe is handy, the work can be accomplished with
nothing more than the aid of a file and a little patience. It should be
cleaned out until it is perfectly round and measures one and
five-sixteenths of an inch in diameter.
Any rough spots on the casting should be smoothed up with a file.
*The Armature* is illustrated in Figure 125. The centre of the armature
should be bored out to fit a three-sixteenths inch shaft.
The shaft is a piece of steel rod four inches long. The outside of the
armature should be turned down to a diameter of one and one-quarter
inches, making it one-sixteenth of an inch smaller in diameter than the
tunnel in the field.
*The Commutator* is illustrated in Figure 126. It has two sections and
consists of a short piece of brass tubing fitted on a fibre core and
split lengthwise on two opposite sides so that each section is insulated
from the other.
It is not very difficult to make such a commutator. A hole is drilled
through the fibre, which fits very tightly on the shaft. The shaft is
then placed in a lathe with the fibre in position and the latter turned
down until a piece of seven-sixteenths inch brass tubing can be driven
on. The tube should be five-eighths of an inch long. Then mark two lines
along the tube at points diametrically opposite. Bore two small holes to
receive two small screws, a short distance away from each side of these
lines and on each side of them. Make certain that the screws do not go
into the fibre far enough to touch the shaft. The commutator may then be
split along each side of the lines with a hacksaw, continuing the cut
right through the brass and slightly into the insulating core. The heads
of the screws should be filed off flush with the surface of the
commutator and the latter trued up and made perfectly smooth.
[Illustration: FIG. 126.—The Commutator.]
If each section of the commutator is provided with a small brass machine
screw near the back edge as shown in Figure 126, it will greatly
facilitate connection with the ends of the armature windings.
The armature, shaft and commutator, as they should appear when assembled
are shown in Figure 127.
Those portions of the armature and shaft which will come into contact
with the armature wire should be insulated with shellaced paper. Soak
the paper in the shellac until it is soft and it can be very easily
pressed into proper shape to fit the armature. Allow the shellac to dry
and harden before winding on the wire.
[Illustration: FIG. 127.—The Armature and Commutator Assembled on the
Shaft ready for winding.]
The armature will not need to be fastened on the shaft if it is a tight
fit and cannot be twisted. If it is loose, it may be fastened by means
of a small set screw or pin. The commutator should fit the shaft very
tightly so that it will not slip or twist.
*The Armature Winding* is No. 20 B. & S. Gauge single-cotton covered
magnet wire. Sufficient wire should be put on to fill up the winding
space completely. Do not, however, put on too much wire or it will
interfere with the field magnets and prevent the armature from
revolving. Test the winding after it is finished to see that the wire is
not "grounded" or connected to the armature at any point. If the
insulation is perfect, give the winding a good coat of shellac and allow
it to dry. The ends of the winding are each connected to one of the
commutator sections as shown in Figure 127.
*The Field Winding* is No. 20 B. & S. Gauge single cotton covered wire.
The wire should be wound on in smooth, even layers, and the winding
space between the flanges completely filled up. The winding space in the
field frame should be insulated with shellaced paper by covering the
core and the flanges. The flanges are best insulated with paper disks
cut in two halves so that they will slip around the core.
[Illustration: FIG. 128.—Details of the Wooden Base.]
The details of the wooden base are shown in Figure 128. It is a
rectangular shaped piece of wood, five inches long, four inches wide and
five-eighths of an inch thick. The corners are slightly rounded.
*The Bearings* are small brass castings. They are both alike. The
details are illustrated in Figure 129. It will be necessary to make a
wooden pattern and send it to a brass foundry for castings. The castings
should be smoothed up with a file and then drilled. The shaft hole
should be three-sixteenths of an inch in diameter and the screw holes
just large enough to pass an 8-32 screw. The bearings are fastened to
the projecting arms on the field casting by means of round headed 8-32
brass machine screws. The armature should revolve exactly in the centre
of the tunnel in the field and should be free in the bearings so that it
runs easily and without binding.
*The Brushes* are illustrated in Figure 131. They are cut out of spring
copper and bent according to the shape and dimensions shown.
The field is fastened to the base by means of two large machine screws
passing upwards through the base into threaded holes in the bottom of
the casting. The brushes are bent at right angles and mounted on the
base on either side of the commutator with small round headed wood
screws. They should bear firmly against the commutator. The commutator
should be in such a position on the shaft, in relation to the armature,
that the dividing lines between the two sections are directly opposite
the centre of the iron faces of the armature as shown in Figure 127.
[Illustration: FIG. 129.—Details of the Bearings.]
[Illustration: FIG. 130.—The Pulley.]
[Illustration: FIG. 131.—The Brushes.]
The shaft should be fitted with a small grooved pulley to accommodate a
small round belt. The completed dynamo is shown in Figure 132. The
dynamo is connected in what is known as "shunt." One terminal of the
field magnet is connected to one brush and the other terminal to the
other brush. A wire is then led from each one of the brushes to a
binding post.
Before the dynamo will generate current it will be necessary to
magnetize the field by connecting the terminals to several strong
batteries and allowing the current to flow through for several seconds.
A shunt wound dynamo will only generate when run in a certain direction.
In order to make it generate when run in the opposite direction, it is
necessary to reverse the field connections to the brushes.
[Illustration: FIG. 132.—The Completed Dynamo.]
The dynamo will be found to operate as a very powerful little motor, but
on account of having only a two pole armature, it must be started when
the current is turned on by giving the shaft a twist.
The dynamo may be driven by a small water motor or from the driving
wheel of a sewing machine. It may be used as a generator for lighting
lamps, ringing bells, electroplating, etc.