The continuous current and the alternating current generators invented
by Dr. J. Hopkinson and Dr. Alexander Muirhead are peculiarly
interesting as being probably the first in which the bobbins of the
armature were wound with copper ribbon and arranged on a disk armature
much in the same way as was afterward done by Sir William Thomson and by
Mr. Ferranti. In the Muirhead-Hopkinson machine the armature coils are
attached to a soft iron ring, whereas in the Ferranti the iron core is
dispensed with, and a gain of lightness in the armature or rotating part
effected; this advantage is of considerable importance, though Messrs.
Hopkinson and Muirhead can of course reduce the weight of this iron core
to insignificant proportions.
[Illustration: HOPKINSON & MUIRHEAD'S DYNAMO-ELECTRIC GENERATOR.]
The general form of this generator is clearly shown by the side and end
elevation.
The armature is made by taking a pulley and encircling it with a rim of
sheet-iron bands, each insulated from the other by asbestos paper. On
one or both sides of the rim thus formed, radial slots are cut to admit
radial coils of insulated copper wire or ribbon, so that they lie in
planes parallel to the plane of the pulley. In the continuous current
machine coils are placed on both sides of the iron rim and arranged
alternately, that on the one side always covering the gap between two on
the other side. In this way, when a coil on one side of the rim is at
its "dead point" and yields its minimum of current, the corresponding
coil on the other side is giving out its maximum.
The field magnets are made in a similar manner to the armature and run
in circles parallel to the rim of the latter. The cores may be built up
of wrought iron as the rim of the armature is; but it is found cheaper
to make them of solid wrought or cast iron. To stop the local induced
currents in the core, however, Messrs. Muirhead and Hopkinson cut
grooves in the faces of the iron cores, and fill them up with sheet-iron
strips insulated from each other, similar to the sheet-iron rim of the
armature.
The coils, both in the armature and electro-magnets, are packed as
closely as they may to each other, and have thus a compressed or
quadrilateral shape. The arrangement is shown in Figs. 1 and 2, which
represent, in side view and plan, the armature pulley with the soft iron
rim and coils attached. There a is the pulley which is keyed to the
shaft of the machine, and is encircled with bands of sheet iron, b,
insulated from each other by ribbons of asbestos paper laid between
every two bands. When the rim has been built up in this way, radial
holes are drilled through it from the outer edge inward, and the whole
rim is bound together by bolts, d, inserted in the holes and secured by
cottars, e. Radial slots are then cut on each side of the rim all round,
and the coils of wire mounted on them.
Figs. 3 and 4 show the armature of the continuous current dynamo, with
the coils on one side of the rim, half way between the coils on the
other side, so as to give a more continuous current. In the alternating
current machine the slots on the opposite faces are face to face.
Figs. 5 and 9 illustrate the complete continuous current machine, Fig.
9 showing the internal arrangement of the field magnets, and Fig. 5 the
external frame of cast iron supporting them. In these figures a is the
armature already described, b b are the cores of the electro-magnets
with a strong cast iron backing, c c; d d are the exciting coils or
field magnets, so connected that the poles presented to the armature are
alternately north and south, thus bringing a south pole on one side of
the armature opposite a north pole on the other side.
The commutator, e, is arranged to prevent sparking when the brushes
leave a contact piece. This is done by splitting up the brushes into
several parts and inserting resistances between the part which leaves
the contact piece last and the rest of the circuit. This resistance
checks the current ere the final rupture of contact takes place.
Figs. 6 and 7 will explain the structure of the commutator. Here a a a
are the segments or contact pieces insulated from each other, and b' b
b are the collecting brushes carried on a spindle, c c'. One of these
brushes, b', is connected to the spindle, c, through an electrical
resistance of plumbago, arranged as shown in Fig. 7, where d e are metal
cylinders, d being in contact with the brush, b', while e is in contact
with the spindle, c. The space, f, between these two cylinders, d e, is
filled with a mixture of plumbago and lampblack of suitable resistance,
confined at the ends by ivory disks. The brush, b', is adjusted by
bending till it remains in contact with any segment of the commutator
for a short time after the other brushes have left contact with that
segment, and thus instead of sudden break of circuit and consequent
sparking, a resistance is introduced, and contact is not broken until
the current has been considerably reduced.
The contact segments are supported at both ends by solid insulating
disks; but they are insulated from each other by the air spaces between
them, where the brushes rub upon them.
The alternating current dynamo of Drs. Hopkinson and Muirhead differs
little in general construction from that we have described; except that
the commutator is very much simplified, and the armature bobbins are
placed opposite each other on both sides of the rim. Instead of forming
the coils into complete bobbins, Dr. Muirhead prefers to wind them in a
zigzag form round the grooved iron rim after the manner shown in Fig. 8,
which represents a plan and section of the alternating current armature.
This arrangement is simpler in construction than the bobbin winding, and
is less liable to generate self-induction current in the armature. Sir
William Thomson has adopted a similar plan in one of his dynamos. In
Fig. 8, a is the pulley fixed to the spindle of the machine, b b is
the iron rim, and c c are the zigzag coils of copper ribbon. The field
magnets are also wound in a similar manner.
It will be seen from our description that Drs. Hopkinson and Muirhead
have scarcely had sufficient credit given them for this interesting
machine, which so closely approximates to the Ferranti. One of their
alternating dynamos has been built, and was shown at the Aquarium
Exhibition. It works well, and is capable of supporting 300 Swan lights,
while in size and appearance it resembles the Ferranti machine in a
very striking manner. Drs. Muirhead and Hopkinson have also designed
a magneto-electric alternating current machine; but as it closely
resembles the machines described, with the exception that permanent
magnets are employed as field magnets, we need not dwell upon it
further.--_Engineering_.
* * * * *