Download Component Parts of a Dynamo

Component Parts of a Dynamo
We have seen that a dynamo consists of two
essential parts: the field magnet which produces the
magnetic field, and the armature which carries the
conductors, which, by their motion in the magnetic
field, have e.m.f. s induced in them. Also, in the case
of a direct-current dynamo, a commutator is
required in order to rectify the alternating e.m.f. s
induced in the armature conductors.
Construction of the Field Magnet. Except for
very small machines, all modern direct-current
dynamos are of multi-polar construction, that is,
they have more than two poles. Of the many forms
which were in common use some years ago only
one has survived, and that is the circular ring or yoke, provided with inwardly
projecting poles, as shown in Fig. 21. In this figure Y is the yoke, which is usually
divided on a horizontal diameter, the two halves being held by bolts B. Each
pole P carries an exciting coil, and is fitted with an extension S called the
pole shoe. In the figure the dotted lines indicate the paths of the magnetic fluxes,
and it will be seen that the flux from each pole divides into two halves after it has
entered the armature, and also in the yoke, the cross sections of the armature
core and yoke, therefore, only carrying one-half of the total flux per pole.
Materials Used.— The materials used in the construction of the field
magnets are cast iron or cast steel for the yokes of small machines, rolled steel fo r
large machines, the cho ice of mater ial depending on its cost, its magnetic
properties, and on the cost of working it to the required shape, e.g. whether
machining is necessary or not. The poles may be cast steel or wrought iron,
although laminated poles are also in common use. The cost of cast steel suitable
for dynamo construction is more than twice the cost of the same weight of cast
iron, but it has a permeability at least twice as great as cast iron, which means
that to carry the same total magnetic flux, only one -half the cro ss sectio n is
r eq uired. Thus the weight o f cast steel r eq uired will only be about half that
of cast iron, so that from the point of view of the cost of these materials there is
little to choose. The above considerations apply to the yoke. In the case of the
poles there is another consideration, namely, the cost of the copper in the
exciting coils, this obviously depending on the length of a mean turn and, there fore,
on the cross section of the pole. This consideration rules out cast iron for any
but small machines, the poles of medium-sized and large machines being either
cast steel or of laminations. If steel poles are used, the poles and yoke can be cast
together, and also the pole cores can be of circular cross section, which reduces
the amount of field copper to a minimum, since the circle is the shape which
has the minimum perimeter for a given area. If the armature has wide slots the
tufting of the flux into the tops of the teeth may cause such variations in magnetic
flux density in the air gap that excessive eddy currents may be set up in the pole
faces, if these are solid. In such a case laminated poles can be used with
advantage. It is quite common to employ laminated poles for small as well as large
machines. Sometimes the pole core is solid, and only the pole shoe laminated,
the shoe being secured to the pole core by means of countersunk screws.
It is unnecessary to deal at length with the exciting coils. In small sizes it is
usual to wind the coils on a former, and to interlace the different layers with
tape, so that, when the former is removed the coil is self supporting. The ends
and inside are then covered with paper or other insulating material, depending on
the voltage to frame which the coil has to withstand, and the whole coil is finally
wrapped all over with tape. For large machines it is usual t o wind the coil on to a
bobbin, which may be either of metal or of some insulating material.
A common, and annoying, fault with shunt coils, which are wound with smallsection wire, is for the lead -in to the bottom layer of wire to break off close to
the coil*. To obviate this it is a good plan to solder a strip of copper to the
first turn at the bottom, bringing out this strip between the coil and the inner
face of the bobbin (if one is used). The voltage between this strip and the
nearest turn on the outer layer of the coil will be practically the whole of the
voltage across the coil, and in the event of the field circuit being broken when carrying
the full current, it may reach a very high value. It is, therefore, advisable to
insulate this strip with mica.
Series coils consist of comparatively few turns of thick wire, or for heavycurrent work, of copper strip wound on edge.
Construction of the Armature Core.— The functions of the armature core
are twofold: first, to provide a path of low magnetic reluc tance to the magnetic
lines of force; second, to act as a rigid struc ture on which the armature winding
is secured. In order that it may provide an easy path for the magnetic lines of
force it must, of course, be composed of iron, and since iron is a cond uctor of
electricity, it must be built up of thin discs, that is, it must be laminated. The
reason for this is as follows: when the core is rotated it cuts the lines of force,
and in consequence has e.m.f.'s induced in it in the same direction as the e.m.f's
induced in the armature conductors, namely, in an axial direction. If the core
were solid it would form a closed path of very low electrical resistance, so that
heavy parasitic currents, called eddy currents, would be set up in it. Very serious
heating and loss of power would result. It is impossible to reduce these eddy
currents to zero, but the loss caused by them can be made very small by building
up the core of thin laminations lightly insulated from one another by varnish or
by the oxide on their surface. This lamination of the core being carried out in a
plane perpendicular to the direction of the induced e.m.f' results in a very small
e.m.f. being induced in each core disc; the induced e.m.f. per disc is obviously
proportional to the thickness of the disc. Also the lamination splits up the eddy
currents, which are now confined to a small path through each disc, the result
being that the loss of power due to this cause is reduced to very small
proportions. The core discs are usually from 16 to 25 mil s in thickness.
In all modern machines the armature winding is housed in slots at the
periphery of the core.
М.А. Беляева и др. «Сборник технических текстов на англ. языке»