TEXT 17. CONSTRUCTION OF THE FIELD MAGNET
The materials used in the construction of the field magnetsare cast iron or cast steel for the yokes of small machines, rolled steel for large machines, the choice of material 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 cross section is required. Thus the weight of cast steel required 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, therefore, 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. 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 withpaper 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 to 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 small-section 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.
Construction of the Armature Core.— The functions of the armature core are twofold: first, to provide a path of low magnetic reluctance to the magnetic lines of force; second, to act as a rigid structure 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 conductor 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's 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 mills in thickness.
In all modern machines the armature winding is housed in slots at the periphery of the core.
Notes
yoke -скоба(для скріплення)
to work (wrought) - приводити в дію
laminated -листовий; розщеплений
cast - форма; зразок; тип
permeability -проникливість
slot -щілина, паз
gap -зазор
to eddy -вирувати, крутитися
screw -гвинт
to interlace -переплітатися
tape -стрічка; рулетка
to obviate -уникати
to solder -паяти
core -жила (кабеля ел.)
mica -слюда
reluctance -магнітний опір
to vanish -зникати
oxide -окис
excitation -збудження; піднесення
shunt -шунт
dynamo -генератор, динамо-машина
to induce -спонукати
Comprehension questions:
1. What do you know about materials used in the construction of the field magnets?
2. Why is the length important in the exciting coils?
3. Describe the construction of the armature core with your partner.
4. What can you say about the methods for the excitation of the field magnets?