Action of the Commutator.We have seen that the e.m.f. induced in the simple coil rotating in a magnetic field is an alternating e.m.f. Hence if the two ends of the coil are connected to insulated slip rings mounted on the shaft, and the external circuit is connected to brushes which press on these slip rings, the electrical polarity of each ring will alternate as the coil rotates, so that the current set up will also be alternating. In order that the current may be unidirectional, it is necessary for one brush always to be positive and the other always negative. In other words, the negative parts of the induced voltage wave must be rectified. This is done by using one ring which is split into two halves, the halves insulated from one another, and each connected to one end of the coil. In this way the positive end of the coil is always under the same brush, because, as the polarity of one end is about to reverse that end passes to the next brush. Similarly, the negative end of the Coil is always under the other brush. The voltage across the brushes therefore always acts in the same direction, and the current produced in the external circuit always flows in one direction. This current will obviously pulsate with the variations in magnitude of the rectified voltage curve, so that, although it will be unidirectional, it can hardly be termed a continuous current. It must be remembered that this rectification of the current and voltage takes place in the external circuit only. The current and voltage in the rotating coil remain alternating.

 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 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. 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 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.

Series coils consist of comparatively few turns of thick wire, or for heavy-current 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 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' 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 mils 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»