This section is from the book "Modern Buildings, Their Planning, Construction And Equipment Vol3", by G. A. T. Middleton. Also available from Amazon: Modern Buildings.
If a coil of wire, such as C, Fig. 130, be revolved between the poles of a horseshoe magnet, a current will be induced in the coil. To utilise this current some means of conducting it away must be devised. An early device for this purpose consists of a tube of metal split into two pieces, as R and T mounted upon the same axle as the coil C. The fixing of R and T to the axle must be made with some substance which will not conduct an electric current. Two springs or brushes of metal, A and B, are mounted to press upon R and T at diametrically opposite points, and in a particular position relative to the poles of the magnet, and the external circuit is connected to these springs.
It will be seen that, as the coil C revolves, the current passing through it is reversed twice in every revolution, but the direction of the current in the external circuit is always the same, for, when the current is flowing in the direction indicated by the arrows in the coil C, the half of the split tube R is in contact with A, and when the current flows through the coil in the opposite direction the other portion of the split tube is in contact with A; so that the current in the external circuit is uni-directional. A current so produced is called a Continuous or Direct current.
Dynamos are sometimes constructed on the principle shown in Fig. 131, with two complete rings of copper upon which the brushes press, and with such an arrangement it will be seen that the current in the external circuit changes its direction of flow twice in every revolution of the coil. Such a dynamo produces alternating current, and is usually called an Alternator. The axle of a dynamo with the coils and split tube mounted upon it is commonly called the Armature, and the magnet is called the Field magnet, because it creates the magnetic field in which the coils are revolved. Sometimes the field magnets of an alternator are revolved, while the armature remains stationary.
It should also be observed that, as the coil revolves between the poles of the magnet, the number of lines of force cut by the coil will vary with every position of the coil. Thus as the coil passes through the position shown in Fig. 130 a maximum number of lines of force for any constant speed of the coil will be cut; and as the plane of the coil becomes more horizontal, so the number of lines of force cut thereby is decreased. It has already been said that the current induced in a coil varies in the direct proportion to the number of lines cut, and it follows, therefore, that the current will not be steady if only one coil is used. To avoid this defect a dynamo is made with a great number of coils spinning between the magnets, the ends of which are connected to a number of pairs of hard copper strips in place of the two halves of the split tube R and T. Such an arrangement of copper strips is called a Commutator.
In practice it is found that electromagnets - i.e. magnets produced by passing a current round a coil of wire having an iron core - are more suitable for field magnets than are large permanent horseshoe magnets. The iron used for the core is of such a nature that it retains a certain small amount of magnetism, so that a small amount of current may be generated in the armature coils, which is passed round the field magnet coils. This current strengthens the field magnets, and the current in the armature coils increases accordingly. By this process the maximum strength of the field magnet is reached.
The methods of winding the field magnets of a dynamo give rise to the following classification: - Series-wound dynamos, Shunt-wound dynamos, and Compound-wound dynamos.
In these the winding is so arranged that the whole of the current generated in the armature coils passes round the field magnets before it reaches the external circuit, as shown dia-grammatically in Fig. 132. This type of dynamo is very-little used in practice, except for lighting arc lamps in series, and it is quite unsuitable for changing accumulators or for any purpose where the resistance of the external circuit is not constant, and the dynamo should be designed to work with a particular external resistance.
In this type of machine two paths are provided for the current, one from one brush A, as shown in Fig. 133 (the whole connection is not shown), through the external circuit to the other brush B, and the other from brush A round the field magnets to the other brush B. When a circuit is divided into two or more paths the amount of current passing in each path is inversely proportional to the resistance of the paths. In a shunt-wound dynamo only a small proportion, usually from two to three per cent, of the total current, passes round the field magnets, and therefore the windings must be composed of much smaller wire of a high resistance compared with the external circuit and winding of the armature than is necessary in a series-wound dynamo.
It is usual to run dynamos at a constant speed, and it would be very awkward if the dynamo varied its pressure according to the number of lights which are switched on and off from time to time. It is therefore necessary, where this inconvenience is to be avoided, to wind a dynamo with series as well as shunt windings, so that should the resistance of the external circuit vary, the pressure remains constant. This is called the compound method of winding. It will be clear from the diagram (Fig. 134) that, as the resistance of the external circuit decreases, the series windings strengthen the field magnet, and the proportion of series and shunt windings should be such that a constant pressure is maintained.