This section is from the book "The Principles And Practice Of Modern House-Construction", by G. Lister Sutcliffe. Also available from Amazon: How Your House Works: A Visual Guide to Understanding & Maintaining Your Home.
The explanation is that, no matter how soft the iron of which the magnets are made, they generally retain sufficient residual magnetism to cause them, to a slight extent, to act as permanent magnets, and (without the winding upon them having a current passed through it) to produce a sufficient number of lines
Fig. 614 - Armature and commutator.
Dynamo-builders occasionally produce a machine, which, on being driven by an engine for the first time (to test it), refuses to show the slightest magnetism, and it is then necessary to magnetize it from another source, perhaps the current from a battery, or from another dynamo which may be at work and producing current.
There are three kinds of dynamos - " Series", "Shunt", and "Compound"t which are all built in various patterns. It will suffice to describe simple diagrams of the three kinds, as the theory, which will be readily followed, can be applied to any machine, no matter who the builder or what the pattern.
1. The "Series" dynamo is so called in consequence of the current from the armature passing in series round the magnets and outer circuit. By the outer circuit is meant the path which the current traverses, after leaving the machine. Fig. 615 shows the commutator in section with the brushes bearing thereupon. By following the path from the upper brush, it will bc seen that the current, on leaving the armature, passes through the wire wound on the limbs of the magnet, and then proceeds through the external circuit and back to the other brush.
In this particular diagram there are three paths in the outer circuit through which the current can pass, and the quantity taken by any path will depend upon its resistance compared with the resistance of the others. Naturally, the lower the resistance of any path the greater will be the current through it. With a constant pressure and known resistance of any path, the current through it can be calculated by Ohm's law. as already described.
2. A "Shunt" dynamo is so called in consequence of the circuit of the magnets offering a "shunt" path to the external circuit. As will be seen from the diagram (Fig. 616), the currenl leaving by one brush can go direct to the external circuit, and return therefrom to the other brush without having to go through the magnet- winding, as in the series machine The shunt or magnet-winding also offers, similarly to the external circuit, a complete path, and so the current also passes through it. Since, then, there are two paths for the current from the armature - viz. the external circuit and the magnetic-winding, - the current through each will depend upon their relative resistances, and the shunt-winding, being made of higher resistance, takes only a little of the energy.
Fig 615. - Diagram showing series Dynamo and circuit.
Fig. 616 - Diagram showing Shunt Dynamo and Circuit.
In a series dynamo, all the current that penes from the armature goes around the magnet, and the wire forming the magnet-winding must therefore be of a larger section than that on a shunt machine, which only requires a small quantity of current to pass around the magnet. For the purpose of explaining this point, we will suppose that we have a series and a shunt dynamo, each of which has an armature with wire upon it of sufficient diameter to give 100 amperes with a certain excitation of the magnets. Each machine requires a certain amount of excitation, and supposing, in the case of the series machine, that, by providing 50 turns of wire on the magnets, the loo amperes which must pass through them produces the amount of excitation in question, that will be 50 turns multiplied by 100 amperes, or 5000 ampere-turns. Now a similar number of ampere-turns on the shunt machine will give in its case the excitation required, but we cannot pass 100 amperes (the total current from the machine) through it. So we take a much finer gauge of wire, which will, with (say) 2000 turns, offer such a resistance as to allow only 2½ amperes to pass through it, when it will be found we have a similar excitation to that of theseries machine, since 2000 turns multiplied by 2.½ ampere-= 5000 ampere-tun or a similar amount of exciting energy.
I mentioned above that the armature of each machine gave 100 amperes, and should like to make it clear that the limit of current which a wire will carry, whether on an armature or elsewhere, is that amount which will create in the wire just such sufficient heat as not to damage its insulation. The insulation may eventually suffer without the heat at any particular time appearing excessive, but upon this point I shall be able to say more later.
Comparing the two types of dynamo described, "Series and Shunt", we find the former has this peculiarity: that the more the paths offered to the current in the external circuit, or, in other words, the less the resistance of the external circuit, the greater will be the quantity of current which will pess. through that external circuit, and consequently through the magnet winding. But the greater the quantity of current through the magnet -windling, the greater will be the number of lines of force created to act upon the armature; so the less the external resistance, the stronger the machine. To see this plainly, we have only to turn to the diagram of the "series" dynamo, and cover up the external circuit with a piece of paper, and imagine a series machine without any circuit, when it will be obvious that no current can pass through the magnet-winding at all, since there is no path for it back to the armature; and it will also be obvious that, if we only provide an external circuit of very high resistance, only a little current can pass through it and consequently around the magnet.