By permission of
Lancashire Dynamo & Motor Co. Ltd.
A TYPICAL DYNAMO AND ITS PARTS.
In actual practice a dynamo has a set of electro-magnets, and the armature consists of many coils of wire mounted on a core of iron, which has the effect of concentrating the lines of force. The armature generally revolves in small dynamos, but in large ones it is usually a fixture, while the electro-magnets revolve. [Plate IV]. shows a typical dynamo and its parts.
As we saw in an earlier chapter, an electro-magnet has magnetic powers only while a current is being passed through its winding, and so some means of supplying current to the electro-magnets in a dynamo must be provided. It is a remarkable fact that it is almost impossible to obtain a piece of iron which has not some traces of magnetism, and so when a dynamo is first set up there is often sufficient magnetism in the iron of the electro-magnets to produce a very weak field. The rapid cutting of the feeble lines of force of this field sets up a weak current, which, acting upon the electro-magnets, gradually brings them up to full strength. Once the dynamo is generating current it keeps on feeding its magnets by sending either the whole or a part of its current through them. After it has once been set going the dynamo is always able to start again, because the magnet cores retain enough magnetism to set up a weak field. If there is not enough magnetism in the cores to start a dynamo for the first time, a current from some outside source is sent round the magnets.
The foregoing remarks apply to continuous current dynamos only. Alternating current can be used for exciting electro-magnets, but in this case the magnetic field produced is alternating also, so that each pole of the magnet has north and south magnetism alternately. This will not do for dynamo field magnets, and therefore an alternating current dynamo cannot feed its own magnets. The electro-magnets in such dynamos are supplied with current from a separate continuous current dynamo, which may be of quite small size.
It is a very interesting fact that electric current can be generated by a dynamo in which the earth itself is used to provide the magnetic field, no permanent or electro-magnets being used at all. A simple form of dynamo of this kind consists of a rectangular loop of copper wire rotating about an axis pointing east and west, so that the loop cuts the lines of force of the Earth’s magnetic field.
The dynamo provides us with a constant supply of electric current, but this current is no use unless we can make it do work for us. If we reverse the usual order of things in regard to a dynamo, and supply the machine with current instead of mechanical power, we find that the armature begins to revolve rapidly, and the machine is no longer a dynamo, but has become an electric motor. This shows us that an electric motor is simply a dynamo reversed. Let us suppose that we wish to use the dynamo in [Fig. 20] as a motor. In order to supply the current we will take away the lamp and substitute a second continuous-current dynamo. We know from [Chapter VII]. that when a current is sent through a coil of wire the coil becomes a magnet with a north and a south pole. The coil in our dynamo becomes a magnet as soon as the current is switched on, and the attraction between its poles and the opposite poles of the magnet causes it to make half a revolution. At this point the commutator reverses the current, and consequently the polarity of the coil, so that there is now repulsion where previously there was attraction, and the coil makes another half-revolution. So the process goes on until the armature attains a very high speed. In general construction there is practically no difference between a dynamo and a motor, but there are differences in detail which adapt each to its own particular work. By making certain alterations in their construction electric motors can be run with alternating current.
The fact that a dynamo could be reversed and run as a motor was known probably as early as 1838, but the great value of this reversibility does not seem to have been realized until 1873. At an industrial exhibition held at Vienna in that year, it so happened that a workman or machinery attendant connected two cables to a dynamo which was standing idle, and he was much surprised to find that it at once began to revolve at a great speed. It was then seen that the cables led to another dynamo which was running, and that the current from this source had made the first dynamo into a motor. There are many versions of this story, but the important point in all is that this was the first occasion on which general attention was drawn to the possibilities of the electric motor.
The practical advantages afforded by the electric motor are many and great. Once we have installed a sufficiently powerful dynamo and a steam or other engine to drive it, we can place motors just where they are required, either close to the dynamo or miles away, driving them simply by means of a connecting cable. In factories, motors can be placed close to the machines they are required to drive, anywhere in the building, thus doing away with all complicated and dangerous systems of shafting and belts. In many cases where it would be either utterly impossible or at least extremely inconvenient to use any form of steam, gas, or oil engine, electric motors can be employed without the slightest difficulty. In order to realize this, one only has to think of the positions in which electrically-driven ventilating fans are placed, or of the unpleasantly familiar electric drill of the dentist. An electric motor is small and compact, gives off no fumes and practically no heat, makes very little noise, is capable of running for very long periods at high speed and with the utmost steadiness, and requires extremely little attention.