We abstract the following from a paper on electric lighting by Prof. J.A. Fleeming, read before the Iron and Steel Institute, Manchester. The illustration is from Engineering.
One of the questions which most frequently occurs in reference to mill and factory lighting is whether the factory engines can be used to run the dynamo. As a broad, general rule, there can be no question that the best results are obtained by using a separate dynamo engine, controlled by a good governor, set apart for that purpose. With an ordinary shunt dynamo, the speed ought not to vary more than 2 or 3 per cent. of its normal value on either side of that value. Hence, if a dynamo has a normal speed of 1,000, it should certainly not vary over a greater range than from 970 to 980 to 1,020 to 1,030. In many cases there may be shafting from which the necessary power can be taken, and of which the speed is variable only within these limits. There are several devices by which it has been found possible to enable a dynamo to maintain a constant electromotive force, even if the speed of rotation varies over considerable limits. One of these is that (see illustration) due to Messrs. Trotter & Ravenshaw, and applicable to shunt or series machines.
In the circuit of the field magnet is placed a variable resistance. This resistance is thrown in or out by means of a motor device actuated by an electromotive force indicator. A plunger of soft iron is suspended from a spring, and hangs within a solenoid of wire, which solenoid is in connection with the terminals of the dynamo. Any increase or diminution of the electromotive force causes this iron to move in or out of the core, and its movement is made to connect or disconnect the gearing which throws in the field magnet resistance with a shaft driven by the engine itself. The principle of the apparatus is therefore that small variations of electromotive force are made to vary inversely the strength of the magnetic field through the intervention of a relay mechanism in which the power required to effect the movement is tapped from the engine.
With the aid of such a governor it is possible to drive a dynamo from a mill shaft providing the requisite power, but of which the speed of rotation is not sufficiently uniform to secure alone efficient regulation of electromotive force. Another device, patented by Mr. Crompton, is a modification of that method of field magnet winding commonly known as compound winding. The field magnets are wound over with two wires, one of which has a high resistance and is arranged as a shunt, and the other of which has a low resistance and is arranged in series. Instead, however, of the magnetizing powers of these coils being united in the same direction as an ordinary compound winding, they are opposed to one another. That is to say, the current in the shunt wire tends to magnetize the iron of the field magnets in an opposite direction to that of the series wire. It results from this that any slight increase of speed diminishes the strength of the magnetic field, and vice versa. Accordingly, within certain limits, the electromotive force of the dynamo is independent of the speed of rotation.