The Series Dynamo
By referring to the diagram, it will be seen that the current of a series dynamo issues from the armature mains, and passes through the coils of the field magnets before passing into the external circuit to do its work. The residual magnetism, or the magnetism left in the iron cores of the field magnets from its last charge, provides the initial excitation, when the machine is started. As the resistance of the external circuit is lowered, by turning on more and more lights, more and more current flows from the armature, through the field magnets. Each time the resistance is lowered, therefore, the current passing through the field magnets becomes more dense in amperes, and makes the field magnets correspondingly stronger.
We have seen that the voltage depends on the number of lines of magnetic force cut by the armature conductors in a given time. If the speed remains constant then, and the magnets grow stronger and stronger, the voltage will rise in a straight line. When no current is drawn, it is 0; at full load, it may be 100 volts, or 500, or 1,000 according to the machine. This type of machine is used only in street lighting, in cities, with the lights connected in "series," or one after another on the same wire, the last lamp finally returning the wire to the machine to complete the circuit. This type of dynamo has gained the name for itself of "mankiller," as its voltage becomes enormous at full load. It is unsuitable, in every respect, for the farm plant. Its field coils consist of a few turns of very heavy wire, enough to carry all the current of the external circuit, without heating.
Connections of a series dynamo
The Shunt Dynamo
The shunt dynamo, on the other hand, has field coils connected directly across the circuit, from one wire to another, instead of in "series." These coils consist of a great many turns of very fine wire, thus introducing resistance into the circuit, which limits the amount of current (amperes) that can be forced through them at any given voltage. As a shunt dynamo is brought up to its rated speed, its voltage gradually rises until a condition of balance occurs between the field coils and the armature. There it remains constant. When resistance on the external circuit is lowered, by means of turning on lamps or other devices, the current from the armature increases in working power, by increasing its amperes. Its voltage remains stationary; and, since the resistance of its field coils never changes, the magnets do not vary in strength.
Connections of a shunt dynamo
The objection to this type of machine for a farm plant is that, in practice, the armature begins to exercise a de-magnetizing effect on the field magnets after a certain point is reached—weakens them; consequently the voltage begins to fall. The voltage of a shunt dynamo begins to fall after half-load is reached; and at full load, it has fallen possibly 20 per cent. A rheostat, or resistance box on the switchboard, makes it possible to cut out or switch in additional resistance in the field coils, thus varying the strength of the field coils, within a limit of say 15 per cent, to keep the voltage constant. This, however, requires a constant attendance on the machine. If the voltage were set right for 10 lights, the lights would grow dim when 50 lights were turned on; and if it were adjusted for 50 lights, the voltage would be too high for only ten lights—would cause them to "burn out."