The three main characteristics of a DC shunt generator are the open circuit or magnetic characteristic, the internal or total characteristic, and the external characteristic. These characteristics are represented by graphed curves.
As the generator turns, the spinning armature creates an electromagnetic field. The EMF is proportionate in strength to the speed of the spinning armature and creates a resistance within the generator. This effect is the open circuit or magnetic characteristic. It shows that at a fixed speed, there is a relationship between the generated EMF and the field current with zero load on the generator.
When a load is applied to the generator, its voltage output is decreased. Therefore, generated voltage under a load drops below that of the electromagnetic field when there is no load. The relation of the generated voltage and the load current is referred to as the internal or total characteristic. When graphed, this curve drops below the open circuit characteristic curve.
The relationship between the load current and the terminal voltage is the external characteristic. The external characteristic curve shows a greater drop than the internal one because it includes the voltage drop of the armature circuit while under a load. Determining a generator's external characteristic, or its output compared to its load, is very important when deciding which application it is best suited for.