- Definition of Separately Excited DC Generator: A separately excited DC generator is defined as a DC generator where the field winding is powered by an external source.
- Magnetic or Open Circuit Characteristic: This characteristic shows the relationship between the field current and generated voltage without any load.
- Internal Characteristic Curve: This curve shows the actual generated voltage after accounting for the armature reaction drops.
- External Characteristic Curve: This curve shows the terminal voltage, considering the losses in the armature, and lies below the internal characteristic curve.
- Advantages and Disadvantages: Separately excited DC generators provide stable operation and a wide voltage range but are costly due to the need for an external power source.
A separately excited DC generator receives its field current from an independent external source. Its behaviour is described by open-circuit, internal and external characteristic curves.
Magnetic or Open Circuit Characteristic of Separately Excited DC Generator
The open-circuit characteristic relates field current (If) to generated voltage (E0) at no load and constant speed. It is the machine’s magnetisation or saturation curve. The same test principle applies to the different types of generators, although a self-excited generator’s operating point also depends on its field circuit. This graph is also called the no-load saturation curve of DC generator.
The figure shows how generated emf changes with field current at several fixed armature speeds. At a given field current, a higher speed produces a higher emf. Residual flux in the poles produces the small initial emf OA when field current is zero.
At constant speed, let the no-load voltage be E0 for a fixed field current. If armature reaction and the armature-circuit drop are ignored, the voltage remains at E0 as load current changes. The ideal AB line is therefore horizontal.
In a loaded generator, the terminal voltage drops for two reasons:
- Armature reaction changes the main flux and reduces the generated emf under normal brush conditions.
- Armature current produces the resistive drop IaRa.
Internal or Total Characteristic of Separately Excited DC Generator
The internal characteristic relates load current to the generated emf Eg after allowing for armature reaction. It lies below the ideal no-load AB line and normally falls as load current rises. The AC line represents this total characteristic of a separately excited DC generator.
External Characteristic of Separately Excited DC Generator
The external characteristic of the separately excited DC generator relates terminal voltage to load current. Terminal voltage is generated emf Eg minus the armature resistance drop Ia Ra.
Terminal voltage (V) = Eg – Ia Ra.
The curve plots terminal voltage against load current. The external characteristic curve lies below the internal characteristic curve because it includes the armature-circuit voltage drop. Line AD therefore falls as load current rises. Increasing field current can compensate for this fall within the machine’s excitation, heating and commutation limits.
Compared with self-excited DC generators, separate excitation lets the field be controlled independently of terminal voltage. These kinds of generators require an external field supply, which adds equipment and losses.
