Characteristic of Separately Excited DC Generator
The apparent drawback of a separately excited DC generator is that we tend to need an external DC supply for excitation. However, since the output voltage can also be controlled very simply and over a wide range (from zero to a maximum), this sort of excitation finds several applications.
Open Circuit Characteristic
The open circuit characteristic of a separately excited generator is set in a manner delineated in the previous post. The figure shows the variation of generated e.m.f. on no load with field current for a number of constant speeds. Note that if the constant speed value increases, the curve’s abruptness will also increase. Once the curve current is zero, the residual magnetism within the poles can contribute to the little initial e.m.f., as shown in Fig.
Internal and External Characteristics
To determine the external characteristic curve of a separately excited generator, the curve is drawn between the terminal voltage (V) and the load current IL, shown in Fig.
Due to the load current will increase, the terminal voltage falls due to the following reasons:
- The armature reaction weakens the main flux. Hence, the actual e.m.f. Generated E on load is smaller than generated (E0) on no load.
- There’s the voltage drop across armature resistance (ILRa = IaRa).
Because of this reason, the external characteristic could be a drooping curve, as shown in Fig. Note that within the absence of armature reaction and armature drop, the generated e.m.f. Would are E0.
Adding ILRa drop to the external characteristic will determine the internal characteristic. It’s because armature reaction drop is built-in within the external characteristic. Curve 2 is the internal characteristic of the generator and will lie on top of the external characteristic.
