Nov 29, 2014

Characteristic of Separately Excited DC Generator

Speed Characteristic The apparent drawback of a separately excited DC generator is that we have a tendency to need an external DC supply for excitation. However ever since the output voltage could also be controlled a lot of 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 during a manner delineated in the previous post. The fig shows the variation of generated e.m.f. on no load with field current for a number of constant speeds. Note that if the value of constant speed is increased, the abruptness of the curve additionally will increase. Once the curve current is zero, the residual magnetism within the poles can make to the little initial e.m.f. as shown in the fig.

Internal and External Characteristic:

In order to determine the external characteristic curve of a separately excited generator, then the curve is drawn between the terminal voltage (V) and the load current IL which is shown in the fig. 
circuit diagram of separately excited generator

Due to the load current will increase, the terminal voltage falls due to the following reasons:
(a) The armature reaction weakens the main flux. Hence the actual e.m.f. generated E on load is a smaller amount than that generated (E0) on no load.
(b) There's voltage drop across armature resistance (ILRa = IaRa).

Because of this reasons, the external characteristic could be a drooping curve which is also shown in the fig. Note that within the absence of armature reaction and armature drop, the generated e.m.f. would are E0.
armature drop characteristic

By adding ILRa drop to the external characteristic the internal characteristic will be determined. It’s as a result of armature reaction drop is built-in within the external characteristic. Curve 2 is that the internal characteristic of the generator and will clearly lie on top of the external characteristic.


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