# Characteristics of Separately Excited DC Generators

- Link: https://studyelectrical.com/2014/08/characteristics-of-separately-excited-dc-generator.html
- Published: 2014-08-07T16:33:00+00:00
- Author: 

In a separately excited dc generator, the field coils are energized from an independent
source. 

The disadvantage of a separately excited DC generator is the same that we require
an external DC source for excitation. But since the output voltage may be controlled
more easily and over a wide range (from zero to a maximum), this type of excitation
finds many applications.

Read: [Working Principle of Alternator](https://studyelectrical.com/2015/12/working-principle-of-alternator.html)

## Open circuit characteristic

The [O.C.C. of a separately excited generator](https://studyelectrical.com/2014/08/dc-generator-characteristics.html)
is determined in a manner described in the previous section. It shows the variation
of generated e.m f. on no load with field current for various fixed speeds.

Note that if the value of constant speed is increased, the steepness of the curve
also increases. When the field current is zero, the residual magnetism in the poles
will give rise to the small initial e.m.f. as shown.

## Internal and External Characteristics

The external characteristic of a separately excited generator is the curve between
the terminal voltage (V) and the load current IL (which is the same as armature 
current in this case).

In order to determine the external characteristic, the circuit set up is as shown
in Fig (i). As the load current increases, the terminal voltage falls due to two
reasons:

![preview file+(2)](http://1.bp.blogspot.com/-fDndusl6mT0/U-MltB1olbI/AAAAAAAABEM/
nGqdUG-aBXg/s400/preview_file+(2).png)

Internal and External Characteristics

 * (a) The armature reaction** weakens the main flux** so that actual e.m.f. generated
   E on load is less than that generated (E0) on no load.
 * (b) There is a voltage drop across armature resistance (= ILRa = IaRa).

Due to these reasons, the external characteristic is a drooping curve [curve 3 in
Fig (ii)]. Note that in the absence of armature reaction and armature drop, the 
generated e.m.f. would have been E0 (Curve 1).

The internal characteristic can be determined from an external characteristic by
adding ILRa drop to the external characteristic. It is because the armature reaction
drop is included in the external characteristic.

Curve 2 is the internal characteristic of the generator and should obviously lie
above the external characteristic.

### Advantages and Disadvantages

The separately excited dc generators have a decided advantage over the self-excited
generators – it operates in a stable condition with any field excitation. Thus a
wide range of output voltage may be obtained.

The **main disadvantage** of a separately excited generator lies in the inconvenience
and expense of providing the separate excitation source. For this reason, the use
of this type of generator is limited to experimental and testing laboratories where
such a source is available and a wide variation of the output voltage is desirable.

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