# Characteristics of Series Wound DC Generators

- Link: https://studyelectrical.com/2014/08/characteristics-of-series-wound-dc-generator.html
- Published: 2014-08-11T05:19:00+00:00
- Author: 

In this post, you will learn the characteristics of a series wound dc generator.
This article is the continuation of the [dc generator characteristics](https://studyelectrical.com/2014/08/dc-generator-characteristics.html).

The connection diagram of a series-wound generator is shown in the figure below.

![dc series generator connection](https://studyelectrical.com/wp-content/uploads/
2014/08/dc-series-generator-connection.png)

Dc series generator connection

Since there is only one current (that which flows through the whole machine), the
load current is the same as the exciting current.

The open circuit, internal and external characteristics of series wound dc generators
are discussed here.

### Open circuit characteristic

Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator. Curve
2 shows the total or internal characteristics of a series generator. Curve 3 shows
the external characteristic of a series generator.

It can be obtained experimentally by disconnecting the field winding from the machine
and exciting it from a separate DC source as discussed [in dc generator characteristics](http://www.studyelectrical.com/2014/08/dc-generator-characteristics.html).

![Dc series generator characteristics](https://studyelectrical.com/wp-content/uploads/
2014/08/dc-series-generator-characteristics.png)

Dc series generator characteristics

### Internal characteristic

Curve 2 shows the total or internal characteristics of a series generator. It gives
the relation between the generated e.m.f. E. on load and armature current.

![internal characteristics](https://studyelectrical.com/wp-content/uploads/2014/
08/internal-characteristics.png)

Internal characteristics

Due to the [armature reaction in the dc generator](https://studyelectrical.com/2014/09/armature-reaction-in-dc-machines.html),
the flux in the machine will be less than the flux at no load.

Hence, e.m.f. E generated under load conditions will be less than the e.m.f. E0 
generated under no-load conditions.

Consequently, the internal characteristic curve lies below the OCC curve; the difference
between them representing the effect of armature reaction.

This curve also gives the relation between emf Eg and armature current Ia since 
Ia=If. 

### External or Load characteristic

Curve 3 shows the external characteristic of a series generator.

It gives the relation between terminal voltage and load current IL.

V = E – Ia (Ra + Rse )

Therefore, external characteristic curve will lie below internal characteristic 
curve by an amount equal to ohmic drop [i.e., Ia(Ra + Rse)] in the machine.

[⌊preview file+(1)⌉⌊preview file+(1)⌉[

External vs Internal Characteristics

This voltage drop for different values of load current may be represented by straight-
line OC. 

 

The internal and external characteristics of a dc series generator can be plotted
from one another as shown in the figure above.

Suppose we are given the internal characteristic of the generator. Let the line 
OC represent the resistance of the whole machine i.e. Ra + Rse.

If the load current is OB, drop in the machine is AB i.e. AB = Ohmic drop in the
machine = OB(Ra + Rse)

 

Now raise a perpendicular from point B and mark a point b on this line such that
ab = AB. Then point b will lie on the external characteristic of the generator.

Following a similar procedure, other points of the external characteristic can be
located.

It is easy to see that we can also plot internal characteristics from the external
characteristic. So external characteristic is what we obtain by deducting ohmic 
drop from internal characteristics. 

 _Note:_

From the external characteristic, it is observed that the terminal voltage first
increases with the increase in load, reaches the maximum and finally decreases.

If the load resistance is reduced sufficiently, the terminal voltage may fall to
zero. So if the series generator is operated on the initial straight-line portion
of the characteristic, it gives voltage approximately proportional to the load current.

If it is operated on the drooping portion of the characteristic, it gives approximately
constant current irrespective of the external load circuit resistance.

 

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