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Characteristics of Series Wound DC Generators


Related Topics
Construction of DC Generator
Types of DC Generators
Losses in DC Machine
Armature Reaction in DC Machines
Compensating Windings and Interpoles
Commutation in DC Machines
DC Generator Characteristics
Characteristics of Separately Excited Generators
Characteristics of DC Series Generator
Characteristics of Shunt DC Generator
Characteristics of Compound DC Generators
Parallel Operation of DC Generators
Parallel Operation of DC Shunt Generators
Parallel Operation of DC Series Generators
Parallel Operation of DC Compound Generators


The connection diagram of a series wound generator is shown in figure (i) below. 
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.
Characteristics of Series Wound DC Generators

Open circuit characteristic

Curve 1 shows the open circuit characteristic (O.C.C.) of a series generator. It can be obtained experimentally by disconnecting the field winding from the machine and exciting it from a separate d.c. source as discussed here.

Internal characteristic


Curve 2 shows the total or internal characteristic of a series generator. It gives the relation between the generated e.m.f. E. on load and armature current. Due to armature reaction, 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, internal characteristic curve lies below the O.C.C. 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. This voltage drop for different values of load current may be represented by straight line OC. 

The internal and external characteristics of a d.c. series generator can be plotted from one another as shown in Figure right. 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 similar procedure, other points of external characteristic can be located. It is easy to see that we can also plot internal characteristic from the external characteristic. So external characteristic is what we obtain by deducting ohmic drop from internal characteristic. 

Note:
From the external characteristic  it is observed that the terminal voltage first increases with increase in load, reaches the maximum and finally decreases. If load resistance is reduced sufficiently, the terminal voltage may fall to zero. So if series generator is operated on initial straight line portion of the characteristic, it gives voltage approximately proportional to the load current and if it is operated on the drooping portion of the characteristic, it gives approximately constant current irrespective of the external load circuit resistance.