DC Generator Characteristics
DC generator characteristics are the relations between excitation, terminal voltage and load exhibited graphically by means of curves. These characteristics of dc generators are very important in the design and operation of dc generators. This article describes the different characteristics of a dc generator in brief.
One question comes into our mind is that why the DC generator characteristics are plotted between excitation, terminal voltage, and load. Here is the answer.
The speed of a dc machine operated as a generator is fixed by the prime mover to which the dc generator is coupled. The prime mover may be a turbine or diesel engine etc. The prime mover is equipped with a speed governor so that the speed of the generator is practically constant.
The speed of a dc machine operated as a generator is fixed by the prime mover to which the dc generator is coupled. The prime mover may be a turbine or diesel engine etc. The prime mover is equipped with a speed governor so that the speed of the generator is practically constant.
So the speed is practically constant for a generator, then the generator performance mainly deals with the relation between excitation, terminal voltage, and load.
These relations can be best exhibited graphically by means of curves known as dc generator characteristics. These characteristics of a dc generator show the behavior of the generator under different load conditions.
These relations can be best exhibited graphically by means of curves known as dc generator characteristics. These characteristics of a dc generator show the behavior of the generator under different load conditions.
DC Generator Characteristics
The three most important characteristics of a dc generator are given below
Magnetic or Open C– This curve shows the relation between the generated e.m.f. at no-load (E0) and the field current (If) at a constant speed.
Internal or Total characteristic (E/Ia)– This curve shows the relation between the generated e.m.f. on load (E) and the armature current (Ia).
External characteristic (V/IL) – This curve shows the relation between the terminal voltage (V) and load current (IL).
For a self-excited or separately excited DC generator, all the above three dc generator characteristics can be drawn. Of these characteristics, the shape of open circuit characteristic (OCC) is practically the same for all type dc generators.
Next, we are going to discuss the characteristics of all types of dc generators as given below:
Before going to these topics, we will discuss about the common characteristic of all generators, that is the open circuit characteristics or magnetic characteristics.
Open Circuit Characteristics
The Open Circuit Characteristic is one of the dc generator characteristics which is practically same for all dc generators. The OCC of even self-excited dc generator is obtained by running it as a separately excited generator. The procedure is explained below,
The field winding of the dc generator (series or shunt) is disconnected from the machine and is separately excited from an external dc source as shown in Figure (ii).
The field winding of the dc generator (series or shunt) is disconnected from the machine and is separately excited from an external dc source as shown in Figure (ii).
The generator is run at a fixed speed (i.e., normal speed). The field current (If) is increased from zero in steps and the corresponding values of generated e.m.f. (E0) read off on a voltmeter connected across the armature terminals.
On plotting the relation between E0 and If, we get the open circuit characteristic as shown in Figure (i).
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The following points may be noted from OCC:
- When the field current is zero, there is some generated emf OA.
- This is due to the residual magnetism in the field poles.
- Over a fairly wide range of field current (up to point B in the curve), the curve is linear.
- It is because, in this range, the reluctance of iron is negligible as compared with that of the air gap. The air gap reluctance is a constant and hence linear relationship.
- After point B on the curve, the reluctance of iron also comes into the picture.
- It is because, at higher flux densities, μr for iron decreases and the reluctance of iron is no longer negligible. Consequently, the curve deviates from the linear relationship.
- After point C on the curve, the magnetic saturation of poles begins and E0 tends to level off.
The Open Circuit Characteristic of even self-excited dc generator is obtained by running it as a separately excited generator.
Next: Read the characteristics of all types of dc generators given below:
Characteristics of Series Wound DC Generators
In this post, we will learn the characteristics of a series wound dc generator. This article is the continuation of the dc generator characteristics.
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.
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 in dc generator characteristics.
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 the armature reaction in dc generator, 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 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.
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 a similar procedure, other points of the 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 the increase in load, reaches the maximum and finally decreases.
If 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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