Show Mobile Navigation

Featured post

How to Make a Digital Lock Using Arduino?

Interesting Articles

,

DC Generator Characteristics


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

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. 

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 shows the behavior of the generator under different load conditions.

DC Generator Characteristics

The three most important characteristics of a dc generator are given below

  1. Magnetic or Open Circuit Characteristic (O.C.C.) - This curve shows the relation between the generated e.m.f. at no-load (E0) and the field current (If) at constant speed.
  2. 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).
  3. 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 of 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 Characteristic

The Open Circuit Characteristic is one of the characteristics of a dc generator 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 generator is run at 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).

The following points may be noted from OCC:
(i) When the field current is zero, there is some generated e.m.f. OA. This is due to the residual magnetism in the field poles.
(ii) Over a fairly wide range of field current (upto point B in the curve), the curve is linear. It is because in this range, reluctance of iron is negligible as compared with that of air gap. The air gap reluctance is constant and hence linear relationship.
(iii) After point B on the curve, the reluctance of iron also comes into picture. It is because at higher flux densities, μr for iron decreases and reluctance of iron is no longer negligible. Consequently, the curve deviates from linear relationship.
(iv) 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.