The internal characteristic of a DC generator is a curve between the-

Concept

Internal characteristic

• It is the graph plotted between generated EMF() on load and the armature current().
• Because of the effect of the armature reaction, the magnetic flux on load will be less than the flux at no load.
• Therefore, the generated EMF () is under loaded conditions. will be less than the EMF generated () at no-load.
• As a result of this, the internal characteristics curve lies just below the open circuit characteristics.

 Additional Information

Characters of DC generator

1. Open circuit characteristic().
2. Internal characteristics ().
3. External characteristics().​

Open Circuit Characteristic (O.C.C.) (E0 / If):

• Open circuit characteristic is also known as magnetic characteristic or no-load saturation characteristic.
• This characteristic shows the relation between generated emf at no load (E0) and the field current (If) at a given fixed speed.
• The O.C.C. curve is just the magnetization curve and it is practically similar for all type of generators.
• The data for O.C.C. curve is obtained by operating the generator at no load and keeping a constant speed.
• Field current is gradually increased and the corresponding terminal voltage is recorded.
• The connection arrangement to obtain O.C.C. curve is as shown in the figure below.

• O.C.C. of different types of dc generator is shown below

• For shunt or series excited generators, the field winding is disconnected from the machine and connected across an external supply.

External Characteristic (V / IL):

• An external characteristic curve shows the relation between terminal voltage (V) and the load current (IL).
• Terminal voltage V is less than the generated emf Eg due to voltage drop in the armature circuit.
• So that external characteristic curve lies below the internal characteristic curve.
• External characteristics are very important to determine the suitability of a generator for a given purpose.
• Hence this type of characteristic is sometimes also called as performance characteristic or load characteristic.