5. CHARACTERISTICS OF D.C. GENERATORS
The properties of generators are analysed with the aid of characteristics which give the relations between fundamental quantities determining the operation of a generator. These include the voltage across the generator terminals V, the field or exciting current If, the armature current Ia, and the speed of rotation N.
The three most important characteristics of D.C. generators are given below:
circuit characteristic (O.C.C.). It shows the relationship between the no-load generated e.m.f. in armature, E0 and field or exciting current If at a given fixed speed. The shape of the curve is practically the same for all types of generators whether they are separately excited or self-excited. It is just the magnetisation curve for the material of the electromagnets.
- This characteristic is also referred to as performance characteristic or sometimes voltage-regulating curve.
- It gives relation between the terminal voltage V and load current I .
- The curve lies below the internal characteristic because it takes into account the voltage drop over the armature circuit resistance. The values of V are obtained by subtracting IaRa from corresponding values of E.
- The characteristic is of great importance in judging the suitability of a generator for a particular purpose.
- The external characteristic can be obtained by the following two ways:
- i. By making simultaneous measurements with a suitable voltmeter and an ammeter on a loaded generator.
- ii. Graphically from the O.C.C. provided the armature and field resistances ate known and also if the demagnetising effect of the armature reaction is known.