**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 I_{f}, the armature current I_{a}, 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, E_{0} and field or exciting current I_{f} 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.

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- 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 I
_{a}R_{a}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.

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