02-05-2013, 12:52 PM
D.C. Generator Characteristics
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Introduction
The speed of a d.c. machine operated as a generator is fixed by the prime mover.
For general-purpose operation, the prime mover is equipped with a speed
governor so that the speed of the generator is practically constant. Under such
condition, the generator performance deals primarily with the relation between
excitation, terminal voltage and load. These relations can be best exhibited
graphically by means of curves known as generator characteristics. These
characteristics show at a glance the behaviour of the generator under different
load conditions.
D.C. Generator Characteristics
The following are the three most important characteristics of a d.c. generator:
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. It is also known as magnetic characteristic
or no-load saturation curve. Its shape is practically the same for all generators
whether separately or self-excited. The data for O.C.C. curve are obtained
experimentally by operating the generator at no load and constant speed and
recording the change in terminal voltage as the field current is varied.
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). The e.m.f. E is less than E0 due to the demagnetizing effect
of armature reaction. Therefore, this curve will lie below the open circuit
characteristic (O.C.C.). The internal characteristic is of interest chiefly to the
designer. It cannot be obtained directly by experiment. It is because a voltmeter
cannot read the e.m.f. generated on load due to the voltage drop in armature
resistance. The internal characteristic can be obtained from external
characteristic if winding resistances are known because armature reaction effect
is included in both characteristics.
External characteristic (V/IL)
This curve shows the relation between the terminal voltage (V) and load current
(IL). The terminal voltage V will be less than E due to voltage drop in the
armature circuit. Therefore, this curve will lie below the internal characteristic.
This characteristic is very important in determining the suitability of a generator
for a given purpose. It can be obtained by making simultaneous measurements
of terminal voltage and load current (with voltmeter and ammeter) of a loaded
generator.
Open Circuit Characteristic of a D.C. Generator
The O.C.C. for a d.c. generator is determined as follows. The field winding of
the d.c. generator (series or shunt) is disconnected from the machine and is
separately excited from an external d.c. source as shown in Fig. (3.1) (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
Fig. (3.1) (i).