Characteristics of Separately Excited DC Generator


     In separately excited dc generator, field windings are energised by a separate external source, rather than using same supply used to supply the armature.



No-Load Characteristic : 


     It shows the relation between no-load generated e.m.f. Eo and the field current  at a given speed. It is also known as Magnetisation characteristics or open-circuit characteristics ( O.C.C. )         
                                      
No load e.m.f.,

  Where 

     K  = constant  =  ZNP/60

     Eo ∝ field current

     φ  ∝ field current


     It is clear from the above expression that when field current is increased by varying the potential divider from zero, the flux φ and hence Eo increases. The readings of Eo and field current should be tabulated and the relation between the two should be plotted. The flux increases until the poles become saturated, after that a greater increase in field current is required to produce a given φ and hence Eo.
Characteristics of Separately Excited DC Generator

     That is why, the upper portion of the curve bends over as shown. The O.C.C  is useful in finding the required voltage, Eo of the generator at any particular conditions such as no-load and speed.



Load characteristics :


     The relation between the e.m.f. actually induced, E and the armature current Ia gives the Internal characteristics.
Characteristics of Separately Excited DC Generator

     First the generator should be brought up to its rated speed and then field should be increased until the voltmeter reads rated voltage.  Now the switch S2 is closed. Then the initial readings of load current (  = O at this instant ) and V should be recorded and tabulated. Now the load is adjusted to increase load current. The load resistance should be decreased further, until the ammeter, load current reads full load current or as desired. 


     Now the graph between V and load current should be plotted by curve ab. This is known as load or external  characteristic curve. It can be noticed from the graph that the voltage decreases slowly with an increase in load current. While taking the readings the generator speed should be  maintained constant by varying the prime mover speed.

Characteristics of Separately Excited DC Generator

     The voltage recorded at zero load current is Eo. It is parallel to abscissa ( X-axis ) and is shown by dotted lines because, on no-load current is zero. Now determine IRa drop at every reading and add to the load curve. Thus the curve obtained is a.c. and is known as internal characteristics. The relation between the terminal voltage V and load current gives the external characteristics.


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