When the field winding is supplied from external, separate d.c. supply i.e. excitation of field winding is separate then the generator is called separately excited generator. Schematic representation of this type is shown in the Fig.1.
|Fig. 1 Separately excited generator|
The field winding of this type of generator has large number of turns of thin wire. So length of such winding is more with less cross-sectional area. So resistance of this field winding is high in order to limit the field current.
1.1 Voltage and Current Relations
The field winding is excited separately, so the field current depends on supply voltage and resistance of the field winding.
For armature side, we can see that it is supplying a load, demanding a load current of IL at a voltage of Vt which is called terminal voltage.
Now Ia = IL
The internally induced e.m.f. E is supplying the voltage of the load hence terminal voltage Vt is a part of E. But E is not equal to Vt while supplying a load. This is because when armature current Ia flows through armature winding, due to armature winding resistance Ra ohms, there is a voltage drop across armature winding equal to Ia Ra volts. The induced e.m.f. has to supply this drop, along with the terminal voltage Vt. To keepIa Ra drop to minimum, the resistance Ra is designed to be very very small. In addition to this drop, there is some voltage drop at the contacts of the brush called brush contact drop. But this drop is negligible and hence generally neglected. So in all, induced e.m.f. E has three components namely,
i) Terminal voltage Vt
ii) Armature resistance drop Ia Ra
iii) Brush contact drop Vbrush
So voltage equation for separately excited generator can be written as,
E = Vt + Ia Ra + Vbrush
Where E = (?PNZ)/(60A)
Generally Vbrush is neglected as is negligible compared to other voltages.