Separately Excited Generator

       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 I_L at a voltage of V_t which is called terminal voltage.

       Now   I_a = I_L

       The internally induced e.m.f. E is supplying the voltage of the load hence terminal voltage V_t is a part of E. But E is not equal to V_t while supplying a load. This is because when armature current I_a flows through armature winding, due to armature winding resistance R_a ohms, there is a voltage drop across armature winding equal to I_a

R_a volts. The induced e.m.f. has to supply this drop, along with the terminal voltage V_t. To keep I_a R_a drop to minimum, the resistance  R_a 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 V_t

ii) Armature resistance drop  I_a R_a

iii) Brush contact drop V_brush

       So voltage equation for separately excited generator can be written as,

       E = V_t + I_a R_a + Vbrush

       Where E = (ΦPNZ)/(60A)

       Generally V_brush  is neglected as is negligible compared to other voltages.