Separately Excited Generator

Separately Excited Generator

AU: Dec.-17,18. May-04, 05, 08

 • 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. 3.17.1.

• 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. 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 + V_brush

Where E = ϕ P N Z / 60 A

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

Ex. 3.17.1 A separately excited D.C. generator, when running at 1000 r.p.m. supplied 200 A at 125 V. What will be the load current when speed drops to 800 r.p.m., if I is unchanged?

Given that the armature resistance 0.40 Ω and brush drop 2 V.

AU: Dec.-17. May-08, Marks 8

Sol.:

Ex. 3.17.2 A 250 V, 10 kW, separately excited generator has an induced e.m.f. of 255 V at full load. If the brush drop is 2 V per brush, calculate the armature resistance of the generator.

Sol. Consider separately excited generator as shown in the Fig. 3.17.3.

Ex. 3.17.3 A separately excited DC generator, when unning at 1200 rpm supplies 200 A at 125 V to a circuit of constant resistance. What will be the current when the peed is dropped to 1000 rpm and the field current is educed to 80 %. Armature resistance 0.4 Ω and total Irop at brushes = 2 V. Ignore saturation and armature reaction.

AU: Dec.-18, Marks 13

Sol.

Review Questions

1. Explain the voltage and current relations for seperately excited generator. AU: May-04, Marks 4

2. A separately excited D.C. generator, when running at 1000 r.p.m. supplied 200 A at 125 V. What will be the load current when speed drops to 800 r.p.m., if^f I is unchanged?

Given that the armature resistance 0.40 Ω and brush drop = 2 V.

[Ans. 159.398 A] 

3. A separately excited generator when running at 1200 r.p.m. supplies a current of 150 A at 125 V to circuit of constant resistance. What will be the current when the speed drops to 800 r.p.m. if the field current is unaltered? Armature resistance is 0.05 ohm and the total voltage drop at the brushes is 2 V. Ignore the change in armature reaction. 

[Ans. 99.2381 A]