Electrical Engineering ⇒ Topic : Self-excited Generator

In this type of generators, field magnets are energized by the current produced by the generators themselves. This type of generators are classified into the following categories depending upon their field connections :

(i) Shunt wound
The field winding is connected in parallel to the armature. So the full voltage of the generator is applied across the field winding as shown in Figure

(ii) Series wound
The field winding is connected in series with the armature. Since the field carries the full-load current, it consists of relatively few turns of thick wires as shown in Figure

(iii) Compound generators
This type of generator is a combination of series and shunt generators. It can be either long shunt as shown in Figure below

or short shunt as shown in Figure below

         Self-excited  generator                            

                        Self-excited shunt generator

         Fig.  Building up the voltage of a shunt generator.

Critical resistance (R_c). The critical field circuit resistance, R_c, is shown as tangent to the saturation curve passing through the origin, 0, of the axes of the curve of Fig. (b). Field circuit resistance above the critical field resistance will fail to produce build-up.

Reasons for failure of self-excited shunt generator to build-up voltage :

• No residual magnetism ;
• Field connections reversed ; and
• Field circuit resistance too high