Compound Generator

In this type, the part of the field winding is connected in parallel with armature and part in series with the armature. Both series and shunt field windings are mounted on the same poles. Depending upon the connection of shunt and series field winding, compound generator is further classified as : i) Long shunt compound generator, ii) Short shunt compound generator.

1.1 Long Shunt Compound Generator

       In this type, shunt field winding is connected across the series combination of armature and series field winding as shown in the Fig. 1.

Fig. 1 Long shunt compound generator

       Voltage and current relations are as follows.

       From the Fig. 1.

       I_a = I_se 

       and  I_a = I_sh +  I_L

       Voltage across shunt field winding is V_t.

       I_sh = V_t /R_sh

      where R_sh = Resistance of shunt field winding

       And voltage equation is,

       E = V_t + I_a R_a + I_a R_se + V_brush

       Where R_se = Resistance of series field winding

1.2 Short Shunt Compound Generator

       In this type, shunt field winding is connected, only across the armature, excluding series field winding as shown in the Fig. 2.

Fig. 2  Short shunt compound generator

      Voltage and current relations are as follows.

       For the Fig. 2, I_a =  I_se + I_sh 

       and I_se = I_L

.^..    I_a =  I_L + I_sh 

       The drop across shunt field winding is drop across the armature only and not the total V_t, in this case. So drop across shunt field winding is E -I_a R_a .

        I_sh   = (E - I_a R_a ) / ( R_sh)

        Now the voltage equation is E = V_t + I_a R_a + I_se R_se + V_brush

.^..      I_se = I_L

.^..      E = V_t + I_a R_a + I_L R_se + V_brush

       Neglecting V_brush , we can write,

       E = V_t + I_a R_a + I_L R_se

       E - I_a R_a = V_t + I_L R_se

       I_sh = (V_t + I_L R_se) / ( R_sh)

       Any of the two above expression of I_sh can be used, depending on the quantities known while solving the problems.

1.3  Cumulative and Differential Compound Generator

       It is mentioned earlier that the two windings, shunt and series field are wound on the same pole. Depending on the direction of winding on the pole, two fluxes produced by shunt and series field may help or may oppose each other. This facts decides whether generator is cumulative or differential compound. If the two fluxes help each other as shown in Fig. 3 the generator is called cumulative compound generator.

Fig. 3 Cumulative compound generator

       Φ_T = Φ_sh + Φ_se

      Where  Φ_sh = Flux producd by shunt

                 Φ_se = Flux produced by series, field winding

       If the two windings are wound in such a direction that the fluxes produced by them oppose each other then the generator is called differential compound generator. This is shown in the Fig. 4.

       Φ_T = Φ_sh - Φ_se

       Where   Φ_sh = Flux produced by shunt field winding.

                    Φ_se = Flux produced by series field winding.

Fig. 4  Differential compound generator