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_aR_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

Ф_T = Ф_sh + Ф_se

Where  Ф_sh = Flux producd by shunt

Ф_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.

Ф_T = Ф_sh – Ф_se

Where   Ф_sh = Flux produced by shunt field winding.

Ф_se = Flux produced by series field winding.

Fig. 4  Differential compound generator