Wednesday, August 29, 2012

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.
       Ia = Ise 
       and  Ia = Ish +  IL
       Voltage across shunt field winding is Vt.
       Ish = Vt /Rsh
      where Rsh = Resistance of shunt field winding
       And voltage equation is,
       E = Vt + Ia Ra + Ia Rse + Vbrush
       Where Rse = 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, Ia Ise + Ish 
       and Ise = IL
...    Ia IL + Ish 
       The drop across shunt field winding is drop across the armature only and not the total Vt, in this case. So drop across shunt field winding is E -Ia Ra .
        Ish   = (E - Ia Ra ) / ( Rsh)
        Now the voltage equation is E = Vt + Ia Ra + Ise Rse + Vbrush
...      Ise = IL
...      E = Vt + Ia Ra + IL Rse + Vbrush
       Neglecting Vbrush , we can write,
       E = Vt + Ia Ra + IL Rse
       E - Ia Ra = Vt + IL Rse
       Ish = (Vt + IL Rse) / ( Rsh)
       Any of the two above expression of Ish 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
       ΦT = Φsh + Φse
      Where  Φsh = Flux producd by shunt
                 Φse = 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.
       ΦT = Φsh - Φse
       Where   Φsh = Flux produced by shunt field winding.
                    Φse = Flux produced by series field winding.
Fig. 4  Differential compound generator
Read solved examples

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