Types of DC Generator


     Based on the methods of interconnection of windings the following types of dc generators are available.

(i). Separately excited dc generator 

(ii). Self-excited dc Generators with

      (a) Shunt-field connection,

      (b) Series-field connection and

      (c) Compound-field connection.



Separately excited DC Generator :


     Separately excited dc generators are those whose field winding are excited by from an external d.c. source.


E   =   V + Ia Ra + BCD

where

     BCD  =  brush contact drop ( usually 1 V / brush ) 

     Ra   =  armature resistance



Self-excited DC Generators :


     Self-excited dc generators are those whose field windings are excited by the same current which as connected to the armature winding terminals. These generators consists of permanent magnetic poles, in which there is always some magnetic flux is present and is known as residual magnetism. Due to this residual magnetism, when the armature is rotated, some e.m.f. and hence current is produced which is partially or fully passed through the field coils thereby strengthening the residual flux. 


      There are three types of self excited generators and named according to the manner in which their field windings are connected to the armature.

(a) Shunt generator 

(b) Series generator 

(c) Compound generator  

    (i) Short shunt compound generator  

    (ii) Long shunt compound generator



Shunt Generator :


     A dc shunt generator, the field winding is connected across the armature terminals. 


     Field current flows due to the emf induced in the armature because of the rotation of the armature by the prime mover. When the armature just starts rotating a small amount of emf is induced in the armature due to the residual magnetism of the field. This emf causes some current to flow in the field winding which causes a rise in the magnitude of the emf induced. This in turn increases the field current which causes more flux and hence more induced emf. 


     A shunt generator will excite only if the poles have some residual magnetism and the resistance of the field circuit winding has some value less than a critical value. The actual value depends on the machine and its speed.


         Iₐ =  Iₗ + Iₛₕ

         Iₛₕ =  V / Rₛₕ

         E  =  V + IₐRa + BCD

where
           Iₛₕ  =  shunt field current
           Rₛₕ  =  shunt field resistance



Series Generator :


     In this case, the field windings are connected in series with the armature terminals. Due to series connection armature is made with few turns as the full-load current passes through it. These generators are used for special purpose such as boosters etc.


         Iₐ =  Iₛₑ + Iₗ

         E =  V + Iₐ(Ra + Rₛₑ) BCD

where
        Iₛₑ =  series field current,
        Rₛₑ =  series field resistance


Compound Generator :


     In a compound generator there are two field windings, namely a shunt-field winding and a series-field winding. The shunt-field winding is connected in parallel with the armature and the series-field winding is connected in series with the combinations.

      The series winding will carry a large armature current Ia or Il and therefore it is made of wires of large cross-section and has a few turns only. The resistance of a series winding is very small. The shunt-winding is made up of wires of small cross-sections and has high resistance. Since the resistance of the shunt-field winding is high, the current flowing through it is small in comparison with the generator armature current. A compound generator therefore combines the best features of shunt generator and series machines.


     In a compound generator the shunt winding can be connected in two ways, namely   
 

(i) across the armature winding only (short-shunt).

(ii) across both the armature winding and the series winding (long-shunt).


Short-shunt Generator :



             Iₗ  =  Iₛₑ

             Iₐ  =  Iₗ +  Iₛₕ

             Iₛₕ  =  V + IₛₑRₛₑ / Rₛ

             E  =  V + IₐRa + IₛₑRₛₑ + BCD


Long-shunt Generator :



              Iₐ  =  Iₛₑ  =  Iₗ  + Iₛₕ

              Iₛₕ  =  V / Rₛₕ


              E   =  V +  Iₐ (Ra + Rₛₑ) + BCD

In all the types of generators discussed above, the power developed  =  EIₐ

Power delivered to the external load  = VIₗ 



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