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144                                  Chapter 7. DC machines


                           7.6   Types of dc generators


                           DC generators are classified according to the method of excitation of
                           their field windings. The production of the magnetic flux in the gener-
                           ator by circulating current in the field winding is called excitation. DC
                           generators are classified as given in the chart 7.8.

                                         DC generators



                                Separately excited      Self excited
                                  DC generator     DC generator




                                        Series        Shunt          Compound
                                      generator     generator          generator



                                                              Long shunt         short shunt
                                                              compound        compound
                                                           DC generator      DC generator


                                       Figure 7.8: Different types of dc generators



                           7.7   Separately excited dc generator


                           In this type of machine, the field winding is excited by a separate dc
                           source. The schematic diagram of separately excited dc machines is
                           shown in figure 7.9.

                              Let I a = Armature current
                                R a = Armature resistance

                                E = Generated emf
                                I L = Current through the load



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