Introduction
 Act of drawing or state of being drawn propulsion of vehicle 
 is  called  tractions.  There  are  various  systems  of  traction 
 prevailing in our country such as steam engine drive, electric 
 drive. These systems of tractions may be classified broadly 
 into groups namely
 1)  The  traction  system  which  do  not  involve  the  use  of 
 electricity  at  any  stage  and  called  as  non-electric  tractions 
 system such as steam engine drive, IC engine drive etc.
 2) The tractions system which involves the use of electricity 
 at some stage and called as electric tractions. System such a 
 diesel electric drive, electric drive etc.
  In India electrification in tractions are conducted 
  with three types of locomotives.
  1.  Using  single  phase  A.C.  series  commutator 
   motor.
  2.  Using  D.C.  motor  with  tapped  transformer 
   and rectifier.
  3. Using phase converter and induction motors.
        Existing Tractions System
   Existing tractions system uses D.C. motors.
   a) The 25 KV over head voltage is step down to 2000 V with the 
     help of step down transformer.
   b) Rectifier rectifies this A.C. voltage to D.C. voltage.
   c) This rectified D.C. voltage is used to operate the D.C. motors 
     in existing system engine.
   Causes favouring the DC motors
   1) D.C. series motors are less costly, however for some H.P more 
    efficient and requires less maintenance than A.C. series motor.
   2)  Rail  conductor  system of  track  electrifications  which  is  less 
    costly with D.C. system than with A.C. system
  Future Trends Of Tractions System
  There  are  some  disadvantages  of  D.C.  series 
  motor used in system.
  1) D.C. motors commutator which prove to failure 
   because of vibrations and shock. This results in 
   lots of sparking and corrosion.
  2) It is hard to use a D.C. motor for regenerative 
   braking and for this purpose extra switchgear is 
   required, which adds to the bulks and increases 
   the complexity of the locomotives.