<P> Bird - nesting due to overspeed can occur either in operating traction motors of powered locomotives or in traction motors of dead - in - consist locomotives being transported within a train traveling too fast . Another cause is replacement of worn or damaged traction motors with units incorrectly geared for the application . </P> <P> Damage from overloading and overheating can also cause bird - nesting below rated speeds when the armature assembly and winding supports and retainers have been damaged by the previous abuse . </P> <P> Because of the high power levels involved, traction motors are almost always cooled using forced air . </P> <P> Typical cooling systems on U.S. diesel - electric locomotives consist of an electrically - powered fan blowing air into a passage integrated into the locomotive frame . Rubber cooling ducts connect the passage to the individual traction motors and cooling air travels down and across the armatures before being exhausted to the atmosphere . </P>

A dc series motor is best suited for electrical traction because