A question about electric traction?

Tr active effort comes via the weight of the locomotive. With electric engines not having the weight of the prime mover and other gear found with diesel electric, how was the difference in weight compensated for?

 

Tractive effort itself has nothing to do with the weight of the locomotive; it is the torque generated at the wheel rim which becomes a force put into the rail to move the locomotive and its train. This force can exceed the wheels' adhesion with the rail, which is a function of the weight on the wheel and the coefficient of friction between the wheel and rail, in which case the wheel will slip. But the TE is still being applied.

 

While weight does not actually set tractive effort, it does limit how much can be usefully applied to the train (rather than wasted on spinning the wheels), and most designs are somewhere near this limit; traditionally 25% of the weight was considered the maximum useful, but creep control has somewhat increased this.

Hence, while electric power allows lightweight high power locos (e.g. compare a 91 with a HST), these have a low tractive effort and can only pull light loads, though they can do so very fast (power = tractive effort x speed).

When more low-speed tractive effort is needed, one option is to deliberately add weight to the loco, but this increases energy consumption and track wear. An alternative is to put the motors under the load and use its weight, i.e. a unit; this is particularly suited to frequently stopping services, which need a high ratio of tractive effort (and hence weight on powered axles) to total train weight for accelerating away from stops.