The compensation winding in our ET traction motors, which is in series
with the armature, weakens the field when the motor is running forward.
Without a compensation winding DC motors, running in the forward
direction, will slow the heavier the load (current draw) is. If the
compensation is set to weaken the field under load the effect is to
speed the motor up. GE just had to size the compensation winding to
weaken the field just enough that the motor only slowed down a little
under load, not speed up.
GE by using the compensation this way reduced tractor speed changes due
to hills and the like. I know my E15 doesn't slow all that much going up
hill, at least in the lower motor speeds of 1, 2, and 3.