voltage and current
current
voltage over a wide load range
voltage over a narrow load range
parallel with the field windings
series with the field windings
parallel with the load
series with the armature winding
copper loss
eddy current loss
hysteresis loss
windage loss
reduce the overall mass
insulate core
reduce the eddy current loss
all the above
imperfect contact
severe sparking
rough commutator surface
any one of the above