imperfect contact

severe sparking

rough commutator surface

any one of the above

low voltage, low current

high voltage, high current

low voltage, high current

high voltage, low current

shunt generator

series generator

long shunt compound generator

short shunt compound generator

60-80%

75-85%

80-90%

85-95%

to avoid unequal distribution of current at the brushes

to help in reducing sparking

both the above

none of the above