A. abrasion from dust
B. excessive spring pressure
C. rough commutator bars
D. high mica insulation between com-mutation bars
E. all of the above factors
A. 2 : 3
B. 3 : 1
C. 3 : 2
D. 1 : 3
A. severe sparking
B. rough commutator surface
C. imperfect contact
D. any of the above
A. mainly to reduce the eddy currents by providing local short-circuits
B. to provide path for the circulation of cooling air
C. to neutralise the cross-magnetising effect of the armature reaction
D. none of the above
A. brushes should be of proper grade and size
B. brushes should smoothly run in the holders
C. smooth, concentric commutator properly undercut
D. all of the above
A. any even number of conductors
B. any odd number of conductors
C. that even number which is exact multiple of poles + 2
D. that even number which is exact multiple of poles
A. demagnetisation only
B. cross magnetisation as well as mag¬netisation
C. crossmagnetisation as well as demagnetising
D. cross magnetisation only