Brushes of D.C. machines are made of_________?

Related Mcqs:

1. If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be________________?

   A. demagnetisation only
   B. cross magnetisation as well as mag¬netisation
   C. crossmagnetisation as well as demagnetising
   D. cross magnetisation only

2. In lap winding, the number of brushes is always_____________?

   A. double the number of poles
   B. same as the number of poles
   C. half the number of poles
   D. two

3. Copper brushes in D.C. machine are used_____________?

   A. where low voltage and high currents are involved
   B. where high voltage and small cur-rents are involved
   C. in both of the above cases
   D. in none of the above cases

4. The material for commutator brushes is generally_______________?

   A. mica
   B. copper
   C. cast iron
   D. carbon

5. In D.C. generators, the brushes on commutator remain in contact with conductors which_________________?

   A. lie under south pole
   B. lie under north pole
   C. lie under interpolar region
   D. are farthest from the poles

6. To achieve sparkless commutation brushes of a D.C. generator are rockedm ahead so as to bring them?

   A. just ahead of magnetic neutral axis
   B. in magnetic neutral axis
   C. just behind the magnetic neutral axis
   D. none

7. Armature coil is short circuited by brushes when it lies_____________?

   A. along neutral axis
   B. along field axis
   C. in any of the above positions
   D. in none of the above positions

8. The rapid wear of brushes takes place due to____________?

   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

9. The number of brushes in a commutator depends on______________?

   A. speed of armature
   B. type of winding
   C. voltage
   D. amount of current to be collected

10. In a D.C. generator in order to reduce sparking at brushes, the self-induced e.m.f. in the coil is neutralised by all of the following except______________?

    A. interpoles
    B. dummy coils
    C. compensating winding
    D. shifting of axis of brushes