Which method of braking is generally used in elevators ?

In Ward-Leonard control the lower limit of speed is imposed by________?

A. residual magnetism of the generator
B. core losses of motor
C. mechanical losses of motor and generator together
D. all of the above

The speed of a D.C. shunt motor more than its full-load speed can be obtained by_____________?

A. decreasing the field current
B. increasing the field current
C. decreasing the armature current
D. increasing the armature current

The main disadvantage of the Ward-Leonard control method is______________?

A. high initial cost
B. high maintenance cost
C. low efficiency at Hght loads
D. all of the above

D.C. generators are normally designed for maximum efficiency around_____________?

A. full-load
B. rated r.p.m.
C. rated voltage
D. all of the above

Speed control by Ward Leonard method gives uniform speed variation_____________?

A. in one direction
B. in both directions
C. below normal speed only
D. above normal speed only

In case of D.C. shunt motors the speed is dependent on back e.m.f. only because____________?

A. back e.m.f. is equal to armature drop
B. armature drop is negligible
C. flux is proportional to armature current
D. flux is practically constant in D:C. shunt motors