771. 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
772. 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
773. Regenerative method of braking is based on that___________?
A. back e.m.f. is less than the applied voltage
B. back e.m.f. is equal to the applied voltage
C. back e.m.f. of rotor is more than the applied voltage
D. none of the above
774. The hysteresis loss in a D.C. machine least depends on____________?
A. Frequency of magnetic reversals
B. Maximum value of flux density
C. Volume and grade of iron
D. Rate of flow of ventilating air
775. In a D.C. generator all of the following could be the effects of iron losses except___________?
A. Loss of efficiency
B. Excessive heating of core
C. Increase in terminal voltage
D. Rise in temperature of ventilating air
776. The losses occurring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator ?
A. hysteresis loss
B. field copper loss
C. armature copper loss
D. eddy current loss
777. Which of the following loss in a D.C. generator varies significantly with the load current ?
A. Field copper loss
B. Windage loss
C. Armature copper loss
D. None of the above
778. Torque developed by a D.C. motor depends upon_____________?
A. magnetic field
B. active length of the conductor
C. current flow through the conductors
D. number of conductors
E. radius of armature
F. all above factors
779. D.C. shunt motors are used for driving____________?
A. trains
B. cranes
C. hoists
D. machine tools
780. In a manual shunt motor starter?
A. over load relay is connected in series and no volt relay in parallel with the load
B. over load relay is connected in parallel and no volt relay in series with the load
C. over load relay and no volt relay are both connected in series with the load
D. over load relay and no volt relay are both connected in parallel with the load
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