Tuesday, February 5, 2013

DC Machines Test Questions Answers

1. The output power of any electrical motor is taken from the:
a) Armature
b) Field
c) Coupling mounted on the shaft
d) Motor frame

2. If the back emf in a dc motor varnishes suddenly, then the motor:
a) Run at very high speeds
b) Start hunting
c) Burn
d) Come to stall

3. The direction of the armature current in a dc motor is:
a) The same as the generated emf
b) Opposite to that of generated emf
c) Not dependent upon the direction of the armature emf
d) None of the above

4. When a dc machine is connected to the dc supply main it will produce:
a) EMF in opposition to the applied voltage
b) EMF in phase with the applied voltage
c) EMF decreases with time
d) None of the above

5. The direction of rotation of a dc motor can be determined by:
a) Fleming's right hand rule
b) Fleming's left hand rule
c) Lenz's law
d) Ampere's law

6. A thick wire is used in dc series motor field winding than that in dc shunt motor:
a) To create more flux
b) To reduce the resistance
c) To carry large load current
d) Both (b) and (c)

7. An electrical train employing a dc series motor is running at the fixed speed, when a sudden slight drop in the mains voltage occurs. This would result in:
a) Drop in the speed and rise in the current
b) Rise in the speed and drop in the current
c) Rise in the speed and rise in the current
d) Drop in speed with current unaltered

8. A DC shunt motor is driving a constant torque load. On inserting an additional resistance in the armature circuit, the speed of the motor will:
a) Remains unchanged
b) Increases
c) Decreases
d) Be Zero

9. A DC series motor is best suited for electrical traction because:
a) It develops high starting torque
b) It runs at very high speed
c) It is cheaper in cost
d) It is more rugged in construction like induction motor

10. A DC series motor is used for overhauling load. It can work stable if:
a) The armature is shunted by a resistor
b) The field winding is revered
c) A resistor is put in series with the machine
d) A divertor is put across the field

11. Which of the following dc motor has maximum self relieving property:
a) Shunt
b) Series
c) Cumulative compound
d) Differential compound

12. When the direction of power flow in a differential compounded motor reverses, it will operate as a:
a) Cumulative compound generator
b) Differential compound generator
c) Series motor
d) Shunt motor

13. A separately excited dc generator is feeding  dc shunt motor. If the load torque on the motor is halved approximately, then:
a) Armature current of both motor and generator are halved
b) Armature current of motor is halved and that of the generator is unaltered
c) Armature current of generator is halved and that of the motor is unaltered
d) Armature current of both motor and generator are unaltered

14. A flywheel is employed with a dc compound motor to reduce the peak demand, the compound motor should be:
a) Level compound
b) Differential compound
c) Cumulative compound
d) None of the above

15. The flux leakage coefficient in dc machine is usually:
a) 0.6 to 0.8
b) 1.05
c) 1.1 to 1.3
d) 2 to 2.5


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