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1.0.0
MCQ
1. When the speed of DC motor is increased-
Back E.M.F increases and current drawn decreases
Back E.M.F decreases and current drawn increases
Back E.M.F and current drawn both increases
Back E.M.F and current drawn both decreases
2. Which DC motor is preferred for machine tools?
Series motor
Cumulative compound motor
Differentially compound motor
Shunt motor
Shunt characteris is the speciality of DC shunt motor The speed of the motor almost constant on various loads, thus it suits perfectly to the application, where speed requirement is constant like in machine tools.
3. In a DC shunt motor if field current is decreased then the speed of the motor is-
Decrease
Increase
Remain same
None of the above
4. For highly explosive atmosphere which of the following motor is generally preferred?
Battery operated motor
Series motor
Air mesur
Shunt motor
5. The direction of rotation of DC series motor can be changed by-
Interchanging field terminal
Interchanging supply terminal
Both a & b
None of the above
The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.
6. . A 250V, DC shunt motor takes a line current of 20A. Resistance of shunt field winding is 2002 and resistance of the armature is 0.30. Find the armature current and the back e.m.f.
18.75A, 244V
25.32A, 225V
15.65A, 100V
10A, 150V
7. The flux density under leading pole tip in DC motor tends to -
Decrease
Remain same
Increas
None of the abovee
8. What will happen when the DC motor is connected to A.C supply?
DC motor will run at rated speed
DC motor will burn
DC motor will run at slow speed
Both b & c
(i) In case of series connection of armature winding and field winding, it may run. (ii) But, In case of parallel connection, it won't rotate at all and will start humming and will create vibrations, us a torque produced by positive and negative cycle will cancel out each onther D.C motor will be heated up.
9. In D.C motors diverters are used in-
Cumulative compound motors
Differentially compound motors
Shunt motors
Series motors
10. When the ratio of back EMF to applied voltage is -- then the mechanical power developed by the shunt generator is maximum.
1.0
0.5
2
3
11. The starting resistance of DC motor is generally-
High
Infinite
Low
None of the above
12. The flux density under trailing pole tip in DC motor tends to-
Decrease
Remain same
Increase
None of the above
13. The amount of back emf of a shunt motor will Increase when
the load is increased
the field is weakened
the field is strengthed
None of the above
14. The function of yoke in a DC machine is-
To provide mechanical protection
To reduce eddy current
Flux path completion
Both a & c
15. When the speed of the DCtor increases its Armature current-
Increases
Decreases
Remain constant
Any of the above
16. What will happen if the 220 volts DC series motor is connected to 220 Volt AC supply?
The motor will not run
The armature winding of the motor will burn
The motor will run with less efficiency and more sparking
Either b & c
17. A 4 pole de shunt generator with a shunt field resistance of 100 ohm and armature resistance of 1 ohm has 378 wave connected conductors in its armature. The flux per pole is 0.02Wb. If a load resistance of 10 ohm connected across the armature terminals and the generator is driven at 1000 rpm then power absorbed by the load is-
6321W
4315W
5153W
2314W
18. Which DC motor is preferred for elevator?
Differentially compound motor
Series motor
Shunt motor
Cumulative compound motor
DC cumulative compound motor having high starting torque up to 450% depending upon the degree of compounding. The speed regulation is varying up to 25 -30% That's why these motors are used in elevators.
19. In DC motor the percentage of residual magnetism is
2-3%
10-20%
50%
80-90%
20. The function of dummy coil is to-
Reduce eddy current
Reduce hysteresis loss
Increase flux
Mechanical balancing