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1.0.0
MCQ
2421. In D.C motors diverters are used in-
Cumulative compound motors
Differentially compound motors
Shunt motors
Series motors
2422. Counter EMF of the DC motor is-
Less than the applied voltage
More than the applied voltage
Equal to the applied voltage
None of the above
Counter emf or back emf makes the DC motor a self- regulating machine. Le. It makes the DC motor to draw armature current which is just enough to develop the required torque. Thus it helps in energy conversion and it is always less than applied voltage.
2423. The starting resistance of DC motor is generally-
High
Infinite
Low
None of the above
2424. 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
2425. 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.
2426. In DC motor the percentage of residual magnetism is
2-3%
10-20%
50%
80-90%
2427. 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.
2428. 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.
2429. The flux density under leading pole tip in DC motor tends to -
Decrease
Remain same
Increas
None of the abovee
2430. 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
2431. The function of dummy coil is to-
Reduce eddy current
Reduce hysteresis loss
Increase flux
Mechanical balancing
2432. If the back EMF of DC motor vanishes then-
The motor continues to run
Motor will stop
Armature will burn
The motor continues to run in slow speed
If back emf vanishes suddenly, motor circuit will try to retain emf by drawing more current from supply. If supplying unit didn't trip down by this time, excess current in armature may heat up the armature.
2433. The flux density under trailing pole tip in DC motor tends to-
Decrease
Remain same
Increase
None of the above
2434. 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.
2435. Why is the speed of DC shunt motor dependent on Back EMF?
Because flux is proportional to the armature current
Because armature drop is negligible
Because Back EMF is equal to armature current
Because flux is constant in DC shunt motor
The field winding is connected across the armature. The resistance offered by the field is constant so the current is constant. Therefore the speed of the DC shunt motor depends on the back EMF.
2436. 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
2437. The application of differentially compound motor is-
Frequent on-off cycle
Low-starting torque
High-starting torque
Variable speed
Differential compound motor gives low starting torque, examined by torque current characteristic.
2438. The function of yoke in a DC machine is-
To provide mechanical protection
To reduce eddy current
Flux path completion
Both a & c
2439. For highly explosive atmosphere which of the following motor is generally preferred?
Battery operated motor
Series motor
Air mesur
Shunt motor
2440. 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