MCQ
3801. Ampere-hour is the unit of-
quantity of electricity
strength of current
power
energy
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ব্যাখ্যা: Power is measured in watt. Joules. Strength of current measured in Amperes. Ampere-hour is the unit of quantity of electricity of electric charge.
3802. The application of differentially compound motor is-
Frequent on-off cycle
Low-starting torque
High-starting torque
Variable speed
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ব্যাখ্যা: Differential compound motor gives low starting torque, examined by torque current characteristic.
3803. The efficiency of the DC motor at maximum power is-
90%
100%
Around 80%
Less than 50%
3804. Which DC motor is preferred for elevator?
Differentially compound motor
Series motor
Shunt motor
Cumulative compound motor
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ব্যাখ্যা: 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.
3805. 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
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ব্যাখ্যা: 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.
3806. 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
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ব্যাখ্যা: 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.
3807. The current drawn by a 120 volts d.c. motor of armature resistance 0.50 and back e.m.f. 110 --- volts is ampere.
20
240
220
5
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ব্যাখ্যা: E = V- laRa
⇒ 110 = 120-la x0.5
la = 20 A
3808. Which DC motor has the least percentage increase in input current for the same percentage increase in torque?
Separately excited motor
Series motor
Shunt motor
Compound motor
3809. Which part of DC motor can sustain maximum temperature rise?
Armature winding
Field winding
Slip ring
Commutator
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ব্যাখ্যা: Field winding in a DC motor is not a rotating part. It is present away from the motating part of the machine. So, temperature rise in the machine will not produce any effect on the machine rotation. Thuse field winding can sustain the maximum temperature rise.
3810. 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
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ব্যাখ্যা: 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.
3811. The basic electrical requirement is art. Welding is that there should be-
coated electrodes
high open-circuit voltage
no arc blow
de power supply
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ব্যাখ্যা: The art of welding refers to a fabrication process that causes coalescence between materials such as metals, thermoplastics and others. In welding, heat is used to melt the workpieces and a filler materials is used to form a weld pool or molten material.
3812. The ratio of starting torque to full load torque is least in-
Differential compound motor
Shunt motor
Series motor
Cumulative compound motor
3813. 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
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ব্যাখ্যা: The direction of rotation of DC series motor may be reversed by changing the polarity of either field or the armature winding.
3814. The starting winding of a single phase motor is placed in the-
rotor
stator
armature
field
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ব্যাখ্যা: The starting winding also known as the auxiliary winding is used to create the torque needed to start a single phase motor. The starting winding is placed in the stator and also parallel of rotor of the motor.
3815. The energy is stored in a lead acid battery in the form of
Electromagnetic energy
Electrostatic Energy
Charged ions
Chemical energy
3816. Which DC motor is preferred for machine tools?
Series motor
Cumulative compound motor
Differentially compound motor
Shunt motor
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ব্যাখ্যা: 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.
3817. In the DC machine, the fractional pitch winding is used-
To reduce the harmonic in generated EMF
Improve cooling
Increase EMF
To reduce the copper losses
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ব্যাখ্যা: They improve the waveform of generated EMF Le. generated EMF can be made to approximate to sine wave more easily and the distorting harmonics can be reduced or totally eliminated.
3818. For which of the following protection from negative sequence currents is provided?
Generators
Motors
Transmission line
Transformers
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ব্যাখ্যা: Negative sequence over current relays are used to detect the unbalanced load on a generator which may cause excessive rotor heating. The relay is also used to detect unbalanced load currents in motors.
3819. The direction of induced e.m.f can be found by -
Lenz's law
Fleming's Right hand rule
Kirchoff's law
Ohm's law
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ব্যাখ্যা: The direction of the induced emf is always such as to result in opposition to the change producing it. This law is known as Lenz's law
3820. 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
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ব্যাখ্যা: (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.