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৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]

পরীক্ষা৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]তারিখতারিখ অনির্ধারিতসময়18 minutes
মোট প্রশ্ন৩০
সিলেবাস
Exam 4 i) DC Motors ii) Synchronous Generators [Source: Class–3 and relevant books]
প্রশ্ন দেখুনঅনুশীলন
ঘনত্ব
উত্তর
উত্তরিতবর্তমানপুনরায় দেখুনঅসম্পূর্ণ

৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২]

৪৯তম বিসিএস ⎯ তথ্য ও যোগাযোগ প্রযুক্তি (EEE) [ ৮৯২] · তারিখ অনির্ধারিত · ৩০ প্রশ্ন

১.
In a DC motor, the back EMF is:
  1. ক
    Proportional to field flux and speed 
  2. খ
    Inversely proportional to speed
  3. গ
    Proportional to current
  4. ঘ
    Independent of flux
ব্যাখ্যা

Eb = PΦZN/60A 

২.
If the back EMF of a DC motor becomes equal to the applied voltage, then the armature current is:
  1. ক
    Maximum
  2. খ
    Zero
  3. গ
    Constant
  4. ঘ
    Infinite
ব্যাখ্যা

Ia = V−Eb/Ra 
​If V = Eb, then Ia = 0

৩.
When load increases on a DC motor:
  1. ক
    Speed increases
  2. খ
    Back EMF decreases
  3. গ
    Armature current decreases
  4. ঘ
    Torque decreases
ব্যাখ্যা

When load increase back emf will decrease 

৪.
A series motor has:
  1. ক
    High starting torque
  2. খ
    Low starting torque
  3. গ
    Constant speed
  4. ঘ
    Cannot run on load
ব্যাখ্যা

The main properties of a series motor is that it has high starting torque 

৫.
A shunt motor is preferred where
  1. ক
    Variable speed required
  2. খ
    Constant speed required
  3. গ
    Very high torque required
  4. ঘ
    Frequent starting required
ব্যাখ্যা

Shunt motor provides constant speed 

৬.
Which motor is used for electric traction?
  1. ক
    Shunt
  2. খ
    Series
  3. গ
    Compound
  4. ঘ
    Universal
ব্যাখ্যা

High starting torque is required for that purpose. Series motor deliver this.

৭.
Speed of a series motor at no-load becomes:
  1. ক
    Infinite
  2. খ
    Zero
  3. গ
    Constant
  4. ঘ
    Reduced
ব্যাখ্যা

Field flux ∝ load current . At no-load flux ~ 0 so, Speed goes to dangerously high.

৮.
Why DC motors require starters?
  1. ক
    To avoid high speed
  2. খ
    To limit starting current
  3. গ
    To increase torque
  4. ঘ
    To reduce losses
ব্যাখ্যা

Starter is used in DC motor to limit starting current 

৯.
Which method is used for speed control of DC shunt motor above rated speed?
  1. ক
    Armature control
  2. খ
    Field control
  3. গ
    Voltage control
  4. ঘ
    Resistance control
ব্যাখ্যা

Weakening field flux will cause high speed.

১০.
In armature control method of speed control, speed is:
  1. ক
    Below rated speed
  2. খ
    Above rated speed
  3. গ
    Constant
  4. ঘ
    Independent of load
ব্যাখ্যা

Armature control method is applied when motor operates at below rated speed 

১১.
The starter used in DC shunt motor:
  1. ক
    2-point starter
  2. খ
    3-point starter
  3. গ
    4-point starter
  4. ঘ
    Any of above
ব্যাখ্যা
DC shunt motor uses → 3-point starter
2-point starter → for DC series motor
3-point starter → for DC shunt motor (Line, Armature, Field)
4-point starter → improved type, used when field weakening is needed
১২.
In a compound motor, cumulative compounding means:
  1. ক
    Series flux aids shunt flux
  2. খ
    Series flux opposes shunt flux
  3. গ
    Series flux is zero
  4. ঘ
    Both fluxes cancel
ব্যাখ্যা

Cumulative compound is the process of adding flux from series and shunt field windings 

১৩.
A shunt motor runs at 1000 rpm at 220 V. If terminal voltage is reduced to 110 V (flux constant), speed becomes:
  1. ক
    500 rpm
  2. খ
    1000 rpm
  3. গ
    220 rpm
  4. ঘ
    550 rpm
ব্যাখ্যা

N2/N1=V2/V1
N2=N1×V2/V1 
​        = (1000×110)220 
​        = 500 rpm

১৪.
Which is not a method of DC motor braking?
  1. ক
    Plugging
  2. খ
    Rheostatic braking
  3. গ
    Regenerative braking
  4. ঘ
    Dynamic speed control
ব্যাখ্যা

DC motor braking methods are-
​Plugging
​Rheostatic braking
​Regenerative braking

১৫.
Plugging in DC motor means:
  1. ক
    Connecting motor across supply
  2. খ
    Reversing supply terminals
  3. গ
    Reducing field flux
  4. ঘ
    Adding external resistance
ব্যাখ্যা

Plagging is a process of electrical braking which attained by reversing supply terminal 

১৬.
Which braking returns energy to supply?
  1. ক
    Rheostatic
  2. খ
    Plugging
  3. গ
    Regenerative
  4. ঘ
    Dynamic
ব্যাখ্যা

During regenerative braking, machine act as generator 

১৭.
A 10 kW motor has 500 W constant losses and 500 W copper losses. Efficiency?
  1. ক
    85%
  2. খ
    90%
  3. গ
    92%
  4. ঘ
    95%
ব্যাখ্যা

Output = 10 kW
​Input = 10,000 + (500 + 500) = 11,000 W
​η = (10,000/11,000) × 100 = 90.9%

১৮.
A 220 V motor delivers 10 kW at 85% efficiency. Input power?
  1. ক
    10 kW
  2. খ
    12 kW
  3. গ
    15 kW
  4. ঘ
    20 kW
ব্যাখ্যা

η = Output/Input ⇒
​Input = 10/0.85 = 11.76 ≈ 12kW

১৯.
Synchronous machines operate at a speed which is:
  1. ক
    Proportional to load
  2. খ
    Proportional to supply voltage
  3. গ
    Independent of load
  4. ঘ
    Proportional to armature current
ব্যাখ্যা

Synchronous speed depends only on frequency and numberof poles, not load.

২০.
Which part of a synchronous machine carries field winding?
  1. ক
    Armature
  2. খ
    Rotor
  3. গ
    Stator
  4. ঘ
    Yoke
ব্যাখ্যা

Insynchronous machines, the rotor carries field winding supplied with DC.

২১.
In synchronous machines, armature winding is generally placed on:
  1. ক
    Rotor
  2. খ
    Shaft
  3. গ
    Stator
  4. ঘ
    Both rotor & stator
ব্যাখ্যা

For better cooling & insulation, armature windings on stator

২২.
Synchronous machine power factor can be controlled by:
  1. ক
    Rotor resistance
  2. খ
    Stator current
  3. গ
    Field excitation
  4. ঘ
    Armature resistance
ব্যাখ্যা

By increasing/decreasing excitation, machine operates at lagging/ leading power factor.

২৩.
A synchronous generator works on the principle of:
  1. ক
    Lenz’s Law
  2. খ
    Faraday’s Law
  3. গ
    Coulomb’s Law
  4. ঘ
    Fleming’s Left Hand Rule
ব্যাখ্যা

Voltage is induced in stator conductors due to rotatiing magnetic field. 

২৪.
A 3-phase, 4-pole alternator has flux/pole = 0.05 Wb, 60 turns/phase, runs at 1500 rpm. Find emf per phase (assume Kw=1)
  1. ক
    300 V
  2. খ
    377 V
  3. গ
    444 V
  4. ঘ
    500 V
ব্যাখ্যা

f = PN120 = 4 × 1500120 = 50 Hz
​E = 4.44fΦT = 4.44 × 50 × 0.05 × 60 = 444 V

২৫.
If No-load voltage = 250 V, terminal voltage = 230 V at full load, find voltage regulation.
  1. ক
    5%
  2. খ
    8.7%
  3. গ
    10%
  4. ঘ
    12%
ব্যাখ্যা

VR = (250−230/230) × 100 = 8.7%

২৬.
Core loss in synchronous machine mainly depends on:
  1. ক
    Current
  2. খ
    Voltage & frequency
  3. গ
    Speed
  4. ঘ
    Load power factor
ব্যাখ্যা

Hysteresis Loss ,Ph​=Kh​fBmaxV 
​            Depends on frequency (f) and maximum flux density (Bmax). 
​            Bmax​ is proportional to voltage applied.
​Eddy Current Loss
​            Pe=Ke f2 Bmax2 V 
​            Depends on square of frequency (f²) and flux density (∝ voltage).
​So, total core loss = hysteresis loss + eddy current loss, both of which are dependent on voltage & frequency

২৭.
Before connecting alternators in parallel, following must be same:
  1. ক
    Frequency
  2. খ
    Phase sequence
  3. গ
    Voltage magnitude
  4. ঘ
    All of the above
ব্যাখ্যা

following must be same for alternators in parallel-
​Frequency
​Phase sequence
​Voltage magnitude
​Phase angle

২৮.
Power equation of synchronous machine is P = (EV/Zs) × sinδ. Here δ is-
  1. ক
    Torque angle
  2. খ
    Load angle
  3. গ
    Power angle
  4. ঘ
    All of the above
ব্যাখ্যা

Explanation: δ can betermed as-
​Torque angle
​Load angle
​Power angle

২৯.
For maximum power transfer in synchronous machine, δ=
  1. ক
    0°
  2. খ
    45°
  3. গ
    90°
  4. ঘ
    180°
ব্যাখ্যা

P = (EV/Zs) * sinδ
​Maximum value of sinδ=1
​So δ=90°

৩০.
If synchronous reactance increases, maximum power transfer capability:
  1. ক
    Increases
  2. খ
    Decreases
  3. গ
    Remains same
  4. ঘ
    Becomes zero
ব্যাখ্যা

Power is inversely proportional to synchronous reactance