EEE MCQ — Electrical Machines | ৫০টি প্রশ্ন ও উত্তর
Electrical Machines BCS EEE পরীক্ষার অন্যতম গুরুত্বপূর্ণ বিষয়। Transformer, DC Motor/Generator, Induction Motor ও Synchronous Machine থেকে প্রতি বছর প্রশ্ন আসে। নিচে ৫০টি নির্বাচিত MCQ দেওয়া হলো।
⚡ Transformer
1. A transformer works on the principle of:
a) Self-induction b) Mutual induction c) Electromagnetic force d) Electrostatic induction
✅ Answer: b
2. The core of a power transformer is made of:
a) Soft iron laminations b) Steel casting c) Copper d) Aluminium
✅ Answer: a
3. The purpose of laminating the core of a transformer is to reduce:
a) Copper losses b) Eddy current losses c) Hysteresis losses d) Stray losses
✅ Answer: b
4. The no-load current of a transformer is used to supply:
a) Copper loss only b) Core loss and magnetizing current
c) Load current d) Stray losses only
✅ Answer: b
5. In a transformer, if the turns ratio (N1/N2) = 2, then the voltage ratio (V1/V2) is:
a) 1 b) 4 c) 2 d) 0.5
✅ Answer: c
6. The efficiency of a transformer is maximum when:
a) Copper loss = Iron loss b) Copper loss > Iron loss
c) Iron loss > Copper loss d) No-load
✅ Answer: a
7. An auto-transformer has one winding that acts as both:
a) Primary and core b) Primary and secondary
c) Secondary and core d) Primary and load
✅ Answer: b
8. The short-circuit test of a transformer is conducted to determine:
a) Core losses b) Copper losses c) Efficiency d) Magnetizing current
✅ Answer: b
9. The open-circuit test of a transformer determines:
a) Copper losses b) Core (iron) losses c) Short-circuit impedance d) Load regulation
✅ Answer: b
10. Voltage regulation of a transformer is defined as:
a) (V_NL − V_FL) / V_FL × 100% b) (V_FL − V_NL) / V_FL × 100%
c) V_NL / V_FL × 100% d) V_FL / V_NL × 100%
✅ Answer: a
⚙️ DC Machines
11. In a DC generator, the function of a commutator is to:
a) Increase voltage b) Convert AC to DC c) Reduce losses d) Increase speed
✅ Answer: b
12. The EMF equation of a DC generator is:
a) E = PΦNZ / 60A b) E = PΦNZ / 120A c) E = PΦN / ZA d) E = ZΦN / PA
✅ Answer: a
13. A DC motor works on the principle of:
a) Faraday's law b) Fleming's left-hand rule (motor action)
c) Lenz's law d) Ampere's circuital law
✅ Answer: b
14. Back EMF in a DC motor:
a) Aids the applied voltage b) Opposes the applied voltage
c) Has no effect on current d) Is equal to zero at full speed
✅ Answer: b
15. The speed of a DC shunt motor can be controlled by:
a) Changing frequency b) Varying field resistance or armature resistance
c) Changing the number of poles d) Reversing the supply
✅ Answer: b
16. A DC series motor should never be started on:
a) Full load b) Half load c) No load d) Overload
✅ Answer: c (At no load, it runs at dangerously high speed)
17. The starting torque of a DC series motor is:
a) Zero b) Very low c) Very high d) Constant
✅ Answer: c
18. Ward-Leonard method is used for speed control of:
a) Induction motors b) Synchronous motors c) DC motors d) Universal motors
✅ Answer: c
19. In a lap winding DC machine, the number of parallel paths equals:
a) 2 b) Number of poles (P) c) Number of conductors d) P/2
✅ Answer: b
20. In a wave winding DC machine, the number of parallel paths is always:
a) P b) 4 c) 2 d) Z/P
✅ Answer: c
🌀 Induction Motor
21. The rotating magnetic field in a three-phase induction motor is produced by:
a) DC excitation b) Three-phase AC supply c) Permanent magnets d) Single-phase supply
✅ Answer: b
22. The synchronous speed of an induction motor is:
a) Ns = 120f/P b) Ns = 60f/P c) Ns = fP/120 d) Ns = 2f/P
✅ Answer: a
23. The slip of an induction motor is defined as:
a) (Ns − N) / N b) (Ns − N) / Ns c) N / Ns d) (N − Ns) / Ns
✅ Answer: b
24. At starting, the slip of an induction motor is:
a) 0 b) 0.5 c) 1 d) Greater than 1
✅ Answer: c
25. At synchronous speed, the slip of an induction motor is:
a) 1 b) 0.5 c) 0 d) Infinity
✅ Answer: c
26. The rotor frequency of an induction motor is:
a) f b) sf c) f/s d) f(1−s)
✅ Answer: b (where s = slip)
27. Which motor is used in ceiling fans?
a) Three-phase induction motor b) DC series motor
c) Single-phase induction motor d) Synchronous motor
✅ Answer: c
28. DOL (Direct On Line) starter is used for motors with power up to:
a) 100 kW b) 50 kW c) 5 kW d) 500 kW
✅ Answer: c
29. Star-delta starter reduces starting current by a factor of:
a) 2 b) √3 c) 3 d) 1/√3
✅ Answer: c
30. The torque of an induction motor is proportional to:
a) Rotor resistance only b) Slip only
c) Square of rotor EMF and slip d) Stator voltage only
✅ Answer: c
🔄 Synchronous Machine
31. A synchronous motor runs at:
a) Sub-synchronous speed b) Synchronous speed only
c) Super-synchronous speed d) Variable speed
✅ Answer: b
32. A synchronous motor is NOT self-starting because:
a) It has no starting torque at standstill b) It requires DC excitation
c) It has high resistance d) The rotor cannot follow the rotating magnetic field at start
✅ Answer: d
33. When a synchronous motor is over-excited, it operates at:
a) Lagging power factor b) Unity power factor c) Leading power factor d) Zero power factor
✅ Answer: c
34. A synchronous condenser is a synchronous motor running at:
a) Full load b) No load — used for power factor correction
c) Half load d) Overload
✅ Answer: b
35. The phenomenon of a synchronous motor falling out of step is called:
a) Hunting b) Surging c) Pulling out of synchronism d) Slipping
✅ Answer: c
36. Hunting in synchronous machines is controlled by:
a) Reducing excitation b) Damper windings c) Increasing load d) Reducing frequency
✅ Answer: b
37. The frequency of voltage generated by a synchronous generator is:
a) f = NP/60 b) f = NP/120 c) f = 120N/P d) f = 60N/P
✅ Answer: b (f = NP/120, where N = rpm, P = poles)
38. Salient pole synchronous machines are used for:
a) High-speed (turbo) generators b) Low-speed hydro generators
c) DC generators d) Induction generators
✅ Answer: b
39. Cylindrical rotor synchronous machines are suitable for:
a) Low-speed operation b) High-speed steam turbine driven generators
c) Hydro generators d) DC generators
✅ Answer: b
40. The voltage regulation of a synchronous generator is:
a) (E₀ − V) / V × 100% b) (V − E₀) / V × 100%
c) E₀ / V × 100% d) V / E₀ × 100%
✅ Answer: a
🔧 General & Mixed
41. Eddy currents in a machine core cause:
a) Increased efficiency b) Heating and power loss c) Increased torque d) Reduced noise
✅ Answer: b
42. Hysteresis loss in a magnetic core depends on:
a) Frequency and flux density (Steinmetz equation) b) Current only
c) Resistance only d) Temperature only
✅ Answer: a
43. A universal motor can run on:
a) AC only b) DC only c) Both AC and DC d) Three-phase AC only
✅ Answer: c
44. The starting torque of a squirrel cage induction motor compared to slip ring motor is:
a) Higher b) Lower c) Equal d) Zero
✅ Answer: b
45. External resistance can be added to the rotor circuit of a:
a) Squirrel cage induction motor b) Slip ring (wound rotor) induction motor
c) Synchronous motor d) DC shunt motor
✅ Answer: b
46. In a transformer, the leakage flux:
a) Links both primary and secondary b) Links only one winding
c) Reduces core losses d) Increases efficiency
✅ Answer: b
47. A stepper motor is used where:
a) High speed is required b) Precise angular positioning is required
c) High torque at low speed is needed d) Power factor correction is needed
✅ Answer: b
48. BLDC (Brushless DC) motor has advantages over conventional DC motor because:
a) It uses brushes for commutation b) No brushes — less maintenance, longer life
c) It runs at lower speed d) It requires more maintenance
✅ Answer: b
49. The efficiency of a large power transformer is typically:
a) 50–70% b) 70–85% c) 85–95% d) 95–99%
✅ Answer: d
50. In a DC generator, armature reaction causes:
a) Increase in terminal voltage b) Distortion and weakening of the main field flux
c) Reduction in speed d) Increase in efficiency
✅ Answer: b
গুরুত্বপূর্ণ সূত্র সংক্ষেপ
| বিষয় | সূত্র |
|---|---|
| Transformer turns ratio | N1/N2 = V1/V2 = I2/I1 |
| DC Generator EMF | E = PΦNZ/60A |
| Synchronous speed | Ns = 120f/P |
| Slip | s = (Ns−N)/Ns |
| Rotor frequency | fr = s×f |
| Generator frequency | f = NP/120 |
পরবর্তী পোস্ট: Power Systems MCQ — ৫০টি প্রশ্ন ও উত্তর
Published by Electricity School | www.esbangla.com
.jpg)
0 Comments