electrical machines transformers induction motors synchronous machines dc machines characteristics starting control losses efficiency - Question Bank

1. What is the main purpose of the oil in a power transformer?
A) To provide mechanical support to the core
B) To act as a coolant and insulating medium
C) To reduce the noise level
D) To prevent corrosion of the windings
2. The maximum efficiency of a transformer occurs when:
A) Load is zero
B) Iron losses equal copper losses
C) Copper losses are zero
D) Load is at its maximum rating
3. Which component is responsible for maintaining the direction of torque in a DC motor?
A) Stator
B) Rotor
C) Commutator and brushes
D) Field winding
4. A cumulative compound DC motor has:
A) Series field assisting the shunt field
B) Series field opposing the shunt field
C) Only a series field
D) Only a shunt field
5. What is the function of the interpoles in a DC machine?
A) To increase the main field flux
B) To reduce the armature reaction and improve commutation
C) To provide a constant speed characteristic
D) To provide a high starting torque
6. In a DC generator, the induced EMF is proportional to:
A) Field flux and speed
B) Armature current and speed
C) Armature voltage and field flux
D) Speed and number of poles
7. The V-curves of a synchronous machine represent the variation of:
A) Armature current with field current
B) Torque with speed
C) Power factor with load
D) Efficiency with load
8. When a synchronous motor is operated at its rated load and unity power factor, its excitation is:
A) Under-excited
B) Normally excited
C) Over-excited
D) Zero
9. A synchronous motor is inherently:
A) Self-starting
B) Not self-starting
C) Starting with a high starting current
D) Starting with a low power factor
10. Which of the following is a method of starting a large three-phase induction motor to reduce starting current?
A) Direct-on-line (DOL) starting
B) Star-delta starting
C) Auto-transformer starting
D) All of the above
11. The maximum torque of an induction motor occurs at a slip value that is:
A) Always 1
B) Always 0
C) Dependent on the ratio of rotor resistance to rotor reactance
D) Independent of rotor parameters
12. What is the slip of an induction motor when it is running at synchronous speed?
A) 1
B) 0
C) 0.5
D) Negative
13. In an induction motor, the rotor's magnetic field rotates at:
A) Synchronous speed
B) Rotor speed
C) Slip speed
D) A speed slightly less than synchronous speed
14. What is the unit of magnetic flux density?
A) Weber (Wb)
B) Tesla (T)
C) Ampere-turn (At)
D) Henry (H)
15. The losses that vary with the square of the load current in a transformer are called:
A) Iron losses
B) Copper losses
C) Dielectric losses
D) Stray losses
16. Which type of transformer has a single winding that serves as both primary and secondary?
A) Isolation transformer
B) Step-up transformer
C) Autotransformer
D) Distribution transformer
17. In a transformer, the ratio of output voltage to input voltage is called:
A) Current ratio
B) Turns ratio
C) Voltage regulation
D) Transformation ratio
18. The efficiency of a DC machine is maximum when:
A) Variable losses are equal to constant losses
B) Variable losses are half of constant losses
C) Constant losses are half of variable losses
D) Variable losses are zero
19. Which DC motor has the poorest speed regulation?
A) Shunt motor
B) Series motor
C) Differential compound motor
D) Cumulative compound motor
20. What is the purpose of a starter in a DC motor?
A) To increase the speed of the motor
B) To limit the starting current
C) To reverse the direction of rotation
D) To improve the power factor
21. The back EMF in a DC motor opposes:
A) The applied voltage
B) The armature current
C) The field flux
D) The mechanical load
22. A DC shunt motor is generally preferred for:
A) Traction applications
B) Cranes and hoists
C) Applications requiring constant speed
D) High starting torque applications
23. Which type of DC motor has a speed that increases significantly with the decrease in load?
A) Series motor
B) Shunt motor
C) Compound motor
D) Permanent magnet DC motor
24. The torque produced by a DC motor is directly proportional to:
A) Armature voltage and field flux
B) Armature current and field flux
C) Armature voltage and armature current
D) Field flux and speed
25. A DC motor converts:
A) Electrical energy into mechanical energy
B) Mechanical energy into electrical energy
C) AC electrical energy into DC electrical energy
D) DC electrical energy into AC electrical energy
26. What is the role of a commutator in a DC machine?
A) To convert AC to DC in the armature winding
B) To convert DC to AC in the armature winding
C) To reverse the direction of current in the armature winding
D) To provide excitation to the field poles
27. The field winding of a synchronous motor is located on the:
A) Stator
B) Rotor
C) Commutator
D) Brush assembly
28. Hunting in a synchronous motor is caused by:
A) Sudden changes in load
B) Fluctuations in supply voltage
C) Oscillations of the rotor around its synchronous speed
D) All of the above
29. What is the purpose of damper windings in a synchronous machine?
A) To increase the field flux
B) To provide starting torque and prevent hunting
C) To improve ventilation
D) To reduce eddy current losses
30. A synchronous motor can be used for:
A) Power factor correction
B) Variable speed drive
C) High starting torque applications
D) Converting AC to DC
31. What is the power factor of a synchronous motor when over-excited?
A) Lagging
B) Leading
C) Unity
D) Zero
32. A synchronous motor operates at:
A) A speed less than synchronous speed
B) A speed equal to synchronous speed
C) A speed greater than synchronous speed
D) Variable speed depending on load
33. What is the starting torque of a three-phase induction motor proportional to?
A) Square of the supply voltage
B) Square of the rotor resistance
C) Square of the rotor frequency
D) Square of the number of poles
34. Which method is commonly used to control the speed of a three-phase induction motor?
A) Varying the stator voltage
B) Varying the rotor resistance (for wound rotor motors)
C) Varying the frequency of the supply
D) All of the above
35. What is the primary disadvantage of a wound rotor induction motor compared to a squirrel cage motor?
A) Lower starting torque
B) Higher efficiency
C) More complex construction and higher maintenance
D) Lower power factor
36. In a squirrel cage induction motor, the rotor bars are:
A) Wound with insulated copper wire
B) Connected to slip rings
C) Short-circuited by end rings
D) Connected to a commutator
37. The difference between the synchronous speed and the actual rotor speed of an induction motor is called:
A) Speed variation
B) Torque ripple
C) Slip
D) Rotor frequency
38. What is the synchronous speed of an induction motor?
A) The speed of the rotating magnetic field
B) The speed of the rotor
C) The speed of the stator current
D) The speed of the supply frequency
39. The principle of operation of a transformer is based on:
A) Ampere's law
B) Faraday's law of electromagnetic induction
C) Lenz's law
D) Coulomb's law
40. What is the purpose of the conservator in a power transformer?
A) To cool the windings
B) To store lubricating oil
C) To provide space for oil expansion and contraction
D) To prevent moisture ingress
41. Which test is performed on a transformer to determine its copper losses?
A) Open-circuit test
B) Short-circuit test
C) Polarity test
D) Load test
42. A three-phase transformer can be connected in Delta-Delta configuration. What is the phase shift introduced between primary and secondary line voltages?
A) 0 degrees
B) 30 degrees
C) 90 degrees
D) 180 degrees
43. What is the unit of apparent power in an AC circuit?
A) Watts (W)
B) Volt-Amperes (VA)
C) Volt-Amperes Reactive (VAR)
D) Joules (J)
44. The efficiency of a transformer is generally highest when:
A) Copper losses are maximum
B) Iron losses are maximum
C) Copper losses are equal to iron losses
D) The load is very light
45. Eddy current losses in a transformer core can be reduced by:
A) Increasing the thickness of the core laminations
B) Using a core material with high permeability
C) Laminating the core
D) Increasing the frequency of the supply
46. What is the main cause of hysteresis loss in a transformer core?
A) Eddy currents induced in the core
B) Resistance of the windings
C) Rapid reversal of magnetization in the core material
D) Leakage flux
47. In an ideal transformer, which of the following is assumed to be zero?
A) Primary voltage
B) Secondary voltage
C) Core losses
D) Copper losses
48. The ratio of the number of turns in the secondary winding to the number of turns in the primary winding of a transformer is called the:
A) Voltage ratio
B) Current ratio
C) Turns ratio
D) Power ratio
49. Which type of transformer is commonly used for stepping down voltage in power distribution systems?
A) Autotransformer
B) Isolation transformer
C) Step-down transformer
D) Step-up transformer
50. What is the primary function of a transformer?
A) To increase the frequency of AC power
B) To convert AC power to DC power
C) To change the voltage level of AC power
D) To reduce the power factor of AC circuits
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