Objective questions (AC Machines)

 

Unit‑1: Three‑Phase Induction Motor (50 Questions)

  1. The stationary part of a three‑phase induction motor is called: A) Rotor B) Stator C) Shaft D) Frame Answer: B) Stator

  2. The rotating part of a three‑phase induction motor is: A) Stator B) Rotor C) Armature D) Commutator Answer: B) Rotor

  3. A three‑phase induction motor works on the principle of: A) Mutual induction B) Electromagnetic induction C) Self induction D) Resonance Answer: B) Electromagnetic induction

  4. Slip is defined as: A) Ns/N B) (Ns – N)/Ns C) N/Ns D) N – Ns/N Answer: B) (Ns – N)/Ns

  5. Slip speed is: A) Ns B) N C) Ns – N D) N – Ns Answer: C) Ns – N

  6. The most commonly used rotor type is: A) Slip ring B) Squirrel cage C) Wound rotor D) DC rotor Answer: B) Squirrel cage

  7. Slip ring induction motors are preferred for: A) Low cost B) High starting torque C) Small size D) Domestic fans Answer: B) High starting torque

  8. The equivalent circuit of an induction motor is similar to: A) Transformer B) DC motor C) Alternator D) Rectifier Answer: A) Transformer

  9. The phasor diagram of induction motor represents: A) Voltage‑current relation B) Speed‑torque relation C) Losses D) Efficiency Answer: A) Voltage‑current relation

  10. The torque equation depends on: A) Slip B) Rotor resistance C) Stator voltage D) All of these Answer: D) All of these

  11. Maximum torque occurs when: A) R2 = X2 B) R2 = S C) R2 = Ns D) R2 = V Answer: A) R2 = X2

  12. Starter is necessary in induction motor to: A) Increase torque B) Reduce starting current C) Reduce losses D) Increase efficiency Answer: B) Reduce starting current

  13. DOL starter is used for: A) Small motors B) Large motors C) Slip ring motors D) High voltage motors Answer: A) Small motors

  14. Star‑Delta starter reduces: A) Voltage B) Current C) Torque D) Speed Answer: A) Voltage

  15. Autotransformer starter is suitable for: A) Small motors B) Large motors C) Fans D) Pumps Answer: B) Large motors

  16. Rotor resistance starter is used for: A) Slip ring motors B) Squirrel cage motors C) DC motors D) Alternators Answer: A) Slip ring motors

  17. No‑load test is performed to find: A) Copper losses B) Iron losses C) Efficiency D) Torque Answer: B) Iron losses

  18. Blocked rotor test is performed to find: A) Copper losses B) Iron losses C) Efficiency D) Speed Answer: A) Copper losses

  19. Efficiency of induction motor is: A) Input/Output B) Output/Input C) Losses/Input D) Output – Losses Answer: B) Output/Input

  20. Speed control by stator voltage is suitable for: A) Fans B) Pumps C) Compressors D) All of these Answer: A) Fans

  21. Pole changing method is used for: A) Speed control B) Torque control C) Efficiency improvement D) Loss reduction Answer: A) Speed control

  22. Rotor resistance method of speed control is used for: A) Slip ring motors B) Squirrel cage motors C) DC motors D) Alternators Answer: A) Slip ring motors

  23. VVVF method controls: A) Speed B) Torque C) Efficiency D) Losses Answer: A) Speed

  24. Maintenance procedure includes: A) Cleaning B) Lubrication C) Insulation check D) All of these Answer: D) All of these

  25. Rewinding of induction motor is required when: A) Shaft breaks B) Bearings fail C) Winding burns D) Rotor bends Answer: C) Winding burns

  26. Motor selection depends on: A) Load torque B) Speed C) Application D) All of these Answer: D) All of these

  27. Fans require: A) Low starting torque B) High torque C) Constant speed D) Variable speed Answer: A) Low starting torque

  28. Crushers require: A) High starting torque B) Low torque C) Constant speed D) Low speed Answer: A) High starting torque

  29. Pumps require: A) Medium torque B) High torque C) Low torque D) Constant speed Answer: A) Medium torque

  30. Slip in induction motor is always: A) Positive B) Negative C) Zero D) Infinite Answer: A) Positive

  31. At synchronous speed, the torque of induction motor is: A) Maximum B) Zero C) Infinite D) Constant Answer: B) Zero

  32. The rotor frequency at standstill is equal to: A) Supply frequency B) Zero C) Slip × Supply frequency D) Rotor speed Answer: A) Supply frequency

  33. The rotor copper loss is proportional to: A) Slip B) Speed C) Voltage D) Torque Answer: A) Slip

  34. The mechanical power developed is maximum when slip is: A) 0 B) 1 C) 0.5 D) 2 Answer: C) 0.5

  35. The synchronous speed depends on: A) Frequency and poles B) Voltage and current C) Resistance and reactance D) Torque and slip Answer: A) Frequency and poles

  36. The rotor of squirrel cage motor is made of: A) Copper bars short‑circuited B) Slip rings C) Winding with brushes D) Laminated iron only Answer: A) Copper bars short‑circuited

  37. The air gap in induction motor is: A) Large B) Small C) Zero D) Infinite Answer: B) Small

  38. The magnetizing current in induction motor is: A) Large B) Small C) Zero D) Constant Answer: A) Large

  39. The power factor of induction motor at no load is: A) High B) Low C) Unity D) Zero Answer: B) Low

  40. The rotor input power is equal to: A) Mechanical power + Rotor copper loss B) Mechanical power only C) Copper loss only D) Iron loss only Answer: A) Mechanical power + Rotor copper loss

  41. The torque is directly proportional to: A) Slip at small values B) Speed C) Voltage D) Frequency Answer: A) Slip at small values

  42. The induction motor is called: A) Constant speed motor B) Variable speed motor C) DC motor D) Universal motor Answer: A) Constant speed motor

  43. The starting torque of squirrel cage motor is: A) Low B) High C) Medium D) Infinite Answer: A) Low

  44. The starting torque of slip ring motor is: A) High B) Low C) Medium D) Zero Answer: A) High

  45. The efficiency of induction motor is generally: A) 60–70% B) 80–90% C) 40–50% D) 30–40% Answer: B) 80–90%

  46. The induction motor is also called: A) Asynchronous motor B) Synchronous motor C) DC motor D) Universal motor Answer: A) Asynchronous motor

  47. The torque developed in induction motor is due to: A) Interaction of stator and rotor fields B) Voltage only C) Current only D) Resistance only Answer: A) Interaction of stator and rotor fields

  48. The blocked rotor test is similar to: A) Short circuit test of transformer B) Open circuit test of transformer C) Load test D) Efficiency test Answer: A) Short circuit test of transformer

Unit‑2: Single‑Phase Induction Motor (Objective Questions)

  1. A single‑phase induction motor is not self‑starting because: A) No rotating magnetic field is produced B) High rotor resistance C) Low supply voltage D) High iron losses Answer: A) No rotating magnetic field is produced

  2. The double revolving field theory explains: A) Efficiency improvement B) Starting torque production C) Loss reduction D) Speed control Answer: B) Starting torque production

  3. The equivalent circuit of a single‑phase induction motor is similar to: A) Transformer B) DC motor C) Alternator D) Rectifier Answer: A) Transformer

  4. Split‑phase induction motor uses: A) Resistance start method B) Capacitor start method C) Shaded pole method D) Rotor resistance method Answer: A) Resistance start method

  5. Capacitor start motor provides: A) High starting torque B) Low torque C) Constant speed D) Low efficiency Answer: A) High starting torque

  6. Capacitor start‑capacitor run motor has: A) Better efficiency and performance B) Low torque C) High losses D) Low speed Answer: A) Better efficiency and performance

  7. Shaded pole motor is commonly used in: A) Fans B) Pumps C) Compressors D) Crushers Answer: A) Fans

  8. The torque‑speed characteristic of capacitor start motor shows: A) High starting torque B) Low torque C) Constant speed D) Low efficiency Answer: A) High starting torque

  9. Applications of single‑phase induction motors include: A) Fans B) Mixers C) Pumps D) All of these Answer: D) All of these

  10. Maintenance of single‑phase motors includes: A) Cleaning B) Lubrication C) Insulation check D) All of these Answer: D) All of these

  11. Rewinding of single‑phase motor is required when: A) Shaft breaks B) Bearings fail C) Winding burns D) Rotor bends Answer: C) Winding burns

  12. Split‑phase motor is suitable for: A) Medium torque loads B) High torque loads C) Low torque loads D) Variable speed loads Answer: A) Medium torque loads

  13. Shaded pole motor is suitable for: A) Low torque loads B) High torque loads C) Medium torque loads D) Variable speed loads Answer: A) Low torque loads

  14. Capacitor start motor is used in: A) Compressors B) Fans C) Mixers D) Toys Answer: A) Compressors

  15. Double revolving field theory assumes: A) Two fields rotating in opposite directions B) One field only C) No field D) Infinite fields Answer: A) Two fields rotating in opposite directions

  16. The auxiliary winding in split‑phase motor is: A) High resistance winding B) Low resistance winding C) Rotor winding D) Field winding Answer: A) High resistance winding

  17. The main winding in split‑phase motor is: A) Low resistance winding B) High resistance winding C) Rotor winding D) Field winding Answer: A) Low resistance winding

  18. Capacitors start‑capacitor run motor is used in: A) Refrigerators B) Toys C) Ceiling fans D) Mixers Answer: A) Refrigerators

  19. Shaded pole motor has: A) Low starting torque B) High starting torque C) Medium torque D) Constant torque Answer: A) Low starting torque

  20. The shaded pole is created by: A) Copper ring around part of pole B) Extra winding C) Rotor resistance D) Capacitor Answer: A) Copper ring around part of pole

  21. The efficiency of single‑phase induction motor is generally: A) 40–60% B) 70–80% C) 90–95% D) 20–30% Answer: A) 40–60%

  22. The power factor of single‑phase induction motor is: A) Low B) High C) Unity D) Zero Answer: A) Low

  23. The starting current of single‑phase induction motor is: A) High B) Low C) Zero D) Constant Answer: A) High

  24. The capacitor in single‑phase motor improves: A) Starting torque and power factor B) Efficiency only C) Speed only D) Losses only Answer: A) Starting torque and power factor

  25. The direction of rotation in single‑phase motor can be reversed by: A) Reversing auxiliary winding connections B) Reversing rotor C) Changing supply frequency D) Changing voltage

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