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Electrical Machine

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Electrical Machine

shape Introduction

What is an Electrical Machine? Electrical Machine is a device which converts mechanical energy into electrical energy or vice versa (generators and motors) and also includes transformers. Electric motor converts electricity to mechanical power while an Electric Generator converts mechanical power to electricity. As per Wikipedia, "Electric machines, in the form of generators, produce virtually all electric power on Earth, and in the form of electric motors consume approximately 60% of all electric power produced".

shape Quiz

Q1. For an induction motor, operating at a slip s, the ratio of gross power output to air gap power is equal to
    A. [latex](1 – s)^{2}[/latex] B. (1 – s) C. [latex]\sqrt{(1-s)}[/latex] D. [latex](1- \sqrt{s})[/latex] Answer: B

Q2. Which three phase connection can be used in a transformer to introduce a phase difference of 30° between its output and corresponding input time voltages?
    A. Star-Star B. Star-Delta C. Delta-Delta D. Delta-Zigzag Answer: B

Q3. The equivalent circuit of a transformer has leakage reactances [latex] X_{1}[/latex],[latex]X_{1}[/latex] and magnetizing reactance [latex]X_{M}[/latex]. Their magnitudes satisfy
    A. [latex]X_{1}[/latex]>>[latex]X_{2}[/latex]>>[latex]X_{M}[/latex] B. [latex]X_{1}[/latex] <<[latex]X_{2}[/latex] <>[latex]X_{M}[/latex] D. [latex]X_{1}[/latex] [latex]X_{2}[/latex]<<[latex]X_{M}[/latex] Answer: D

Q4. A synchronous generator is feeding a zero power factor (lagging) load at rated current. The armature reaction is
    A. magnetizing B. demagnetizing C. cross-magnetizing D. ineffective Answer: B

Q5. For a single phase capacitor start induction motor which of the following statements is valid
    A. The capacitor is used for power factor improvement. B. The direction of rotation can be changed by reversing the main winding terminals. C. The direction of rotation cannot be changed. D. The direction of rotation can be changed by interchanging the supply terminals. Answer: B

Q6. In a transformer, zero voltage regulation at full load is
    A. not possible B. possible at unity power factor load C. possible at leading power factor load D. possible at lagging power factor load Answer: C

Q7. The DC motor, which can provide zero-speed regulation at full load without any controller, is
    A. series B. shunt C. cumulative compound D. differential compound Answer: B

Q8. A 4-point starter is used to start and control the speed of a
    A. dc shunt motor with armature resistance control B. dc shunt motor with field weakening control C. dc series motor D. dc compound motor Answer: A

Q9. A negative sequence relay is commonly used to protect
    A. an alternator B. a transformer C. a transmission line D. a bus bar Answer: A

Q10. The slip of an induction motor normally does not depend on
    A. rotor speed B. synchronous speed C. shaft torque D. core-loss component Answer: D
Q11. Leakage flux in an induction motor is
    A. flux that leaks through the machine B. flux that links both stator and rotor windings C. flux that links none of the windings D. flux that links the stator winding or the rotor winding but not both Answer: D

Q12. In dc machines, the armature windings are placed on the rotor because of the necessity for
    A. electromechanical energy conversion B. generation of voltage C. commutation D. development of torque Answer: C

Q13. In normal dc machines operating at full-load conditions, the most powerful electromagnet is
    A. field winding B. armature winding C. interpole winding D. interpole and compensating windings together Answer: A

Q14. In a synchronous generator operating at zero power factor lagging, the effect of armature reaction is
    A. magnetizing B. demagnetizing C. cross-magnetizing D. both magnetizing and cross-magnetizing Answer: B

Q15. In double delta transformation, a double delta refers to the case where there are two delta transformations in
    A. Parallel B. Series C. Both series and parallel D. Neither series nor parallel Answer: D

Q16. Most familiar application of zig - zag transformer is as
    A. Ground reference on an ungrounded system B. Converting single phase to two phase C. Reducing harmonics D. All of these Answer: A

Q17. In Scott connection, the voltage across the teaser leads the mains by
    A. 30 degree B. 60 degree C. 90 degree D. 120 degree Answer: C

Q18. Th e load carried by V - V connection is
    A. 47.7% of the original load B. 57.7% of the original load C. 67.7% of the original load D. 87.7% of the original load Answer: B

Q19. The most commonly used connections for power systems as a step - up and step - down transformers are
    A. Star - delta, star - star B. Delta - star, star - delta C. Star - star, delta - delta D. Star - delta, delta - star Answer: D

Q20. For large low voltage transformers, the most commonly used connections are
    A. Star - star connection B. Delta - delta connection C. V connection D. All of these Answer: B
Q21. Which test is also called as back to back test?
    A. Retardation test B. Sumpner's test C. Field test D. Voltage drop test Answer: B

Q22. Open circuit test is to be performed on transformer to find
    A. Constant losses B. Copper losses C. Both (a) and (b) D. None of these Answer: A

Q23. In washing machines, most commonly used motors are
    A. Split phase induction motors B. Slip ring induction motors C. Capacitor start induction motors D. Shaded pole induction motors Answer: A

Q24. The phase shift in a star-star connected three phase transformer is
    A. 0 degree B. 30 degree C. 60 degree D. 120 degree Answer: A

Q25. In a single phase induction motor, the motor speed on no load is
    A. Almost equal to its synchronous speed B. Less than its synchronous speed C. Greater than synchronous speed D. None of these Answer: A

Q26. In refrigerators the commonly used motors are
    A. Split phase induction motors B. Capacitor induction motors C. Shaded pole induction motors D. None of these Answer: B

Q27. The constant voltage transformers are most commonly used in
    A. Personal computers B. Arc welding C. Power distribution D. Electric cars Answer: A

Q28. For the parallel operation of three phase transformers, which among the following connection is not applicable?
    A. D- Dto Y - Y B. Y - D to D - Y C. Y - Y to Y - Y D. None of these Answer: D

Q29. The spit phase induction motor has
    A. Low starting current and high starting torque B. Moderate starting current and moderate starting torque C. Low starting current and moderate starting torque D. Moderate starting current and low starting torque Answer: C

Q30. If field current is decreased in shunt dc motor, the speed of the motor
    A. remains same. B. increases. C. decreases. D. none of the above. Answer: C
Q31. Eddy current loss will depends on
    A. Frequency B. Flux density C. Thickness D. All of the above Answer: A

Q32. Thin laminations are used in a machine in order to reduce
    A. Eddy current losses B. Hyster esis losses C. Both 1 and 2 D. Copper losses Answer: A

Q33. Hysteresis loop represents the area of
    A. Copper loss B. Eddy current loss C. Hysteresis loss D. Total iron losses Answer: C

Q34. Hysteresis loss will depends on
    A. Frequency B. Flux density C. Thickness D. All of the above Answer: A

Q35. The emf induced in the dc generator armature winding is
    A. AC B. DC C. AC and DC D. None of the above Answer: A

Q36. A DC generator without commutator is a
    A. AC generator B. DC motor C. DC generator D. Induction motor Answer: A

Q37. In DC generators brushes are used for
    A. Collecting of current with minimum sparking B. Collecting of voltage C. Reduce eddy current loss D. Convert ac armature current in to dc Answer: A

Q38. Which of the following bearings and their uses are correct
    A. Ball bearings small machines B. Roller bearings large machines C. Both A and B D. Neither 1 nor 2 Answer: A

Q39. Which of the following windings are necessary in case of all dc machines?
    A. Closed winding B. Lap winding C. Wave winding D. Open type winding Answer: A

Q40. DC machine windings are
    A. Full pitched B. Short pitched C. Either of these D. None of the above Answer: A
Q41. Inter pole winding is connected in_______?
    A. Series with armature B. Series with main poles C. Parallel with armature D. Parallel with main poles Answer: A

Q42. Equalizer rings are used in which of the followings?
    A. Lap winding B. Wave winding C. Both 1 and 2 D. None of the above Answer: A

Q43. For a dc machine shunt resistance and armature resistance values are
    A. High and high B. High and low C. Low and low D. Low and high Answer: B

Q44. The effect of ________ on main field flux is armature reaction?
    A. Armature mmf B. Armature current C. Armature flux D. All of the above Answer: C

Q45. Shunt field of DC generators consists of--------number of turns and ---------- conductors respectively
    A. Large and thick B. Large and thin C. Less and thick D. Less and thin Answer: B

Q46. No load saturation characteristics are plotted between
    A. No load voltage and field current B. No load voltage and armature current C. Short circuit current and field current D. Short circuit current and armature current Answer: A

Q47. Cross magnetization affects which of the following?
    A. Commutation B. Reduction in main field flux C. Reduces the terminal voltage D. Both 2 and 3 Answer: A

Q48. Series field of DC generators consists of______number of turns and ______ conductors respectively
    A. Large and thick B. Large and thin C. Less and thick D. Less and thin Answer: A

Q49. DC Motor Yoke is generally made of
    A. Wood B. Copper C. Aluminium D. Steel Answer: D

Q50. The transformer noise is mainly because of
    A. Cooling oil B. Sinusoidal current C. Magnetic flux D. All of the above Answer: C
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