[amp_mcq option1=”small induction motor” option2=”D.” option3=”series motor C.” option4=”double winding A.C./D.C. motor” correct=”option1″]
Synchronous motors
52. If the field circuit of an unloaded salientpole synchronous motor gets suddenly open-circuited, then
[amp_mcq option1=”it runs at a slower speed” option2=”the motor stops” option3=”it continues to run at the same speed” option4=”it runs at a very high speed” correct=”option3″]
53. Under which of the following conditions hunting of synchronous motor is likely to occur ?
[amp_mcq option1=”Periodic variation of load” option2=”Over-excitation” option3=”Over-loading for long periods” option4=”Small and constant load” correct=”option1″]
54. Which of the following motors will be used in electric clocks ?
[amp_mcq option1=”D.C. shunt motor” option2=”D.” option3=”shunt motor B.” option4=”A.C. synchronous motor” correct=”option1″]
Detailed SolutionWhich of the following motors will be used in electric clocks ?
55. A synchronous motor connected to infinite bus-bars has at constant full load, 100% excitation and unity power factor. On changing the excitation only, the armature current will have
[amp_mcq option1=”no change of power factor” option2=”lagging power factor with over-excitation” option3=”leading power factor with under-excitation” option4=”leading power factor with over-excitation” correct=”option3″]
56. The maximum power developed in a synchronous motor will depend on
[amp_mcq option1=”the rotor excitation only” option2=”the supply voltage only” option3=”the rotor excitation and supply volt-age both” option4=”the rotor excitation, supply voltage and maximum value of coupling angle (90°)” correct=”option3″]
Detailed SolutionThe maximum power developed in a synchronous motor will depend on
57. In a synchronous motor, the damping winding is generally used to
[amp_mcq option1=”prevent hunting and provide the starting torque” option2=”reduce the eddy currents” option3=”provide starting torque only” option4=”reduce noise level” correct=”option1″]
Detailed SolutionIn a synchronous motor, the damping winding is generally used to
58. If in a synchronous motor, driving mechanical load and drawing current at lagging power factor from constant voltage supply, its field excitation is increased, then its power factor
[amp_mcq option1=”become more” option2=”become less” option3=”remain constant” option4=”none of the above” correct=”option1″]
59. In case one phase of a three-phase synchronous motor is short-circuited the motor will
[amp_mcq option1=”Not start” option2=”Run at $$\frac{2}{3}$$ of synchronous speed” option3=”Run with excessive vibrations” option4=”Take less than the rated load” correct=”option1″]
60. A synchronous motor is running with normal excitation. When the load is increased, the armature current drawn by it increases because
[amp_mcq option1=”Speed of the motor is reduced” option2=”Power factor is decreased” option3=”Eb (back e.m.f.) becomes less than V (applied voltage)” option4=”Er (net resultant voltage) in armature is increased” correct=”option4″]