Polyphase Induction Motors

91. The vafcie of transformation ratio of an induction motor can be found by

1. open-circuit test only
2. short-circuit test only
3. stator resistance test
4. none of the above

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92. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero?

1. The slip of the motor will be 5%
2. The rotor will not run
3. The rotor will run at very high speed
4. The torque produced will be very large

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93. When /?2 is the rotor resistance, .X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be

1. sR2X2 = 1
2. sR2 = X2
3. R2 = sX2
4. R2 = s2X2

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94. When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered

1. in series
2. in parallel
3. in series-parallel
4. in parallel with stator

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95. Which is of the following data is required to draw the circle diagram for an induction motor?

1. Block rotor test only
2. No load test only
3. Block rotor test and no-load test
4. Block rotor test, no-load test and stator resistance test

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96. Which motor is preferred for use in mines where explosive gases exist?

1. Air motor
2. Induction motor
3. D.C. shunt motor
4. Synchronous motor

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97. Which of the following component is usually fabricated out of silicon steel?

1. Bearings
2. Shaft
3. Statorcore
4. None of the above

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98. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor?

1. By changing the number of stator poles
2. Rotor rheostat control
3. By operating two motors in cascade
4. By injecting e.m.f. in the rotor circuit

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99. Which type of bearing is provided in small induction motors to support the rotor shaft?

1. Ball bearings
2. Cast iron bearings
3. Bush bearings
4. None of the above

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