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Polyphase Induction Motors

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71Electrical EngineeringMedium

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

AAir motor

BSynchronous motor

CD.C. shunt motor

DInduction motor

Correct answer

A. Air motor

Explanation

The correct answer is Air motor.

72Electrical EngineeringMedium

Slip ring motor is recommended where________?

Aall above features are required

Bhigh starting torque is needed

Cspeed control is required

Dfrequent starting, stopping and reversing is required

Correct answer

A. all above features are required

Explanation

The correct answer is all above features are required.

73Electrical EngineeringMedium

In an induction motor if air-gap is increased________?

Acopper loss will reduce In an induction motor

Bthe power factor will be low

Cbearing friction will reduce

Dwindage losses will be more

Correct answer

B. the power factor will be low

Explanation

The correct answer is the power factor will be low.

74Electrical EngineeringMedium

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

ABall bearings

BBush bearings

CCast iron bearings

DNone of the above

Correct answer

A. Ball bearings

Explanation

The correct answer is Ball bearings.

75Electrical EngineeringMedium

The purpose of using short-circuit gear is________?

Ato short circuit the starting resistances in the starter

Bto short circuit the stator phase of motor to form star

Cto short circuit the rotor at slip rings

Dnone of the above

Correct answer

C. to short circuit the rotor at slip rings

Explanation

The correct answer is to short circuit the rotor at slip rings.

76Electrical EngineeringMedium

Size of a high speed motor as compared to low speed motor for the same H.P. will be________?

Asame

Bany of the above

Cbigger

Dsmaller

Correct answer

D. smaller

Explanation

The correct answer is smaller.

77Electrical EngineeringMedium

In three-phase squirrel-cage induction motors________?

Arotor conductors are short-circuited through end rings

Brotor conductors are kept open

Crotor conductors are connected to insulation

Drotor conductor ends are short-circuited through slip rings

Correct answer

A. rotor conductors are short-circuited through end rings

Explanation

The correct answer is rotor conductors are short-circuited through end rings.

78Electrical EngineeringMedium

In an induction motor, no-load the slip is generally________?

Aless than 1%

B2%

C4%

D1.5%

Correct answer

A. less than 1%

Explanation

The correct answer is less than 1%.

79Electrical EngineeringMedium

Insertion of reactance in the rotor circuit________?

Areduces starting torque as well as maximum torque

Bincreases starting torque but maxi-mum torque remains unchanged

Cincreases starting torque but maxi-mum torque decreases

Dincreases starting torque as well as maximum torque

Correct answer

A. reduces starting torque as well as maximum torque

Explanation

The correct answer is reduces starting torque as well as maximum torque.

80Electrical EngineeringMedium

As load on an induction motor goes on increasing________?

Aits power factor goes on increasing even after full load

Bits power factor goes on decreasing

Cits power factor goes on increasing up to full load and then it falls again

Dits power factor remains constant

Correct answer

C. its power factor goes on increasing up to full load and then it falls again

Explanation

The correct answer is its power factor goes on increasing up to full load and then it falls again.

The purpose of using short-circuit gear is________?

Ato short circuit the starting resistances in the starter

Bto short circuit the stator phase of motor to form star

Cto short circuit the rotor at slip rings

Dnone of the above

Correct answer

C. to short circuit the rotor at slip rings

Explanation

The correct answer is to short circuit the rotor at slip rings.

In three-phase squirrel-cage induction motors________?

Arotor conductors are short-circuited through end rings

Brotor conductors are kept open

Crotor conductors are connected to insulation

Drotor conductor ends are short-circuited through slip rings

Correct answer

A. rotor conductors are short-circuited through end rings

Explanation

The correct answer is rotor conductors are short-circuited through end rings.

Insertion of reactance in the rotor circuit________?

Areduces starting torque as well as maximum torque

Bincreases starting torque but maxi-mum torque remains unchanged

Cincreases starting torque but maxi-mum torque decreases

Dincreases starting torque as well as maximum torque

Correct answer

A. reduces starting torque as well as maximum torque

Explanation

The correct answer is reduces starting torque as well as maximum torque.

As load on an induction motor goes on increasing________?

Aits power factor goes on increasing even after full load

Bits power factor goes on decreasing

Cits power factor goes on increasing up to full load and then it falls again

Dits power factor remains constant

Correct answer

C. its power factor goes on increasing up to full load and then it falls again

Explanation

The correct answer is its power factor goes on increasing up to full load and then it falls again.