Quiz - Three Phase Induction Motor

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  • 1/10 Questions

    The maximum torque in an induction motor depends on

    • Frequency
    • Rotor inductive reactance
    • Square of supply voltage
    • All of the above
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About This Quiz

This quiz on Three Phase Induction Motor explores foundational concepts, focusing on construction, operation, and performance characteristics. It assesses understanding of motor mechanics, including frame materials, rotor design, speed calculations, and torque relations, essential for students and professionals in electrical engineering.

Quiz - Three Phase Induction Motor - Quiz

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  • 2. 

    The frame of an induction motor is usually made of

    • Silicon steel

    • Cast iron

    • Aluminium

    • Bronze

    Correct Answer
    A. Cast iron
    Explanation
    The frame of an induction motor is usually made of cast iron because it provides high mechanical strength and excellent durability. Cast iron is known for its ability to withstand high temperatures and vibrations, making it suitable for the harsh operating conditions of an induction motor. Additionally, cast iron has good magnetic properties, which helps in reducing losses and improving the efficiency of the motor.

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  • 3. 

    The starting torque of a squirrel-cage induction motor is

    • Low

    • Negligible

    • Same as full-load torque

    • Slightly mode than full load torque

    Correct Answer
    A. Low
    Explanation
    The starting torque of a squirrel-cage induction motor is low. This means that the motor requires a relatively small amount of torque to start rotating. The squirrel-cage design of the motor's rotor, with its shorted-circuited bars, creates a magnetic field that interacts with the rotating magnetic field produced by the stator. Initially, this interaction results in a low torque, which gradually increases as the motor gains speed. This low starting torque is advantageous for applications where a gradual and smooth start is required, such as in conveyor belts or pumps.

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  • 4. 

    The 'cogging' of an induction motor can be avoided by

    • Proper ventilation

    • Using DOL starter

    • Auto-transformer starter

    • Having number of rotor slots more or less than the number of stator slots (not equal)

    Correct Answer
    A. Having number of rotor slots more or less than the number of stator slots (not equal)
    Explanation
    Having a different number of rotor slots compared to the number of stator slots can avoid the cogging phenomenon in an induction motor. Cogging is the tendency of the motor to vibrate or jerk when it starts, due to the magnetic attraction between the stator and rotor teeth. By having a different number of slots, the magnetic forces between the stator and rotor are distributed unevenly, reducing the likelihood of cogging. This design feature helps to ensure a smoother and more stable motor operation.

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  • 5. 

    In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

    • Ns

    • S. N

    • (l-s)Ns

    • (Ns-l)s

    Correct Answer
    A. (l-s)Ns
    Explanation
    The actual running speed of an induction motor can be calculated using the formula (l-s)Ns, where Ns is the synchronous speed and s is the slip. The slip represents the difference between the synchronous speed and the actual speed of the motor. By subtracting the slip from 1 (l-s), we can determine the actual running speed of the motor. Multiplying this value by the synchronous speed Ns gives us the correct answer.

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  • 6. 

    A three phase induction motor is

    • Self-starting with zero torque

    • Self-starting with high torque

    • Self-starting with low torque

    • Non-self starting

    Correct Answer
    A. Self-starting with low torque
    Explanation
    A three-phase induction motor is self-starting, which means it can start running on its own without the need for any external assistance or manual intervention. However, it typically generates low torque during the starting process. This means that the motor may not have enough initial force to overcome the inertia and start rotating heavy loads or resistive forces. As the motor gains speed and momentum, the torque gradually increases, allowing it to handle higher loads efficiently.

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  • 7. 

    In a three-phase induction motor, the number of poles in the rotor is always

    • Zero

    • More than the number of poles in stator

    • Less than the number of poles in stator

    • Equal to number of poles in stator

    Correct Answer
    A. Equal to number of poles in stator
    Explanation
    The number of poles in the rotor of a three-phase induction motor is always equal to the number of poles in the stator. This is because the rotor poles are magnetically coupled with the stator poles, and the interaction between them is what creates the rotating magnetic field necessary for the motor to operate. If the number of poles in the rotor is not equal to the number of poles in the stator, the motor will not function properly.

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  • 8. 

    In three-phase squirrel-cage induction motors

    • Rotor conductor ends are short-circuited through slip rings

    • Rotor conductors are short-circuited through end rings

    • Rotor conductors are kept open

    • Rotor conductors are connected to insulation

    Correct Answer
    A. Rotor conductors are short-circuited through end rings
    Explanation
    In three-phase squirrel-cage induction motors, the rotor conductors are short-circuited through end rings. This allows for the creation of a rotating magnetic field in the rotor, which interacts with the stator's magnetic field to produce torque and enable the motor to operate. Short-circuiting the rotor conductors through end rings ensures that the rotor currents are able to flow freely, allowing for efficient operation of the motor.

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  • 9. 

    In case of the induction motors the torque is

    • Inversely proportional to (V * slip)

    • Directly proportional to (slip)2

    • Inversely proportional to slip

    • Directly proportional to slip

    Correct Answer
    A. Directly proportional to slip
    Explanation
    The torque in induction motors is directly proportional to slip. Slip refers to the difference between the synchronous speed and the actual rotor speed. As slip increases, the torque also increases. This is because the rotor needs to rotate at a slower speed than the rotating magnetic field in order to generate torque. Therefore, the greater the slip, the greater the torque produced by the motor.

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  • 10. 

    In squirrel cage induction motors, the rotor slots are usually given slight skew in order to

    • Reduce windage losses

    • Reduce eddy currents

    • Reduce accumulation of dirt and dust

    • Reduce magnetic locking

    Correct Answer
    A. Reduce magnetic locking
    Explanation
    In squirrel cage induction motors, the rotor slots are usually given slight skew in order to reduce magnetic locking. Magnetic locking refers to the phenomenon where the rotor and stator magnetic fields align perfectly, causing the rotor to lock in place and preventing it from rotating. By giving the rotor slots a slight skew, the magnetic field is disrupted, reducing the magnetic locking effect and allowing the rotor to rotate freely.

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  • Current Version
  • Mar 20, 2023
    Quiz Edited by
    ProProfs Editorial Team
  • Apr 18, 2020
    Quiz Created by
    Saradhadevi.a
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