Single-Phase Induction Motor
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Questions and Answers

What is the purpose of making the stator flux rotating type in a single-phase induction motor?

  • To increase the motor's speed
  • To make the motor self-starting (correct)
  • To decrease the motor's torque
  • To reduce the motor's efficiency

    • Why does a single-phase induction motor require some starting means?

  • Because the alternating flux cannot produce rotation in a stationary squirrel-cage rotor (correct)
  • Because the motor is not self-sustaining
  • Because the stator windings produce a rotating magnetic field
  • Because the rotor is not laminated

    • What is the main problem with the pulsating flux produced by the main winding?

  • It is too weak
  • It is alternating and does not rotate (correct)
  • It is rotating in both directions
  • It is too strong

    • What happens if the rotor of a single-phase I.M is rotated in one direction by some mechanical means?

    <p>The rotor quickly accelerates until it reaches a speed slightly below the synchronous speed</p> Signup and view all the answers

    What is the direction of rotation of the motor due to the forward rotating field?

    <p>Forward direction</p> Signup and view all the answers

    What is the percentage of positive slip generated by the forward rotating field?

    <p>1-3%</p> Signup and view all the answers

    What is the purpose of the aluminum bars in the squirrel cage rotor?

    <p>To short-circuit the slots at both ends with a ring</p> Signup and view all the answers

    What is the reason for a single-phase induction motor not being self-starting?

    <p>The magnetic field produced is pulsating but not rotating</p> Signup and view all the answers

    What is the formula to calculate the slip (Sp) in a single-phase induction motor?

    <p>(Ns - Nr)/Ns</p> Signup and view all the answers

    What is the percentage of negative slip generated by the reverse rotating field?

    <p>1.95%</p> Signup and view all the answers

    What is the basis of the behavior of a single-phase induction motor?

    <p>Double-revolving field theory</p> Signup and view all the answers

    What is the characteristic of a single-phase induction motor when connected to a single-phase a.c. supply?

    <p>It does not develop any starting torque</p> Signup and view all the answers

    What is the formula to calculate the negative slip (Sneg) in a single-phase induction motor?

    <p>(Ns + Nr)/Ns</p> Signup and view all the answers

    What is the theory used to describe the operation of a single-phase induction motor?

    <p>Double Revolving Field Theory</p> Signup and view all the answers

    What is the consequence of rotating the rotor of a single-phase I.M by some mechanical means?

    <p>The motor will start and continue to rotate even though single-phase current is flowing through the stator winding</p> Signup and view all the answers

    Why is this method of starting not convenient for large motors?

    <p>Because it requires some mechanical means</p> Signup and view all the answers

    What is the range of small positive slip that generates larger torque than a large negative slip?

    <p>0.01-0.03</p> Signup and view all the answers

    What is the purpose of the auxiliary winding in a single-phase IM?

    <p>To shift the EMF and aid in starting</p> Signup and view all the answers

    What happens to the auxiliary winding when the rotor reaches synchronous speed?

    <p>It is disconnected from the supply</p> Signup and view all the answers

    According to the Double Revolving Field Theory, what is the magnitude of each rotating component?

    <p>One-half of the maximum stator flux</p> Signup and view all the answers

    What is the direction of rotation of the forward component of the stator flux?

    <p>Anti-clockwise</p> Signup and view all the answers

    What is the relationship between the synchronous speed and the supply frequency?

    <p>The synchronous speed is directly proportional to the supply frequency</p> Signup and view all the answers

    What is the resultant of the two components Φf and Φb at start?

    <p>ΦR = 0</p> Signup and view all the answers

    Why are single-phase induction motors not self-starting according to double revolving field theory?

    <p>Because the two torque components are equal in magnitude, but opposite in direction</p> Signup and view all the answers

    What is the effect of the rotating field on the rotor, according to the Double Revolving Field Theory?

    <p>The torque due to the rotating field is more in the direction of initial rotation</p> Signup and view all the answers

    What is the resultant of the two rotating components of the stator flux at any instant?

    <p>The instantaneous value of the stator flux</p> Signup and view all the answers

    What happens to the two components Φf and Φb after 90°?

    <p>They point in the same direction</p> Signup and view all the answers

    What is the direction of the torque produced by the interaction of the rotor flux with the forward component Φf?

    <p>Anti-clockwise direction</p> Signup and view all the answers

    What is the instantaneous value of the stator flux at θ = 90°?

    <p>ΦR = Φ1m</p> Signup and view all the answers

    What is the effect of the rotor flux interacting with the backward component Φb?

    <p>Produces a torque in the clockwise direction</p> Signup and view all the answers

    What is the net torque experienced by the rotor at starting?

    <p>Zero</p> Signup and view all the answers

    What can be used to show the two oppositely directed torques and the resultant torque?

    <p>Torque-speed characteristics</p> Signup and view all the answers

    According to the double-field revolving theory, how many constant-amplitude fluxes are produced in a single-phase induction motor?

    <p>Two</p> Signup and view all the answers

    What is the relationship between the equivalent circuits of the two motors in a single-phase induction motor?

    <p>They are connected in series</p> Signup and view all the answers

    What is the input power equation for a single-phase induction motor?

    <p>Pin = Is * Vs * p.f</p> Signup and view all the answers

    What is the significance of the double-field revolving theory in understanding single-phase induction motors?

    <p>It explains the production of pulsating flux in single-phase induction motors</p> Signup and view all the answers

    What is the relationship between the rotor values in the equivalent circuit of a single-phase induction motor?

    <p>The rotor resistance and reactance are half the actual values</p> Signup and view all the answers

    What is the condition for a single-phase induction motor to operate in the running condition?

    <p>Ef &gt;&gt; Eb</p> Signup and view all the answers

    What is the purpose of the capacitor in a single-phase induction motor?

    <p>To provide a starting torque</p> Signup and view all the answers

    What is an advantage of single-phase induction motors?

    <p>Simple and robust construction</p> Signup and view all the answers

    Study Notes

    Single-Phase Induction Motor

    • A single-phase induction motor is not self-starting, unlike a three-phase induction motor, and requires some starting means such as electric or electronic means.
    • The single-phase stator windings produce a magnetic field that pulsates in strength in a sinusoidal manner, which does not rotate and therefore cannot produce rotation in a stationary squirrel-cage rotor.

    Squirrel Cage Rotor

    • The rotor has a laminated iron core with slots, and aluminum bars are molded on the slots and short-circuited at both ends with a ring.
    • The rotor quickly accelerates until it reaches a speed slightly below the synchronous speed once it is rotated in one direction by some mechanical means.

    Basics of Single-Phase Induction Motor

    • A single-phase induction motor inherently does not develop any starting torque and therefore will not start to rotate if the stator winding is connected to a single-phase ac supply.
    • However, if the rotor is started by auxiliary means, the motor will quickly attain the rated speed.

    Double Revolving Field Theory

    • The pulsating flux produced by the main winding can be resolved into two counter-rotating components (forward and backward components).
    • The motor can be started in the direction of the forward rotating field, which generates a small (≈1-3%) positive slip.
    • The reverse rotating field generates a larger (≈1.95%) negative slip.
    • The interaction between the fields and the current induced in the rotor bars generates opposing torque.

    Solution to Starting Torque Problem

    • The single-phase induction motor can be made self-starting by adding an auxiliary winding in the slots of the stator to shift the EMF.
    • The auxiliary winding and main winding are connected in parallel together and with the supply.
    • When the rotor moves with speed until the synchronous speed, the auxiliary winding will open from the connection after a few seconds.

    Torque/Speed Characteristics

    • The two oppositely directed torques and the resultant torque can be shown effectively with the help of torque-speed characteristics.

    Equivalent Circuit of Single-Phase Induction Motor

    • A single-phase induction motor can be imagined to be consisting of two motors, having a common stator winding, but with their respective rotors revolving in opposite directions.
    • Each rotor has resistance and reactance half the actual rotor values.
    • The two equivalent circuits are connected in series.
    • The current, power, and torque can be calculated from the combined equivalent circuit using network basics.

    Review Questions

    • What is a single-phase induction motor?
    • How does a single-phase induction motor work?
    • How do you test a single-phase motor?
    • Which capacitor is used in a single-phase motor?
    • What are the advantages of a single-phase induction motor?
    • What are the applications of a single-phase induction motor?

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    Description

    Learn about the working of single-phase induction motors, their differences from three-phase motors, and their starting mechanisms.

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