Induction Machine Speed Control
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Questions and Answers

When the rotor resistance increases, what happens to the motor speed?

  • The speed increases
  • The speed decreases (correct)
  • The motor stops
  • The speed remains constant

    • What is the condition for maximum torque at starting in a three-phase WRIM?

  • Slip equals zero
  • Slip equals one (correct)
  • Slip equals three
  • Slip equals two

    • What is the value of the resistance to be inserted in the rotor circuit per phase to provide maximum torque at starting?

  • 0.07 ohm (correct)
  • 0.08 ohm
  • 0.06 ohm
  • 0.05 ohm

    • What is the purpose of adding rotor resistance per phase externally?

    <p>To achieve 80% of the maximum torque at starting</p> Signup and view all the answers

    What is the type of cascading where the induced torque produced by the main motor acts in the same direction as the auxiliary motor?

    <p>Cumulative Cascading</p> Signup and view all the answers

    What is the effect of cumulative cascading on the number of poles?

    <p>The number of poles is the sum of the poles of the two motors</p> Signup and view all the answers

    What is the type of cascading where the induced torque produced by the main motor acts in the opposite direction to the auxiliary motor?

    <p>Differential Cascading</p> Signup and view all the answers

    What is the effect of differential cascading on the number of poles?

    <p>The number of poles is the difference of the poles of the two motors</p> Signup and view all the answers

    What is the speed of the main induction motor?

    <p>Ns1 = 120f/p1</p> Signup and view all the answers

    What is the speed of the auxiliary induction motor?

    <p>Ns2 = 120f/p2</p> Signup and view all the answers

    What is the expression for the actual speed of a 3-phase I.M?

    <p>$$N_r = (1 - s)N_s$$</p> Signup and view all the answers

    What is the speed of the cascade connection of three-phase induction motors when cumulative cascading is done?

    <p>Ns = 120f/(p1 + p2)</p> Signup and view all the answers

    What is the formula for the synchronous speed of a 3-phase I.M?

    <p>$$N_s = 120 f / (2P)$$</p> Signup and view all the answers

    What is the speed of the cascade connection of three-phase induction motors when differential cascading is done?

    <p>Ns = 120f/(p1 - p2)</p> Signup and view all the answers

    What happens to the motor speed if the induced torque is less than the load torque?

    <p>The motor speed will decrease.</p> Signup and view all the answers

    What is the method of speed control that involves changing the stator voltage?

    <p>Stator Voltage Control</p> Signup and view all the answers

    What is the rated voltage applied to the stator in the slip power recovery scheme?

    <p>Rated voltage is applied to the stator</p> Signup and view all the answers

    What is the purpose of the diode bridge rectifier in the slip power recovery scheme?

    <p>To rectify the AC voltage</p> Signup and view all the answers

    What is the expression for the induced torque in a 3-phase I.M?

    <p>$$T_{ind} = \frac{3V_{TH}^2}{R_{TH}^2 + (X_{TH} + X_2)^2}$$</p> Signup and view all the answers

    What is the air-gap power that is not converted into mechanical power?

    <p>Slip power</p> Signup and view all the answers

    What are the two main methods of controlling the speed of an I.M?

    <p>Stator and rotor parameters</p> Signup and view all the answers

    What is the purpose of the slip power recovery scheme?

    <p>To recover the slip power from the motor</p> Signup and view all the answers

    What is the effect of increasing the stator voltage in a 3-phase I.M?

    <p>The motor speed will increase.</p> Signup and view all the answers

    What is the term for the difference between the synchronous speed and the actual speed of a 3-phase I.M?

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

    What is the purpose of connecting external variable resistors in speed control using external rotor resistance?

    <p>To vary the rotor resistance</p> Signup and view all the answers

    What is the effect of increasing the rotor resistance on the maximum torque occurrence?

    <p>It shifts to higher slip values</p> Signup and view all the answers

    What type of machine is required for rotor resistance control?

    <p>Slip-ring machine</p> Signup and view all the answers

    Why is the method of rotor resistance control not very efficient?

    <p>Due to power loss in the resistor</p> Signup and view all the answers

    What is the effect of increasing the external rotor resistance on the torque-slip characteristics?

    <p>The critical slip Sm increases</p> Signup and view all the answers

    What happens to the operating point of the induction machine when the rotor resistance is increased?

    <p>It shifts from point A to point B</p> Signup and view all the answers

    What type of load torque benefits from the speed control range of this method?

    <p>Both constant torque type and fan-type</p> Signup and view all the answers

    What is an advantage of using rotor resistance control for speed control?

    <p>Wide speed control range</p> Signup and view all the answers

    Study Notes

    Speed Control Techniques of Induction Motors

    • There are several speed control techniques of induction motors, including:
      • Stator Voltage Control
      • Supply Frequency Control
      • Volt/Hertz Control
      • Stator Current Control
      • Pole Changing Methods

    Speed Control Using External Rotor Resistance

    • Speed control can be achieved by changing the rotor resistance of an induction motor
    • The maximum torque point shifts to higher slip values with an increase in rotor resistance, while retaining a constant torque
    • The starting torque increases with an increase in rotor resistance
    • The speed control range is more with this method

    Drawbacks of Speed Control Using External Rotor Resistance

    • The method requires a slip-ring machine
    • The additional resistance and operation at high slips entail power loss in the resistor

    Speed Control of WRIM using Cascade Connection

    • Cascade connection of two motors allows for speed control
    • The torque produced by the main and auxiliary motors can act in the same direction (Cumulative Cascading) or opposite directions (Differential Cascading)
    • Four different speeds can be extracted using cascade connection

    Slip Power Recovery Scheme

    • The scheme is applied to WRIM only
    • Rated voltage is applied to the stator, and the rotor voltage is rectified and inverted to feed back to the supply
    • Slip power is recovered from the motor, and the speed is reduced

    Actual Speed of a 3-Phase I.M

    • The actual speed of a 3-phase induction motor depends on two factors: synchronous speed (Ns) and slip (s)
    • Changing either of these factors can vary the speed of the motor

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    Description

    Learn about the relationship between rotor resistance and torque in induction machines, including how it affects maximum torque and slip values.

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