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  • Alternators
    • Basic Principle of Alternators
    • Advantages of stationary armature
    • Basic Construction of Alternator
    • Detailed Construction of Alternator
    • Damper Windings & Speed and Frequency of Alternator
    • Armature Windings
    • Concentric or Chain Windings
    • A.C. Armature Windings of Alternator
    • Pitch factor of alternator
    • Distribution Factor
    • E.M.F. Equation of an Alternator
    • Armature Reaction in Alternator
    • Summary of Armature reaction in alternator:
    • Alternator on Load
    • Synchronous Reactance
    • Phasor Diagram of a Loaded Alternator
    • Voltage Regulation of alternators
    • Determination of Voltage Regulation
    • EMF method
    • MMF method for voltage regulation determination
    • Procedure for mmf Method
    • Potier method
    • Procedure for potier method
    • Two reaction theory
    • Effect of Salient Poles
    • Analysis by two reaction theory
    • Modified phasor diagram by two reaction theory
    • Reluctance Power
    • Power angle characteristic of salient pole machines
    • Losses and efficiency of an alternator

  • Synchronous generator
    • Parallel Operation of synchronous generator
    • Advantages and condition of Parallel Operation of synchronous generator
    • Methods of Synchronization
    • Synchronising Action
    • Effects on synchronising action
    • Synchronizing Current
    • Synchronizing Power
    • Synchronous generator Connected to Infinite Busbars
    • Alternators Connected to Infinite Busbars
    • Two identical synchronous generators in parallel
    • Alternators on Infinite Busbars
    • Load Sharing
    • Effect of Change in Excitation on an alternator connected to an infinite busbars
    • Effect of change of fuel supply to alternators connected to infinite busbar
    • Governor characteristics
    • Electrical load diagram

  • Synchronous motor
    • Introduction to Synchronous Motor
    • Principle of Operation of synchronous motor
    • Method of Starting of synchronous motor
    • Construction of synchronous motor
    • Motor Starting by Reducing the supply Frequency
    • Motor Starting with an External Motor
    • Motor Starting by Using damper (Amortisseur) Winding
    • Motor on Load with Constant Excitation
    • Power Flow within a Synchronous Motor
    • Equivalent circuit model and phasor diagram of a synchronous motor
    • Synchronous-motor power equation
    • Synchronous Motor with Different Excitations
    • Effect of Increased Load with Constant Excitation
    • Effect of Changing Excitation on Constant Load
    • Different Torques of a Synchronous Motor
    • Salient Pole Synchronous Motor
    • Effect of changes in load on armature current, power angle, and power factor of synchronous motor
    • Effect of changes in field excitation on synchronous motor performance
    • Constant-power Lines
    • Construction of V-curves
    • V curves
    • O-Curves and V -Curves
    • Hunting
    • Methods and procedure of Starting a Synchronous Motor
    • Comparison Between Synchronous and Induction Motors
    • Synchronous Motor Applications
    • Synchronous Condenser
    • Synchronous-motor losses and efficiency

  • Induction machines
    • Theory of induction machines
    • Universal motor
    • Three phase induction motors
    • Construction of Induction motors
    • Principle of operation of induction motor
    • Rotating Magnetic Field Due to 3-Phase Currents
    • Properties of rotating magnetic field
    • Alternate Mathematical Analysis for Rotating Magnetic Field
    • slip and rotor frequency of induction motor
    • Effect of Slip on The Rotor Circuit
    • Rotor Current
    • Rotor torque and Starting Torque of induction machines
    • Condition for Maximum Starting Torque
    • Starting Torque of 3-Phase Induction Motors
    • Behaviour of 3-phase induction motor on load
    • Torque Under Running Conditions
    • Maximum Torque under Running Conditions
    • Torque-Slip Characteristics
    • Full-Load, Starting and Maximum Torques
    • comparison of induction motor and transformer
    • Speed Regulation of Induction Motors
    • Speed Control of 3-Phase Induction Motors
    • Power Factor of Induction Motor
    • Power Stages in an Induction Motor
    • Induction Generator
    • No-load Test
    • Blocked Rotor Test
    • Construction of the Circle Diagram
    • Double Squirrel Cage Motor
    • single phasing
    • Time Harmonics of Induction motors
    • Effects of air gap flux harmonics
    • Construction & Working of Double Squirrel-Cage Motors
    • Equivalent Circuit of Double Squirrel-Cage Motor
    • cogging
    • crawling
    • Line excited and self excited induction generator
    • principle of operation of induction generator
    • Applications of Induction generator
    • Induction generator controller technology

  • Speed control of Induction Motors
    • Direct-switching or Line starting of Induction Motors
    • Stator resistance starting of induction motors
    • Primary resistors starting of Induction motor
    • Autotransformer starting of Induction motor
    • Star-delta Starter of induction motor
    • Rotor resistance starting of induction motor
    • Speed Control of Induction Motors
    • Speed control by changing applied voltage
    • Rotor resistance speed control of Induction motors
    • Cascade speed control of induction motor
    • Pole changing speed control scheme of induction motor
    • Stator frequency control of induction motor

Branch : Electrical and Electronics Engineering
Subject : Electrical Machines II (AC Machines)
Unit : Speed control of Induction Motors

Rotor resistance speed control of Induction motors


Rotor resistance speed control of Induction motors:

From the expression for the torque of the induction machine

Clearly, it is dependent on the rotor resistance. Further, equation

shows that the maximum value is independent of the rotor resistance. The slip at maximum torque equation

 is dependent on the rotor resistance. Therefore, we may expect that if the rotor resistance is changed, the maximum torque point shifts to higher slip values, while retaining a constant torque. Figure28 shows a family of torque-speed characteristic obtained by changing the rotor resistance.

Fig: Speed-torque curves: rotor resistance variation

 

  • While the maximum torque and synchronous speed remain constant, the slip at which maximum torque occurs increases with increases in rotor resistance, and so does the starting torque. Whether the load is of constant torque type or fan-type, it is evident that the speed control range is more with this method. Further, rotor resistance control could also be used as a means of generating high starting torque.
  • For all its advantages, the scheme has two serious drawbacks. Firstly, in order to vary the rotor resistance, it is necessary to connect external variable resistors (winding resistance itself cannot be changed). This therefore necessitates a slip-ring machine, since only in that case rotor terminals are available outside. For cage rotor machines, there are no rotor terminals.
  • Secondly, the method is not very efficient since the additional resistance and operation at high slips entails dissipation. The resistors connected to the slip-ring brushes should have good power dissipation capability. Water based rheostats may be used for this. A ‘solid-state’ alternative to a rheostat is a chopper controlled resistance where the duty ratio control of the chopper presents a variable resistance load to the rotor of the induction machine.

Questions of this topic


  • Write a short note on the rotor resistance speed control of induction motor.

    Answer this
  • State the advantages and disadvantages of rotor resistance speed control of induction motors.

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