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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 : Alternators

Armature Reaction in Alternator


Armature Reaction in Alternator:

When an alternator is running at no-load, there will be no current flowing through the armature winding. The flux produced in the air-gap will be only due to the rotor ampere-turns. When the alternator is loaded, the three-phase currents will produce a totaling magnetic field in the air-gap. Consequently, the air-gap flux is changed from the no-load condition. The effect of armature flux on the flux produced by field ampere-turns (i. e., rotor ampere-turns) is called armature reaction. Two things are worth noting about the armature reaction in an alternator. First, the armature flux and the flux produced by rotor ampere-turns rotate at the same speed (synchronous speed) in the same direction and, therefore, the two fluxes are fixed in space relative to each other. Secondly, the modification of flux in the air-gap due to armature flux depends on the magnitude of stator current and on the power factor of the load. It is the load power factor which determines whether the armature flux distorts, opposes or helps the flux produced by rotor ampere-turns. To illustrate this important point, we shall consider the following three cases:

(i) When load p.f. is unity

(ii) When load p.f. is zero lagging

(iii) When load p.f. is zero leading

 

(i) When load p.f. is unity

Fig. (1 (i)) shows an elementary alternator on no-load. Since the armature is on open-circuit, there is no stator current and the flux due to rotor current is distributed symmetrically in the air-gap as shown in Fig. (1 (i)). Since the direction of the rotor is assumed clockwise, the generated e.m.f. in phase R1R2 is at its maximum and is towards the paper in the conductor R1 and outwards in conductor R2. No armature flux is produced since no current flows in the armature winding. Fig. (1 (ii)) shows the effect when a resistive load (unity p.f.) is connected across the terminals of the alternator. According to right-hand rule, the current is “in” in the conductors under N-pole and “out” in the conductors under S-pole. Therefore, the armature flux is clockwise due to currents in the top conductors and anti-clockwise due to currents in the bottom conductors. Note that armature flux is at 90° to the main flux (due to rotor current) and is behind the main flux. In this case, the flux in the air-gap is distorted but not weakened. Therefore, at unity p.f., the effect of armature reaction is merely to distort the main field; there is no weakening of the main field and the average flux practically remains the same. Since the magnetic flux due to stator currents (i.e., armature flux) rotate; synchronously with the rotor, the flux distortion remains the same for all positions of the rotor.

 

 (ii) When load p.f. is zero lagging

When a pure inductive load (zero p.f. lagging) is connected across the terminals of the alternator, current lags behind the voltage by 90°. This means that current will be maximum at zero e.m.f. and vice-versa. Fig. (2 (i)) shows the condition when the alternator is supplying resistive load. Note that e.m.f. as well as current in phase R1R2 is maximum in the position shown. When the alternator is supplying a pure inductive load, the current in phase R1R2 will not reach its maximum value until N-pole advanced 90° electrical as shown in Fig. (2 (ii)). Now the armature flux is from right to left and field flux is from left to right All the flux produced by armature current (i.e., armature flux) opposes be field flux and, therefore, weakens it. In other words, armature reaction is directly demagnetizing. Hence at zero p.f. lagging, the armature reaction weakens the main flux. This causes a reduction in the generated e.m.f.

 

(iii) When load p.f. is zero leading

When a pure capacitive load (zero p.f. leading) is connected across the terminals of the alternator, the current in armature windings will lead the induced e.m.f. by 90°. Obviously, the effect of armature reaction will be the reverse that for pure inductive load. Thus armature flux now aids the main flux and the generated e.m.f. is increased. Fig. (3 (i)) shows the condition when alternator is supplying resistive load. Note that e.m.f. as well as current in phase R1R2 is maximum in the position shown. When the alternator is supplying a pure capacitive load, the maximum current in R1R2 will occur 90° electrical before the occurrence of maximum induced e.m.f. Therefore, maximum current in phase R1R2 will occur if the position of the rotor remains 90° behind as compared to its position under resistive load. This is illustrated in Fig. (3 (ii)). It is clear that armature flux is now in the same direction as the field flux and, therefore, strengthens it. This causes an increase in the generated voltage. Hence at zero p.f. leading, the armature reaction strengthens the main flux. For intermediate values of p.f, the effect of armature reaction is partly distorting and partly weakening for inductive loads. For capacitive loads, the effect of armature reaction is partly distorting and partly strengthening. Note that in practice, loads are generally inductive.

Questions of this topic


  • Write a short note on the armature reaction in alternator.

    Answer this
  • Explain the armature reaction in alternator in the following cases: (i) when load of pf is unity (ii) when pf is zero lagging and (iii) when pf is zero leading.

    Answer this
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