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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

Basic Construction of Alternator


Basic construction of Alternator:

           

Fig: 1  Salient pole type                                                                      Fig: 2 Non-salient Pole Type

An alternator has 3,-phase winding on the stator and a d.c. field winding on the rotor.

1. Stator

It is the stationary part of the machine and is built up of sheet-steel laminations having slots on its inner periphery. A 3-phase winding is placed in these slots and serves as the armature winding of the alternator. The armature winding is always connected in star and the neutral is connected to ground.

2. Rotor

The rotor carries a field winding which is supplied with direct current through two slip rings by a separate d.c. source. This d.c. source (called exciter) is generally a small d.c. shunt or compound generator mounted on the shaft of the alternator. Rotor construction is of two types, namely;

 

(i) Salient (or projecting) pole type

(ii) Non-salient (or cylindrical) pole type

(i) Salient pole type

In this type, salient or projecting poles are mounted on a large circular steel frame which is fixed to the shaft of the alternator as shown in Fig. (1). The individual field pole windings are connected in series in such a way that when the field winding is energized by the d.c. exciter, adjacent poles have opposite polarities. Low and medium-speed alternators (120-400 r.p.m.) such as those driven by diesel engines or water turbines have salient pole type rotors due to the following reasons:

(a)    The salient field poles would cause .an excessive windage loss if driven at high speed and would tend to produce noise.

(b)   Salient-pole construction cannot be made strong enough to withstand the mechanical stresses to which they may be subjected at higher speeds.

Since a frequency of 50 Hz is required, we must use a large number of poles on the rotor of slow-speed alternators. Low-speed rotors always possess a large diameter to provide the necessary spate for the poles. Consequently, salient-pole type rotors have large diameters and short axial lengths.

(ii) Non-salient pole type

In this type, the rotor is made of smooth solid forged-steel radial cylinder having a number of slots along the outer periphery. The field windings are embedded in these slots and are connected in series to the slip rings through which they are energized by the d.c. exciter. The regions forming the poles are usually left unslotted as shown in Fig. (2). It is clear that the poles formed are non-salient i.e., they do not project out from the rotor surface.

High-speed alternators (1500 or 3000 r.p.m.) are driven by steam turbines and use non-salient type rotors due to the following reasons:

(a)    This type of construction has mechanical robustness and gives noiseless operation at high speeds.

(b)   The flux distribution around the periphery is nearly a sine wave and hence a better e.m.f. waveform is obtained than in the case of salient-pole type.

Since steam turbines run at high speed and a frequency of 50 Hz is required, we need a small number of poles on the rotor of high-speed alternators (also called turboalternators). We can use not less than 2 poles and this fixes the highest possible speed. For a frequency of 50 Hz, it is 3000 r.p.m. The next lower speed is 1500 r.p.m. for a 4-pole machine. Consequently, turboalternators possess 2 or 4 poles and have small diameters and very long axial lengths.

Questions of this topic


  • Write a short note on the basic construction of alternator.

    Answer this
  • Write short notes on i. Stator ii. Rotor iii. Salient Pole type alternator iv. Non salient pole type alternator

    Answer this
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