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# Electrical Machines By SK Sahdev

1914

Electrical Machines By SK Sahdev covers the fundamental concepts of electrical machines, highlighting transformers, motors, generators and magnetic circuits. It provides in-depth discussion on construction, working principles and applications of various electrical machines. The design of transformers, functioning of generators and performance of induction motors are explained through descriptive illustrations, step-by-step solved examples and mathematical derivations. A separate chapter on special purpose machines offers important topics such as servomotors, brushless motors and stepper motors, which is useful from industrial perspective to build a customized machine.

Supported by 400 solved examples, 600 figures, and more than 1000 self-assessment exercises, this is an ideal text for one or two-semester undergraduate courses on electrical machines under electrical and electronics engineering.

Electrical Machines By SK Sahdev follows a structured approach to explain construction and functioning of electrical machines. Section-wise practice problems including short answer type questions and numerical questions are provided at regular intervals. Numerous solved examples, multiple choice questions, review, and open book questions are offered in the book.

Book Name – Electrical Machines By SK Sahdev
Author –
SK Sahdev
Publisher –
Cambridge University Press
Language – English

Edition – 1st, 2017
Paperback – 974 Pages

## Book Contents

1. Electro Magnetic Circuits
2. Single-Phase Transformers
3. Three-Phase Transformers
4. DC Generator
5. DC Motors
6. Synchronous Generators or Alternators
7. Parallel Operation of Alternators
8. Synchronous Motors
9. Three-Phase Induction Motors
10. Starting Methods and Speed Control of Three-phase Induction Motors
11. Single-Phase Motors
12. Special Purpose Machines

### 1. Electro Magnetic Circuits

Introduction
1.1 Magnetic Field and its Significance
1.2 Magnetic Circuit and its Analysis
1.3 Important Terms
1.4 Comparison between Magnetic and Electric Circuits
1.5 Ampere-turns Calculations
1.6 Series Magnetic Circuits
1.7 Parallel Magnetic Circuits
1.8 Leakage Flux
1.9 Magnetisation or B-H Curve
1.10 Magnetic Hysteresis
1.11 Hysteresis Loss
1.12 Importance of Hysteresis Loop
Section Practice Problems
1.13 Electro Magnetic Induction
1.14 Faraday’s Laws of Electromagnetic Induction
1.15 Direction of Induced emf
1.16 Induced emf
1.17 Dynamically Induced emf
1.18 Statically Induced emf
1.19 Self Inductance
1.20 Mutual Inductance
1.21 Co-efficient of Coupling
1.22 Inductances in Series and Parallel
1.23 Energy Stored in a Magnetic Field
1.24 AC Excitation in Magnetic Circuits
1.25 Eddy Current Loss
Section Practice Problems
1.26 Electro-mechanical Energy Conversion Devices
1.27 Torque Development by the Alignment of Two Fields
1.27.1 Soft Iron Piece Placed in the Magnetic Field
1.27.2 Permanent Magnet Placed in the Magnetic Field
1.27.3 Electromagnet Placed in the Magnetic Field
1.28 Production of Torque
1.28.1 In Case of Permanent Magnet
1.28.2 In Case of Electromagnet
1.29 Production of Unidirectional Torque
1.29.1 By Rotating the Main Magnets
1.29.2 By Changing the Direction of Flow of Current in the Conductors of
Electromagnet
1.30 emf Induced in a Rotating Coil Placed in a Magnetic Field
1.31 Elementary Concept of Electrical Machines
1.31.1 Operation of Machine as a Generator (Conversion of Mechanical
Energy into Electric Energy)
1.31.2 Operation of Machine as a Motor
Section Practice Problems
Review Questions
Multiple Choice Questions

### 2. Single-Phase Transformers

Introduction
2.1 Transformer
2.2 Working Principle of a Transformer
2.3 Construction of Transformer
2.3.1 Core Material
2.3.2 Core Construction
2.3.3 Transformer Winding
2.3.4 Insulation
2.3.5 Bushings
2.3.6 Transformer Tank
2.4 Simple Construction of Single-phase Small Rating (SAY 2 kVA) Transformers
2.5 An Ideal Transformer
2.6 Transformer on DC
2.7 emf Equation
Section Practice Problems
2.9 Effect of Magnetisation on No-load (Exciting) Current
2.10 Inrush of Magnetising Current
Section Practice Problems
2.12 Phasor Diagram of a Loaded Transformer
2.13 Transformer with Winding Resistance
2.14 Mutual and Leakage Fluxes
2.15 Equivalent Reactance
Section Practice Problems
2.16 Actual Transformer
2.17 Simplified Equivalent Circuit
2.18 Short Circuited Secondary of Transformer
2.19 Expression for No-load Secondary Voltage
2.20 Voltage Regulation
2.21 Approximate Expression for Voltage Regulation
2.22 Kapp Regulation Diagram
Section Practice Problems
2.23 Losses in a Transformer
2.24 Effects of Voltage and Frequency Variations on Iron Losses
2.25 Efficiency of a Transformer
2.26 Condition for Maximum Efficiency
2.28 Efficiency vs Power Factor
2.29 All-day Efficiency
Section Practice Problems
2.30 Transformer Tests
2.31 Polarity Test
2.32 Voltage Ratio Test
2.34 Separation of Hysteresis and Eddy Current Losses
2.35 Short Circuit Test
2.36 Back-to-back Test
Section Practice Problems
2.37 Classification of Transformers
2.38 Parallel Operation of Transformers
2.39 Necessity of Parallel Operation
2.40 Conditions for Parallel Operation of One-phase Transformers
2.41 Load Sharing between Two Transformers Connected in Parallel
Section Practice Problems
2.42 Auto-transformer
2.43 Auto-transformer vs Potential Divider
2.44 Saving of Copper in an Auto-transformer
2.45 Advantages of Auto-transformer over Two-winding Transformer
2.47 Phasor Diagram of an Auto-transformer
2.48 Equivalent Circuit of an Auto-transformer
2.49 Simplified Equivalent Circuit of an Auto-transformer
2.50 Conversion of a Two-winding Transformer to an Auto-transformer
2.51 Comparison of Characteristics of Auto-transformers and Two-winding Transformers
2.52 Applications of Auto-transformers
Section Practice Problems
Review Questions
Multiple Choice Questions

### 3. Three-Phase Transformers

Introduction
3.1 Merits of Three-phase Transformer over Bank of Three Single-phase Transformers
3.2 Construction of Three-phase Transformers
3.3 Determination of Relative Primary and Secondary Windings in Case of Three-phase Transformer
3.4 Polarity of Transformer Windings
3.5 Phasor Representation of Alternating Quantities in Three-phase Transformer Connections
3.6 Three-phase Transformer Connections
3.7 Selection of Transformer Connections
3.7.1 Star-Star (Yy0 or Yy6) Connections
3.7.2 Delta-Delta (Dd0 or Dd6) Connections
3.7.3 Star-Delta (Yd1 or Yd11) Connections
3.7.4 Delta-Star (Dy1 or Dy11) Connections
3.7.5 Delta-Zigzag Connections
Section Practice Problems
3.8 Parallel Operation of Three-phase Transformers
3.9 Necessity of Parallel Operation of Three-phase Transformers
3.10 Conditions for Parallel Operation of Three-phase Transformers
3.11 Load Sharing between Three-phase Transformers Connected in Parallel
3.12 Three Winding Transformers (Tertiary Winding)
3.12.1 Stabilisation Provided by Tertiary Winding in Star-Star Transformer
3.13 Tap-changers on Transformers
3.14 Types of Tap-changers
Section Practice Problems
3.15 Transformation of Three-phase Power with Two Single-phase Transformers
3.16 Open-Delta or V-V Connections
3.17 Comparison of Delta and Open Delta Connections
3.18 T-T Connections or Scott Connections
3.19 Conversion of Three-phase to Two-phase and vice-versa
3.20 Difference between Power and Distribution Transformers
3.21 Cooling of Transformers
3.22 Methods of Transformer Cooling
3.23 Power Transformer and its Auxiliaries
3.24 Maintenance Schedule of a Transformer
3.25 Trouble Shooting of a Transformer
Section Practice Problems
Review Questions
Multiple Choice Questions

### 4. DC Generator

Introduction
4.1 DC Generator
4.2 Main Constructional Features
4.3 Simple Loop Generator and Function of Commutator
4.4 Connections of Armature Coils with Commutator Segments and Location of Brushes
4.5 Armature Winding
4.6 Types of Armature Winding
4.7 Drum Winding
4.8 Lap Winding
4.9 Numbering of Coils and Commutator Segments in Developed Winding Diagram
4.10 Characteristics of a Simplex Lap Winding
4.11 Characteristics of a Multiplex Lap Winding
4.12 Equalising Connections and their Necessity
4.13 Simplex Wave Winding
4.14 Dummy Coils
4.15 Applications of Lap and Wave Windings
Section Practice Problems
4.16 emf Equation
4.17 Torque Equation
4.18 Armature Reaction
4.19 Calculations for Armature Ampere-turns
4.20 Commutation
4.21 Cause of Delay in the Reversal of Current in the Coil going through Commutation and its Effect
4.22 Magnitude of Reactance Voltage
4.23 Good Commutation and Poor Commutation
4.24 Interpoles and their Necessity
4.25 Compensating Winding and its Necessity
4.26 Methods of Improving Commutation
Section Practice Problems
4.27 Types of DC Generators
4.28 Separately-excited DC Generators
4.29 Self-excited DC Generators
4.30 Voltage Regulation of a DC Shunt Generator
4.31 Characteristics of DC Generators
4.32 No-load Characteristics of DC Generators or Magnetisation Curve of DC Generator
4.33 Voltage Build-up in Shunt Generators
4.34 Critical Field Resistance of a DC Shunt Generator
4.35 Load Characteristics of Shunt Generator
4.36 Load Characteristics of Series Generators
4.37 Load Characteristics of Compound Generator
4.38 Causes of Failure to Build-up Voltage in a Generator
4.39 Applications of DC Generators
4.40 Losses in a DC Generator
4.41 Constant and Variable Losses
4.42 Stray Losses
4.43 Power Flow Diagram
4.44 Efficiency of a DC Generator
4.45 Condition for Maximum Efficiency
Section Practice Problems
Review Questions
Multiple Choice Questions

### 5. DC Motors

Introduction
5.1 DC Motor
5.2 Working Principle of DC Motors
5.3 Back emf
5.4 Electro-magnetic Torque Developed in DC Motor
5.5 Shaft Torque
5.6 Comparison of Generator and Motor Action
5.7 Types of DC Motors
5.8 Characteristics of DC Motors
5.9 Characteristics of Shunt Motors
5.10 Characteristics of Series Motors
5.11 Characteristics of Compound Motors
5.12 Applications and Selection of DC Motors
5.12.1 Applications of DC Motors
5.12.2 Selection of DC Motors
5.13 Starting of DC Motors
5.14 Necessity of Starter for a DC Motor
5.15 Starters for DC Shunt and Compound Wound Motors
5.16 Three-point Shunt Motor Starter
5.17 Four-point Starter
5.18 Calculation of Step Resistances Used in Shunt Motor Starter
5.19 Series Motor Starter
Section Practice Problems
5.20 Speed Control of DC Motors
5.21 Speed Control of Shunt Motors
5.21.1 Field Control Method
5.21.2 Armature Control Method
5.22 Speed Control of Separately Excited Motors
5.23 Speed Regulation
5.24 Speed Control of DC Series Motors
5.24.1 Armature Control Method
5.24.2 Field Control Method
5.24.3 Voltage Control Method
5.25 Electric Braking
5.26 Types of Electric Braking
5.26.1 Plugging
5.26.2 Rheostatic Braking
5.26.3 Regenerative Braking
Section Practice Problems
5.27 Losses in a DC Machine
5.28 Constant and Variable Losses
5.29 Stray Losses
5.30 Power Flow Diagram
5.31 Efficiency of a DC Machine
5.32 Condition for Maximum Efficiency
5.33 Test Performed to Determine Efficiency of DC Machines
5.34 Brake Test
5.35 Swinburne’s Test
5.36 Hopkinson’s Test
5.37 Testing of DC Series Machines
5.38 Inspection/maintenance of DC Machines
5.39 Faults in DC Machines
5.40 Trouble Shooting in a DC Motor
Section Practice Problems
Review Questions
Multiple Choice Questions

### 6. Synchronous Generators or Alternators

Introduction
6.1 General Aspects of Synchronous Machines
6.2 Basic Principles
6.3 Generator and Motor Action
6.4 Production of Sinusoidal Alternating emf
6.5 Relation between Frequency, Speed and Number of Poles
6.6 Advantages of Rotating Field System over Stationary Field System
6.7 Constructional Features of Synchronous Machines
6.8 Excitation Systems
6.8.1 DC Exciters
6.8.2 Static-Excitation System
6.8.3 Brushless Excitation System
Section Practice Problems
6.9 Armature Winding
6.10 Types of Armature Winding
6.11 Important Terms Used in Armature Winding
Section Practice Problems
6.12 Coil Span Factor
6.13 Distribution Factor
6.14 Winding Factor
6.15 Generation of Three-phase emf
6.16 emf Equation
6.17 Wave Shape
6.18 Harmonics in Voltage Wave Form
Section Practice Problems
6.19 Production of Revolving Field
6.20 Ferrari’s Principle (Vector Representation of Alternating Field)
6.21 Production of Two-phase Rotating Magnetic Field
6.22 Production of Three-phase Rotating Magnetic Field
6.23 Rating of Alternators
6.24 Armature Resistance
6.25 Armature Leakage Reactance
6.26 Armature Reaction
6.27 Effect of Armature Reaction on emf of Alternator
6.28 Synchronous Reactance and Synchronous Impedance
6.29 Equivalent Circuit of an Alternator and Phasor Diagram
6.30 Expression for No-load Terminal Voltage
6.31 Voltage Regulation
6.32 Determination of Voltage Regulation
6.33 Synchronous Impedance Method or emf Method
6.33.1 Determination of Synchronous Impedance
6.33.2 Determination of Synchronous Reactance
6.34 Modern Alternators
6.35 Short-Circuit Ratio (SCR)
Section Practice Problems
6.36 Assumptions Made in Synchronous Impedance Method
6.37 Ampere-turn (or mmf) Method
6.38 Zero Power Factor or Potier Method
Section Practice Problems
6.39 Power Developed by Cylindrical Synchronous Generators
6.39.1 Power Output of an AC Generator (in Complex Form)
6.39.2 Real Power Output of an AC Generator
6.39.3 Reactive Power Output of an AC Generator
6.39.4 Power Input to an AC Generator (in Complex Form)
6.39.5 Real Power Input to an AC Generator
6.39.6 Reactive Power Input to an AC Generator
6.39.7 Condition for Maximum Power Output
6.39.8 Condition for Maximum Power Input
6.39.9 Power Equations, when Armature Resistance is Neglected
6.40 Two-Reactance Concept for Salient Pole Synchronous Machines
6.40.1 Determination of Xd and Xq by Low Voltage Slip Test
6.41 Construction of Phasor Diagram for Two-Reac tion Concept
6.42 Power Developed by a Salient Pole Synchronous Generator
Section Practice Problems
6.43 Transients in Alternators
6.43.1 Sub-transient, Transient and Direct Reactance
6.44 Losses in a Synchronous Machine and Efficiency
6.45 Power Flow Diagram
6.46 Necessity of Cooling
6.47 Methods of Cooling
6.48 Preventive Maintenance
Section Practice Problems
Review Questions
Multiple Choice Questions

### 7. Parallel Operation of Alternators

Introduction
7.1 Necessity of Parallel Operation of Alternators
7.2 Requirements for Parallel Operation of Alternators
7.3 Synchronising Alternators
7.4 Conditions for Proper Synchronising
7.5 Synchronising Single-phase Alternators
7.5.1 Dark Lamp Method
7.5.2 Bright Lamp Method
7.6 Synchronising Three-phase Alternators
7.6.1 Three Dark Lamps Method
7.6.2 Two Bright and One Dark Lamp Method
7.7 Synchronising Three-phase Alternators using Synchroscope
7.9 Load Sharing between Two Alternators
Section Practice Problems
7.10 Two Alternators Operating in Parallel
7.11 Synchronising Current, Power and Torque
7.12 Effect of Change in Input Power of One of the Alternators
7.13 Effect of Change in Excitation of One of the Alternators
7.14 Effect of Reactance
7.15 Effect of Governors’ Characteristics on Load Sharing
7.16 Hunting
Section Practice Problems
Review Questions
Multiple Choice Questions

### 8. Synchronous Motors

Introduction
8.1 Working Principle of a Three-Phase Synchronous Motor
8.2 Effect of Load on Synchronous Motor
8.3 Equivalent Circuit of a Synchronous Motor
8.4 Phasor Diagram of a Synchronous Motor (Cylindrical Rotor)
8.5 Relation between Supply Voltage V and Excitation Voltage E
8.6 Different Torques in a Synchronous Motor
8.7 Power Developed in a Synchronous Motor (Cylindrical Rotor)
8.8 Phasor Diagrams of a Salient-pole Synchronous Motor
8.9 Power Developed in a Salient-pole Synchronous Motor
8.10 Power Flow in a Synchronous Motor
Section Practice Problems
8.11 Effect of Change in Excitation
8.12 V-Curves and Inverted V-Curves
8.13 Effect of Change in Load on a Synchronous Motor
8.14 Methods of Starting of Synchronous Motors
8.15 Synchronous Condenser
8.16 Characteristics of Synchronous Motor
8.17 Hunting
8.18 Applications of Synchronous Motors
8.19 Comparison between Three-phase Synchronous and Induction Motors
8.20 Merits and Demerits of Synchronous Motor
Section Practice Problems
Review Questions
Multiple Choice Questions

### 9. Three-Phase Induction Motors

Introduction
9.1 Constructional Features of a Three-Phase Induction Motor
9.2 Production of Revolving Field
9.3 Principle of Operation
9.4 Reversal of Direction of Rotation of Three-Phase Induction Motors
9.5 Slip
9.6 Frequency of Rotor Currents
9.7 Speed of Rotor Field or mmf
9.8 Rotor emf
9.9 Rotor Resistance
9.10 Rotor Reactance
9.11 Rotor Impedance
9.12 Rotor Current and Power Factor
9.13 Simplified Equivalent Circuit of Rotor
Section Practice Problems
9.14 Stator Parameters
9.17 Induction Motor vs Transformer
9.18 Reasons of Low Power Factor of Induction Motors
9.19 Main Losses in an Induction Motor
9.20 Power Flow Diagram
9.21 Relation between Rotor Copper Loss, Slip and Rotor Input 745
9.22 Rotor Efficiency
Section Practice Problems
9.23 Torque Developed by an Induction Motor
9.24 Condition for Maximum Torque and Equation for Maximum Torque
9.25 Starting Torque
9.26 Ratio of Starting to Maximum Torque
9.27 Ratio of Full Load Torque to Maximum Torque
9.28 Effect of Change in Supply Voltage on Torque
9.29 Torque-slip Curve
9.30 Torque-speed Curve and Operating Region
9.31 Effect of Rotor Resistance on Torque-slip Curve
Section Practice Problems
9.32 Constant and Variable Losses in an Induction Motor
9.33 Main Tests Performed on an Induction Motor
9.33.1 Stator Resistance Test
9.33.2 Voltage-ratio Test
9.33.4 Blocked Rotor Test
9.33.5 Heat Run Test
9.34 Equivalent Circuit of an Induction Motor
9.35 Simplified Equivalent Circuit of an Induction Motor
9.36 Maximum Power Output
9.37 Circle Diagram
9.38 Circle Diagram for the Approximate Equivalent Circuit of an Induction Motor
9.39 Construction of a Circle Diagram for an Induction Motor
9.40 Results Obtainable from Circle Diagram
9.41 Maximum Quantities
9.42 Significance of Some Lines in the Circle Diagram
Section Practice Problems
9.43 Effect of Space Harmonies
9.43.1 Cogging in Three-phase Induction Motors
9.43.2 Crawling in Three-phase Induction Motors
9.44 Performance Curves of Induction Motors
9.45 Factors Governing Performance of Induction Motors
9.46 High Starting Torque Cage Motors
9.46.1 Deep Bar Cage Rotor Motors
9.46.2 Double Cage Induction Motor
9.47 Motor Enclosures
9.48 Standard Types of Squirrel Cage Motor
9.48.1 Class A Motors
9.48.2 Class B Motors
9.48.3 Class C Motors
9.48.4 Class D Motors
9.48.5 Class E Motors
9.48.6 Class F Motors
9.49.1 Squirrel Cage Induction Motors
9.49.2 Slip-ring Induction Motors
9.50 Applications of Three-phase Induction Motors
9.51 Comparison of Squirrel Cage and Phase Wound Induction Motors
9.52 Comparison between Induction Motor and Synchronous Motor
9.53 Installation of Induction Motors
9.54 Preventive Maintenance of Three-phase Induction Motors
Section Practice Problems
Review Questions
Multiple Choice Questions

### 10. Starting Methods and Speed Control of Three-phase Induction Motors

Introduction
10.1 Necessity of a Starter
10.2 Starting Methods of Squirrel Cage Induction Motors
10.2.1 Direct on Line (D.O.L.) Starter
10.2.2 Stator Resistance (or Reactance) Starter
10.2.3 Star-Delta Starter
10.2.4 Auto-transformer Starter
10.3 Rotor Resistance Starter for Slip Ring Induction Motors
Section Practice Problems
10.4 Speed Control of Induction Motors
10.5 Speed Control by Changing the Slip
10.5.1 Speed Control by Changing the Rotor Circuit Resistance
10.5.2 Speed Control by Controlling the Supply Voltage
10.5.3 Speed Control by Injecting Voltage in the Rotor Circuit
10.6 Speed Control by Changing the Supply Frequency
10.7 Speed Control by Changing the Poles
10.8 Speed Control by Cascade Method
10.9 Speed Control by Injecting an emf in the Rotor Circuit
10.9.1 Kramer System of Speed Control
10.9.2 Scherbius System of Speed Control
Section Practice Problems
Review Questions
Multiple Choice Questions

### 11. Single-Phase Motors

Introduction
11.1 Classification of Single-phase Motors
11.2 Single-phase Induction Motors
11.3 Nature of Field Produced in Single Phase Induction Motors
11.4 Torque Produced by Single-phase Induction Motor
11.5 Equivalent Circuit of Single-phase Induction Motor
11.6 Rotating Magnetic Field from Two-phase Supply
11.7 Methods to make Single-phase Induction Motor Self-starting
11.8 Split Phase Motors
11.9 Capacitor Motors
Section Practice Problems
11.11 Reluctance Start Motor
11.12 Single-phase Synchronous Motors
11.13 Reluctance Motors
11.14 Hysteresis Motors
11.15 AC Series Motor or Commutator Motor
11.16 Universal Motor
11.17 Comparison of Single-phase Motors
11.18 Trouble Shooting in Motors
Section Practice Problems
Review Questions
Multiple Choice Questions

### 12. Special Purpose Machines

Introduction
12.1 Feedback Control System
12.2 Servomechanism
12.3 Servomotors
12.4 DC Servomotors
12.4.1 Field-controlled DC Servomotors
12.4.2 Armature-controlled DC Servomotors
12.4.3 Series Split-field DC Servomotors
12.4.4 Permanent-magnet Armature-controlled DC Servomotor
12.5 AC Servomotors
12.6 Schrage motor
Section Practice Problems
12.7 Brushless Synchronous Generator
12.7.1 Brushless DC Generator
12.8 Brushless Synchronous Motor
12.9 Three-brush (or Third-brush) Generator
12.10 Brushless DC Motors
12.11 Stepper Motors
12.11.1 Permanent-magnet (PM) Stepper Motor
12.11.2 Variable-reluctance (VR) Stepper Motor
Section Practice Problems
12.12 Switched Reluctance Motor (SRM)
12.13 Linear Induction Motor (LIM)
12.14 Permanent Magnet DC Motors
12.15 Induction Generator
12.16 Submersible Pumps and Motors
12.17 Energy Efficient Motors
Section Practice Problems
Review Questions
Multiple Choice Questions
Open Book Questions
Index

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