unit 3 pdf

Page 1: Introduction

• Lecturer: Ashima Airan

• Institution: Bharati Vidyapeeth Pune, Department of Applied Science

• Subject: Electrical Science

• Course Code: ES-107/108

Page 2: Overview of Unit 3

• Focus: D.C. Generators & Motors

• Department: Applied Science, BVCOE New Delhi

Page 3: DC Machines

• Electric Generator: Converts mechanical energy into electrical energy through electromagnetic induction.

• Electrical Motor: Converts electrical energy into mechanical energy, allowing for movement and power generation in various applications.

• Classification:

  • D.C. Generators

  • D.C. Motors

• Construction: The basic construction shares similarities for both generators and motors, including key components that define their operation.

• Key Concepts: Understanding principles of magnetism and electromagnetism is crucial as they form the foundational concepts for the operation of D.C. machines.

Page 4: Magnetism

• Definition: Magnetism is a fundamental property of certain materials that allows them to exert forces on other materials, particularly iron and magnetic metals.

• Natural Magnet: A solid body that naturally exhibits magnetism with defined magnetic poles (North and South).

• Behavior: Magnets will align themselves with the Earth's magnetic field when suspended freely, demonstrating an inherent magnetic orientation.

Page 5: Laws of Magnetism

• Law 1: Like poles repel (North-North or South-South), whereas unlike poles attract (North-South), a principle integral to understanding magnetic interactions.

• Law 2 (Coulomb's Law): The magnetic force (F) between two poles is directly proportional to the product of their strengths and inversely proportional to the square of the distance separating them:

  • Mathematically: F = k(M1 * M2) / d² where M1 and M2 are the strengths of the poles, k is a constant of proportionality, and d is the distance between the poles.

Page 6: Magnetic Field & Flux

• Magnetic Field: The area surrounding a magnet where magnetic forces are exerted and felt by other materials.

• Magnetic Lines of Force: Visual representations of a magnetic field; these lines illustrate the direction and strength of the field and are measured in webers (1 weber = 10^8 lines of force).

• Properties: Magnetic lines of force never intersect; they have a fixed direction (from North to South outside the magnet and from South to North inside it).

Page 7: Diagram Representation

• A diagram showing the flux direction for a bar magnet should include lines of force, demonstrating the characteristics and behavior of magnetic fields visually.

Page 8: Electromagnetism Basics

• Current and Magnetic Field: A conductor carrying an electric current induces a magnetic field around itself, fundamental to the operation of electric machines.

• Direction: The direction of the induced magnetic field can be determined using the right-hand rule: with the right thumb pointing in the direction of the current, the curled fingers point in the direction of the magnetic field.

Page 9: Convention of Current Direction

• Symbols: Current flowing into the paper is denoted by a cross (X), whereas current coming out of the paper is indicated by a dot (•).

• Right-hand Rule Application: This rule is crucial in indicating the directions of the magnetic field for currents moving towards or away from the observer, providing clarity in analysis.

Page 10: Principle & Operation of a DC Generator

• Function: D.C. generators convert mechanical energy into electrical energy through the process of electromagnetic induction, requiring a mechanical rotation to generate e.m.f.

Page 11: Fleming's Right Hand Rule

• Concept: In applying Fleming's right-hand rule, the thumb, index finger, and middle finger are oriented at right angles to each other to determine the directions of motion (thumb), magnetic field (index), and induced electromotive force (e.m.f.) (middle finger).

Page 12: Verifying Current Direction

• Scenarios: Different configurations of current movement will yield different directions for the induced e.m.f., showcasing various practical situations encountered in circuit analysis.

Page 13: Induced E.M.F. Equation

• Formula: E = B * L * v, where B is the magnetic flux density, L is the length of the conductor within the magnetic field, and v is the velocity of the conductor perpendicular to the magnetic field. The active length plays a vital role in determining the value of induced e.m.f.

Page 14: Angular Relationship in E.M.F.

• Conditions: The amount of induced e.m.f. is contingent upon the angle between the conductor's direction of motion and the magnetic flux direction. If these two vectors are parallel, no e.m.f. is generated, emphasizing the importance of angle in design.

Page 15: Nature of Induced E.M.F.

• Characteristics: For a D.C. generator to continuously produce D.C. voltage, a commutator is essential. This device rectifies the alternating e.m.f. generated due to the nature of rotation, ensuring a stable unidirectional output.

Page 16-27: Construction of DC Machines

• Components: Detailed exploration of essential components and their functions, including:

  • Yoke: Provides mechanical support to the machine and serves as a magnetic conductive path for the magnetic flux.

  • Poles: Produce the magnetic flux necessary for operation, typically composed of core and shoe structures to optimize magnetic performance.

  • Field Winding: Generates the magnetic field vital for machine operations by creating magnetic forces when current flows through the coils.

  • Armature: Houses the conductors; it generates e.m.f. when rotated within the magnetic field.

  • Commutator: Converts alternating e.m.f. induced in the armature windings into a unidirectional flow suitable for D.C. applications.

  • Brushes: Function to conduct electrical current from the rotating commutator to the external electrical circuit, allowing for external load connection.

Page 28-67: Deep Dive into Specific Components and Functions

• Focus on Winding Terminologies: Detailed definitions and roles of various winding configurations such as conductors, turns, coils, and armature winding methodologies.

• E.M.F. Calculations: Comprehensive derivations for generated e.m.f. under various armature configurations (e.g., lap winding vs. wave winding).

• Examples and Numerical Problems: Provide practical calculations illustrating how to derive e.m.f. values based on critical parameters such as speed, pole strength, and winding configuration, facilitating better comprehension of operational principles.

Page 68-72: Losses in DC Machines

• Types of Losses: Identification and explanation of major loss types such as copper losses (I²R losses), iron losses (including hysteresis and eddy current losses), and mechanical losses arising from friction.

• Constant vs Variable Losses: Differentiation between losses that remain constant regardless of load conditions and those that vary based on changing load requirements, providing insights into efficiency management.

Page 73-81: Operation Principles of DC Motors

• Principle: Current-carrying conductors experience a force when placed in magnetic fields, leading to torque production that causes armature rotation and performs useful work.

• Fleming's Left-Hand Rule: Used to determine the direction of force acting on conductors residing within magnetic fields, crucial for understanding motor behavior.

• Back E.M.F.: Interaction between the generating action of the motor and the applied voltage leads to the generation of a back e.m.f., which acts to oppose the supply voltage, regulating the current drawn by the motor and influencing performance.

Page 82-82: Torque and Power Equations

• Torque Equation: Presents the relationship between the torque produced, the current supplied, and the rotational speed, thus illustrating the principles behind mechanical power generation in motors.

Page 83-108: Characteristics of Various DC Motors

• Machine Types: An overview of the operational characteristics of different D.C. motor configurations, including shunt, series, and compound motors.

• Starting Characteristics: Detailed examination of how torque, speed regulation, and back e.m.f. influence the performance and operational behavior of various motor designs.

Page 109-119: Induction Motors

• AC Motors: Introduction to basic components (stator and rotor) and their respective functions in the operation of induction motors.

• Types: Breakdown of three-phase and single-phase induction motor structures, highlighting differences in rotor designs and overall operational methodologies.

Page 120-156: Specific Induction Motor Types and Their Operations

• Squirrel Cage: Explanation of the common design, including advantages such as ease of operation and maintenance, low cost, and robustness in various applications.

• Slip Ring Type: Detailed description of this design, including its distinct advantages, such as the ability to introduce external resistance for speed control and improved starting performance.

Page 157-161: Synchronous Motors

• Characteristics: Maintaining a steady operational speed under varying load conditions necessitates specific designs and starting mechanisms, particularly due to inertia challenges.

• Power Factor Improvement: Application examples where synchronous motors are employed for load balancing and enhancing the efficiency of electrical systems.

Page 162-176: Starting Methods and Systems

• Star Delta Starter: A widely used method for starting large motors, designed to minimize initial torque and prevent electrical spikes during startup, ensuring smoother operation.

• DOL Starter: Explanation of functionality, advantages, and systems utilized in direct online motor starting configurations, ensuring immediate application of full voltage.

Page 177-191: Control Mechanisms and Motor Characteristics

• VFD (Variable Frequency Drive): The primary mechanism implemented for speed control of induction motors, highlighting diverse applications across various industrial sectors.

• Comparative Analysis of Star Delta vs DOL Starters: Thorough outline of operational differences, advantages, and disadvantages of the various starting methods, aiding in the selection process based on specific application requirements.