Chapter FOur

Chapter Overview

  • This chapter focuses on DC machines and their operational principles, along with an overview of induction machines.

DC Machines and Their Principles of Operation

Introduction and Application

  • Electromechanical Energy Conversion: Utilizes three electrical machines: DC, induction, and synchronous.

  • Key Electromagnetic Phenomena:

    • Generator Action: Inducing voltage in a conductor moving through a magnetic field.

    • Motor Action: Mechanical force on a current-carrying conductor within a magnetic field.

DC Machines

  • Electrical System: Voltage and current are key quantities.

  • Mechanical System: Torque and speed are analogous.

  • Modes of Operation:

    • Generator Mode: Mechanical power is converted into electrical power.

    • Motor Mode: Electrical power is converted into mechanical power to drive a load.

  • Applications: DC motors are increasingly used where controlled torque is critical (e.g., rolling mills, cranes, electric vehicles).

Construction of DC Machines

Main Parts

  • Three essential components:

    1. Field System (Stator)

    2. Armature (Rotor)

    3. Commutator

Field System (Stator)

  • Location: The stationary part of the machine.

  • Function: Produces magnetic flux for machine excitation.

  • Components:

    • Main Poles

    • Inter-poles

    • Frame (Yoke)

Frame (Yoke)

  • Serves as support and part of the magnetic circuit.

Main Poles

  • Support field coils on pole bodies, reducing air gap reluctance.

Inter Poles

  • Improve commutation under load conditions, positioned between main poles.

Armature

  • Function: Rotates within the magnetic field, enabling electromechanical energy conversion.

  • Comprises:

    • Armature core (with slots)

    • Armature windings

Commutator

  • Function: Mechanical rectification of power; connects armature coils and helps collect current.

Types of DC Machines

Field Excitation Methods

  • Separatedly Excited Machines: Field winding powered by an external DC source.

  • Self-Excited Machines: Field winding energized by its armature, reliant on residual magnetism.

    1. Series Excitation: Field winding in series with the armature.

    2. Shunt Excitation: Field winding connected in parallel with the armature.

    3. Compound Excitation: Combines series and shunt windings.

DC Generators

  • Converts mechanical energy into electrical energy using Faraday’s principles of electromagnetic induction.

  • Key Components: Magnetic Field and Moving Conductors (Armature).

  • Induced emf dependent on coil flux cutting and given by: e=βLVsinθ.

Induction Machines

Induction (Asynchronous) Machines

  • Most commonly used in industries due to their robust design.

  • Can operate as both motors and generators, predominantly used for motor applications.

Advantages of Induction Motors

  • Simple construction, low cost, high efficiency, and no need for brushes, minimizing maintenance.

Disadvantages of Induction Motors

  • Speed variation sacrifices efficiency; inferior starting torque compared to DC shunt motors.

Applications of Induction Motors

  • Large Motors: Used for pumps, fans, compressors.

  • Small Motors: Common in home appliances.

  • Two-Phase Motors: Primarily used as servomotors.

Construction

Key Components

  1. Stationary Stator: Steel frame with a cylindrical core; supports stator winding.

  2. Revolving Rotor: Composed of laminated layers, can be squirrel-cage or wound type.

Comparison of Squirrel Cage and Wound Rotors

  • Squirrel cage motors: simple, efficient, and cost-effective compared to wound rotor machines.

Principle Operation of 3-Phase Induction Motor

  • When fed with 3-phase supply, magnetic flux rotates at synchronous speed.

  • Induced emf produced in stationary rotor due to relative speed between rotating flux and rotor current.

  • Motor adjusts speed under varying load conditions through this induced emf and current relationship.