8 Key excerpts on "Electric Generators"

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  eBook - ePub

  Power System Fundamentals

  • Pedro Ponce, Arturo Molina, Omar Mata, Luis Ibarra, Brian MacCleery(Authors)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  2

  Power Flow and Electric Machinery Basics

  As a first step into the electric grid, the fundamental topics about power generation, transmission, distribution, and consumption must be established. Each phase must then be first described and appropriately placed among the others so the power flow is thoroughly understood. This chapter aims to introduce the electric grid by presenting how the power flow occurs and by offering the modeling basics around its major electromagnetic components.

  An electric machine is a device that can convert either mechanical energy to electrical energy or electrical energy to mechanical energy. When such a device is used to convert mechanical energy to electrical energy, it is called a generator. When it converts electrical energy to mechanical energy, it is called a motor. Since any given electric machine can convert power in either direction, any machine can be used as either a generator or a motor. Almost all practical motors and generators convert energy through the interaction of a magnetic field, and only machines using magnetic fields to perform such conversions are considered in this book.

  Many concepts must be established before performing any analysis of electric machines. The principle of electromechanical energy conversion is the most important law of machine analysis. This theory allows us to establish an expression of electromagnetic torque in terms of machine variables, like the currents and the displacement of the mechanical system. In this chapter, basic principles that will be mentioned are the equivalent circuit representations of magnetically coupled circuits, the concept of a sinusoidally distributed winding, and the winding inductances, among others.

   

  2.1 A glance into power flow

  2.1.1 Generators

  The generators produce the electrical energy distributed by a power system. Almost all of the generators in use today produce electrical energy by converting mechanical energy to electrical energy through the action of a magnetic field. The mechanical energy comes from a prime mover, which is the device that spins the generator. Prime movers are usually some form of steam or water turbines, but diesel engines are sometimes used in remote locations. Modern generators generate electrical power at voltages of

  13.8 - 24 [ k V ]

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  eBook - ePub

  Introductory Electrical Engineering With Math Explained in Accessible Language

  • Magno Urbano(Author)
  • 2019(Publication Date)
  • Wiley

    (Publisher)

  12 Generators : And Motors

  12.1 Introduction

  In this chapter, we will examine Electric Generators that are direct applications of electromagnetic induction and their cousins, electric motors.

  12.2 Electric Generators

  Electric Generators are devices that can generate alternating current (AC) by the movement of a wire winding inside a magnetic field, for example.

  Figure 12.1 shows the idea behind a simple generator: a winding of wire that rotates inside a magnetic field generated by two static magnets.

  Figure 12.1

  Electric generator (winding at 0°).

  In real life, windings are attached to an axis that rotates by external force like the turbine of a hydroelectric power plant.

  As the winding rotates, its turns of wire cut the magnetic field lines, represented by the arrows in Figure 12.1 . This process induces electrical current in the winding.

  12.2.1 How It Works

  Suppose the winding is, initially, at the position 0°, represented by Figure 12.1 . The winding is cutting the magnetic field at a position of minimal induced current. The induced current at this point is 0.

  Moved by external forces, the winding rotates anticlockwise and cuts more magnetic field lines. The induced current increases gradually across the winding. When the rotation reaches 90°, as shown in Figure 12.2 , current reaches its maximum positive value.

  Figure 12.2

  Electric generator (winding at 90°).

  Rotation continues from 90° to 180°. Current decreases gradually to 0, because the winding moves toward a position where the magnetic field is inverted, compared with what it was at 0°.

  Rotation continues and the winding goes from 180° to 270°. Now the winding position is inverted compared with how it was at 90°. Consequently the magnetic field and the current are inverted. The current is at its negative peak.

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  eBook - ePub

  Electrical Machines

  A Practical Approach

  • Satish Kumar Peddapelli, Sridhar Gaddam(Authors)
  • 2020(Publication Date)
  • De Gruyter

    (Publisher)

  1 DC Generators 1.1 Introduction A machine that changes mechanical power into electrical power by using the principle of magnetic induction is called generator. Whenever a conductor cuts the magnetic field, an emf(Electromotive Force) is generated in that conductor. The magnitude of the generated emf is proportional to dϕ/dt and the polarity depends on the direction of the flux and the conductor. Fleming’s right hand rule is used to determine the current direction. 1.2 Operation of DC generator The construction of a simple DC generator is shown in Fig. 1.1. Whenever a conductor is kept in a magnetic field, an emf is induced in the conductor. In a DC generator, magnetic field is generated by field coils and the armature conductors are rotated in the magnetic field. Thus, an emf is produced in the armature conductors. The voltage/current output waveform is shown in Fig. 1.2. Fig. 1.1: Principle of the generator. Fig. 1.2: Voltage across the load. 1.3 Voltage equation of a generator Let P = Number of poles Φ = Flux/pole in weber Z = Number of conductors (Armature) = Number of slots × Number of conductors/slot N = No. of rotations in rpm; A = No. of parallel paths E g = emf generated in any one parallel path in the armature (1.1) A v e r a g e v o l t a g e p r o d u c e d p e r c o n d u c t o r = d ϕ d t V o l t The change in flux per conductor in one revolution dϕ = ϕ × P. webers T h e f l u x c h a n g e s N 60 r o t a t i o n s / s e c Therefore, the time taken to complete one complete rotation is. dt = 60 N second (1.2) V o l t a g e g e n e r a t e d / c o n d u c t o r (E g) = d ϕ d t = ϕ P N 60 w e b e r s For a simple wave-wound generator A = 2 ; The number of conductors per. path = Z 2 (1.3) E g / p a t h = ϕ P N 60 × Z 2 = ϕ Z N P 120 V o l t For a simple lap-wound generator A = P ; The number of conductors per

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  eBook - ePub

  Aircraft Electrical and Electronic Systems

  • David Wyatt, Mike Tooley(Authors)
  • 2018(Publication Date)
  • Routledge

    (Publisher)

  Chapter 4 Generators and motors

  Engine-driven generators are a primary source of electrical power in an aircraft. Generators can supply either direct or alternating current (DC or AC), as appropriate to the needs of an individual aircraft type. Motors (which can also be either DC or AC types) are fitted in aircraft in order to satisfy a wide range of needs. Generators and motors share many similarities and this chapter provides an introduction to their operating principles by looking in more detail at the construction and operation of these indispensable aircraft electrical components. The chapter also includes a brief introduction to three-phase AC supplies, including the theoretical and practical aspects of their generation and distribution.

  4.1 Generator and motor principles

  In Chapter 1 we introduced the concept of electromagnetic induction. Put simply, this is the generation of an e.m.f. across the ends of a conductor when it passes through a change in magnetic flux. In a similar fashion, an e.m.f. will appear across the ends of a conductor if it remains stationary whilst the field moves. In either case, the action of cutting through the lines of magnetic flux results in a generated e.m.f. – see Fig. 4.1 . The amount of e.m.f., e, induced in the conductor will be directly proportional to:

  • the density of the magnetic flux, B, measured in tesla (T)
  • the effective length of the conductor, l, within the magnetic flux
  • the speed, v, at which the lines of flux cut through the conductor measured in metres per second (m/s)
  • the sine of the angle, θ between the conductor and the lines of flux.

  The induced e.m.f. is given by the formula:

  e = Blv sin θ

  Figure 4.1 A conductor moving inside a magnetic field

  Note that if the conductor moves at right angles to the field (as shown in Fig. 4.1

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  eBook - ePub

  Practical Electricity for Aviation Maintenance Technicians

  • Dale Crane, Dennis Wilt(Authors)
  • 2017(Publication Date)
  • Aviation Supplies & Academics, Inc.

    (Publisher)

  8 Electric Motors and Generators

  Electric motors have become such a standard part of our lives that they are usually taken for granted. They are made in all sizes and power outputs, from the tiny motors that move the hands in analog wrist watches to the motors that drive ocean-going ships. Regardless of their size, all electric motors work on the same principle. One magnetic fields reacts with another magnetic field to produce a physical force.

  Figure 8-1 shows the basic way an electric motor works. The conductor (represented by the circle) in view A has no current flowing in it, and the lines of flux pass straight across the space from the north pole of the magnet to the south pole. But when current flows in the conductor as in view B, it produces a magnetic field, which surrounds the conductor.

  Figure 8-1 . When the magnetic field surrounding the conductor distorts the lines of flux between the poles of the magnet, a force is produced that tries to move the conductor out of the magnetic field.

  The lines of flux between the poles of the magnet try to remain as short as possible, and when they are distorted by the field surrounding the conductor, they produce a physical force that tries to move the conductor to the left, out of their field.

  The right-hand rule for motors helps understand this action. Hold the fingers of your right hand as shown in Figure 8-2, with the forefinger pointing in the direction of the lines of flux (from the north pole of the magnet to the south pole) and the second finger pointing in the direction of electron flow in the conductor (from negative to positive); the thumb will point in the direction the conductor will move. The amount of force that acts on the conductor is determined by the strength of the two magnetic fields.

  Figure 8-2

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  eBook - ePub

  Electrical Machines

  Fundamentals of Electromechanical Energy Conversion

  • Jacek F. Gieras(Author)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  1 INTRODUCTION TO ELECTROMECHANICAL ENERGY CONVERSION

  1.1What is electromechanical energy conversion?

  Electromechanical energy conversion is a conversion of mechanical energy into electrical energy (generator) or vice versa (motor) with the aid of rotary motion (rotary machines) or translatory (linear) motion (linear machines and actuators).

  Electrical machines, solenoid actuators and electromagnets are generally called electromechanical energy conversion devices (Fig. 1.1 ).

  Fig. 1.1.Electromechanical energy conversion.

  Transformers and solid-state converters do not belong to the group of electromechanical energy conversion devices because they only convert one kind of electrical energy into another kind of electrical energy with different parameters (change in voltage, current, frequency, number of phases, conversion of DC into AC current, etc.) without any motion.

  1.1.1Block diagrams of electromechanical energy conversion devices

  Fig. 1.2a shows a block diagram of a motor, while Fig. 1.2a shows a block diagram of a generator. An example of application of an electric motor is

  shown in Fig. 1.3 . An example of application of an electric generator is shown in Fig. 1.4 .

  Fig. 1.2.Block diagrams of electromechanical energy conversion devices: (a) motor; (b) generator.

  Fig. 1.3.Power tool: an example of conversion of electrical energy into mechanical energy.

  Fig. 1.4.Wind turbine generator: an example of conversion of mechanical energy into electrical energy.

  1.1.2Left-hand and right-hand rule

  The left-hand rule (Fig. 1.5a ) indicates the direction of the phasor of the electrodynamic force (EDF), i.e.,

  d F = I d l × B

      ( 1.1 )

  Fig. 1.5.Left-hand and right-hand rules: (a) left-hand rule shows the direction of electrodynamic force (EDF); (b) right-hand rule shows the direction of electromotive force (EMF).

  or, in scalar form

  F = B I l

      ( 1.2 )

  The right-hand rule (Fig. 1.5b

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  eBook - ePub

  Handbook of Power Systems Engineering with Power Electronics Applications

  • Yoshihide Hase(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Chapter 13 The Generator as Rotating Machinery

  We have studied the generator's characteristics from the viewpoint of electrical theory in the previous chapters. In this chapter we examine the generator as a rotating mechanical machine. This base knowledge is essential in order to understand the dynamic characteristics of a generator and of the power system as a congregation of generators.

  Note that we omit the overbar symbol for the per unit value below and for all subsequent chapters, even though most of all the quantities are in per unit values.

  13.1 Mechanical (Kinetic) Power and Generating (Electrical) Power

  We examine first the relation between the generator's mechanical input (driving force by steam turbine or by water wheel) and electrical output (generating power).

  13.1.1 Mutual Relation Between Mechanical Input Power and Electrical Output Power

  A generator connected to a power system network is operating under three-phase-balanced conditions. The generator's electrical quantities are given by Equation (10.43) in the d–q–0 domain, and the apparent power is given by Equation (11.25) , all in per unit values.

  Substituting Equation (10.43) into Equation (11.25) to eliminate voltage variables, the following equation is derived:

  (13.1a)

  (13.1b)

  The angular velocity in the equation is the instantaneous value of the rotor shaft, which fluctuates and does not necessarily coincide with the electrical angular velocity of the power network

  Equation 13.1b is a very important equation which combines the concepts of mechanical power and electrical power. Now, we investigate each term on the right-hand side of this equation.

  13.1.1.1 The First Term

  From the dynamic theory of rotating devices in physics,

  (13.2)

  Comparing the first term on the right-hand side of Equation 13.1b with Equation (13.2) , the part in corresponds to the mechanical torque

  Tm

  , and the first term itself corresponds to mechanical power

  Pm

  which is given from the prime-mover. This mechanical power

  Pm

  intervenes from flux linkages and finally is transferred from the rotor to the stator armature windings in the form of electrical power across the air gap. In other words, the first term on the right-hand side is the mechanical power

  Pm

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  eBook - ePub

  Electrical Equipment

  A Field Guide

  • B. Koti Reddy(Author)
  • 2021(Publication Date)
  • Wiley-Scrivener

    (Publisher)

  3 Generators

  3.1 Introduction

  An Electrical Generator, which is driven by a prime-mover, converts mechanical energy into electrical energy. There are various types of conventional generators, as shown in Figure 3.1 . At present, most of the generators are of AC, which will be explained in subsequent sections.

  3.2 Alternator

  Unlike DC generators, alternators have a stationary armature. The main advantages of stationary armature are:

  1. The output can be taken easily from armature which avoids brush gear thus avoids sparking and wear & tear problems.
  2. Ease in insulation and can built generators up to 30 KV or more.
  3. No slip rings and less maintenance.
  4. Low voltage is supplied to rotating field, thus avoids brush gear and other problems.
     Figure 3.1

     Types of generators.
  5. The armature can be braced which prevents the deformation of windings due to mechanical forces on occurrence of severe faults such as short circuits.

  3.3 Field Poles

  There are two type of poles, which are explained in Table 3.1 .

  3.4 Construction of Field Poles

  The basic construction model of salient pole and cylindrical pole type generators are shown in Figures 3.2 and 3.3 , respectively.

  Table 3.1

  Types of field poles of AC generators.

  S. no.	Salient pole	Cylindrical pole
  1	Projected or protruding poles.	Wound rotor type poles.
  2	Used for low speeds like diesel driven Generators.	Used for high speeds and large capacities like turbo Generators.
  3	Needs damper winding.	No need of damper winding since rotor body itself acts as a damper.
  4	Has larger diameter of order of 2 to 6 meters to accommodate several poles and short axial length of around 1M.	Has a small diameter of order of 1 meter and long axial length of order of 2 to 5 meters.
  5	Little vibrating and noisy operation.	Better balance of rotor and quite operation.
  6	Have different axis reactances (xd and xq ) which are to be analysed based on two reaction theory. Here,	Have only one direct axis reactance. Here,

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