- •Textbook Series
- •Contents
- •1 DC Electrics - Basic Principles
- •Introduction
- •Electromotive Force (EMF)
- •Current
- •Resistance
- •Factors Affecting the Resistance
- •Units of Resistance
- •Resistors
- •Power
- •Series and Parallel Circuits
- •Kirchoff’s Laws
- •Annex A
- •2 DC Electrics - Switches
- •Switches
- •Proximity Detectors
- •Time Switches
- •Centrifugal Switches
- •3 DC Electrics - Circuit Protection and Capacitors
- •Electrical Faults
- •Circuit Protection Devices
- •Fuses
- •The Cartridge Fuse
- •Spare Fuses
- •High Rupture Capacity (HRC) Fuses
- •Dummy Fuses
- •Current Limiters
- •Circuit Breakers
- •Reverse Current Circuit Breakers
- •Capacitors
- •Capacitance
- •Capacitor in a DC Circuit
- •Capacitor in an AC Circuit
- •Capacitors in Parallel
- •Capacitors in Series
- •4 DC Electrics - Batteries
- •Batteries
- •Secondary Cells
- •Lead Acid Battery
- •Alkaline Battery (Nickel Cadmium, NiCad)
- •Battery Checks
- •Battery Charging
- •Secondary Batteries Summary
- •5 DC Electrics - Magnetism
- •Magnetism
- •Temporary Magnets
- •Permanent Magnets
- •Permeability
- •Magnetism
- •The Molecular Structure of Magnets
- •The Magnetic Effect of a Current
- •The Corkscrew Rule
- •The Magnetic Field of a Solenoid
- •The Right Hand Grasp Rule
- •The Strength of the Field of a Solenoid
- •Solenoid and Relay
- •The Forces on a Conductor Which is Carrying a Current
- •Questions
- •Answers
- •6 DC Electrics - Generators and Alternators
- •Electromagnetic Induction
- •Fleming’s Right Hand Rule
- •Faraday’s Law
- •Lenz’s Law
- •Simple Generator
- •Simple DC Generator
- •Characteristics of the Series Wound DC Generator
- •Commutator Ripple
- •Characteristics of the Shunt Wound DC Generator
- •A Compound Wound DC Generator
- •Flashing the Generator Field
- •Alternators
- •Voltage Control
- •Voltage Regulator Operation
- •Layout of a Generator System
- •Load Sharing Circuits
- •Operation of Load Sharing Circuit
- •7 DC Electrics - DC Motors
- •Electric Motors
- •Fleming’s Left Hand Rule
- •Practical DC Motor
- •Back EMF
- •Slow Start Resistor
- •Commutation
- •Series Wound Motors
- •Shunt Wound Motors
- •Starter-generator Systems
- •Actuators
- •Solenoid Actuators
- •Motor Actuator Construction
- •The Split Field Series Actuator
- •The Split Field Series Actuator Operation
- •Motor Actuators
- •Rotary Actuators
- •Linear Actuators
- •Actuator Brakes
- •Actuator Clutches
- •Visual Indicators Used with Linear Actuators
- •Visual Indicators Used with Rotary Actuators
- •Indicator Lights
- •Electromagnetic Indicators
- •Questions
- •Answers
- •8 DC Electrics - Aircraft Electrical Power Systems
- •Aircraft Electrical Power Systems
- •Dipole or Two Wire System
- •Single Pole (Unipole or Earth Return) System
- •Generators and Alternators
- •Voltage Regulators
- •Overvoltage Protection Unit
- •Generator Cut-out or Reverse Current Relay
- •Rectifiers
- •Inverters
- •The Generator Differential Cut-out
- •Generator (or Alternator) Warning Light
- •Generator (or Alternator) Master Switch
- •Monitoring Instruments
- •Ammeters and Voltmeters
- •The Battery
- •Bus Bars
- •Bus Bar Systems
- •Parallel Bus Bar System
- •Load Shedding
- •Generator or Alternator Failure
- •9 DC Electrics - Bonding and Screening
- •Bonding
- •The Static Discharge System or Static Wicks
- •Discharge of Static on Touchdown
- •Screening
- •Questions
- •Answers
- •10 DC Electrics - Specimen Questions
- •Questions – General 1
- •Questions – General 2
- •Answers – General 1
- •Answers – General 2
- •11 AC Electrics - Introduction to AC
- •Introduction
- •The Nature of Alternating Current
- •Terms
- •The Relationship of Current and Voltage in an AC Circuit
- •Resistance in AC Circuits
- •Inductance in AC Circuits
- •Inductive Reactance
- •Capacitance in AC Circuits
- •Capacitive Reactance
- •Impedance
- •Resonant Circuits
- •Summary
- •Power in AC Circuits
- •Power in a Purely Resistive Circuit
- •Power in a Purely Inductive Circuit
- •Power in a Capacitive Circuit
- •Power in a Practical AC Circuit
- •Power Factor
- •Power Factor Resume
- •Questions
- •Answers
- •12 AC Electrics - Alternators
- •Introduction to Aircraft Power Supplies
- •Generators / Alternators
- •Rotating Armature Alternator
- •Rotating Field Alternator
- •Alternator Output Rating
- •A Single Phase Alternator
- •Polyphase Circuits
- •Three Phase Alternator Connections
- •The Four Wire Star Connection
- •Delta Connected Alternator
- •Practical AC Generators
- •Brushed Alternators
- •Brushless Alternators
- •Frequency Wild Alternators
- •Obtaining a Constant Frequency Supply from a Frequency Wild System
- •Constant Frequency Alternators
- •Constant Speed Generator Drive Systems
- •CSDU Fault Indications in the Cockpit
- •The Drive Disconnect Unit (Dog Clutch Disconnect)
- •Variable Speed Constant Frequency Power Systems (VSCF)
- •Self-excited Generators
- •Load Sharing or Paralleling of Constant Frequency Alternators
- •Real Load
- •Reactive Load
- •Parallel Connection
- •Before Connecting in Parallel
- •Layout of a Paralleled System
- •Real Load Sharing
- •Reactive Load Sharing
- •Load Sharing General
- •Alternator Cooling
- •Generator Fault Protection
- •Bus Tie Breakers (BTBs)
- •Discriminatory Circuits
- •Differential Fault Protection
- •Synchronizing Units
- •Generator Failure Warning Light
- •Load Meters
- •Voltage and Frequency Meters
- •Generator Control Unit (GCU)
- •Emergency Supplies
- •The Ram Air Turbine (RAT)
- •The Auxiliary Power Unit (APU)
- •The Static Inverter
- •Ground Power Constant Frequency Supply System
- •Typical Controls and Indications
- •Questions
- •Answers
- •13 AC Electrics - Practical Aircraft Systems
- •Power Distribution
- •The Split Bus System
- •Parallel Bus Bar System
- •Questions
- •Answers
- •14 AC Electrics - Transformers
- •Transformers
- •Transformation Ratio
- •Power in a Transformer
- •Three Phase Transformers
- •Autotransformers
- •Rectification of Alternating Current
- •Half Wave Rectification
- •Full Wave Rectification
- •Three Phase Rectifiers
- •Transformer Rectifier Units (TRUs)
- •Inverters
- •Questions
- •Answers
- •15 AC Electrics - AC Motors
- •Alternating Current Motors
- •The Principle of Operation of AC Motors
- •The Synchronous Motor
- •The Induction Motor
- •The Squirrel Cage Rotor
- •The Induction Motor Stator
- •Slip Speed
- •Starting Single Phase Induction Motors
- •Fault Operation
- •Questions
- •Answers
- •16 AC Electrics - Semiconductors
- •An Introduction to Semiconductors
- •Conductors and Insulators
- •Semiconductors
- •N-Type Material
- •P-Type Material
- •Current Flow
- •The P-N Junction
- •Reverse Bias
- •Forward Bias
- •The Junction Diode
- •The Bipolar or Junction Transistor
- •Summary
- •17 AC Electrics - Logic Gates
- •An Introduction to Logic Gates
- •Binary Logic
- •Truth Tables
- •Gate Symbols
- •Positive and Negative Logic
- •The ‘AND’ Gate
- •The ‘OR’ Gate
- •The ‘INVERT’ or ‘NOT’ Gate
- •The ‘NAND’ Gate
- •The ‘NOR’ Gate
- •The ‘EXCLUSIVE OR’ Gate
- •Questions
- •Answers
- •18 Index
Chapter
11
AC Electrics - Introduction to AC
Introduction |
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161 |
The Nature of Alternating Current |
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Terms |
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The Relationship of Current and Voltage in an AC Circuit . . |
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165 |
Resistance in AC Circuits . . . . . . . . . . . . . . |
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Inductance in AC Circuits |
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166 |
Inductive Reactance . . . . . . . . . . . . . . . . |
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Capacitance in AC Circuits . . . . . . . . . . . . . . |
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Capacitive Reactance . . . . . . . . . . . . . . . . |
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.171 |
Impedance |
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172 |
Resonant Circuits . . . . . . . . . . . . . . . . . |
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172 |
Summary |
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173 |
Power in AC Circuits |
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173 |
Power in a Purely Resistive Circuit . . . . . . . . . . . |
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174 |
Power in a Purely Inductive Circuit |
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174 |
Power in a Capacitive Circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .175 |
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Power in a Practical AC Circuit . . . . . . . . . . . . |
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176 |
Power Factor . . . . . . . . . . . . . . . . . . |
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177 |
Power Factor Resume |
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177 |
Questions . . . . . . . . . . . . . . . . . . . . |
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.178 |
Answers . . . . . . . . . . . . . . . . . . . . |
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184 |
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11 AC Electrics -Introduction to AC
AC to Introduction - Electrics AC 11
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AC Electrics - Introduction to AC
Introduction
Alternating current (AC) is used in most large modern transport aircraft because of the following advantages that it holds over direct current (DC) supplies:
•AC generators are simpler and more robust in construction than DC machines.
•The power to weight ratio of AC machines is better than comparable DC machines.
•The supply voltage can be converted to a higher or lower value with almost 100% efficiency using transformers.
•Any required DC voltage can be obtained simply and efficiently using transformer rectifier units. (TRUs).
•Three phase AC motors which are simpler, more robust and more efficient than DC motors, can be operated from a constant frequency source, (AC generators).
•AC machines do not suffer from the commutation problems associated with DC machines and consequently are more reliable, especially at high altitude.
•High voltage AC systems require less cable weight than comparable power low voltage DC systems.
The Nature of Alternating Current
If the electrons flowing in a circuit move backwards and forwards about a mean position then the current produced is known as alternating current, AC. The simple AC generator shown in Figure 11.1 shows that a loop of wire (armature) rotated in a magnetic field experiences a continuously changing flux through it so that a voltage will be induced as long as rotation continues.
Figure 11.1 Simple AC generator
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AC Electrics - Introduction to AC 11
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11 AC Electrics -Introduction to AC
AC to Introduction - Electrics AC 11
The magnitude of the voltage depends on the speed of rotation and the field strength (i.e. rate of change of flux).
When an armature is connected to a load (resistor) in a closed circuit through slip rings and carbon brushes a current will flow around the circuit in proportion to the induced voltage.
If this armature is rotated as in Figure 11.2 then the flux is constantly changing. In positions 1, 3 and 5 the two sides of the loop are moving parallel to the field and so there is no voltage induced as there is no rate of change of flux. In positions 2 and 4 the two sides of the armature are moving at right angles to the field and the maximum voltage is induced as there is maximum rate of change of flux. In between these positions the induced voltage is between maximum and zero.
The polarity of the induced voltage changes as it passes through zero because the direction that each side of the armature moves through the field is reversed. If the polarity reverses then so must the current through the external circuit, and current flowing backwards and forwards about a mean position is alternating current. The direction of current flow through each side of the armature at any point can be determined by using Fleming’s Right Hand Rule for generators.
Figure 11.2 shows one complete revolution of the generator armature and the associated rise and fall of induced voltage.
Figure 11.2 Production of AC
Figure 11.3 illustrates the production of AC. The blue vector arrow OP represents one half of the coil of the generator, pivoted at O and rotating in an anti-clockwise direction. The EMF induced in the coil is proportional to the ordinate ON, or can be calculated by multiplying the max value by the sine of the Phase Angle at that point.
Successive ordinates plotted to a time scale corresponding to the rate of rotation of OP produce a sine wave which represents an alternating current or voltage.
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