- •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
3
DC Electrics - Circuit Protection and Capacitors
Electrical Faults . . . . . . . . . . . . |
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Circuit Protection Devices |
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Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 |
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The Cartridge Fuse . . . . . . . . . . |
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Spare Fuses . . . . . . . . . . . . . |
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High Rupture Capacity (HRC) Fuses . . . . |
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Dummy Fuses . . . . . . . . . . . . |
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Current Limiters |
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Circuit Breakers . . . . . . . . . . . . |
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Reverse Current Circuit Breakers |
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Capacitors . . . . . . . . . . . . . . |
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Capacitance . . . . . . . . . . . . . |
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Capacitor in a DC Circuit . . . . . . . . |
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Capacitor in an AC Circuit . . . . . . . . |
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Capacitors in Parallel . . . . . . . . . . |
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Capacitors in Series . . . . . . . . . . |
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Questions - Circuit Breakers . . . . . . . |
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Questions - Fuses . . . . . . . . . . . |
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Answers - Circuit Breakers . . . . . . . . |
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Answers - Fuses . . . . . . . . . . . . |
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DC Electrics - Circuit Protection and Capacitors |
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Capacitors and Protection Circuit - Electrics DC 3
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DC Electrics - Circuit Protection and Capacitors |
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Electrical Faults
In an electrical circuit, abnormal conditions may arise for a variety of reasons, which can cause overcurrent or overvoltage conditions.
If allowed to persist, these abnormal conditions or faults will lead to damage or destruction of equipment and in extreme cases, loss of life. Certainly the essential power supplies will fail, and it is therefore necessary to protect circuits against all such faults, by the use of fuses and circuit breakers.
Circuit Protection Devices
There are a number of protection devices used in aircraft electrical systems but only 2 basic types are discussed here:
•Fuses
•Circuit breakers
The fundamental difference in the type of protection provided by fuses and circuit breakers is in their time of operation relative to the attainment of maximum fault current.
A fuse normally opens the circuit before full fault current is reached, whereas the circuit breaker opens after the full fault current is reached.
This means that when circuit breakers are used as the protection device, both the circuit breaker and the component must be capable of withstanding the full fault current for a short time.
The circuit breaker has the capability, which the fuse has not, of opening and closing the circuit, and can perform many such operations before replacement is necessary. It may also be used as a circuit isolation switch.
Fuses
There are 3 basic types of fuse currently in use on aircraft:
•Cartridge fuse
•High rupture capacity (HRC) fuse
•Current limiter fuse
The Cartridge Fuse
The cartridge type fuse consists of a tubular glass or ceramic body, 2 brass end caps and a fuse element.
The element may be one of the following:
•Tinned copper wire
•Silver wire
•A strip of pure zinc - electro tinned
DC Electrics - Circuit Protection and Capacitors 3
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Capacitors and Protection Circuit - Electrics DC 3
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Figure 3.1 Typical fuses |
a. A light duty circuit fuse |
b. A high rupturing capacity fuse |
The latter type element is generally used in heavy duty circuits, the zinc strip being cut to a specified width.
A fuse operates when the current flowing through it is sufficient to melt the wire or strip element, the time taken varying inversely with the current.
All fuses are rated at a specific current value, i.e. the rating indicates the current they will carry continuously or intermittently without unduly heating up or deteriorating.
The rating of a fuse for a particular circuit is such that it is not less than the normal current flowing in the circuit, but that it operates (‘blows’) at a current level below the safety limit of the equipment or cable used.
For this reason only the specified fuse should be used in a particular circuit. The diagram shows typical aircraft fuses; the ratings can vary between 0 .5 and 500 amps, the higher ratings being limited to the HRC or current limiter types.
Fuses are made of a type of wire which has a low melting point, and when it is placed in series with the electrical load it will melt, blow or rupture when a current of higher value than its ampere rating is placed upon it.
Fuses are rated in ‘amps’.
A blown fuse may be replaced with another of the correct rating once only. If it blows again when switching on, there is a defect in the system and the fuse must not be changed again until the circuit has been investigated.
Spare Fuses
The carriage of spare fuses is mandatory, the quantity of spares being at least 10% of the number of each rating installed, with a minimum of 3 of each.
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