- •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
DC Electrics - Aircraft Electrical Power Systems |
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Figure 8.6 shows both how current is measured with an ammeter placed in the current flow so that it measures the current flowing through it and how EMF and pd are measured with a voltmeter connected to the two points between which the potential difference is to be measured.
Voltmeters have a high internal resistance and are connected in parallel to measure the voltage between two points. It may have a multiplier fitted in series with the meter to increase the indicating range of the instrument.
Ammeters have a low internal resistance and are placed in series to measure current through the load. An ammeter may have a shunt fitted in parallel with the meter to increase the indicating range of the instrument.
DC Electrics - Aircraft Electrical Power Systems 8
Figure 8.6 Ammeter and voltmeter connections
The Battery
The battery would normally be a 12 or 24 volt lead acid or alkaline and can be used to start the engines, or to supply electrical power in the event of generator or alternator failure.
Figure 8.7 Lead Acid Battery (Absorbed Liquid Type)
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DC Electrics - Aircraft Electrical Power Systems |
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Bus Bars |
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Systems Power Electrical Aircraft - Electrics DC 8
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GEN. FAILURE |
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WARNING |
L |
L |
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LIGHT |
O |
O |
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A |
A |
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D |
D |
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LOAD METER |
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CENTRE ZERO |
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AMMETER |
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BUS BAR |
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GEN. |
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CUT-OUT ALTERNATOR |
BATTERY |
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SWITCH |
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SWITCH |
14 V |
VOLTAGE |
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REGULATOR |
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OVERVOLTAGE |
VOLTMETER |
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12 V |
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PROTECTION |
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FIELD |
UNIT |
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Figure 8.8 General arrangement - single-engine light aircraft
In most types of aircraft, the output from the generating sources is coupled to one or more low impedance conductors referred to as bus bars.
The bus bars are the collection and distribution centre for a generator or alternator power supply. They use solid copper bars which can be drilled to permit supply and distribution cables to be attached to them.
Bus bars are usually situated in junction boxes or distribution panels located at central points within the aircraft, and they provide a convenient means for connecting power supplies to the various consumer circuits; in other words, they perform a ‘carry-all’ function.
Bus bars vary in form dependent on the methods to be adopted in meeting the electrical power requirements of a particular aircraft type.
In its simplest form a bus bar can take the form of a strip of interlinked terminals, while in the more complex systems main bus bars are thick metal (usually copper) strips or rods to which input and output supply connections can be made.
The strips or rods are insulated from the main structure and are normally provided with some form of protective covering. Flat, flexible strips of braided copper wire are also used in some aircraft and serve as subsidiary bus bars.
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DC Electrics - Aircraft Electrical Power Systems |
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Bus Bar Systems
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ALTERNATOR |
SEPARATE BATTERY |
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SOURCE-POWER |
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STARTER |
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RELAY ENERGISING |
& ALTERNATOR |
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CIRCUIT |
SWITCHES |
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MOTOR |
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M |
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MASTER INTERLOCK |
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BATTERY |
BATTERY & |
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ALTERNATOR SWITCH |
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STARTER |
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STARTER |
SWITCH |
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LAMP |
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ALT |
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SOLENOID |
TEST |
MASTER |
OVER |
RADIO |
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SOLENOID |
INTERFERENCE |
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VOLTAGE |
CAPACITOR |
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PROTECTOR |
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B |
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G |
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VOLTAGE |
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U |
ALTERNATOR |
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REGULATOR |
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5A FIELD |
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S |
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AMMETER |
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B |
5A |
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EXTERNAL |
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POWER |
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ALT |
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SOLENOID |
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R |
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WARN |
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LIGHT CABIN |
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LIGHT |
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15A |
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EXTERNAL |
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POWER |
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CIGAR |
RECEPTACLE |
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LIGHTER |
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Figure 8.9 A typical light aircraft single alternator DC system
The function of a distribution system is primarily a simple one, but it is complicated by having to meet additional requirements which concern a power source, or a power consumer system operating either separately or collectively, under abnormal conditions.
The requirements and abnormal conditions may be considered in relation to three main areas, which are summarized as follows:
•Power-consuming equipment must not be deprived of power in the event of power source failures unless the total power demand exceeds the available supply.
•Faults on the distribution system (e.g. fault currents, grounding or earthing at a bus bar) should have the minimum effect on system functioning and should constitute minimum possible fire risk.
•Power-consuming equipment faults must not endanger the supply of power to other equipment.
These requirements are met in a combined manner by paralleling generators where appropriate, by providing adequate circuit protection devices, and by arranging for failed generators to be isolated from the distribution system.
The operating principle of these methods is concerned with the additional one of arranging bus bars and distribution circuits so that they may be fed from different power sources.
In adopting this arrangement it is usual to categorise all consumer services into their order of importance and, in general, they fall into three groups:
•Vital
•Essential
•Non-essential
DC Electrics - Aircraft Electrical Power Systems 8
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Systems Power Electrical Aircraft - Electrics DC 8
Vital services are those which would be required after an emergency wheels-up landing, e.g. emergency lighting and crash switch operation of fire extinguishers. These services are connected directly to the battery.
Essential services are those required to ensure safe flight in an in-flight emergency situation. They are connected to DC and AC bus bars, as appropriate, and in such a way that they can always be supplied from a generator or from batteries.
Non-essential services are those which can be isolated in an in-flight emergency for load shedding purposes (see below), and are connected to DC and AC bus bars, as appropriate, and are supplied from a generator.
Figure 8.10 illustrates, in a very simplified form, the principle of dividing categorized consumer services between individual bus bars; this is an example of a parallel bus bar system.
In this example, the power distribution system is one in which the power supplies are 28 volts DC, from engine-driven generators operating in parallel, 115 volts 400 Hz AC from inverters, and 24 volts DC from batteries.
Parallel Bus Bar System
Figure 8.10 shows that each generator has its own bus bar to which are connected the non-essential consumer services.
Both bus bars are in turn connected to a single bus bar which supplies power to the essential services. Thus, with both generators operating, all consumers requiring DC power are supplied.
The essential services bus bar is also connected to the battery bus bar so ensuring that the batteries are maintained in the charged condition.
The battery bus bar may be referred to as a ‘hot bus’ or ‘hot battery bus’ because it is always connected to the battery.
In the event that one generator should fail it is automatically isolated from its respective bus bar and all bus bar loads are then taken over by the operating generator. In the event of a generator failure the pilot will commence “LOAD SHEDDING” (page 131).
Should both generators fail, however, non-essential consumers can no longer be supplied, but the batteries will automatically supply power to the essential services and keep them operating for a predetermined period calculated on the basis of consumer load requirements and battery state of charge. (Normally a minimum of 30 minutes).
In the case of the system represented in Figure 8.10, the DC supply to power the inverters is taken from bus bars appropriate to the importance of the AC operated consumers.
Thus, essential AC consumers are operated by the No. 3 inverter and so it is supplied with DC from the essential services bus bar.
No. 1 and No. 2 inverters supply AC to non-essential services and so they are powered by DC from the No. 1 and No. 2 bus bars.
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