AT 202 Unit 1-8 to 1-11 Electrical Systems

what does a B777 use for emergency power?

RAT (VSVF)

what do a/c use for power distribution?

a solid copper bar (a bus bar)

what would happen to a gen/alt if the shunt field opened (wire got cut)?

alt/gen would stop putting out voltage

what's APU stand for?

alternator control unit

what's a loadmeter measure?

alternator output current

what type of armature winding do most turbine engines use?

four phase

what's a GCB?

generator control breaker

what's GCU stand for?

generator control unit

how does a zener diode help with voltage? (simple answer)

it controls the current through the field coil

tell me about starter generators

~normally used in turboprop or turbine powered a/c ~usually on APU ~all one unit ~direct drove (never disengages from engine) ~both a starter motor and a generator (becomes normal generator after start)

why chose an alternator and not a generator? (short answer)

~lighter ~cheaper ~better ~more reliable (~all thanks to diodes)

tell me about the B-747-400 bus system

~like 2 systems (left and right) that can operate together or independently ~during normal flight, all generators operate in parallel ~split system breaker (SSB) used to connect left and right side busses/generators ~2 APUs are operable in flight ~4 IDGs ~2 external power sources (EXT 1 and EXT 2) ~1 BPCU ~4 GCUs ~many CTs, GCBs, and BTBs

what does the cirrus DC dual bus consist of?

~main distribution bus ~essential distribution bus ~2 essential bus bars ~2 main bus bars ~non-essential bus bar

what is MTBF?

~mean time between failures ~a measure of how reliable a hardware product or component is

what are the two basic types of alternator regulators?

~mechanical (vibrating points) ~solid-states (transistorized)

how do GCUs/ACUs respond to an emergency?

~monitor bus voltage, amperage, and interference (voltage spikes) ~if there's a problem, they disconnect

tell me about parallel circuits

~more than 1 path for current to flow (electron flow divided into different paths) ~*constant voltage*

tell me about the hot battery bus

~most critical bus, connected to most critical systems ~always connected to the a/c battery

tell me about series circuits

~only 1 path for current to flow (all electrons must flow through all components of the circuit) ~*constant current*

what are the 3 common AC power distribution systems?

~parallel bus ~split bus ~split-parallel bus

tell me about the B-727 bus system

~parallel system ~3 engine a/c ~during normal flight, all 3 generators are connected to a paralleling or synchronizing bus (synch bus) ~all generators share equal load ~all generators must be in phase ~operation is labor intense (flight engineer controls power distribution system) ~APU only operable on ground

what's an RAT?

~ram air turbine ~AC backup power source ~provides limited electrical power (if all engines are lost)

what do all generators/alternators employ? some type of...

~regulation system (either a voltage regulator or a GCU/ACU)

what are the three types of circuits?

~series circuit ~parallel circuit ~series/parallel circuit

what's a semiconductor made of? what's its main characteristic?

~silicon ~conducts under some conditions and not under others

tell me about zener diodes

~solid state voltage control unit ~voltage sensitive diode ~controls transistors (hence controlling the field current and therefore the alternator output) ~stops flow once voltage reaches a certain threshold (so it stops/starts flow to equalize voltage on both sides of it) ~controls output in generators and alternators by controlling magnetic

what is the most common type of alternator regulator? why?

~solid-state ~they're more reliable because there's fewer moving parts

tell me about the frequency of AC voltage

~speed at which polarity reverses ~must be constant ~measured in Hertz or cycles per second ~a/c = 400 Hz ~US = 60 Hz ~ Europe = 100 Hz

tell me about the A-320 bus system

~split bus *four AC generators* ~2 are powered by the main engines ~1 is powered by the APU ~1 Emergency Generator powered by a RAT *2 main (engine-driven) generators* ~supply power to least critical loads ~a failure of one generator would cause only a momentary loss of gallery power

tell me about the field windings of a generator

~stationary ~commutator is split ~brushes ware out and need to be replaced

tell me about the armature winding in an alternator

~stationary ~where voltage/current is produced (high current) using electromagnetic induction ~3 phase winding (each 120º apart) ~output power can go directly to a/c bus (no brushes, no rotating connection, no commutator, etc.)

what would happen if a rheostat shorted out?

~there'd be a high voltage output, overcharging the battery, producing high voltage in electric busses, and shorting out some circuits (like bulbs burning out)

why do we use diodes in alternators?

~they only allow DC to pass, not AC

tell me about how the CSD and IDK operate

~they're both hydraulic units ~usually share oil with AC generator ~CSD uses oil to operate the hydraulic "automatic transmission" ~generator uses oil for cooling

tell me about the B-787 power distribution

~traditional systems now electric ~SG (starter generator) ~XFR (trans rectifier) ~P (hydraulic pump) ~M (motor) ~NGS (nitrogen generating system)

what are the 3 common methods for starting a turbine engine?

~turn the starter (uses the battery) ~bleed air from APU ~ground power (hook up a ground power cart)

tell me about parallel generators

~twin engine = 2 generators ~safer ~more power ~operated in parallel (both generators connected to one bus, both carry load equally)

tell me about the power distribution of a King Air

~twin turbo powered a/c ~split bus 28V DC ~2 AC inverters provide AC

tell me about the parallel bus system

~typically found on older a/c (3 engine a/c, like B-727, MD-11, L-1011, early B-747) ~electrical load shared equally by all working generators ~all generators are connected to a paralleling (synch) bus during operation ~APU is NOT operable in flight (so there's no backup)

define volt

~unit of electrical pressure ~amount of pressure required to force one amp of flow through one ohm of resistance

tell me about vibrator type voltage regulators

~used for a/c in the 1960s-1980s ~relays with contact points control field current ~contact points opened/closed by an electromagnet

tell me about a full wave rectifier

~used in alternators to change AC to DC ~changes "full" AC wave into ripple DC ~employs 6 silicon diodes (3 positive, 3 negative)

how do circuits change from AC to DC?

~used rectifiers and diodes ~full wave is most efficient ~rectifier coverts negative parts of AC to positive

how many KVA is 2,562 volt amps?

2.562KVA

why does an inverter in a turbine a/c put out 2 different types of AC voltage?

*115V AC* ~provides power for pax and crew ~phone chargers *26V AC* ~used for avionics systems

what are the advantages and disadvantages of VSCF generators (variable speed constant frequency)

*advantages* ~no CSD required ~smaller/lighter ~more versatile installation (electronics can be mounted anywhere) *disadvantages* ~relatively low power output ~not yet proven through long term use (reliability and operating cost)

what are the differences between alternators and generators?

*alternators* ~use field windings that rotate driven by engine (rotor) ~armature windings are stationary ~supply power output through *direct contact* to a/c power bus (no rotating connection required) *generators* ~can use stationary field windings or a permanent magnet field ~armature windings rotate driven by engine (rotor) ~supply power output through *commutator and brush assembly* (unreliable connection)

what are the inputs and outputs for a generator/alternator?

*inputs* (what we're using to drive the power) ~mechanical energy (engine drive/RPM, APU driven, RAT driven) ~electrical energy (field current, main bus) *outputs* ~voltage/current from armature to power systems

what are the differences between a loadmeter and an ammeter?

*loadmeter* ~scale starting at 0 ~shows the load being placed on the alt/gen *ammeter* ~scale goes negative to positive with 0 in center ~positive indication means battery is charging, alternator is putting out too much ~negative indication means battery is discharging, alternator isn't putting out enough

what are the power distribution differences between a single wire system and a two wire system?

*single wire* ~aluminum a/c, metal airframe = ground system *two wire* ~composite a/c, need a ground conductor to complete the loop (since the airframe isn't a conductor) ~both positive and negative voltage must have a distribution system

how do large a/c maintain a constant 400 Hz AC frequency?

1. use a constant speed drive (CSD) (commonly used on main engine driven generators) 2. connect the generator to an engine that turns at a constant speed (common on APUs, which maintain constant speed)

nuclear power plants, coal power plants, aircraft generators, and aircraft alternators all produce...?

AC

why do small a/c use DC alternators?

AC alternators are too heavy with the CSD

what's the most electric airplane?

B-787

what are twin piston engines powered by?

DC alternators

how do you start a twin piston engine?

DC power on ground supplied through an external power receptacle

what's IDG stand for?

Integrated Drive Generator

what do the B787 and A380 use for their primary source of power?

VSVF

what's a BTB?

battery tie breaker

what's CSD stand for?

constant speed drive

what's the best way to control gen/alt output?

control the magnetic field strength

what does the base of a transistor do?

controls flow of current to emitter and collector (so it's the controlling circuit)

what's a CT?

current transformer

what were the first commonly used semiconductors?

diodes

what type of battery does a piston engine use?

lead acid

what's an LED?

light emitting diode

what is polarity?

location of the positive connection compared to the negative connection

what type of battery to turbine a/c use?

ni-cad

what is the advantage of a split bus system?

one entire bus can fail and all other systems will operate normally

what type of circuit do we have in our house?

parallel

what device does an alternator use to output direct current?

rectifier

what's another word for commutator?

slip ring

what type of bus system does a twin piston engine have?

split bus system (one alternator, one bus)

what's an SSB?

split system breaker

what's the main disadvantage of parallel generators?

they're unable to maintain full output at low rpm (which is why we don't use them in twin piston engines) (use parallel alternators instead)

what's the purpose of a bus hierarchy?

to ensure that the most critical distribution bus is the least likely to lose electric power

why do we use flux when soldering?

to prevent oxidation

what components of an alternator can technicians replace?

~brush assembly ~diode assembly ~bearings

tell me about operational theory of zener diodes with transistors

when bus voltage reaches a set value: ~zener diode conducts ~turns transistor on or off ~transistor controls field current, which controls alternator output

what is a gen/alt made up of?

~2 electrical coils ~magnetic field (small input) ~armature (large output)

what are the battery/alternator voltages for most systems?

~24V battery with 28V alternator ~12V battery with 14V alternator

tell me about the power distribution of an A-380

~4 engines ~variable frequency system ~4 main-engine generators output ~115 VAC ~370-770 Hz ~150 KVA max output ~2 flight operable APU generators ~one static inverter

what are the common power outputs of twin piston engine a/c?

~50-amp alternators ~100-am alternators ~150-amp alternators

tell me about the generators of a B-777

~7 engine driven generators ~2 for main engine (AC output) ~2 back ups (AC output) ~2 permanent magnet generators (PMGs) (DC) ~1 APU ~and 1 RAT, but it's not engine driven

tell me about the bus hierarchy of AC systems from most critical to least critical

~AC Static Inverter Bus ~AC Essential Bus ~Essential AC Shed Bus ~Main Left AC Bus and Main Right AC Bus

what happens if an A-320 emergency generator fails?

~AC power available through static inverter powered by ship's batteries

what happens if two main generators in an A-320 fail?

~APU generator would supply all AC power

what happens if a main generator in an A-320 fails?

~BCPU will turn off some non-essential loads ~APU generator gets turned on and everything returns to normal

tell me about the two common methods use to connect three phase armature windings

~Delta winding and Y winding ~produce the same results in a/c DC alternators

tell me about the bus hierarchy of a DC system from most critical to least critical

~Hot Battery Bus #1 and Hot Battery Bus #2 ~DC Essential Bus ~DC Essential Shed Bus ~DC Battery Bus ~DC Left Bus #1 and DC Right Bus #2

what does LRU stand for? give me some examples of them

~Line-replaceable unit ~IDG and GCU are LRUs

what happens if the two main generators and the APU in an A-320 fail?

~RAT would be used to drive an emergency power generator

what is RAT?

~Ram Air Turbine ~wind drives a tiny prop and supplies electrical power

what are the two main types of AC generators?

~Type 1: Constant RPM Generators ~Type 2: Variable RPM Generators

tell me about the 2 engine driven backup generators of a B-777

~VSCF ~115 VAC ~400 Hz ~20 KVA max output

what do metal a/c use for power distribution?

~a single wire system ~airframe is ground and distributes the negative voltage

what's a rheostat? how's it work?

~a variable resistor ~if the load (output) is too high, the rheostat will increase resistance, weakening the shunt field, hence weakening the magnetic field and decreasing the output

what's a simple voltage regulator?

~a variable resistor in the field circuit that controls the field strength (change current to magnetic field)

what are the advantages of a DC alternator over a DC generator? (long answer)

~all advantages are a result of the use of *diodes* as the alternator's rectifier (diodes are solid-state devices that are stationary, so less likely to ware and fail, last longer) ~field rotates ~armature is stationary (carries more current) ~requires less maintenance (inspected every 100 hours or annually) ~A&P tech can repair an alternator (only the factory can "rebuild" it though) ~light, cheaper, and more reliable (again, bc of diodes) ~direct power output from alternator to bus system

why do some a/c monitor voltage?

~alternator system must maintain voltage slightly higher than battery voltage (to make sure battery stays charged)

define watt

~amount of power used in a circuit when one amp of power flows under a pressure of one volt ~unit for electrical power

what are the three parts of a transistor?

~base ~emitter ~collector

what are the advantages of DC?

~batteries can store DC ~portable ~used for a/c backup power ~used to start the engine

how are DC alternators like generators?

~change *mechanical* energy into *electrical* energy ~produce electrical power through the use of *electromagnetic induction* ~produce *AC* in the *armature* and change it to *DC* before it leaves the alternator

tell me about the rectifier assembly of an alternator

~changes AC to DC ~uses diodes

why were vibrating regulators replaced with transistorized regulators? (simple answer)

~cheaper ~better ~lighter ~more reliable ~longer MTBF

what's a series/parallel circuit

~combination of both series and parallel circuit (so part of it has one path but another has multiple)

tell me about a split-parallel bus

~combination type system ~found in B-747-400 ~hybrid parallel and split system ~safer (more options) ~parallel during normal operation ~split during malfunctions

tell me about variable speed constant frequency (VSCF) generators

~constant 400 Hz output ~changed by solid-state circuitry ~no need for CSD (more reliable) ~power = 75 to 150 KVA

tell me about the 2 APU generators of an A-380

~constant frequency ~115 VAC ~400 Hz ~RPM kept constant by fuel control unit

tell me about a CSD

~constant speed drive ~separate from the generator (separate units) ~installed between variable speed engine and constant speed generator ~used in olde a/c produced prior to 1980s ~keeps RPM in a generator constant ~controlled using hydraulics

what do alternator regulators do?

~control input to control output ~control field current to control armature output ~more field = more alternator output

what is regulator theory?

~controlling the input (field) current will change the gen/alt output (voltage/current)

what does a regulator do in a generator/alternator?

~controls field strength ~keeps generator power output at the necessary voltage and current for variable load (can change output based on load and demand)

tell me about the 2 main generators of a B-777

~conventional IDGs ~115 VAC ~400 Hz ~120 KVA max output

tell me about the field winding of an alternator

~creates magnetic field ~rotating

tell me about alternating current

~current flows in one direction, reverses, and flows in the opposite direction ~current used in houses and large a/c ~produced by alternators and generators ~polarity of voltage reverses once every cycle

tell me about the cirrus power distribution system

~dual bus, 28V DC ~9 different busses ~2 primary power supply: main bus and essential bus ~2 Lead-acid batteries (12V each) ~24V nominal rating ~12 cells ~if an alternator fails, system automatically diverts all power to essential bus

what are the advantages of AC?

~easily converted to different voltage levels using transformers ~high voltage is easier to transmit through wires (less loss) (higher voltage, lower amperages) ~can use brushless motors ~can use a three phase system (for more power)

which traditional systems are electric in a B-787?

~electric starters ~cabin air conditioning ~pressurization, motor driven hydraulic actuators ~electro/hydraulic control surface actuators ~wing ice protection ~electric nitrogen system for fuel tanks

define EMF

~electromotive force that produces electron movement ~measured in volts

tell me about direct current

~electrons flow in one direction ~produced by batteries ~can be constant or pulsating ~polarity of voltage is constant ~used by portable/moveable systems (like satellites, UAVs, spacecraft)

tell me about the one static inverter of an A-380

~emergency use only ~fed from battery bus ~2.4 KVA max at 115 VAC

why does a B-787 have an electric nitrogen generation system for the fuel tanks?

~emptier tanks produce a lot of flammable fumes ~nitrogen replaces oxygen, so any spark/flame will be snuffed out

tell me about the C check of an IDC

~every 20-24 months ~takes 1-4 hours

tell me about the A check of an IDC

~every 400-600 flight hours ~takes 10 hours

tell me about the D check of an IDC

~every 6 years ~takes 2 moths

tell me about the B check of an IDC

~every 6-8 months ~takes 1-3 days

tell me about a split bus system

~found in modern twin-engine a/c (B-737, 757, 777, 787, A-300, 320, and 310) ~during normal conditions, each generator powers only one main AC bus ~two generators can NEVER power the same bus simultaneously ~if one generator fails, the other is available as a backup (done by closing breakers) ~most APUs are operational in flight

tell me about Variable RPM AC Generators

~found on more modern a/c ~voltage output = 115V or higher ~frequency output may change with engine RPM ~2 types (constant frequency and variable frequency) ~no need for a CSD

tell me about VSVF AC Generators (variable speed variable frequency)

~found on newest a/c (B777, B787, A380) ~output frequency changes with engine RPM ~AC frequency made constant as needed during power distribution ~no CSD or IDG ~high power output

why do large transport a/c use AC generators?

~high power outputs ~produce 3-phase power

what types of systems did the electrical systems replace in an A-380?

~hydraulic and pneumatic systems

what's the different between an insulator and a conductor in terms of atomic structure

~insulators have a full valence shell

tell me about an IDG

~integrated drive generator ~newer than CSD ~integrates generator and speed control into one unit ~smaller and lighter ~higher RPM (12,000 rpm) (higher power output) ~higher alternator speed ~improved cooling features ~gearing controlled by hydraulics

what's KVA?

~kilo-volt amperes ~a unit of apparent power

tell me about transistors

~used to control field current ~replaces the relay (kind of works like a relay in that it can work like a switch to amplify a current) ~works as a control valve (opens/closes circuit) ~smaller input (base/emitter connections) controls larger output (collector/emitter connections)

how do circuits change from DC to AC?

~using inverters (complex with low efficiency, produces heat as a waste product)

tell me about a constant RPM AC Generator

~voltage output = 115V AC ~frequency output = 400 Hz ~requires a constant speed drive unit (either a Constant Speed Drive or an Integrated Drive Generator)

what device controls the output of a generator? how?

~voltage regulator ~controls field current

what's the main difference between an alternator and a generator?

~what rotates and what doesn't ~*generator* = armature rotates ~*alternator* = magnetic field rotates

how can you monitor alternator operation?

~with a loadmeter or an ammeter ~most light a/c use a loadmeter