electrical power supply systems 7705 Flashcards
what do A/C generators do?
generators convert mechanical energy derived from the aircraft engines or rotor heads into electrical energy by electro-magnetic induction
describe batteries
and their role in A/C
batteries produce DC electrical power by chemical reaction.
provide an emergency source of power,
power source for engine starting
limited system testing,
and recharge when the main generators come on line
auxiliary power unit (APU)
capable of providing power during emergencies to augment or replace the aircraft primary power generation system
such units typically consist of a generator powered by a small gas turbine engine
power conversion equipment
aircraft having a 200/115v three-phase AC generation system will often require the AC supply to be ‘stepped down’ to power instruments or rectified to provide a 28v DC supply to DC components
control and protection units
the protection devices will isolate the generator from the bus bars by removing the excitation filed at the generator.
voltage regulation ensures satisfactory operation, the output voltage must be maintained within a set tolerance over a wide range of engine speeds and electrical loads
protection devices
abnormal conditions or faults, may arise in an electricla circuit for a variety of reason,
devices circuit breakers are essential components in an electrical system. additionally we often employ switches as potential isolation devices
electriclal distribution
emergency services - fed diectly by main batteries, services such as flot, extinguishers, hoist cutter
essential services - services required to ensure safe flight, connected to busbars so that they can be fed by generator or batteries, using invertors and TRU’s
non-essesntial/ general services - considered non essential for flight, if they fail they will limit he A/C only,
bonding and earthing
bondng leads are used to achieve good bonding between structures or components,
will be similar in constructuion to an airframe, earth point and protected by blue paint
cable and component marking
coding method applicable to that aircraft type as laid down in the relevent topic 10’s (CIETP) such as:
system / circuit
location on the aircraft
the position within the circuit
type of supply (if AC, which phase it is)
component / equipment it is supplying
describe circuit breakers
a bi- metallic device opens the circuit in event of an electrical overload and release the actuating button
actuating button can be pulled out to isolate a circuit, pushed back in to reset the circuit.
contacts are opened by springs, the actuatting button is released to reveal a white collar to indicate tripped condition
manual switches
consists of two contact surfaces which can be brought together to make a circuit or separated to break a circuit
Types of toggle ships
1 pole, 2 posn. on-off
1 pole, 2 way, 2 posn
1 pole, 2 way, 3 posn
2 pole, 2 way, 2 posn
mechanical switches
these swithces are made as the result of some mechanical action or force
micro switches
float switches
pressure swithces
centrifugal switches
electrical switches
circuits can be made or broken by rapid movement of contacts which reduce the effects of sparking.
relays normally have more than one contact and can therefore provide multi-circuit switching
automatic or semi-automatic cotrol over circuits
heavy current circuits can be completed or isolated using low current control circuits
control circuits can be quite remote, allowing heavy cirrent carrying cables to be of minium length
heavy duty single coil relays
a relay which is used to switch a heavy current necessitates a powerful, fast build-up of electromagnetic force
this causes heat in the coil which may be severe, if the coil is energised for a considerable time
only fitted in circuits which are used to periodically for a short duration
heavy duty double coil relays
where a relay is needed to be energised for a long period of time, two coils are used, a low impedance high current coil is used to energise the relay via an auxilary contact
high impedance coil is inserted in series with the operating coil, reduces the current flow through the two coils reducing the heat
light duty relays
here the contacts are housed in a sealed unit which plugs into a fixed base, connecting the external circuits
inspection practices
servicing or maintenance is carried out whenever a fault has been reported, during flight servicing, periodicities laid down, aircraft, maintenace regime, modification are required or technical instruction
what checks are carried put
maintenance is either prevengtative or corrosive defined within MAPs
cleanliness
sign of corrosion
areas/equipment correctly protected against corrosion
damage/ faults in surrounding are and equipment
security of attachments and electrical connections
damage caused by wear and tear
sign of overheating
signs of fluid leaks
simple construction of battery
positive plate, negative plate and electrolyte
specific substance of which is dependant on battery
what is a cell
two unlike metals in a conducting solution (the electrolyte)
how is a battery made up
number of cells connected together
what is a primary cell
a cell, which once discharged, can only be recharged by replacing the chemicals in it
what is a secondary cell
a cell, which can be restored to its original state, after discharged, by passing a reverse current through it
what is EMF
electro motive force
total electrical pressure produced by a cell
internal resistance
resistance to current flow through the cell itself
if a battery is connected in a circuit the terminal voltage will be less than the EMF
capacity of a batery
the amount of electricity a cell/battery will deliver between the fully charged and discharged state
rated capacity
dependant on electrolyte
the rate at which a battery is discharched affects the amount of electricity it can deliver
efficiency of a battery
this is a measure of the performance of a battery.
as a bettery deteriorates with use and age, it will no longer produce its rated performance
construction of a lead acid batteries
positive plates are made of lead peroxide and the negative plates are made of spongy lead
electrolyte is dilute sulphuric acid
each cell voltage is 2v
construction of an alkaline battery
nickel hydroxide positive plates, cadmium hydroxide negative plates, potassium hydroxide electrolyte, seperated by synthethic fibre. the cell is made of steel nylon
what are the discharge characterisitics of lead acid battery
steadily decreasing
discharge characteristics of a alkaline battery
remains constant until fully discharged
what is thermal runaway? what conditions can cause it
alkaline and li-ion suffer from rare instances of thermal runaway, caused by an increase in internal temperature of the battery causing a chain reaction of temperature rises,
adverse operationg conditions
poor battery maintenance
battery damage
high battery usage
explain battery servicing
batteries are installed removed IAW topic 1, topic 5C and topic 5A1
battery maintenance is staffed by civi’s ashore and navy afloat,
battery maintenance TEM are provided for those personnel who need the experiance and specific detail
what types of battery servicing are there
commission charge: carried out on all new batteries prior to first use
routine charge: carried out on ‘not in use batteries’ not connected to aircraft, but are servicable, or ‘in use’ batteries when charge state is unkown
full maintenance/charge carried out on all ‘in use’ and ‘not in use’ batteries and is carried out at periodicity laid down in topic 5C
chargeing methods depends on the battery type
what are the checks for battery compartments
general cleanliness
electrolyte spillage
corrosion
damage to battery or battery tray
vent pipes attached and secure
explain a capacity test (lead acid and alkaline)
a capacity charge is used to force discharge an A/C battery, at its 1 hour rate until the trip voltage of 20V is reached
a digital counter indicates capacity percentage
a capacity of over 80% shows is fit for A/C use
describe how a slab battery is dishcarged
deep discharge is undesirable
and batteries are discharged for 48 mins, if it doesnt reach its trip voltage in that time, then it is considered to be at least 80% efficient and fit for use
explain the procedure for battery spillage in an aircraft
entry in Mod form 700C pt1 (placing aircraft unservicable
soak up spillage
dispose of contaminated rags IAW local orders
use neutralising agent
wash and dry area to minimise any free fluid and residual contamination
use litmus paper to test for residual contamination
have area surveyed, by an authorised person for any possible skin repairs
refinish area as required
clear the entry Mod form 700C pt 1
battery eye contamination
flush with eye irrigator, or clean water holding eye open for min 15 min until medical help arrives
inform sickbay of all relevent info
do not leave patient alone or let them make there own way to sickbay
what is load sharing
if one output voltage is slightly higher than the other, the high generator will provide a greater share of the load.
an equalising circuit operates via both generator control units (GCU) to adjust the output voltage of the generators to maintain equal load sharing
when inspecting a generator what do we look for
general wear
burring, pitting or scoring of the commutator segments
thrown solder
brushes are checked for:
acceptable length and even wear
cracking, chipping or scoring
frayed or broken pigtail connections
freedom of movement in brush holder
correct spring tension
what does the GCU provide protection from
generator control unit
protect the generators and distribution circuits from:
over and under voltage
over excitation
overspeed
current limiting
reverse current
reverse polarity
short circuit / overload protection
what is the prupose of the GCU
generator control unit
maintains the output voltage of the DC starter generator at a constant level of 28.5V, as well as control of contactors
GCU’s are also responsible for load sharing
what does the PDU do
power distribution unit provides primary distribution of DC electrical power to the aircraft
what does the main battery do
it supplies DC power to the essential busbar in the event of failure of the main generation power
what are the advanatages of using AC generators as a primary power source
- Better power to weight ratio
- AC generators sre robust and simpler in construction
- generating AC at a higher voltage reduces the size and weight of the current carrying cables
- can be stepped up or down efficiently by use of transformers
- easily rectified
- brushless AC generators design removes the servicing and the problems associated with the brushes
what is load shedding
in an aircraft generators are connected to separate busbars, but there has to be a method of ensuring essential loads to incase of a single generator failure
by using bus tie contactors if a generator fails the working generator, APU or GI can have power diverted from them to power essential systems while isolating non-essential
what do GCU’s protect against (additionally)
- over voltage
- under voltage
- under frequency
- over current
- differential current (between load and busbars)
- current unbalance (different current between phases)
- dead bus (prevents a generator being connected to a bus bar if frequncies are out of phase)
What do EPCU monitor
external power control units
incorrect phase
under frequency
under/over voltage
