AIRCRAFT ELECTRICAL POWER SYSTEMS Flashcards
(47 cards)
What is a rectifier ?
The rectifiers in the alternator end frame convert AC to DC and permit the current to flow out
from the alternator but not into it from the battery. They have a low resistance in the direction
of current flow and a high resistance in the other direction.
What is an Inverter ?
Static inverter
Static inverters are solid state devices which covert DC to constant frequency AC. A typical inputs to a static inverter would be 18 - 30 volts DC and the output would be 115 volts AC at 400 hertz frequency. The internal circuitry of a static inverter contains standard electrical and electronic components such as oscillators, diodes, transistors, capacitors and transformers.
Rotary inverter
Rotary inverters convert DC to AC by using a constant speed DC motor to drive an alternator
thereby producing constant frequency AC.
-Voltmeters have a …….. internal resistance and are connected in ………. 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 …….. internal resistance and are placed in ………. to measure current through
the load It may have a shunt fitted in parallel with the meter to increase the indicating range of
the instrument.
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 It may have a shunt fitted in parallel with the meter to increase the indicating range of
the instrument.
What is a Bus Bar ?
–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.
–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.
–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.
–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
The requirements and abnormal conditions concerning an electrical system may be considered in relation to three main areas
The requirements and abnormal conditions may be considered in relation to three main area,
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.
How are the electrical loads on a bus bar categorised ?
In adopting this arrangement it is usual to categorize all consumer services into their order of
importance and, in general, they fall into three groups;
– Vital
– Essential
– Non-essential
–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.
SPLIT BUSBAR SYSTEM - Page 134
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.
–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 pre-determined period calculated on the basis of consumer load requirements and battery
state of charge. (Normally a minimum of 30 minutes).
In an electrical circuit the reverse current cut-out relay will open:
a. when battery voltage exceeds generator voltage.
b. when circuit voltage is less than generator voltage.
c. when the main output C/B is reset.
d. when the batteries are flat.
A
A generator cut-out is provided:
a. to prevent the battery over heating.
b. to prevent the battery from being overcharged.
c. to allow the generator to be isolated in a crash.
d. to prevent discharge of the battery through the generator.
D
A generator cut-out will open when:
a. circuit loads equal the battery voltage.
b. the air temperature reaches 45 C.
c. circuit loads equal the generator voltage.
d. generator voltage falls below battery voltage.
D
A generator cut-out is fitted to prevent:
a. the battery discharging through the generator windings.
b. the generator overcharging the battery.
c. fire in the event of overloading the system.
d. out of phasing.
A
In the event of the cut-out points sticking in the closed position, the most probable results, when
the engine stopped would be:
a. gain of engine power.
b. a burnt out generator.
c. loss of residual magnetism.
d. no apparent reaction.
B
To prevent circulating currents when more than one generator is being connected to the same
bus bar:
a. reverse current relays are fitted.
b. the generators are connected in series.
c. rectifiers are fitted.
d. differential cut-outs are used.
D
A generator cut-out is fitted:
a. in series with the generator output.
b. in the diode circuit.
c. in parallel with the generator output.
d. in the field circuit.
A
On a 28 volt system with a 24 volt battery the cut-out contacts close at approximately:
a. 36 volts.
b. 24 volts.
c. 28 volts.
d. 26 volts.
D
A component whose job is similar to a generator cut out is:
a. a rectifier.
b. a converter.
c. an inverter.
d. a reverse current relay.
D
If the cut-out is open, the battery is feeding the loads which are:
a. in series with the battery.
b. in parallel with the battery.
c. in sequence with the cut-out.
d. cross coupled.
B
In a two engine aircraft with two generators, there would be:
a. one ammeter for each generator and one voltmeter switchable to indicate either
generator voltage or battery voltage.
b. one voltmeter for each generator. and one ammeter switchable to indicate either
generator current or battery current.
c. one ammeter showing the total output and one switchable voltmeter .
d. one ammeter and one voltmeter each showing the average current and voltage output.
A
A generator converts mechanical energy to electrical by:
a. electro magnetic spring action.
b. electro magnetic induction.
c. electrostatic induction.
d. electro dynamic induction.
B
In an aircraft electrical system which incorporates a voltmeter, the voltmeter indicates:
a. the flow in the electrical system before the battery cut-out contacts close.
b. the rate of flow at all times.
c. the pressure in the electrical system before and after the cut-out contacts close.
d. the flow in the electrical system after the battery cut-out contacts close.
C
If the generator warning light comes on in flight it indicates that:
a. the generator is feeding the battery bus bar.
b. the generator is not feeding the battery bus bar.
c. the battery has failed.
d. a rectifier is faulty.
B
A generator failure is usually indicated by:
a. the ammeter reading decreasing or showing a discharge and a red warning lamp
lighting.
b. the voltmeter reading increasing, the ammeter reading showing discharge and a red
lamp lighting.
c. the current consuming devices failing to operate.
d. the motor speed increasing.
A
A generator warning light will be illuminated:
a. when the battery voltage exceeds that of the generator and the cut-out has opened.
b. at night only.
c. when the generator is supplying current to a fully charged battery, and no electrical
loads are switched on.
d. when the battery charge current is lower than required to maintain its fully charged
state.
A
If a generator fails in flight:
a. the voltmeter will read maximum.
b. the ammeter reading will decrease.
c. load sharing circuits will operate.
d. the watt metre will show a increase.
B
