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ATPL Systems and Aerodynamics > electric systems > Flashcards

electric systems Flashcards

1
Q

In flight and after engine start, what form of electrical power is delivered?

A

3 phase alternating current (115V 400Hz)

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2
Q

What does the APU do in regards to electrical supply?

A

The turbine powers an AC generator, and bleed air powers pneumatics (usually used for air-start of main engines)

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3
Q

Where does the APU get power from to start?

A

A DC battery, which can also be used by other systems in the event of a serious emergency

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4
Q

Why is DC seen as inferior to AC in aircraft?

A

DC typically requires heavier equipment, and per unit size is not as efficient as AC, it is also lower voltage (and higher current which requires heavier wires)

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5
Q

What is a series wound generator?

A

generator field in series, as load increases, voltage increases

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6
Q

What is a shunt wound generator?

A

Generator in parallel, as load increases, voltage decreases

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7
Q

What is a compound wound generator?

A

Both series and parallel (both series and shunt), and it maintains a constant voltage for varying loads.

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8
Q

What is the most common type of AC generator?

A

A compound wound generator (because it provides a constant voltage for a range of loads)

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9
Q

What is meant by the term “aircraft generators are self-excited”?

A

They produce their own magnetic field once the circuit is complete (once they’re switched on).

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10
Q

Once the Constant Speed Drive is disconnected, it can only be reconnected….

A

on the ground

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11
Q

total power (Kva) =

A

kW (real power e.g. lights and heaters) + kVAR (reactive power e.g. transformers and motors)

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12
Q

How many generators does a twin engine transport aircraft generally have, and how many generators need to be lost before heavy loads need to be shed?

A

Typically have 4, and loads need to be shed if 2 are lost, unless APU can provide sufficient backup.

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13
Q

If a generator fails, how will power from other generators be supplied to that failed generator’s bus?

A

through a bus tie

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14
Q

AC generators can be split bus or parallel bus, explain the terms.

A

split bus is where only one generator supplies a bus at any one time, and parallel is where multiple generators may be connected to the same bus.

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15
Q

What is the purpose of the sycnhronising bus?

A

Ensures that generator voltage and frequency outputsare in phase.

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16
Q

In monitoring the electrical system, what values are usually monitored?

A

Most of the time it is kw, but to check on the status of the voltage regulator, Kvar can be checked as well.

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17
Q

What is the purpose of a Transformer Rectifier Unit (TRU)?

A

transforms 115V AC to 28V AC, and then converted to 28V DC through Diodes.

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18
Q

Where do DC buses get their power from?

A

The AC generators but THROUGH TRUs and Diodes.

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19
Q

What does the generator field switch do?

A

Automatically trips (opens) the circuit when a fault exists

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20
Q

What are trip free circuit breakers?

A

Circuit breakers which are always free to trip, and cannot be overridden

21
Q

What is a rectifier?

A

Diodes which convert AC to DC

22
Q

What is a static inverter?

A

Converts 28V DC to 115V AC trhough use of transistors.

23
Q

What does a series circuit boost, voltage or current?

A

Voltage

24
Q

What does a parallel circuit boost, voltage or current?

A

Current

25
Q

Are systems usually connected in series or parallel?

A

Parallel

26
Q

What type and voltage are aircraft batteries typically?

A

12 or 24V, and lead acid or Nickel Cadmium (nicad)

27
Q

What are the advantages and disadvantages of nicad batteries to lead acid

A

nicad is better for faster recharge, lighter, tougher, and don’t give off as much toxic gas, but are more expensive, and are more prone to thermal runaway.

28
Q

What is a hot battery bus?

A

A bus that is always connected. Used for items such as the aircraft clock.

29
Q

What is a bus tie?

A

Connects different electrical buses together

30
Q

What is a split bus?

A

Only 1 generator can supply a bus at a time

31
Q

What is a parallel bus?

A

multiple generators can supply one bus

32
Q

What does a split system breaker (SSB) do?

A

Prevents paralleling (multiple generators connecting to same bus) with external devices. If it didn’t exist, damage would result because external devices wouldn’t be synchronised

33
Q

What does a reverse current relay do?

A

In a parallel generator system, a weak generator may be ‘charged’ by the stronger generators, causing a reverse current flow. The reverse current relay prevents this from happening which prevents damage.

34
Q

What is a diode?

A

An electric version of a check valve: Only lets current go in one direction

35
Q

How are batteries typically charged?

A

Using the constant voltage reducing current method

36
Q

What are the typical batteries found on aircraft?

A

12V or 24V lead acid or NiCad battery.

37
Q

What is thermal runaway?

A

Overheating of the battery, causing the electrolytes to boil

38
Q

In the event of a failure of the active sources of AC power generation in a large jet transport category aircraft, the component of the electrical system which supplies AC power is….

A

The static inverter

39
Q

Batteries are often connected in parallel in an aircraft electrical system to….

A

Increase the total amp hour capacity

40
Q

The function of a TRU in an aircraft electrical system is to….

A

Convert AC to DC at a lower voltage (115V to 28V)

41
Q

The type of rectifiers normally used in large aircraft AC generators to provide DC for field excitation of the main generator are…

A

Full wave bridge rectifiers

42
Q

When a generator field breaker trips, the associated generator….

A

excitation field is disconnected from the voltage regulator

43
Q

An advantage of constant frequency alternators in an aircraft electrical system is….

A

load sharing (if frequencies are the same, you can distribute the load)

44
Q

Which is more expensive, lead acid battery or NiCad?

A

NiCad

45
Q

EMF is the same as…

A

Voltage

46
Q

The function of a static inverter in an aircraft electrical system is to….

A

Convert DC to AD at a higher voltage (28V to 115V)

47
Q

The component of an electrical system in which diodes would be found is a….

A

Rectifier (TRU)

48
Q

If the constant speed drive is overheating, will it automatically disconnect?

A

No, the temperature needs to be monitored and disconnected if required

49
Q

An advantage of a lead acid battery over a NICAD battery is…

A

Less chance of thermal runaway

ATPL Systems and Aerodynamics (19 decks)

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