Aeroplane Electrical Systems Basics

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Questions and Answers

What type of generators are used as the principal source of electric power in almost all transport-category aircraft?

Alternators

Transformer Rectifier Units

AC generators (correct)

DC generators

What type of generator is used on light aircraft due to being lighter than an AC generator?

AC generator with constant-speed drive

Auxiliary battery charger

DC alternator (correct)

Transformer Rectifier Unit

What is the purpose of the internal rectifiers in a DC alternator?

Produce AC voltage

Control engine speed

Produce DC output (correct)

Convert AC to DC

Which component is responsible for changing engine-driven AC-generated 115-V AC to 28-V DC on larger aircraft?

Transformer Rectifier Units (TRUs) Signup and view all the answers

In the event of other power sources failing, what serves as the backup DC source on aircraft?

Main battery Signup and view all the answers

What is the function of Auxiliary batteries in aircraft electrical systems?

Backup DC source Signup and view all the answers

What is the function of the shunt connected in series with the output of the transformer rectifier units?

Enable reading of amps at auxiliary terminals Signup and view all the answers

How is the cooling typically provided for Transformer Rectifier Units (TRUs) in modern aircraft?

By fan forced airflow Signup and view all the answers

What is the ripple frequency of the DC voltage produced when 400 Hz, three-phase AC is full wave rectified by six diodes?

2400 Hz Signup and view all the answers

In a star-to-star transformer, are the input and output in phase or out of phase?

In phase Signup and view all the answers

What is the purpose of the thermal switch mentioned in the text?

Provide warning through an overheat light Signup and view all the answers

How many silicon diodes are typically connected in parallel in each bridge rectifier assembly mentioned in the text?

6 Signup and view all the answers

What happens when the output voltage of a generator rises above a critical value?

Contact B opens, reinserting the field rheostat in the field circuit. Signup and view all the answers

How does a vibrating-type voltage regulator ensure a maintained average voltage with load changes?

By opening and closing contact B to adjust generator output. Signup and view all the answers

What role does the dashpot P play in a vibrating-type voltage regulator?

It acts as a dampener to prevent hunting for smoother operation. Signup and view all the answers

What occurs when contact B closes in a vibrating-type voltage regulator?

Shorting of the field rheostat starts to raise terminal voltage. Signup and view all the answers

Why is a capacitor C placed across contact B in a vibrating-type voltage regulator?

To prevent hunting and provide smoother operation by eliminating sparking. Signup and view all the answers

In what scenario does additional force need to be applied to turn the armature of a generator?

When load on the generator increases. Signup and view all the answers

What is the role of the current coil in the relay described in the text?

Assists the voltage coil in keeping the pilot contacts closed Signup and view all the answers

How is the contactor coil energized in the relay system?

Energised from the A+ output of the generator Signup and view all the answers

What initiates the opening of the pilot contacts in the relay system?

Decrease in generator output voltage during engine shut-down Signup and view all the answers

Why does the reverse current circuit breaker remain in a 'locked-out' condition?

To ensure complete isolation of a circuit until a fault is cleared Signup and view all the answers

What leads to the de-energization of the contactor coil in the relay system?

Reverse current condition due to generator output voltage decrease Signup and view all the answers

How do the main and auxiliary contacts behave when the pilot contacts open in the relay system?

Open to disconnect the generator from the battery and main busbar Signup and view all the answers

What is the purpose of the reverse polarity diode in the external DC power circuit?

To prevent incorrect polarity power from being supplied to the aircraft Signup and view all the answers

How is the external AC power receptacle identified?

By six pins, with four large pins for phases and one for neutral Signup and view all the answers

What happens if the external power plug is disconnected while 'live' from the aircraft?

The short pins disconnect first Signup and view all the answers

Which component prevents incorrect polarity power from reaching the aircraft in the external DC power system?

Reverse polarity diode Signup and view all the answers

What is the function of the two smaller pins in the external AC power receptacle?

To provide interlock sensing Signup and view all the answers

Why is it important for the external power supply to be correct before connecting AC to the aircraft?

To prevent arcing at the pins of the external power receptacle Signup and view all the answers

Study Notes

DC Power Generation

DC alternators are used in light aircraft to produce electric power, producing an AC voltage that is then converted to DC using internal rectifiers.
Larger aircraft use Transformer Rectifier Units (TRUs) to convert AC power to 28-V DC.
Aircraft have multiple DC power sources, including main batteries, main battery chargers, and auxiliary batteries and chargers.
These batteries act as a backup DC source if other power sources fail.

AC Power Generation

AC generators (alternators) are used in transport-category aircraft to produce electric power.
Modern aircraft (e.g. 787) have generators that produce 235-V AC, with a frequency governed by engine rotational speed.
The generator consists of a transformer and two three-phase bridge rectifier assemblies, which produce a DC output with a ripple frequency of 2400 Hz.

Voltage Regulation

A vibrating-type voltage regulator uses a solenoid to connect or remove a field rheostat from the circuit, maintaining a consistent output voltage.
When the output voltage rises above a critical value, the solenoid opens the contact, inserting the field rheostat and reducing the field current.
When the output voltage falls below a certain value, the solenoid closes the contact, shorting the field rheostat and increasing the field current.

Reverse Current Circuit Breakers

These circuit breakers protect power supply systems from fault currents greater than those at which cut-outs normally operate.
They remain in a "locked-out" condition until the fault is cleared.

External Power Systems

External DC power systems have a short pin connected to a positive voltage, with a reverse polarity diode to prevent incorrect polarity power being supplied to the aircraft.
External AC power receptacles have six pins: four for phases (ABC) and neutral, and two for interlock sensing.
The external power supply must be correct before connecting to the aircraft, and the system is designed to prevent arcing at the receptacle pins during disconnection.

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Description

Test your knowledge on electrical systems used in modern light aircraft, including DC power generation and DC alternators. Understand the importance of using DC alternators over AC generators in light aircraft.