General Electrical System

Question 1

Inverter (DC — AC)

Answer 1

Converts DC to AC power for use by certain instruments, avionics or other accessories.

Question 2

Rectifier (AC — DC)

Answer 2

Converts AC to DC power. Typically a set of interlocked diodes to redirect the flow of current.

Question 3

Transformer (Changes Voltage)

Answer 3

A device that increases or decreases the voltage of an alternating current.

Question 4

TRU (Transformer rectifier unit)

Answer 4

-Converts AC to DC and steps down the voltage.
- TRU converts the 120V AC power generated by the engine or APU generators (or GPU) down to 28V DC power for use by various electrical components.

Question 5

Relay

Answer 5

Switches that open and close circuits electromechanically/electronically.

They control 1 electrical circuit by opening/closing contacts in another circuit.

Question 6

Solenoid

Answer 6

A switch operated electro-mechanically with a movable core.

Question 7

Fuse

Answer 7

A fuse is a strip of metal that will melt and open a circuit when an excess of current attempt to flow through it.

Helps to protect the circuit and electrical component.

Question 8

Circuit Breaker

Answer 8

A switch that automatically opens and opens electrical circuit when an excess of current attempts to flow through it.

Question 9

How do you convert AC to DC?

Answer 9

Rectifier

Question 10

What is a diode?

Answer 10

An electrical “check valve” that allows current to travel in one direction only.

Question 11

Difference between AC/DC

Answer 11

DC- the electrical charge (current) only flows in one direction. (Ex. Some electrical components is DC depending on voltage requirements/sensitivity)

AC- electric charge (current) changes direction periodically. The voltage in AC circuits also periodically reverses because the current changes direction. (Ex. Avionics, instruments)

Question 12

Types of batteries

Answer 12

1. Nickel Cadmium (NiCAD)
2. Lead Acid Batteries

Question 13

Advantages/disadvantages of Nickel Cadmium batteries (NiCAD)

Answer 13

Advantage- can deliver sustained voltage until almost discharged, short recharge life

Disadvantage- very expensive and risk of overheating when operated above normal temps and at high loads. Most susceptible to battery thermal runaway.

Question 14

Advantages/disadvantages of Lead Acid Batteries

Answer 14

Advantage- cheap and reliable

Disadvantage- voltage can deplete very quickly under heavy loads

Question 15

What is battery thermal runaway and dangers associated with that?

Answer 15

When a battery discharges, it heats up.

If it discharges too rapidly, it can heat to a point where the recharge resistance is less. So when the battery is recharged, it happens at too high a rate, further heating the battery and further reducing resistance.

Eventually this cycle will destroy the battery and melt or explode.

NiCAD batteries most susceptible to battery thermal runaway. Ex. Rotten eggs smell coming from engine compartment during preflight and battery extremely hot to the touch and bulging.

Question 16

Differences between Generators and Alternators

Answer 16

Both convert mechanical energy into electrical energy. Main difference between them is in regard to what spins and what is fixed, and what type of power is produced.

Alternators- rotates an internal set of magnets within a case of conductive wire wrapping to produce alternating current. (AC)

Generator- (water pump) rotates an internal set of conductive wire wrappings within a case of magnets to produce direct current. (DC)

Question 17

Volt

Answer 17

Amount of power

Question 18

Amp

Answer 18

Amount of flow

Question 19

What is a Bus bar?

Answer 19

A bus bar connects power from the battery or generator to the electrical components connected to the particular bus.

Used to distribute power from a central power “collection” point.

Question 20

What is a current limiter?

Answer 20

Reduces or removes voltage from a voltage source (usually a generator) any time the source tries to put out more than its rated current.

“Slow blow fuse” to remove a generator from the system if its output becomes excessive.

Question 21

What is an APU?

Answer 21

Auxiliary Power Unit- a very small gas turbine, located in the aft fuselage and isolated from the cabin by a firewall.

An APU provides a supplemental source of electrical power for ground operations- air conditioning, hydraulic pressure and main engine starting. Or a back up source of electrical power for the main generators in-flight.