Electrical Power sources

Intro to Aero Electrical systems

2 categories of EPS on an aircraft

- batteries (DC)
- generators (AC)

EPS

Electrical power sources

Function of a battery

Stores chemical energy and converts it into electrical energy, doesn't produce energy

Uses of battery

- Provide power when there are no other power sources
- dampen trainsient loads (sudden fluctuations) in DC systems
- provide emergency power

How is capacity of a battery measured

Ampere-hours

When engine-driven generator is working

Batteries are automatically recharged

2 main types of batteries used in aircraft

- lead-acid batteries
- nickel-cadmium batteries

Lead-acid batteries

- common in piston aircraft
- +ve and -ve plates of lead submerged in electrolyte solution
- each cell has 2.2V, rated as 2V
- battery also has corrosive effects

What is electrolyte solution made up of

- sulfuric acid
- water

Frequent total discharge and prolonged discharge leads to

Decrease in battery life,

Nickel-cadmium batteries

- turboprop
- turbojet aircraft

Nickel-cadmium properties

- discharge at a high rate w/o voltage drop
- accept high charge rates that decrease charge time
- may be subject to thermal runway caused by overheating; battery could self-destruct

Generator purpose

Converts mechanical energy into electrical energy through the process of EM induction

How are generators driven

Electrical load

Current required for generator

Which current do generators produce

AC

If a slip ring connects the generator to the circuit, it'll be

AC current. If no slip ring, then DC

Voltage of DC generators

Voltage of 28vdc and current of up to 400A, 270 vdc has been developed

AC generator

Alternator

Alternator current?

3 phase current

Standard aircraft voltage

115vac with 400Hz

2 main types of power generation systems

- Constant frequency integrated drive generators (IDG)
- Variable speed constant frequency (VSCF)

How does IDG work

- for alternator to maintain an output frequency of 400Hz, the rotational speed of the alternator must remain constant
- CSD can be used b/w engine and alternator to provide RPM

CSD

Constant speed drive

Combo of alternator and CSD

IDG

CSD

- hydromechanically operated
- needs to be maintain along oil level and cleanliness
- commonly used in turbine powered aircraft

How does VSCF generator work

- Variable frequency power generated by alternator is converted into constant frequency of 400Hz and 115vac by solid state devices electronically
- no need for CSD
- new tech so limited use

GCU

Generator control unit

GCU functions

- act as a voltage regulator
- direct current to the battery to recharge it
- protect the circuit by disconnecting the generator from the system when electrical faults happen

Conversion devices

- transformers
- rectifiers
- TRUs
- Inverters
- TransistorsT

Transformers function

Change ac voltage level

Rectifiers function

Convert AC into high-amperage, low voltage dc

TRU

Transformer and rectifier

TRU function

Main unit to convert alternator output to dc in an aircraft

Transistors function

Control electron flow and convert ac to dc

Power distribution

Distribution system is needed to convey electrical power to the equipment and systems that need it

Busbar system

- power from generators sent to conductors (busbars) before being distributed
- electrical items hooked up to each bus for power
- bus ties are circuit protection devices which connect the buses to each other
- the emergency lights are usually powered off of battery bus, so they still function w/o generator power
- bus ties isolate failed buses from working ones and send power to buses which lost their normal power source

Conductor material

- copper
- aluminium

Insulation device material

- nylon
- PVC
- fibreglass

Wiring diagrams

- block diagrams
- pictorial diagrams
- schematic diagrams

Electrical loads - once power has been generated and distributed then it can be used by loads

- motors
- lighting servies
- heating services
- avionics