11.6 - Electrical Power Flashcards
What are the parts of a D.C generator?
Field magnet - frame with four electromagnetic poles
Armature
Carbon Brush
What Voltage does a voltage regulator sense and what does it limit?
The voltage regulator senses the generator output voltage, and it’s normally closed contacts vibrate open and closed many times a second, limiting the amount of current that can flow through the field.
What is the purpose of a P wire in a 2 Engine D.C alternator system?
Both regulators are connected through their P, or paralleling terminals.
So circuits inside the regulators compare the field voltages. If one alternator produces more current than the other, it’s field voltage is higher.
This difference is electronically sensed, and the voltage regulator decreases the field current flowing to the high-output alternator and increases the field current to the other.
This adjusts the output voltages so the alternators share the load equally
What’s the purpose of the rotating diodes?
3-phase rectification needing 6 diodes gives more steady output current
What mechanical device is used to keep the frequency constant on an engine generator?
frequency output should range between 380 - 400 Hz
Accomplished with a CSD
Constant Speed Drive which is hydraulically operated
What electronic device is used to keep the frequency constant and what’s it replace?
VSCF (Variable Speed Constant Frequency) Alternator
Utilises a DC alternator which is sent to a solid state device which converts DC to AC voltage at a constant frequency of 400 Hz
It replaced mechanical CSD (Constant Speed Drive) or IDG (Integrated Drive Generator)
When should a C.S.D be engaged or disengaged?
A Constant Speed Drive (CSG) should only be engaged on the ground
It is disengaged by the pilot as the whole IDG (Integrated Drive Generator) consists of both alternator and CSD if:
lf there is a generator or CSD failure, the pilot may
disconnect the input shaft from the drive to prevent any damage.
Can primary cell batteries be recharged?
No Primary cell batteries cannot be recharged
(Secondary cell can be recharged)
What are batteries used for?
Batteries can be used to start engines or to backup avionic system in event of engine and APU loss
What is used to check the charge of a lead acid battery and what is its cell voltage?
The electrolyte in a lead acid battery is sulphuric acid and water
As the battery is used, the specific gravity of the eletrolyte changes
This can be measured with a hydrometer to check the charge
What is the voltage of a nickel-cadmium cell?
The voltage of a nickel cadmium cell is 1.25V
What is thermal runaway?
If a Nickel-Cadmium cell is subjected to an excessively high charging rate, even though it’s internal resistance is low, it can become overheated.
When this occurs, it’s internal resistance drops.
The lower internal resistance
allows the cell to take more current from the charger and more heat is generated. This condition is known as thermal runaway and it can destroy a cell.
What are the two methods of charging a Ni-Cad battery?
Nickel-Cadmium batteries can be charged by either a constant-voltage or a constant current charge
What is deep cycling?
A cell that has had its capacity decreased by repeated shallow charges can be restored to its full capacity by completely discharging it and overcharging it to approximately 140% of its rated ampere-hour capacity
What is the purpose of a transformer rectifier unit?
Rectifiers change the alternating current AC into direct current DC
115/200V @ 400 Hz
to
28V DC
What is the purpose of an inverter?
Changes DC to AC
A typical inverter has a 28-volt DC input and a 115 volt single or three phase, 400 Hz output.
What is the principe of operation of a circuit breaker?
Operate on principe of heat or a magnetic field.
When more current flows than the circuit breaker is rated for, a bimetallic strip inside the housing warps out of shape and snaps the contacts open
How is radio equipment protected at engine shut down?
All radio equipment should be turned off before the engine is shut off.
What type of electrical bus system is used on modern aircraft?
Split but system (twin engine eg 737, A320)
Parallel bus system (+2 engine ie 747, A380)
What is the purpose of an Air Driven Generator? (ADG)
If normal electrical power generation fails, the ADG can be extended into the airstream
The turbine begins to rotate and drives on its shaft directly the generator.
Can a R.A.T provide electrical power?
The Ram Air Turbine (RAT) is primarily for powering hydraulic pump as a backup.
It extends into the airstream
Yes, The R.A.T generator provides sufficient electrical power for a safe flight in emergency configuration
What lights are used for external power and what are their colours?
If ground power is connected and is of the correct voltage, frequency and phase sequence (115/200V 400Hz 3 phase) a blue light will illuminate signalling external power available.
If ground power is connected but not in use by aircraft circuits, a white light will illuminate. Before removing ground power, this light should be illuminated.
What is the purpose of a short pin on an external power cable?
External/ground power has 6 pins, 2 of which are short and marked ‘E’
The short pins are used to force the external power to be switched off automatically at the power source and in the aircraft if the external power plug should be unplugged while supplying power or should it slip out of the receptacle under the weight of the power lead.
How is electricity generated?
Electricity can be generated by the conversion of chemical energy, heat, light, pressure and magnetism
An aircraft used both chemical energy (batteries) and magnetism
Storage batteries convert chemical energy into electricity for starting the engine and for use in emergencies, but the electricity for normal operation is produced by magnetism in a generator or alternator.
A rotating coil in a magnetic field generates _____ and how can it be rectified?
AC (all current generated through magnetism is AC)
Rectified to DC by mechanical means or a semiconductor.
The pole pieces and rotating coil are similar to those in the description of the AC generator, but the ends of the coil are terminated in the two segments of a split-ring commutator. Carbon brushes ride on the commutator to connect the coil to the external circuit.
Adding more coils at 90^ will produce better output DC as per graph
A DC generator /alternator consists of:
Field Magnets
Armature
Carbon brushes
A split ring commutator
What is the purpose of the electromagnetic voltage regulator?
The voltage regulator senses the generator output voltage, and its normally closed contacts vibrate open and closed many times a second, limiting the amount of current that can flow through the field.
What is the difference between a Generator and an Alternator?
Generator = the armature or windings of wire spin inside a fixed magnetic field to generate electricity.
Alternator = Electricity is produced when a magnetic field spins inside the stator (windings of wire)
– Pic shows generator –
What is the output of an AC generator?
115/200V
3-phase
400 Hz
75 kVa (40 - 90 kVa)
What type of electricity is caused by the movement of electrons?
Current electricity is caused by the movement of electrons
How is current created in a self-excited generator?
Generator fields are self-excited. This means that the field current comes from the armature
*Field current is the current supplied to the field windings of a generator or motor to establish the magnetic field for it’s operation,
Where are the electromagnetic poles located on a DC generator?
The frame of a typical aircraft DC generator has four electromagnetic poles
How is differential protection of an electrical system accomplished?
Two 3-phase current transformers located in the generator
- One reads the current to ground (neutral)
- One reads generator current and current to power consumers
If either is off, this indicates an error in grounding or a short circuit somewhere which will cause the generator excitation field to switch off.
Name the advantage of a DC alternator over a DC generator?
- They have many more sets of field poles so they can produce current at a lower speed
- The high current is produced in the stationary component and is rectified by semi-conductor diodes
- An alternator system is lighter in weight than a generator system
How does a CSD work?
Constant speed throughout the engine operating speed range is required to give a constant 400Hz
This is accomplished through a Constant Speed Drive (CSD) which consists of a variable hydraulic pump and motor to keep the alternator output frequency constant.
How is the AC output of the stator in an alternator converted to DC?
(3 phase rectification)
by a 6 diode, 3-phase rectifier
What is the difference between primary and secondary cells?
A primary cell cannot be recharged
A secondary cell can be recharged
Name the cell voltage and the electrolyte of a lead-acid cell and how the charge state is determined
Lead-acid cell voltage = 2.1 V
Hydrometer (measuring specific gravity)
Name the advantage of Ni-Cd cells over lead acid cells?
Nickel-cadmium batteries are used in many aircraft because of their ability to produce large amounts of current for starting turbine engines.
Name the cell voltage and the electrolyte of a Ni-Cd cell and how the charge state is determined?
Nickel-Cadmium Cell
Voltage = 1.25 V
Electrolyte = Potassium Hydroxide and water
Charge state = Capacity test
Draw a schematic how a battery must be connected with a charger
In parallel
I.e positive to positive and negative to negative
For what is a Transformer/Rectifier used, what does it contain?
A transformer/Rectifier is used to change AC current into DC current
What is an inverter used for and how does it work?
An invertor is used to convert DC current into AC current as may be required by some instruments on smaller aircraft which use battery power.
Conventionally it worked by the DC power turning a shaft of an armature and making AC traditional way.
Currently an Oscillator is used however.
Name a melting fuse
Regular glass tubular fuse
What is a current limiter?
A current limiter is a type of fuse that will isolate a complete distribution bus in the event of a short to that bus
What is the advantage of a circuit breaker over a fuse?
A circuit breaker can be reset while a fuse (melts whent too much current passes through it) must be replaced/repaired upon landing
Name all types of circuit breakers: (3)
- Push-to-Reset
- Push-Pull type
- Toggle type
Fuses are installed to protect the:
Wiring in the circuit
Name two ways to protect avionics systems against voltage spikes?
Reverse-biased diode
freewheel diode
Name all protections in the power distribution system: (2)
- Split Bus Power Distribution System (twin engine)
Generator breaker (GB) of failed generator is opened and Bus Tie Breaker (BTB) to that bus is closed. There are 2 separate buses
- Parallel Power Distribution system (3 or 4 engine)
Each generator has an individual bus that feeds on to the main Tie bus, in the event of a generator failure, the individual bus is isolated from the Tie bus and flight continues as normal
What is the meaning of differential current protection?
There is a danger of short circuit between two generator lines or one generator line and ground.
One 3-phase current transformer reads current to ground (neutral)
One 3-phase current transformer reads generator current and power consumer current
If either are off, indicates failure somewhere
What is the mearning of a split bus power distribution system?
A split bus power distribution system is used for twin engine aircraft (eg A320, 737)
Each generator is connected to it’s own bus and in the event of a generator failure, the GB(generator breaker) is opened and the required BTB (Bus Tie Breaker) is closed to allow the APU to supply power to that bus
(Split bus = each generator supplies its separate isolated distribution bus)
What is the meaning of a parallel power distribution system?
In a parallel power distribution system, the main distribution bus (Tie Bus) is fed by each generator’s own individual bus. Each generator shares the load equally from the Tie bus
In the event of a generator failure, that generator’s individual bus is isolated from the Tie bus by opening the GB (Generator Breaker), the rest of the generators compensate for the increased load.
If the aircraft loses all normal generators, the electrical power can be restored from:
ADG (Air Driven Generator)
If all normal generators fail, ADG is extended into airstream which rotates and drives a turbine.
To keep the speed of the generator constant, a built in system adjusts the blade angle accordingly.
The aircraft is parked on the ground. From where is electrical power provided?
On the ground with no generators running, there is only the aircraft battery which provides electric energy however this is not sufficient to run systems over long time.
An external power supply should be used on ground to provide electrical power.
What is the main difference between an AC and DC generator?
A DC generator contains a split-ring commutator at the ends of the coil
The armature is the ____ ___ of the generator
rotating part
The field poles are located on the ____
frame
In a DC generator, the carbon brush would be pressing down on the ____
Commutator
(Note a pair of carbon brushes picks off the output current from the rotating armature)
Generator fields are ___-_____
This means the field current is coming from the _____
Self-excited
Armature
What are the 2 types of voltage regulators?
- Electromagnetic
(The voltage regulator senses the generator output voltage, and it’s normally closed contacts vibrate open and closed many times a second, limiting the amount of current that can flow through the field.
- Solid State Regulator (no moving parts)
This solid-state device provides smooth and consistent control of the system voltage with no moving pans.
Why is a starter-generator used?
A starter generator is directly coupled to the engine, once it starts the engine, it then acts as a generator
When should the Constant Speed Drive (CSD) be disconnected? (pressure and temp)
Low oil pressure
high oil temperature
What device keeps the frequency constant?
CSD (Constant Speed Drive)
A primary cell _____ be recharged
A secondary cell ____ be recharged
A primary cell cannot be recharged
A secondary cell can be recharged
Complete the following for a Nickel-Cadmium cell:
Positive Plate =
Negative Plate =
Electrolyte =
To check charge =
To get rid of cell memory =
Positive Plate = Nickel
Negative Plate = Cadmium
Electrolyte = Potassium Hydroxide and water
To check charge = Capacity test
To get rid of cell memory = perform a deep cycle
How many Diodes are in a Full-wave rectification with bridge type rectifier
4 Diodes
2 working at any instantaneous moment (alternates)
The primary function of circuit breakers it to protect ___
wiring
Induced current protection is found in a _____
Relay
Field windings are on the case in a _____
Generator
A field winding is the insulated current-carrying coils on a field magnet that produce the magnetic field needed to excite a generator or motor
Carbon brushes push against the _____
Commutator
What are the 2 types of voltage regulator?
Electromagnetic (contacts which vibrate…)
Solid State (no moving parts)
Why do you have to flash the field?
Re-magnetise the Permanent magnets
