13.5 Electrical Power Flashcards
What are the main sources of DC and AC power on an aircraft?
- DC alternator
- DC generator
- AC alternator
- Battery
When is an aircraft’s battery typically used?
- No other sources of power are available
- For emergency operations
- Intermittent system overloads
What is a busbar?
A copper bar, located in distribution boxes, designed to carry the entire electrical load and distribute that load to the individual power users
What is the busbar connected to on almost all aircraft?
Positive output terminal of the generator and/or battery
What is the earth return?
The path of the negative voltage, distributed through the metal structure of the aircraft
What are the nominal voltages/frequencies of aircraft power?
- 28v DC
- 115v AC 400 Hz single phase
- 200v AC 400 Hz three phase
What do we do for the earth on a composite airframe?
- Two wires, one positive and one negative
- In other cases, a ground plane is added to the structure of the aircraft
How are busbars categorised, and further subdivided?
- AC and DC
- Subdivided to essential and non-essential
- Further, there are other buses which may be connected directly or indirectly to the battery, known as ‘hot bus’, ‘vital bus’ and ‘emergency bus’
What is a hot busbar?
A busbar that is permanently connected to the battery
What protective devices are used on an aircraft and what are we protecting against?
- Fuses
- Circuit breakers
- Cut-out relays
Protecting against short circuits, hazardous overvoltage and other malfunctions
How many extra fuses should be carried?
10% of the number of each rating (if one fuse is used on the aircraft of a particular rating, three spares should be provided)
or
three of each rating
whichever is greater
What is meant by the term trip-free circuit breaker?
A circuit breaker, which when an overload or circuit fault exists, they will open the circuit irrespective of the position of the operating control
Must not be of the type that can be overridden manually
What are essential and non-essential services?
Essential services are those that are required to ensure safe flight and landing in cases of emergency, such as; essential lighting, flight control systems, and communication and navigation radios
Non-essential services are all of those wouldn’t be included in the class above
What is the circuit protection requirement of essential loads?
Individual circuit protecting
What is the role of a static inverter?
Converts DC, supplied by the battery, into AC power
What is the role of a transformer rectifier unit?
AC power produced by the generator is converted to direct current
What is a split-bus system?
Gearbox driven generators can never be connected to the same distribution bus at the same time
What is a parallel bus system?
Entire electrical load is equally shared by all the working generators
Fuses/circuit breakers for essential circuits must be accessible to whom?
To the pilot or some other member of the crew
What is the physical relationship between batteries and busbars?
Located as near as possible to the main and battery busbars as physically possible
What type of batteries are usually used with metal trays without surface protection (a coating of a PVC)?
Alkaline batteries
If the flight crew are complaining of an acrid smell in the cockpit, what could be wrong as it pertains to the battery system?
Non-return valve is US, allowing corrosive fumes from the battery charging and discharging cycle to escape into the aircraft
Would be detected when using cabin pressurisation as a means to vent battery, and when negative pressure exists
What problems do we monitor batteries for?
- Temperature
- Current output
- Charging rate (linked with temperature monitoring)
What do the regulations state about the capacity of aircraft batteries?
Must provide 30 minutes of power to essential services
Describe the standby battery system?
- Independent of the DC power system
- Provides 12v power to the DC power system protection circuitry
Which circuits, normally supplied by the aircraft’s DC power system, are switched to the standby battery in the event of a DC power failure?
- Emergency exits
- Cabin notices
- Cabin lighting
- Standby artificial horizon
What two methods are used to vent the gasses from a battery?
Venturi action or cabin pressurisation systems
Why should the test function of standby batteries not be repeated at intervals of less than 3 minutes?
May cause the battery temperature to rise and there is the possibility of causing thermal runaway
Why is a ‘cooling-off period’, of approximately 10 to 25 seconds, provided for an on-board battery charging unit, where an overvoltage exists, while no such period is provided for undervoltage conditions?
Overvoltage conditions tend to cause an increase in current, which generally results in an increase in temperature
Undervoltage conditions don’t usually cause excessive current, so no overtemperature conditions exist
In a battery protection circuit, what is the ideal method by which to isolate the battery?
Relays on both terminals of the battery
Why does a battery charger not start unless the voltage sensed at the output terminals of the battery is above approximately 4v DC?
A battery at such a low charge will almost certainly have resulted in damage to the battery
If an attempt is made to charge it, a large current will flow into the battery and there may be a dangerous build-up of temperature or gases
Why is there a difference between a cut-off and cut-on temperature of the battery at which the battery charger will switch on and switch off?
If they were the same temperature, the battery charger will cycle on and off at a fast rate
What is the test provision sequence for a standby battery?
Pressing the charge test pushbuttons causes:
0.5 seconds controlled discharge through the current flow circuit. After this discharge, a measurement circuit checks that the battery voltage remains above 70% of the rated value and test OK light illuminates if successful
Simultaneously, overheat condition is simulated, lighting the fail lights
What is the power supply to the battery charging circuit?
115/200 Volts AC 3 phase
What is the charging current of a battery charging circuit limited to?
Approximately 65A
What two precautions should be taken with regards to aircraft battery switch positions?
- Set the BATT/GND PWR switch to ON unless the aircraft battery or a ground power unit is connected
- Set the NORM/EMERG switch to EMERG unless the aircraft power system is energised, if not, the standby battery will be discharged
Why are alkaline batteries more susceptible to thermal runaway?
Can handle high charge and discharge currents
What is thermal runaway?
At higher than normal temperatures, the heat loss of the battery through radiation and conduction is lower than the heat generated
As a result, internal resistance is lowered which increases current, this increases temperature further and the cycle continues to destruction
What factors can cause overheating in a battery?
- Incorrect voltage regulation
- High load current (multiple engine starts)
- Loose links between cells
- Low electrolyte
- Leakage currents between cells and container
- Cell unbalance
What are the advantages of alkaline over lead acid batteries?
- Can handle higher charge and discharge currents
- Can be left in any state of charge with no detrimental effects
- Normally more robust
- Terminal voltage between charged and discharged changes very little
- Alkaline cell is lighter than lead acid cell (however 24v lead acid is actually lighter than 24v alkaline)
- Normally have a longer life
How are batteries charged in a servicing bay, and how are they charged on an aircraft?
Servicing bay - Constant current method
Aircraft - lead acid charged by constant voltage method, alkaline charged by high initial current then pulse charged to allow for temperature stabilisation before switching to constant voltage
What is the purpose of the suppressor in a general DC generator?
Reduces radio interference, which may be caused by sparking between the brushes and commutator
What is the purpose of the trimmer resistor in a carbon pile regulator?
Adjusted by the engineer during a ground run, to ensure that, at the correct generator output voltage, the current in the voltage coil will produce the correct amount of magnetic force to balance the leaf spring
What is the principle of operation of the carbon pile voltage regulator?
The resistance of a carbon pile is balanced by a voltage coil acting against a leaf spring, which adjusts the excitation voltage in a coil to the generator
In what ways are solid state or transistorised voltage regulator superior to older types of regulators?
- Cost
- Reliability
- Weight
- Less radio interference
If the voltage coil in a carbon pile voltage regulator became open circuit, what would happen to the generator’s output voltage?
The leaf spring would have nothing acting against it, so resistance in the carbon pile decreases, allowing increased current in the excitation field and increased voltage
In an aircraft with fourteen 5A, seven 15A and two 30A fuses, how many fuses should the crew carry as spares?
9
as the rule states ‘10% or 3, whichever is greater’
Resistive loads can be supplied by which what types of AC generating systems?
Frequency wild
What conditions are necessary in the run-up of a generator?
- Allowed to run-up in an off-load situation
- At a specified voltage output, usually slightly higher than the bus bar, before being brought online
In a direct switching (differential cut-out) generator cut-out arrangement, what initially causes the differential cut-out contacts to close?
Thinner ‘differential’ coil
In a direct switching (differential cut-out) generator cut-out arrangement, what is the purpose of the heavy-duty current series coil?
When generator is ‘on-line’ and producing enough current for forward flow, magnetic effects of the coil assist the differential coil
When generator is ‘on-line’ but not producing enough so as to resist reverse flow, reverse current flow through coil creates magnetic field which opposes that of the shunt coil, and so forces the cut-out contacts open, disconnecting the generator from the busbar
What are the components of a basic single engined generator system?
- Generator
- Voltage regulator
- Differential cut-out
- Battery
What is the distinction between a differential relay and an RCCB?
Differential relay will operate on reverse currents of 20-30 amps
RCCBs will operate on reverse currents of approximately 500 amps and at very high speed
What might be the cause of a DC generator having a defect of sparking at all brushes?
- Dirty commutator
- Excessive load
- Incorrect brush position
What is the basic function of hold-off systems?
- Hold off or prevent the generator paralleling with external power
- Prevent aircraft batteries paralleling with external power
In general terms, what controls the hold-off circuits?
Small pin on the NATO 3 pin DC ground supply socket
What are the advantages of paralleling a DC generator system?
- No break in supplies if a generator fails
- System can handle the switching of high transient loads
What are the disadvantages of paralleling a DC generator system?
- Necessitates additional circuitry to ensure that both machines share the loads equally
- A generator side failing may prove difficult to diagnose as its effects can be seen in the other generators
What is the purpose of an ammeter connected to each generator in a parallel DC generator system?
Allows the pilot to regularly check that load sharing is correct
In a parallel DC generator system, what is the purpose of the voltage boost resistor and where is it in the circuit?
Artificially raises the output of generator No 2 above generator No 1, after No 1 has been brought on-line and prior to No 2 being brought likewise
Located in series with the voltage coil of No 2 regulator
Why is it important to ensure that all connections in a parallel DC generator equalising line between generators are secure and have a minimum contact resistance?
Equalising potential differences are small, so small changes in circuit resistance will have drastic effects on load sharing between generators
What is the general acceptable paralleled load difference between generators?
No more out of balance than a 60/40 split
How does a voltage pick-up relay ensure the correct polarity?
A diode connected in series with the voltage pick-up relay
In AC power generation, what are the two main categories on aircraft?
- Frequency wild
- Constant frequency
What can frequency wild AC generation systems be used for?
- To supply resistive AC loads, for example anti-icing/de-icing systems
- Supply DC via a TRU
In order to parallel two AC generators, what properties must be the same?
- Frequency
- Voltage
- Phase rotation
What are the two main types of constant frequency power generation systems?
- Split busbar
- Parallel busbar
What are the disadvantages of constant frequency as compared to frequency wild?
- More expensive
- More complex
- Requires maintenance due to additional components required
What are the advantages of frequency wild AC systems as compared to DC systems?
- Better power to weight ratio
- Much less affected by poor brush performance at high altitude, especially if a brushless generator is used
What is the frequency swing of a frequency wild AC system, connected directly to the engine gearbox?
Approximately 200 Hz either side of 400 Hz
What are the main components of a frequency wild AC generator system?
- AC generator
- Generator control unit (GCU)
- Compounding transformer
- Voltage regulator
Why are compounding transformers and rectifiers incorporated in a frequency wild AC generator system?
Increase the range of regulation
What is the effect of the weight on wheels switch on a frequency wild AC generator system?
Alternator is held to a low level phase voltage of approximately 75v AC
What is the purpose of the interpole windings in a parallel DC generator system?
Aid or oppose the voltage coil, thus increasing or decreasing the generator output voltage
What are the two methods of constant frequency which supply large quantities of power for aircraft?
- Integrated drive generator
- Variable speed constant frequency system
If the springs pushing a carbon pile voltage regulator snap, what would happen to the output voltage?
The only force acting on the carbon pile would be the voltage coil, which forces the pile apart and increases the effective resistance
This reduces the excitation voltage across the field coil, thus reducing the output voltage
On what type of aircraft are frequency wild systems most employed?
Turbo prop and small jet aircraft
What is an integrated drive generator (IDG)?
Constant speed drive unit and brushless generator
What is a variable speed constant frequency (VSCF) system?
A variable speed/frequency generator which feeds an electronic network to produce a constant frequency supply
What is the purpose of the CSDU and how is this achieved?
To drive the brushless generator at a constant RPM for varying engine speeds
Hydro-mechanical unit fitted between the engine gearbox and the brushless generator
What is the principle of operation of a CSDU?
Engine driven hydraulic pump will drive a hydraulic motor at a desired speed, and therefore frequency
Increase of engine speed will cause increase in pump output, while a mechanical governor will sense the increase and port oil to the servo piston which will move a swash plate on the pump to underdrive
Reducing the pump action and therefore the motor drops back to the desired frequency
How is the fine trim of the CSDU set?
Electromagnetic coil receives signals from the electrical system’s frequency control, which modifies the position of the flyweights in the governor
Name the two types of automatic disconnect in a CSDU?
Thermal - internal oil temperature above a threshold is detected which causes a eutectic soldered ring to melt, actuating the disconnect mechanism
Shear - shear disconnect results when a detected torque of greater than a pre-determined inch pounds is present
What parameters can the pilot monitor in order to make a decision to actuate a disconnect of the CSDU from the engine?
Oil pressure and temperature
In the brushless generator of the IDG, what powers the GCU?
The permanent magnet generator (PMG)
What is included in a VSCF generator which provides protection to the accessory gearbox in the event of VSCF generator mechanical failure?
Spindle between the generator input spline and the speed increaser gearbox has a shear section
Premature bearing wear may be caused by an engine due to what type of fault in the electrical distribution system and why?
Earth leakage, leading to current flowing from a phase to the neutral line, resulting in unequal loads being presented between the 3 phases
Unbalanced loads would cause vibration similar to an unbalanced wheel on a motor vehicle passing vibrations on
To discriminate between earth leakage faults and undervoltage conditions, what would the flight crew make reference to?
Voltmeter
An undervoltage will be a slight drop in voltage
An earth leakage will be a very large drop
When can a CSDU be reset?
When the aircraft is on the ground with the engine stopped
Where does a Bus Power Control Unit (BPCU) receive information from?
- The GCUs about any generator malfunction
- Load controllers
- Current transformers
In a three-phase AC generation system, what is the purpose of the capacitor and resistor in the overvoltage protection circuit?
Provide a time delay so that a transient voltage spike will not trip the generator off-line, only sustained overvoltages
In a three-phase AC generation system, how does an over-voltage sensing circuit deal with an overvoltage?
Triggers an SCR which de-excites the generator and removes it from the busbar
In a three-phase AC generation system, what component will sense over-load to protect the system?
Current transformers wrapped around the conductors, which sense when too much current is flowing, and de-excite the generator and remove it from the busbar
In a three-phase AC generation system, how does differential protection work?
Current transformers sense current flow at the generator and at the circuit breaker immediately before the load
Differences in current flow suggest earth or line fault between the current transformers
In a three-phase AC generation system, what lines are monitored?
- Line to line faults
- Line to earth faults
What power supply will a GCU receive, if there is PMG failure of a brushless generator?
Battery backup power supply from an aircraft battery
What tolerance does the GCU in a three-phase AC generation system maintain the generator output to?
115V ± 5 volts
How does a GCU operate in a three-phase AC generation system?
- Samples 3 phase AC output
- Changing to a DC level
- Compares to a reference value
- Adjusts excitation to the field of the generator via the generator field relay (GFR)
What steps are followed when a generator fails, to remove it from the busbar in a three-phase AC generation system?
- GCU senses partial loss of power
- Automatically signals the bus power control unit (BPCU)
- BPCU removes the generator from the busbars
What is the principle of a TRU used on aircraft?
AC voltage input is stepped down by three-phase transformer, rectified by the six rectifier bridge assembly, providing full wave rectified, relatively smooth DC
In an aircraft TRU, what components ensure the smoothest rectified output possible?
2 rectifiers, one delta, the other star
A TRU has what basic protection?
- Overheat
- Voltage
- Reverse current
At what upper and lower voltages might a TRU be automatically removed from the busbar?
33v and 22v
When will the reverse/low current protection disconnect the TRU from the DC busbar automatically?
When fault current reaches approximately 1 amp
In a transistorised AC voltage regulator, how is the strength of the exciter controlled?
Pulse width modulated signal which is a constant amplitude
At high generator voltage, pulse width is small
What is the point of regulation on larger type aircraft?
The point at which the regulator derives its reference is as close to the busbars as possible, but upstream of the generator circuit breaker (GCB)
What are the advantages of a relay over direct switching?
- Remote switching
- Relay can switch many contacts at once
- Low current can be used to switch a large current
What are the advantages of the main power breaker or latching relay?
- Only needs power when changing state
- Not affected by ‘G’ forces
Why is a spike suppression fitted to a power contactor?
Discharge energy in the coil and stop hold-in when power is removed
What are the advantages of the parallel generator system over the split bus system?
- Better able to handle shock loads
- Continuity of electrical supplies; without paralleling, there may be a break in services when switching to a correctly functioning generator
- Generator life is prolonged, since each generator is normally run on part load
What are the disadvantages of the parallel generator system over the split bus system?
- Fault propagation; avoided by adequate circuit protection units
- Parallel operation does not, without modification, meet requirements for totally independent supplies
Why is it usual to incorporate an interlock circuit in a parallel generator system which becomes more prevalent on the ground?
Paralleling of the APU, external supply with each other or the aircaft generators is to be prevented as these sources are not considered compatible
What are the four stages of a VSCF?
- Generator
- Rectifier
- Inverter
- AC filter
What stages does a GCU modify in the VSCF cycle?
Generator - field excitation
Inverter - frequency control
What are the conditions required for synchronising parallel generators?
- Voltages within tolerance
- Frequency within tolerance
- Phase displacement within tolerance
- Phase rotation correct
If generators in a parallel system are not synchronised, what would be the result?
Serious damage to the generators could occur due to high circulating currents
When is the ground handling bus powered and when is it used?
Automatically whenever external power or APU power is available, and mainly to power lights and the refuelling system
What is the typical charging current of a typical battery charger?
45-50 amps
What does protection circuitry in the battery charging system provide protection against?
- Overvoltage
- Overheating
- Battery disconnect
What is active load also known as?
Real load
How is active load sharing achieved by active load controllers?
One active load controller for each IDG, varies the current to the magnetic trim coil of the associated speed governor’s trim head of the CSDU
- Increase the torque of the light active load generator
- Decrease the torque of the highly active load generator
How is reactive load sharing achieved by reactive load controllers?
Output of the reactive load controllers circuit is applied to the voltage regulator GCU as a bias to:
- Increase the excitation of generators carrying too little reactive load
- Decrease the excitation of generators carrying too much reactive load
What is the sensing element in an active load sharing loop?
Current transformers fitted to monitor the current in ‘B’ phase of each of the three generator outputs
How is sensing achieved in a reactive load sharing loop?
Mutual reactor whose secondary winding is fitted in series with one of the phases feeding to the voltage reference detector in the voltage regulator of the GCU
In a parallel AC generator system, with an over excitation (parallel fault) condition, what is the likely fault?
Fault voltage regulator or reactive load sharing circuit
If tripping the BTB cleared the problem, the fault was most likely due to the reactive load sharing circuit
In a parallel AC generator system, what function does the overvoltage protection system operate on?
Inverse time function
Magnitude of voltage determines the time in which the offending generator will be de-energised
What is the configuration of the auxiliary contacts and main contacts in a main power or latched breaker?
When the main contacts are open, the auxiliary contacts are closed
What is the typical twin generator system for a helicopter?
- Number 1 generator supplies the essential AC loads
- Number 2 generator supplies the non-essential AC loads
What is the purpose of the transformers on a helicopter?
Provide different values of AC voltage, typically 26v, for such things as lights and synchros
At what height will a BTB not operate, even if a generator is lost?
Below 200 ft radio altitude
What is the purpose of the auxiliary contacts in an active load sharing loop?
Shorts out the generator, thus ensuring the remaining generators take the active power
At what frequency will instability protection in a parallel generation system delay trip all the BTB?
Approximately 8 Hz
What usually causes frequency instability in a parallel generation system?
Low oil level in the CSD
What part of the reactive load will be damaged with an over frequency in a parallel generation system?
Capacitive
What part of the reactive load will be damaged with an under frequency in a parallel generation system?
Inductive
