Batteries and Charging
What is the purpose of an ammeter?
To monitor battery charging current.
What do we call it when only one generator can power a bus at one time?
Split bus system.
What does a current transformer do and what does it look like?
It monitors current by monitoring the induced voltage that is created by current running through the cable being monitored. It looks like a donut.
What does a power circuit do?
It connects power to the buses (from generation system)
What is the name of the device that connects two buses?
Bus tie breaker.
If a wire is open, what kind of resistance would you see if you tested from one end of the wire to the other.
What do circuit protection devices protect?
The wiring.
An inverter is needed. The two main types are rotary and static.
What voltage is aircraft ground?
Zero.
If an ammeter has a zero in the center, what can it display and what does that tell us?
If the current shows as positive it shows the battery is charging; if it shows negative it shows the battery is discharging?
What do we call it when multiple generators join together to power one bus?
Paralleling.
What does a load circuit do?
Provide power from the bus to components and systems of the aircraft.
What type of bus systems do most smaller business/commuter jets have?
The system usually has split buses, in that each generator has its own bus, but they work in parallel to power both buses together. They need some sort of regulation to keep them in sync.
What type of system is it if multiple generators are connected to a sync bus?
A paralleling system.
If there is a short in a circuit, what kind of resistance would you see on a multimeter if you tested between the power source and the presumed short?
Low resistance.
Why are circuit breakers better than fuses?
Because they can be reset instead of being replaced.
Large transport aircraft use mostly AC current. But some DC is needed, what is the component that provides it? What does it do?
A transformer rectifier. First it steps the current down to 26 volts, then it turns it from AC to DC.
Explain, in general terms, how a relay works.
The relay has an induction coil; when current goes into the coil it turns into an electromagnet and either pulls a switch open or pushes it closed.
What is an ammeter shunt and why do we use it?
It's a way to have the ammeter in parallel with the charging circuit. It provides information about the state of battery charging without having the whole charging load go through it.
When multiple DC generators work together what must be managed to avoid damage?
Voltage must be almost the same coming from each generator?
What are some things that you would need to isolate an avionics bus from?
Starting, and connecting ground power, because of high current involved.
What are the three parts of a component that sits between the two generators of a dual feed bus?
Current limiter. diode, circuit breaker.
If any aircraft system has no generator control or monitoring, what will be the capacity of the generator compared to the needs of the load?
The load will be no more than 80% of the generator's capacity.
What is meant by normally open or normally closed?
A normally open relay will be open when there is no current applied and will close when current is applied.
A normally closed switch will be closed when there is no current and will open when current is applied.
A slow blow fuse.
At least three. Two were pilots and one was an engineer who managed all the systems including managing the power system loads among the generators.
What is the difference between a relay and a solenoid?
A relay converts current into motion to close or open a switch. A solenoid converts current into linear motion to work as a type of actuator.
Most aircraft have a way to protect the battery from unintentional discharge. What must be done before the battery will be connected to the bus?
The battery switch must be turned on.
When multiple AC generators work together to power one bus, what must be managed?
The generators must be in phase with each other.
They can never power one bus together. This is called a split bus system.
If you have a circuit breaker in a circuit before a slow blow fuse, why might the circuit breaker be a lower amperage than the slow blow fuse?
So the circuit breaker will trip before the slow blow fuse.
When DC generators are operated in parallel to supply power for a single load they use an equalizer circuit. How does the equalizer circuit equalize?
By increasing the output of the low generator and decreasing the output of the high generator until they are equal.
How would you get a voltmeter to show a negative number?
By connecting the positive lead to the ground terminal and the positive lead to the ground.
A current limiter has blown. What steps must you take before replacing it?
You must correct the problem that caused it to blow.
What is a RAT?
Ram Air Turbine - a small turbine that will drop into the airstream under emergency conditions; provides a small amount of electrical and hydraulic power.
Briefly describe a single wire system.
It's a system where load is carried on one wire and the circuit is completed by connecting to the frame of the aircraft, which is a common ground.
Why do electrically started aircraft need high amp batteries?
Because a large amount of current is needed to turn the starter.
A 737 (or other modern jetliner) suffers the loss of one of the engine generators. The APU is not operative. What will the power management system do to keep things working normally?
This is a split bus system. If one engine driven generator goes down and the APU can't step in, the other engine driven generator will power both buses.
What is the primary difference between old and new transport aircraft power bus systems?
Old style: generators worked in parallel, each powering both buses - had to be kept in sync. Paralleling system.
New style: generators each feed their own bus. If one generator goes down, the other generator can feed both, but two generators can never feed one bus. Split bus system.
Why might an avionics bus be isolated with a normally closed relay? What process would happen to isolate it?
Normally it's closed, but when current comes from a high current activity like connecting external power or using the starter, it will open and isolate the bus.
What does a current limiter look like on a wiring diagram?
Two arrows pointing to each other.
Define a hot battery bus
Hot battery bus is a bus that's always energized by the battery regardless of flight deck switch position or whether the aircraft is powered on.
It means that is the max amps than can go through it before it trips or blows.
What are the two LRUs that a transport aircraft has to manage power?
Generator control unit (GCU) and Bus Power Control Unit (BPCU).
If you have 2 sets of wires going in and out of a relay, which wire will carry a higher load (generally speaking)
The wire to the coil of the relay will be the low load wire and the wire carrying the current that the relay connects will generally be higher load.