Electrical System Flashcards
Explain the electrical system.
28 volts DC, 24 volt/42-amp hour lead acid gel cell battery
OR
34/36 amp hour nickel-cadmium battery
WITH
Two 250-amp starter-generators
What is DC power supplied to?
Power inverters
How many inverters are there?
2
What power do the inverters provide?
400hz, 115 volt and 26 volt AC power to various avionics (torquemeters and fuel flow meters)
How many DC buses are there?
10
What are the DC buses?
Hot battery bus Main battery bus Left generator bus Right generator bus Isolation bus (current limiters) Number 1-4 dual fed bus Avionics bus
What’s the hot battery bus powered by?
The battery, regardless of switch position
What are the dual fed buses powered by?
Either generator bus through 60-amp limiter, 70-amp diode, and a 50-amp circuit breaker
What type of equipment do the 4 dual fed buses supply mostly?
DC powered equipment
What are inverters powered by?
Generator buses and controlled by the inverter selector switch
Where’s the external power socket located?
Underside of the right wing, outboard of the engine nacelle
What’s the external power DC power limited to?
32 volts, there may not be any voltage protection either
What’s the purpose of the battery ammeter?
Provides a direct reading of the charge or discharge rate of the battery (-60amps to +60amps)
Charge should be 0 to +10amps for takeoff
With the NiCad battery, what is provided to show the charge?
What’s it mean if a BATTERY CHG annunciation illuminates?
A charge current detector
It means the battery charge current is above normal
What should you do if the BATTERY CHG annunciation illuminates during flight?
Turnoff the battery switch, if it remains on, a malfunction is indicated in either the battery system or the current detector and should land ASAP - designed for continuous monitoring of the system
How many generators are there? Explain them.
Two 30-volt, regulated to 28.25 +- .25 volts, 250-amp starter-generators
*optional 300 amps starter-generators are put on some planes
How many generators are required to supply the whole electrical system?
Only one
How is power supplied to each starter generator?
Through the main battery bus through a starter relay.
What does the starter-generator drive during the starting of the engine?
The compressor section through accessory gearing
How much power will the starter-generator draw during engine start?
Initially, 1,100 amps then will drastically drop to 300 amps once N1 reaches 20%. When it reaches 35%, it will then drive the starter.
What can you do after the condition lever is set to 70% N1?
Position the Generator to ON
How do you turn on the generator?
Control switch held up in the GEN REST position for a minimum of 1 second then released to ON
What is power to the bus system from the generators protected by?
The generator control unit
What’s a GCU do?
They maintain a constant voltage during variations in engine speed and electrical load requirements
Where is the load on each generator indicated at? How is it indicated?
Overhead, reads in percent of the generators max capacity.
Normally, max is 250 amps, so therefore 50% is equal to 125 amps.
What would cause the generators to go offline?
If underexcitation, overexcitation, over voltage, or under voltage conditions exist
What’s the required charge on the battery before connecting to ground power?
At least 20 volts, if it’s not then it needs to be charged or replaced before connecting to GPU
How do you know if a generator is offline?
The amber L or R DC GEN annunciation will illuminate
How does the main battery bus connect to the L and R generator buses?
Feeds the isolation bus through two 325-amp isolation limiters (current limiters) and then connects the GEN buses together
When does the isolation bus and L and R generator bus function as one unit?
When the battery, generators, or GPU are providing power and as long as both current limiters are not opened.
What instrumentation occurs if an inverter fails?
A red INST INV annunciation illuminates on the panel
What’s the generators limitations?
Sea level - 31,000ft : 100%
Above 31,000ft : 88%
Ground Ops : 85%
*refer to table in 2-1 for other ground limitations
Starter limitations
40 seconds on, 60 seconds off
40 seconds on, 60 seconds off
40 seconds on, 30 minutes off
Inverter limitations
115 volt inverter output must be 105-120 VAC, 380-420 Hz
How do you know if a current limiter is out?
No voltage on the load meter
Procedurally, what’s the difference between a cross generator start and a generator assisted start?
Cross generator - After first engine start, turning on the generator to that engine and leaving it on
Generator assist - After first engine start, turn the generator on, then turn it off for second engine start and after the second engine is started, the generator to the first engine is brought back online - along with the second generator
