Electrical System

Question 1

Explain the electrical system.

Answer 1

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

Question 2

What is DC power supplied to?

Answer 2

Power inverters

Question 3

How many inverters are there?

Answer 3

2

Question 4

What power do the inverters provide?

Answer 4

400hz, 115 volt and 26 volt AC power to various avionics (torquemeters and fuel flow meters)

Question 5

How many DC buses are there?

Answer 5

10

Question 6

What are the DC buses?

Answer 6

Hot battery bus
Main battery bus 
Left generator bus 
Right generator bus 
Isolation bus (current limiters)
Number 1-4 dual fed bus
Avionics bus

Question 7

What’s the hot battery bus powered by?

Answer 7

The battery, regardless of switch position

Question 8

What are the dual fed buses powered by?

Answer 8

Either generator bus through 60-amp limiter, 70-amp diode, and a 50-amp circuit breaker

Question 9

What type of equipment do the 4 dual fed buses supply mostly?

Answer 9

DC powered equipment

Question 10

What are inverters powered by?

Answer 10

Generator buses and controlled by the inverter selector switch

Question 11

Where’s the external power socket located?

Answer 11

Underside of the right wing, outboard of the engine nacelle

Question 12

What’s the external power DC power limited to?

Answer 12

32 volts, there may not be any voltage protection either

Question 13

What’s the purpose of the battery ammeter?

Answer 13

Provides a direct reading of the charge or discharge rate of the battery (-60amps to +60amps)

Charge should be 0 to +10amps for takeoff

Question 14

With the NiCad battery, what is provided to show the charge?

What’s it mean if a BATTERY CHG annunciation illuminates?

Answer 14

A charge current detector

It means the battery charge current is above normal

Question 15

What should you do if the BATTERY CHG annunciation illuminates during flight?

Answer 15

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

Question 16

How many generators are there? Explain them.

Answer 16

Two 30-volt, regulated to 28.25 +- .25 volts, 250-amp starter-generators

*optional 300 amps starter-generators are put on some planes

Question 17

How many generators are required to supply the whole electrical system?

Answer 17

Only one

Question 18

How is power supplied to each starter generator?

Answer 18

Through the main battery bus through a starter relay.

Question 19

What does the starter-generator drive during the starting of the engine?

Answer 19

The compressor section through accessory gearing

Question 20

How much power will the starter-generator draw during engine start?

Answer 20

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.

Question 21

What can you do after the condition lever is set to 70% N1?

Answer 21

Position the Generator to ON

Question 22

How do you turn on the generator?

Answer 22

Control switch held up in the GEN REST position for a minimum of 1 second then released to ON

Question 23

What is power to the bus system from the generators protected by?

Answer 23

The generator control unit

Question 24

What’s a GCU do?

Answer 24

They maintain a constant voltage during variations in engine speed and electrical load requirements

Question 25

Where is the load on each generator indicated at? How is it indicated?

Answer 25

Overhead, reads in percent of the generators max capacity.

Normally, max is 250 amps, so therefore 50% is equal to 125 amps.

Question 26

What would cause the generators to go offline?

Answer 26

If underexcitation, overexcitation, over voltage, or under voltage conditions exist

Question 27

What’s the required charge on the battery before connecting to ground power?

Answer 27

At least 20 volts, if it’s not then it needs to be charged or replaced before connecting to GPU

Question 28

How do you know if a generator is offline?

Answer 28

The amber L or R DC GEN annunciation will illuminate

Question 29

How does the main battery bus connect to the L and R generator buses?

Answer 29

Feeds the isolation bus through two 325-amp isolation limiters (current limiters) and then connects the GEN buses together

Question 30

When does the isolation bus and L and R generator bus function as one unit?

Answer 30

When the battery, generators, or GPU are providing power and as long as both current limiters are not opened.

Question 31

What instrumentation occurs if an inverter fails?

Answer 31

A red INST INV annunciation illuminates on the panel

Question 32

What’s the generators limitations?

Answer 32

Sea level - 31,000ft : 100%

Above 31,000ft : 88%

Ground Ops : 85%
*refer to table in 2-1 for other ground limitations

Question 33

Starter limitations

Answer 33

40 seconds on, 60 seconds off

40 seconds on, 60 seconds off

40 seconds on, 30 minutes off

Question 34

Inverter limitations

Answer 34

115 volt inverter output must be 105-120 VAC, 380-420 Hz

Question 35

How do you know if a current limiter is out?

Answer 35

No voltage on the load meter

Question 36

Procedurally, what’s the difference between a cross generator start and a generator assisted start?

Answer 36

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