Electrical System

Question 1

Electrical System General Description

Answer 1

The electrical power generation is provided by the:
2 batteries (main and emergency);
2 engine driven DC starter/generators;
2 AC wild frequency generators;
2 external power receptacles (AC and DC);
2 static inverters (DC powered) provide constant frequency AC power.
2 separate networks (left and right) run individually and can be connected by bus tie contactors (BTC).

The ACW system can also supply the DC system through a transformer rectifier unit (TRU).

Electrical distribution is ensured by buses that feed the equipment.

Question 2

DC Power generation

Answer 2

Normal sources:

• 2 engine driven starter/generators
• a ground external power unit

Emergency sources:

• main battery
• emergency battery used only for emer supply excluding start func.
• TRU

Question 3

Batteries

Answer 3

Main Batt:
24V NiCd (43Ahr) for start, emergency supply including prop feathering. 

Emer Batt:
24V NiCd (15Ahr) for avoiding power transients and emergency supply.

Question 4

TRU

Answer 4

Transforms ACW power to DC power for the operation of the DC Emergency System (Basic Mode- DC ESS, DC STBY, DC EMER, INV1 and AC STBY1).

Question 5

Starter/Generator

Answer 5

Nominal power: 400A
Nominal voltage: 27V to 31V
Start sequence ends at 45%NH
Generator on line at 61.5%NH

If both DC Gens fail, DC ESS and DC STBY buses are powered by the Main Batt or the TRU. The DC Emer Bus is powered by the Emer Batt or the TRU.

Question 6

BAT CHG pbs

Answer 6

On (pb depressed): contactor normally closed. Opens in case of thermal runaway, under voltage of DC Main Bus under 25V, Start sequence initiated (in this case, both BCC are opened, and closed when start rotary selector leaves START or CRANK position) or an OVRD signal on BAT switch.

Question 7

OVRD pb

Answer 7

Allows the xfer of the DC STBY Bus and INV 1 from the HOT MAIN BATT BUS to the HOT EMER BUS.

NORM (pb released): DC STBY and INV 1 are powered by HOT MAIN.
OVRD (pb depressed): DC STBY and INV1 are powered by HOT EMER.
UNDV: the light illuminates amber to indicate DC STBY is

Question 8

AC Constant Frequncy

Answer 8

AC Constant Frequency is produced by 2 static inverters.
Power: 500 VA
Output Voltage: 115V +/-4V and 26V +/- 1V
Frequency: 400Hz +/-5Hz
Type: Single phase

Question 9

AC Constant Frequency Distribution

Answer 9

INV1 supplies AC BUS 1 and AC STBY BUS
INV2 supplies AC BUS 2

In the event of INV failure, AC Bus 1 and AC Bus 2 are automatically tied together provided the ISOL pb is not activated. If INV1 fails, the AC STBY BUS is automatically supplied from INV2

Question 10

AC Wild Frequency Generation

Answer 10

2 propeller driven (off the RGB) three phase generators. Brushless and air cooled, rated to deliver 20 KVA.

Nominal set voltage: 115V to 200V
Normal frequency range: 34Hz to 400Hz (70% to 100% NP)

Question 11

ACW Service Bus

Answer 11

Flight attendant controlled, is auto shed when one generator is offline. The coffee gets cold…