Electrical
GENERAL
The electrical system provides 115V and 28V, 400 Hz alternating current (ac) and 28V direct current (dc). Normal operation oft he system is automatic; however, manual control oft he generators and bus distribution is available. The primary 115V, 400 Hz ac power is provided by:
Three engine-driven generators, or
One APU-driven generator, or
One external power unit.
The 24V battery and a standby static inverter provide an emergency standby source of ac and dc power when generator power is not available.
Each engine-driven generator can power ist respective bus through ist GB, and other buses through BTBs and the ac tie bus. The APU generator or external power can also supply the ac tie bus.
Essential ac power is normally supplied by generator 3; however it may be supplied by generator 1 or 2, external power or the APU.
The dc requiremments are supplied by three transformer rectifiers (TRs) which convert 115V ac to 28V dc. Essential dc power is normally supplied by the essential TR.
Standby ac power is normally supplied by the ac essential bus; however, it may be obtained from a battery operated static inverter. Standby dc bus power is normally supplied by the DC ESSENTIAL BUS; however, it may also be obtained from the battery bus (BAT BUS).
(1) DC Meter Selector TR: Indicates transformer-rectifier current and respective dc bus voltage. BAT: Indicates battery current. Battery charging shows positive amperes. Indicates battery voltage when charger is not operating. (2) Battery Switch ON: Battery serves as back-up for battery buses. OFF: Battery continues to charge but is isolated from loads. (3) Galley Power Switch Connects electrical power to galleys. (4) External Power Switch On: External power on the sync-tie bus. When external power is connected, the GBs and APU GB automatically trip. Switch automatically trips to OFF for overvoltage or phase unbalance faults. OFF: If external power is connected, external power dc bus, ac bus and battery transfer bus are powered. GRD SERVICE: In addition to the above three buses, the ac transfer bus is powered. (5) AC Connected Light Illuminates when external power is connected to external power receptactle. External power dc bus, ac bus and battery transferbus are powered.
(1) Essential Power Source Selector Selects power source for essential and standby ac and dc buses. External power or the APU cannot supply essential bus unless they are connected to the sync bus and the No. 3 BTB is closed. (2) Essential Power Failure Warning Light On: Loss of essential power. Off: Essential power is being supplied. (3) Residual Volts Switch Changes ac voltmeter scale from 130 to 30V. Indicates residual generator voltage to check rotation and proper functioning. (4) AC Voltmeter / Frequency Indicator Indicate voltage and frequency selected by AC METERS selector. (5) Synchronizing Lights Compares frequency of selected source with frequency of the sync-tie bus. Blink rate is proportional to frequency difference. (6) AC Meter Selector Selects the AC source and indicates it on AC voltmeter and frequency indicator.
AC ELECTRICAL POWER
GENERATOR
Each generator provides three-phase, 115V ac at a constant 400 Hz. Each engine-driven generator is rated at 40KVa.
CONSTANT-SPEED DRIVE
The CSD maintains generator output at 400 Hz by compensating for changing engine speeds and generator loads.
A self contained oil tank, separate from the engine oil system, supplies the CSD with oil for use in control and lubrication.
CSD Underspeed Switch
The underspeed switch opens the associated GB when the CSD is underspeed. This prevents a three-light under voltage trip during engine shutdown.
CSD Oil Cooler Switch
A three-position switch, located on the flight engineer’s lower panel, provides control over engine bleed augmentation airflow through the CSD oil cooler. In the NORMAL position, augmented airflow through the cooler is off in flight and on when the airplane is on the ground. The FLIGHT ON position turns on engine bleed air ejectors which augment the normal ram air flow through the cooler for additional CSD oil cooling. The GROUND OFF position stops the bleed air augmentation; the switch will automatically resett o NORMAL at takeoff.
CONTROL
Voltage Regulator
A voltage regulator (VR) for each generator is located in an electrical equipment rack in the lower 43 compartment. The VR maintains generator output at 115V ac. Each VR receives power from ist respective generator. The VRs output is routed through a generator field relay (GFR) tot he generator’s field. A voltmeter on the flight engineer’s electrical panel permits checking the voltage output oft he generator.
Load and Frequency Control
Basic frequency is maintained by the CSD; however, a load controller in the lower 43 compartment electrical euipment rack automatically makes fine adjustments tot he CSD to equalize kilowatt (KW) load division between paralleled generators. Limited manual trimming oft he frequency of an isolated generator and balancing oft he KW load of paralleled generators can be accomplished by means of a frequencyy control knob, a frequency meter, and a load meter on the flight engineer’s electrical panel.
Generator Control Panel
Each generator’s ac output is sensed by a respective generator control panel located in the lower 43 compartment electrical equipment rack. The generator control panel contains a GFR which, when closed, energizes the respective generator’s field. The control panel also contains fault-protection circuits which can automatically trip the GFR, GB, and/or BTB oft he respective generator. Each generator control panel is supplied power from ist own generator. The respective DC CONTROL CB on P6-3 provides an alternate power source from the battery.
Syncronizing (Tie) Bus
The sync-tie bus permits paralleling generators to all three ac buses. It also permits external power or the APU to supply all three ac buses.
Bus Tie Breakers
Bus tie breakers (BTBs) permit generators tob e operated completely isolated from one another while supplying only their respective ac bus. They also permit interconnecting generators and their ac buses for parallel operation through the sync-tie bus.
DISTRIBUTION
Load Distribution Center (Lower P6)
All 15V ac power, whether produced by airplane generators or supplied from an external source, is delivered tot he load distribution center, located aft oft he flight engineer’s panel on the lower aft right cockpit wall.
Gen AC Buses.
There is an ac bus on the P6 lower panel for each generator with DISTR X, Y, Z distribution CBs. Certain heavy power cricuits (such as pack cooling fans) use separate distribution CBs from the ac buses on the P6 lower panel.
AC Transfer Bus
The bus supplies power through CBs tot he battery charger, water compressor, equipment cooling blower and various lights for servicing the airplane. The bus is powered whenever the No 3 generator’s ac bus is powered or with the xternal power switch in GRD SERVICE.
FAULT PROTECTION
Automatic fault-protection circuits within the generator control panels protect the ac electrical system from
Overexcitation or underexcitation,
Overvoltage or undervoltage,
Differential fault,
Overspeed or underpseed of CSD, and
Exciter ceiling and instability.
Excitation (over and under) protection refers to a comparison between generator field voltages. Exciter ceiling refers to comparison to ground.
When any abnormal condition is sensed, the fault-protection circuits isloate the affected sections oft he ac electrical system and illuminate the corresponding amber warning trip light(s) on the flight engineer’s panel.
Additionally, the generator control panel incorporates
A circuit that trips the generator’s GFR and GB to deactivate the generator when the respective fire handle is pulled, and
An autoparalleling circuit that prevents the generator GB from closing during a paralleling attempt unless syncrhonous conditions exist.
The sync-tie bus is provided with a short-protection circuit. A short on the sync-tie bus is sensed by a phase unbalance circuit which automatically trips all generator BTBs simulatneously or trips the external power switch to OFF or APU GB OPEN.
DC ELECTRICAL POWER
GENERATION
The primary source of dc power is supplied by transformer rectifier (TR) units that convert 115V ac into 28V dc. An external TR unit converts external ac power into 28V dc and an APU TR unit converts APU ac power into 28v dc. A separate TR is used as a battery charger.
DISTRIBUTION
TR Units 1 and 2
TR units 1 and 2, are powered from their respective generator load buses.
Each TR supplies 28V dc through ist DC BUS CB to their corresponding dc buses. TRs1 and 2 are interconnected by a current limiter. Therefore, ifo ne dc bus is powered, the other dc bus will be powered.
Essential TR Unit
An essential TR unit, powered from the essential ac bus, provides the primary source of 28V dc tot he essential dc bus.
The essential TR supplies 28V dc through ist ESS DC BUS CB tot he essential dc bus. TR unit 1 and the essential TR are interconnected by the ESS BUS TIE CB and a blocking diode. Therefore, iif dc buses 1 or 2 are powered, the essential dc bus will be powered. The essential TR cannot power dc buses 1 or t2.
External TR Unit
An external TR unit provides 28V dc for external power control. The TR is powered whenever the external power plug is connected and the external power unit is on.
Batter Charger TR Unit
The batter charger TR uit is powered from the 115V ac transfer bus. The 115V ac transfer bus is powered whenever the No. 3 generator load bus is powered or whenever the external power switch is in the GRD SERVICE position. Whenever the battery charger TR is powered, it supplies power tot he hot battery bus and charges the battery. In addition, if the essential dc bus is not powered, the TR also powerst he hot battery transfer bus and, if the BATTERY SWITCH is ON, the battery bus and the battery transfer bus.
BATTERY
A nickel cadmium battery, in the lower 43 compartment is connected directly tot he hot battery bus. Whenever essential dc power is not available, the battery provides dc power tot he hot battery bus, the hot battery transfer bus, and if the battery switch is ON, tot he batter bus and the battery transfer bus.
Control
The BATTERY switch on, or above, the flight engineer’s electrical panel ist he only manual control provided; the rest oft he dc electrical system functions automatically. Dc meters and a selector on the same panel permit checking dc load and voltage.
STANDBY POWER SYSTEM
A standby ypower system is available to provide standby 115V ac tot he standby ac bus and battery power tot he standby dc bus.
If all ac power is lost the ESSENTIAL POWER SOURCE selector is moved to STANDBYm connecting the output oft he standby inverter tot he standby ac bus and the battery tot he standby dc bus.
CAUTION: The inverter is powered from the battery bus and, with loss of ac power will run only if the BATTERY switch is ON.
When standby power is selected, batter power closes the standby power relay and transfers:
Standby ac bus 115V ac input from the essential ac bus tot he inverter,
Standby dc bus 28V dc input from the essential dc bus tot he battery bus.
ESSENTIAL POWER
The essential 115V ac bus is powered directly from any one oft he three engine-driven generators selected by the ESSENTIAL POWER SOURCE selector on the flight engineer’s panel. When selected to APU or EXT PWR, the essential ac bus is powered from ac bus No. 3.
AC. The essential ac bus powers the:
Essential TR.
Essential 28V ac bus.
Essential radio and instrument bus.
Standby ac bus.
DC. The essential TR powers the:
Essential dc bus.
Essential radio and instrument bus.
Standby dc bus.
Battery bus.
Battery transfer bus.
Hot battery transfer bus.
EXTERNAL POWER
External power (115/200V ac) can be connected tot he airplane electrical system through the external power receptacle located on the right side oft he fuselage, forward oft he nose wheel well.
External power is monitored and controlled from the flight engineer’s upper panel. A white AC CONNECTED light illuminates when a cis attached. The EXTERNAL POWER switch controls the circuit.
All the buses are powered when the EXTERNAL POWER switch is ON, all BTBs are closed and the ESSENTIAL POWER SOURCE selector iss et to EXT PWR.
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