Descripcionboeing 727 ata 24 - electrical power

1. LINE AND BASE MAINTENACE TRAINING
ELECTRICAL POWER
B727-200
AIRWAYS CONSULTORIA AERONÁUTICA LTDA

2. AIRWAYS TREINAMENTOS ELECTRICAL POWER
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MATERIAL DIDÁTICO
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ELECTRICAL POWER CONTROL AND MONITORING
Each engine driven generator is connected to its engine through a Constant speed drive (CSD)
mechanism.

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The CSD converts the varying speed of the jet engine gearbox to a constant output speed, so that the
generator will produce current at 400 cps within narrow limits. The CSD oil is cooled by engine fan air ducted
through an air-oil heat exchanger. The cooling effectiveness of the heat comparator which measures the rise in
oil temperature through the CSD.
The load on each generator is automatically regulated by a load controller when the generators are
paralleled. Individual frequency control knobs signal the load controllers which in turn can bias the CSD’s
output. When a generator is operating isolated, rotating its frequency control will result in minor changes in the
output speed of the CSD. This speed change is reflected on the frequency meter. When a generator is
paralleled, its frequency control can effect minor changes in the load change is indicated on the generator’s
KW meter.
When any generator is being paralleled by connecting it to a load bus that is being powered from another
generator, an automatic paralleling circuit delays closing of the generator’s output is in phase with that of the
other generator. The automatic paralleling protection circuit is not available when generators are being
paralleled by closing a bus tie breaker. When this is done, the frequency of the generator to be paralleled must
be adjusted manually with the frequency control.
The voltage regulator automatically controls the generator field strength. The field strength is reflected in
one of two indications. When de generator is operating isolated, a change in field strength results in a voltage
change. When operating in parallel, a change in field strength results in a KVAR load change for the
generator. A malfunction of a voltage regulator would result is variations of the voltage (isolated) or KVAR load
(paralleled).
Transformer rectifier units convert 115 volt 3 phase AC to 28 volts DC. When the DC meters selector is
placed in the ESS TR, TR1, or TR2 position the voltage is measured at the load bus. The ammeter indicates
current leaving the transformer rectifier. With the selector in the BAT position, voltage is measured at the
battery. The ammeter indicates current leaving the battery (minus indication) or charging current (plus
indication).

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MAIN POWER DISTRIBUTION
AC power developed by three generators is fed to their respective load busses. The synchronous bus
provides the means for connecting the load busses together and operating all generators in parallel. This
provides a means for powering all busses if one or two generators are inoperative. When APU or external
power is utilized power is supplied to the synchronous bus and thus all the load busses.

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APU ELECTRICAL POWER
When the APU is operating placing the APU generator breaker switch to close will provide power to the
synchronous bus. The amount of load being carried by the APU or external power source is indicated on the
AC ammeter on the APU panel. The electrical system will not accept power of incorrect voltage or frequency
and gives priority to the power system being selected.

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ESSENTIAL POWER
The essential AC bus can receive power from any generator; the essential DC bus normally is powered
by the essential AC bus via a T/R unit. Curtains items which are essential to the operation of the airplane are
powered by these busses. Essential power is obtained at a point between the generator and the generator
breaker. With the Nº 3 bus tie breaker closed, the APU or external power will furnish essential power through
the synchronous bus to load bus Nº 3. For ground operation, normal power transfers from the APU to other
sources can be made without tripping essential power because of a holding relay. DC power is developed by
TR units that are fed by AC load busses Nº 1 and 2 or by the essential AC bus. The essential AC and DC
busses normally feed the standby AC and DC busses.

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BATTERY AND STANDBY POWER
The battery bus has two possible sources of DC power: the battery (through the hot battery bus and the
battery switch) or the essential DC bus. Under normal conditions the essential DC bus will provide power for
the battery bus, disconnecting it from the battery. However, neither the battery nor the battery bus can ever
supply essential DC. With the loss of essential DC, the battery will supply the battery bus if the battery switch
is ON. The battery bus in turn is one of the two possible sources of power for the standby AC and DC busses.
The standby power system is used to supply 115-volt, 400-Hz power to critical communication and
navigation equipment required to maintain safe flight when no other AC power is available on the airplane.
With standby power available, the Captain will have attitude and heading information. The standby power
system contains an AC and a DC bus.
Under normal operating conditions, the standby AC and DC busses receive power from essential AC and
DC. However, whenever the essential power selector is placed in the STANDBY position, the standby busses
are disconnected from the essential busses and the battery bus supplies standby DC directly and standby AC
through the static inverter. In the event of complete loss of AC power the battery could supply the battery bus
and standby busses for a minimum of 30 minutes.

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BATTERY CHARGER
The battery charger will be operative anytime AC power is available to the charger. The charging rate will
slowly decrease until the battery becomes fully charged. Then the battery receives a pulsing charge for about
two minutes. After pulsing, the charge rate will drop to zero until the battery discharges enough to require
further charging. The battery charger will be powered from the AC transfer bus anytime the load bus Nº 3 is
powered or the external power switch is in the GRD SERVICE position with ground power available.

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