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why 400Hz is used for aircraft systems ?
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Answer Posted / adrin
As aircraft fly higher, faster and grow larger, the services
that the power supply has to satisfy also grow more complex.
In civil aircraft this means more power to the galley units,
environmental control and passenger entertainment systems,
while military aircraft require more power sensors and
weapon systems. Both have increased power demands for
actuators, lighting systems, avionics and heating.
There are several different power sources on aircraft to
power the aircraft electrical systems. These power sources
include: engine driven AC generators, auxiliary power units
(APUs), external power and ram air turbines.
The primary function of an aircraft electrical system is to
generate, regulate and distribute electrical power
throughout the aircraft. The aircraft electrical power
system is used to operate (a) aircraft flight instruments,
(b) essential systems such as anti-icing etc. and (c)
passenger services. Essential power is power that the
aircraft needs to be able to continue safe operation.
Passenger services power is the power that is used for cabin
lighting, operation of entertainment systems and preparation
of food.
Aircraft electrical components operate on many different
voltages both AC and DC. However, most of the aircraft
systems use 115 volts (V) AC at 400 hertz (Hz) or 28 volts
DC. 26 volts AC is also used in some aircraft for lighting
purposes. DC power is generally provided by “self-exciting”
generators containing electromagnetics, where the power is
generated by a commutator which regulates the output voltage
of 28 volts DC. AC power, normally at a phase voltage of
115 V, is generated by an alternator, generally in a
three-phase system and at a frequency of 400 Hz.
Higher than usual frequencies, such as 400 Hz, offer several
advantages over 60 Hz – notably in allowing smaller, lighter
power supplies to be used for military hardware, commercial
aircraft operations and computer applications. As aircraft
space is at a premium and weight is critical to aircraft
engine thrust and fuel burn (and thus the aircraft range and
engine horsepower per pound), 115 volts at 400 Hz offers a
distinct advantage and is much better than the usual 60 Hz
used in utility power generation.
However, higher frequencies are also more sensitive to
voltage drop problems. There are two types of drops:
resistive and reactive. Resistive losses are a function of
current flowing through a conductor with respect to the
length and size of the conductor. This is the most
important factor in controlling resistive power loss and
applies regardless of frequency. The short transmission
range of higher frequencies is not a factor in most airborne
applications.
Reactive drops, on the other hand, are caused by the
inductive properties of the conductor. Reactive drops are a
function of both cable length and the AC frequency flowing
through the conductor. With high frequencies such as 400 Hz,
reactive drops are up to seven times greater at 60 Hz.
This raises an interesting question: can you run a 400 Hz
device at 60 Hz.? If you try this, smoke and fire are
certain to result. The lower winding inductance draws a much
higher current at a set voltage, saturates the iron, and
burns up. However, there is a simple workaround using
fundamental principles of flux density. A 400 Hz device will
usually run just fine on 60 Hz if you lower the voltage to
60/400ths or 0.15. The same current will produce the same
magnetic flux, and the device will operate happily.
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