IEEE Electrification Magazine - December 2014 - 7

development, particularly in power electronics. Electrical
systems can now be used for aircraft actuation systems,
wing ice protection systems, environmental control systems, and fuel pumping. The use and versatility of electrical
energy means that these new systems are enabling future
aircraft to be quieter and more fuel efficient improving the
environment for everyone as well as
reducing maintenance costs.
Power electronics is one of the
most important enabling technologies for the More Electric Aircraft.
Without the use of power electronics to convert and control electrical
energy, none of the benefits of the
More Electric Aircraft would be possible. However, aerospace applications present some challenging
operating conditions for power electronics, and there are still a number
of areas where improvements must
be made in terms of the weight, volume, cost, and reliability of power
converters and their associated systems. The operating environment
on an aircraft can be very harsh, and
the hours of operation and expected
lifetime are long in comparison to
many industrial and automotive
applications.

and weight gains should be possible if just one power
source is used for all systems. The chosen single source is
electrical power, as it has a number of advantages in
terms of flexibility and range of applications, hence the
More Electric Aircraft concept. A possible power system
concept for the More Electric Aircraft is shown in Figure 2,
showing the goal of requiring all of
the onboard loads to take their
power from an electrical system.
The aim of this historical step
change in technology is to reduce
operating costs, fuel consumption,
and the environmental impact of
future air travel. The removal of the
pneumatic system eliminates the
need for a bleed air system on the
gas turbine, which leads to a significant improvement in the efficiency
of the turbine. The removal of the
mechanical and hydraulic systems
may also lead to some reductions
in the overall weight at a system
level once the full potential of the
More Electric Aircraft technology
has been realized.
However, more importantly, electrical systems offer far more options
for reconfigurability as well as for
advanced prognostics and diagnostics. These prognostics
and diagnostics systems could help improve aircraft availability and reduce the need for unscheduled maintenance.
There is a potential for equipment on an aircraft to monitor
its use and environmental conditions and use profiles of its
own failure mechanisms to predict end of life, leading to preventative maintenance based on usage rather than simply
age or flying hours.

The removal of the
pneumatic system
removes the need
for a bleed air
system on the
gas turbine, which
leads to a significant
improvement in
the efficiency of
the turbine.

The more electric Aircraft concept
The main source of power for subsystems on large aircraft
is from the gas turbine engines, as shown in Figure 1. While
the gas turbines are primarily used to provide the thrust
needed for the flight of the aircraft, they must also be able
to provide the power sources required for all other loads on
the plane. Figure 1 shows the power sources and loads for a
typical large aircraft power system.
In a conventional large civilian aircraft, there are typically four sources of power derived from the engines: electrical, pneumatic, hydraulic, and mechanical. The electrical system is used to power loads such as the avionics systems, lighting, and in-flight entertainment. The pneumatic
system, taken as an air off-take from the engine, provides
for loads such as the cabin pressurization and air-conditioning as well as wing ice protection. The mechanical system is used for fuel and oil pumping, mainly local to the
engine, and a hydraulic system is used for most of the aircraft actuation systems, both for flight-control and auxiliary systems. The total maximum power level of these
systems is usually in excess of 1 MW, a large power system for a relatively small platform.
Having four separate power systems on an aircraft is a
design that has evolved over time, each system being suitable for their dedicated loads. However, when the aircraft
is considered as a whole, it can be shown that efficiency

more electric Aircraft Power Systems
A number of different voltage standards exist for the electrical system on large civilian aircraft:
xx
28 V dc-low power loads/avionics on large aircraft
and complete electrical system on small aircraft
xx
270 V dc ( ! 135 V)-military aircraft and some subsystems on some larger aircraft
xx
115 V ac at 400 Hz-larger loads on large civilian aircraft.
The use of the More Electric Aircraft concept obviously
puts a significantly larger load on the electrical system.
The typical installed capacity of the electrical system on
an existing medium-range aircraft, such as a Boeing 737, is
about 100 kW; and for the Boeing 787, this power level
increases to more than 1 MW. To reduce the current in the
electrical system and, hence, the cable weight, higher voltage electrical systems are considered. Some of the emerging standards include:
xx
540 V dc (! 270 V)
xx
230 V ac at 400 Hz
IEEE Elec trific ation Magazine / d ec em be r 2 0 1 4

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2014