IEEE Electrification Magazine - December 2014 - 10

Va
Variable-Speed
Shaft
Constant-Output
Speed Gearbox

Constant-Speed
Shaft

Generator

Three Phase
400 Hz, 115 V

(a)
Variable-Speed
Va
Shaft

Power Converter
(ac/ac)

Generator

Three Phase
400 Hz, 115 V

(b)
Variable-Speed
Shaft

Three Phase
320-800 Hz
230 or 115 V

Generator

(c)
Figure 4. (a) A constant-frequency generation system using a constant output speed mechanical gearbox. (b) A constant-frequency generation
system using a power converter. (c) A variable-frequency generation system.

system. The ac/ac power converter can be realized using a
number of different power converter topologies including
matrix converters, cycloconverters, or back-to-back inverters.
This system design has the advantage that no gearbox is
needed between the gas turbine shaft and the generator;
however, the disadvantage is that this power converter must
process all the generated power and,
therefore, must have the full power
rating and high reliability to get the
required level of safety from the aircraft design. Unfortunately, power
electronics has not yet reached the
levels of reliability to make this option
viable, and it remains a rarely chosen
configuration despite being considered, and occasionally used, over the
last 30 years.
If the electrical system and associated loads can be designed to operate with a variable frequency, then it
is possible to connect a generator
directly to a shaft in the gas turbine
and the electrical output directly to
the aircraft's electrical system, as
shown in Figure 4(c). The electrical
output of the generator provides a variable-frequency
supply with the frequency related to the speed of the gas
turbine, typically in the range from 320 to 800 Hz. The
advantage of this variable-frequency system is the direct
connection between the generator output and the electrical power system, giving a simple and potentially very
reliable configuration. The disadvantage is that nearly all

aircraft loads will require power converters for control, as
the variable-frequency supply cannot be used directly for
most applications. However, many applications, actuators, for example, require this power conversion stage for
control even when operated from a fixed frequency supply. Having many distributed power converters gives a lot
more options for a safe aircraft system design as redundancy can be
built in at the systems level, avoiding
any single points of failure within
the design.

The use and
versatility of
electrical energy
means that these
new systems are
enabling future
aircraft to be quieter
and more fuel
efficient.

I E E E E l e c t r i f i c ati o n M agaz ine / december 2014

Actuation Loads

On modern aircraft, hydraulic actuators are used to move the control
surfaces to the control plane. Three
degrees of control are critical for
flight: roll, pitch, and yaw of the
plane. These flight-critical control
surfaces are the rudder, ailerons, and
elevators and are referred to as the
primary flight-control actuators.
Other control surfaces, such as flaps
and slats, are not critical for flight,
and, therefore, the actuators for
these surfaces are referred to as secondary actuators.
These surfaces are useful for the comfort and efficiency of
flight but the aircraft can be flown without these secondary control surfaces if needed.
When replacing hydraulic actuators with electrically
powered actuators, the most obvious choice is to use an
EMA, as shown in Figure 5. Using an EMA system, an aircraft

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