IEEE Electrification Magazine - December 2017 - 26

provide solutions for electrification development of aircraft systems in terms of enhancing the electric powergeneration capacity of the aircraft; reducing the hardware
requirement of the main-engine electric power generation and distribution system; and managing the high
peak and regenerative power flow from the electrohydrostatic actuator/electromechanical actuators (EHA/EMAs).

Background
For the last 30 years, significant progress has been made
in the development of electrification for nonpropulsive
power systems in an aircraft, known as the MEA initiatives, in which the existing aircraft subsystems have
been improved in terms of system weight, volume, complexity, reliability, and maintenance requirements. Such
advancements can be recognized in the adoption of
more electric engine (MEE) concepts, with significant
effort to reduce or eliminate the centralized hydraulic
systems on board. As a result, the EPS in MEA is continuously growing.
The intent of MEA is to replace the pneumatic- and
hydraulic-powered subsystems, i.e., engine start system,
environmental control system (ECS), wing anti-icing system, actuation system, and so on, with the electrical systems as much as possible. As a result, there is an
increase in electrical demand that is to be supplied from
the engine-driven generator. The effect of electrical
power offtake can sometimes have a significant impact
on the dynamics and control of the aircraft engine.
Hence, it is necessary to share the electric power demand
from the loads among multiple spools of the main
engine. Paralleling generators in either a constant frequency or variable frequency (VF) ac primary power generation system are problematic. To mitigate this problem,
dc primary generation systems have been proposed
where the generators can be paralleled on a main dc bus.
However, the ac power from the generator is supplied to
the frequency-insensitive ac loads and transmitted
through a two-stage ac-dc-ac conversion. Such an arrangement adds extra losses and additional hardware to the
system. Neither an ac nor a dc primary generation system is able to meet all of the power requirements with
optimized performance in terms of volume, weight, efficiency, reliability, and cost. Major changes in electrical
power generation and distribution architecture are required
to combine the advantages and address the shortcomings of both types of systems.
The flight control actuation system is one of the most
critical loads in an aircraft. Replacing the centralized
hydraulic system in traditional aircraft with a decentralized actuation system offers reduced weight, volume, production, and maintenance costs with the same level of
safety performance for the aircraft. This replacement
requires substituting hydraulic actuators with either
EHAs or EMAs; however, such a substitution demands
cautious investigation of the high peak and regenerative

I E E E E l e c t r i f i cati o n M a gaz ine / DECEMBER 2017

power management issues. The regenerative energy can
be dissipated by employing resistor banks and associated
cooling devices or restored by using electrical energy storage elements (ESEs) such as ultracapacitors and batteries
to enable energy recovery. Both of these methods require
considerable additional hardware. Returning the regenerative energy to the power source(s) requires minimal
hardware; however, without a separate electric actuation
bus, securing the operation of the main aircraft electrical
power grid within the limits of the specified standards is
quite a challenging task. A new power generation and
management system architecture is necessary to leverage
the existing components in the aircraft for mechanical
energy storage.
The aforementioned technical challenges make optimizing the performance of the new EPS in terms of electric power generation capacity, reliability, fault-tolerance,
size, weight, efficiency, and cost a difficult task. The type
of electric generator also has great impact on the fundamentals of the system architecture and optimization. A
wound-field synchronous generator (WFSG) is the mainengine generator in the most recent commercial transport MEA (e.g., Boeing 787 and Airbus 380); however,
operating WFSGs requires an external brushless exciter,
which limits the maximum generator shaft speed. PM
synchronous generators, a popular solution in recently
published literature (see Sarlioglu and Morris 2015 and
indexed literature therein), do not meet the aerospace
requirements to be used as a main electric power generator in a commercial aircraft due to its vulnerability to
winding short-circuit fault and concerns for potential
corrosion and demagnetization. Using an induction generator for MEA application is not widely reported in the
literature because the excited rotor cage makes induction generators inherently less compact and efficient
compared to PM generators. (However, an induction generator qualifies as a fail-safe machine in an aircraft.) As
a self-excited generator, an induction generator (unlike a
WFSG) does not require external excitation. In addition,
induction generator efficiency is higher as compared to
a WFSG. The adoption of induction generators by aircraft
EPSs can enable new EPS architectures with fewer fundamental limitations.
This article explores and evaluates the option of using
an induction generator for the distributed EPS of MEA.
Induction generator-based electrical power generation
and management system architectures are developed for
both the main-engine generation system and APU system.
Two induction machines are used to develop an ac/dc
hybrid electric power-generation system for main-engine
electrical power generation. In this system architecture, a
high-speed induction starter/generator and a low-speed
induction generator are installed on the high-pressure
(HP) and low-pressure (LP) spools of the engine, respectively. In the generating mode of operation, all of the constant voltage VF (CVVF) ac power is generated by the HP

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2017