IEEE Electrification Magazine - December 2014 - 24

subscale experimental system to imitate the entire power system from the turbine engine to the propulsive fans. First, we
built a system to mimic a gas turbine engine driving a generator, consisting of two permanent-magnet (PM) motors with
brushless motor drives, coupled by a shaft. We programmed
the first motor and its drive to mimic the speed-torque characteristic of the gas turbine engine, while the second motor
and drive act as a generator and produce a torque load on the
first motor. Then we built another system of two PM motors
and drives to simulate a motor driving a propulsive fan. We
programmed the first motor and drive to mimic a woundrotor synchronous motor. The propulsive fan was emulated by
implementing fan maps and flight conditions into the fourth
motor and drive, which produce a torque load on the driving
motor. The stator of each radial flux PM motor is designed to
travel axially to change the coupling between the rotor and
stator. This feature allows the PM
motor to more closely emulate a
wound-rotor synchronous machine.
These techniques can convert the
plain motor system into a unique TeDP
power grid emulator that enables realtime simulation performance using
hardware-in-the-loop (HIL).

Turboelectric Transmissions

A TeDP system using
all ten motors and
drives with closedloop controllers
makes it possible
to simulate a
representative
part of the TeDP
powertrain.

A turboelectric transmission uses
electric generators to convert
mechanical energy from a turboshaft
engine into electrical energy and electric motors to convert it back into
mechanical energy to power the drive
shafts of propulsion fans. The world's
first turboelectric drive was introduced in the U.S. capital ship design of
the USS New Mexico, launched in 1917
(Figure 1). The extra weight of the turboelectric plants has
since made their use prohibitive. But the concept has been
revived for future warship design as a way to provide fuelefficient, survivable, and reconfigurable electrical power for
propulsion systems. Thus, its application has been extended
to ships (steam and, more recently, gas turbines) and some
rail locomotives with gas turbines, e.g., the first train à grande
vitesse (TGV)-French for "high-speed train." An advantage of
the turboelectric drive is that it allows the adaptation of highspeed turboshaft engines to the slowly rotating propellers or
wheels without the need for a heavy and complex gearbox. It
also has the advantage of being able to provide electricity to
the ship or train's electrical systems, such as lighting, computers, radar, and communication equipment.

current research at the NASA
Glenn research center
Recently, the NASA Glenn Research Center, in support of the
Subsonic Fixed Wing (SFW) Project, has begun the evaluation
of new technologies to make large reductions in aircraft

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

energy use, emissions, and noise by 2040. One concept is a
future hybrid wing-body electric airplane employing a TeDP
system (see Figure 2). This aircraft is powered by two turboshaft engines driving two superconducting generators, which
provide electricity to run 15 electric fans mounted on top of
the fuselage. The primary function of the two wingtipmounted superconducting turbogenerators is to make electricity, not thrust. The nozzle of the turbogenerator is sized so
there is enough jet velocity during cruising to produce a small
amount of net thrust to avoid being a source of drag. The turbogenerators are located on the wingtips so the inlets ingest
freestream air. Most of the energy of the gas stream is
extracted by the power turbine to drive the generator. The
electric power from the turbogenerators is distributed along
redundant superconducting electrical cables to an array of
superconducting motor-driven fans. The fans are in a continuous array of propulsors spanning the
entire upper trailing edge of the center
wing-body section. One TeDP system
study on this aircraft estimated that
cryogenic and superconducting turboelectric propulsion systems can be
lightweight and highly efficient, resulting in at least 10% overall net fuel burn
savings before iterating the aircraft sizing. Very recently, Glenn's researchers
have started looking at the development of room-temperature high-power
density motors. In parallel to and in
cooperation with the analytical system
modeling and analyses for both cryogenic and room-temperature applications, it is essential to design a subscale
system to simulate the TeDP power grid
to facilitate in-depth understanding of
the system principles and to validate
system model analyses and performance predictions. Hence,
this article presents research in the development of a propulsion electric grid simulator (PEGS) pursuant to the SFW Project's goal of "greenly" powering future TeDP aircraft. This simulator, composed of relatively small and inexpensive components, would enable rapid analysis and demonstration of the
proof-of-concept of the TeDP electrical system and allow
experimental characterization of components.
A literature survey was done first to investigate similar
research on an electrical-machine-based simulator for a
power grid. There have been some studies on wind-turbine
simulators based on induction motors and on PM synchronous machines. A voltage source converter and vector control were used to simulate the torque characteristics of the
wind turbine. Several researchers have also shown that a
brushless dc motor using direct torque control can serve as
the prime mover of a wind-turbine simulator because it is
easy to control and its dynamic characteristics are excellent.
Also, to provide an economical and flexible testing platform
for the development of generators, several gas turbine

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