IEEE Electrification Magazine - March 2016 - 35

Among various
activities to achieve
the goal of fuel
economy
improvement, vehicle
electrification is
considered to be
an important one.

ac power through inverters, and ac-
dc power through controlled or passive rectification process, at different amplitude and frequency levels
(Mohan et al.). For certain applications involving low and high dc bus
voltages, e.g., 28 V up to 600 V or vice
versa, dc-to-dc power conversion is
needed. Power electronics switches
are also required for vehicle to utility-level (ac) power translation, in
case the vehicle is interfaced with a
utilit y g r id or used in a local
microgrid (Ustun et al., 2013) environment, which can be useful to
meet the power needs of a temporary military base in
the area of operation.
The efficiency of the currently available silicon (Si)based power electronics is high (95% or higher). However,
their tolerance of elevated temperatures is limited. This is
a very important consideration in a military environment,
which can be harsher than regular commercial applications. Hence, adequate cooling is critical to keep the junction temperatures of the solid-state devices within thermal limits (Bennion and Kelly, 2009). The requirement of a
relatively large cooling system increases the overall system size and weight. The cooling burden can be reduced
to a great extent by using high-operating-temperature
switches based on materials with higher temperature
capability, e.g., SiC and gallium nitride, which are also
known as wide-bandgap (WBG) semiconductors (Shenai et
al., 2014; Shenai, 2000).
Although this article includes several separate sections
that could be stand-alone articles, they are all connected
through a common thread involving power electronics,
converters, and also electric drives and generators as end
applications, all ultimately intended toward the implementation of more electrification in military vehicles and
other military equipment.
The activities in the military in general, and U.S. Army
RDECOM-TARDEC in particular, pertaining to electrification include but are not limited to the following:
xx
the HERMIT project, which allows system hardware
testing in a vehicle platform by simulating real-life
field conditions in the laboratory
xx
SiC-based converters
xx
dual-voltage ISG.

Facilitating the Path Toward Vehicular
Electrification with a Generic Test Bed
HERMIT was designed and built to provide a laboratory
vehicle system environment for power and hybrid electric
propulsion systems testing. The propulsion system consists of an engine-driven generator and energy-storage
battery pack. Both the generator and battery provide power
to traction motors and other auxiliary systems through

power-conversion devices, replicating
a field vehicle environment. A systemlevel overview diagram of HERMIT is
shown in Figure 1, and Figure 2 shows
the test bed.
The objective of the HERMIT project
was to study the performance, component interactions, system architecture
with different topologies, and grounding and shielding as well as test
advanced components such as SiC
power-electronics-based traction
motors, inverters, SiC-based dc-dc converters, motors and generators, and
cooling systems.
Major subsystems in the HERMIT system shown in
Figure 1 include a twin turbo-charged 335-kW diesel
engine, a permanent-magnet (PM) generator (three phase,
two windings per phase, 335 kW), dual 250-kW inverters,
traction induction motors (600 V, 410 kW), traction motor
inverters (850-kW maximum power, 600 V), and a 100-kW
dc-dc converter (300-600 V) (Saxon et al., 2007).
HERMIT provides the capability to replicate the behavior of a vehicle by using dynamometers to simulate vehicle mobility (traction) loads on the system. In a real experiment, it was connected to a dc dynamometer, which was
used to replicate the loads encountered by the actual vehicle on the U.S. Army's Aberdeen Proving Grounds courses.
The HERMIT generator system is a special kind of generator, as shown in Figure 3. It is a surface mounted, axial gap
PM machine with two sets of windings, each connected in
a Y configuration to its own inverter.

Generator Inverter
The generator inverter is shown in Figure 4. This converter can be used to translate and regulate an external dc
voltage to ac and also translate the generator ac voltage
to dc. Unlike a passive diode bridge, this method allows
the power flow to be controlled at dc voltages significantly above the line-to-line open circuit voltage (boost
mode), thus allowing the engine/generator set to provide
power at very low r/min. This is commonly accomplished
by a pulsewidth modulation (PWM) algorithm using conventional insulated-gate bipolar transistor (IGBT)/diode,
hard-switched inverter hardware via which it is possible
to gate the IGBTs and boost the ac voltage of the
machine. Another item of interest is that, in addition to
the PWM algorithm, a new, more efficient algorithm,
boost phase control (BPC) (Kajs et al., 2009), was used in
HERMIT. BPC reduces the switching frequency of the
hard-switched inverter to the fundamental frequency of
the generator and results in similar frequency content of
the machine, like silicon control rectifier or diode rectification. Since BPC reduces the switching frequency, the
switching losses are also reduced, resulting in higher
efficiency. Additionally, since the switching losses are
IEEE Electrific ation Magazine / March 2 0 1 6

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2016