IEEE Electrification Magazine - June 2014 - 39

3) power density
4) cooling requirements
5) reuse of already available components
6) efficiency
7) safety.
For off-highway applications and vehicle platforms, it is
very important that the vehicle is versatile to cover various
tasks and functions. For example, the vehicle needs to have
export power for the tools and implements used in the farm field. John
Deere's concept of an infinite-variable
electrical-driven Pto can be obtained
using the Web sites and references listed in the "For Further reading" section.

WBG Technology for HeavyDuty
Vehicle electrification

the increased value of the base speed results in a higher
energy density of the electric motor used in the traction
system and wider constant torque-speed and constant
power regions. Wider constant torque-speed and constant
power regions open up some new applications including
the direct drive system. therefore, the gear system could be
completely eliminated, simplifying the overall drivetrain for
the vehicle traction system. With a higher dc bus voltage, a
higher base speed is possible, which
allows simpler gear systems (twospeed possible versus three-speed
mostly used) to cover a wider speed
range. the higher dc bus voltage at the
inverter offers an increased available
voltage margin, leading to a better
machine control system. With the
higher dc bus voltage at the inverter,
the switching duty cycle of the power
devices could be maintained at
between 30 and 70%. this will result in
better control, improved power quality,
and reduced losses in the electric
machine due to a reduction of the total
harmonic distortion in the motor currents. therefore, it is stated that WBG
power devices could result in an electric drivetrain, which is more efficient,
reduced in size, lower in cost, and
saves more fuel, resulting in far fewer
co2, noX, and soX emissions and particulate matter coming out of vehicle exhaust systems.

Energy density (and
the resulting system
costs) becomes a
very important
design criterion in
vehicle optimization
where a reduction of
the operating
temperature is an
objective function.

WBG power devices, particularly silicon carbide (sic) devices, have some
unique properties that are absent in
silicon (si) power devices. these are:
1) the ability to operate at higher
voltage
2) the ability to switch at higher frequencies
3) the ability to operate at higher
junction temperatures
4) about three times higher thermal
conductivity: 5 W/cm-K (sic) versus 1.5 W/cm-K (si).
these unique properties of sic devices not only offer distinct advantages but also enable some desired functions of
the vehicles that are not possible with the conventional si
power devices. some key points are discussed in the following sections.

WBG Ability to Switch
at Higher Frequencies

WBG Ability to Operate at Higher Voltage

this unique property of WBG power devices offers distinct
advantages that are not possible with si power devices.
Using WBG devices in vehicle electric drivetrain systems, the

in WBG devices, the switching losses do not increase exponentially as the dc bus voltage of the inverter rises; this
allows the electric drivetrain to enjoy easier operation and
fewer thermal management challenges. at a given power
(kilowatts) rating, the higher dc bus voltage of the inverter
enables cable size reduction from 3-aught to 1-aught,
resulting in significant copper savings and an overall
weight reduction of the electric drivetrain. at higher power
(kilowatts), the higher dc bus voltage eliminates the need
for inverter paralleling, resulting in a simpler electric drive
powertrain from an eMc, electromagnetic interference
susceptibility, and vehicle-level simplification standpoint. it
is expected that for inverter operation at a higher dc bus voltage, an increased insulation level of the electric machine is
required; however, this would not be difficult to realize. With
a higher dc bus voltage, a higher base speed for the traction
drive is possible; therefore, an enlarged region of constant
torque is available for the demanding applications and environments in which the vehicle needs to operate.

Figure 12. The 8500 E-Cut hybrid mower.

IEEE Electrific ation Magazine / j une 2 0 1 4

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