IEEE Electrification Magazine - June 2014 - 32

unique challenges of developing and implementing power
electronics and drives in harsh vehicle applications. Field
examples are described in detail to demonstrate the successful implementation of vehicle electrification programs for
heavy-duty off-highway vehicles. the importance of newgeneration wide-bandgap (WBG) technologies is also described to highlight
the emergence of high-temperature
power electronics for heavy-duty, offhighway vehicle applications.
electric drives have been used for
the propulsion systems of large vehicles for decades in the mining and
locomotive industries and have been
very effective in the commercialization of low-volume, extremely highpower vehicles. these applications
typically have used gate turn-off thyristors or silicon-controlled rectifiers.
these devices have good reliability but
have poor switching response, turnoff gain, and efficiency. For mediumpower applications, insulated gate
bipolar transistors (iGBts) are a newer
technology that allow for high-frequency operation, tight and easy control, and significant efficiency gains.
the rising cost of energy and the
falling cost of power electronics have resulted in a revolution in the very cost-competitive automotive industry,
where the power levels are much lower and the volumes
are much higher. today, almost every major car manufacturer is working to bring cost-effective hybrid or electric
vehicles to the market. this has resulted in the growth of
the compact machine for traction applications. the majority of these types of machines are either induction or permanent magnet fed from a standard six-switch inverter.
the greenhouse gas emission norms/standards imposed by
various state and federal governments require that a definite percentage of vehicles sold have either a hybrid or an
electric powertrain. off-highway vehicles, such as those
used for agriculture, construction, and forestry, are not
untouched by the growth and development in the vehicles
for highway applications. however, off-highway vehicles
require a new generation of power electronics and electric
drive technologies to fulfill the high performance expectations under harsh operating conditions and the high reliability goals while reducing fuel consumption and staying
cost competitive. off-highway applications present many
new challenges that need to be overcome. the efforts and
technology used to meet these challenges are resulting in a
new trend in power electronics and electric drives.

drives an electric generator machine that produces
power for a traction electric machine, which provides the
vehicle propulsion. the electric machines are controlled
by power electronic systems, often with a high-voltage
dc link connecting them. the overall architecture is represented in Figure 1.

The E Premium
tractors represent
the first high-power
electrification
approach in a series
production within
agriculture and
already represent a
catalyst for further
electrification in
agricultural
equipment.

electric Drive System Topology
a typical drive system may consist of the following subsystems: an internal-combustion engine (typically diesel)

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

System Structure

the electrical powertrain of the vehicle
consists of multiple inverters and
machines all operating in synchronism
to produce the desired work and
torque profile. the machine types that
are often considered are variable reluctance motors; induction motors; permanent magnet (PM) motors; and
separately excited, fully compensated
dc motors. the choice of electric
machines and power converters
depends on the vehicle's work cycle
and torque/speed requirements. For
example, electric traction applications
may require a high stall torque, a wide
operational speed range, and a constant power range from the traction
motor. the generator machine is optimized for its common operating points
to reduce the size of the machine and
the greenhouse gas emissions from the engine. in the
majority of cases, the generator machine is an ac type. the
generated power is then rectified to create a stable, highvoltage dc bus for the traction drive(s). the traction drives
are inverter-fed, variable-speed machines and have the
ability to change direction, regenerate, and meet the load/
torque demand, resulting in fuel saving during work cycles.
the machine type dictates the type of inverter topology
and control system used as shown in Figures 2 and 3.
there are many possible system architectures, but two
types will be discussed.
1) Single generator/single-traction motor: if the system has a
single generator and a single-traction drive, both
machines and inverters could be identical since they
have similar power ratings. this drives part commonality, which is very beneficial for development, cost, and
service. this architecture does, however, often require
an additional mechanical transmission for adequate
vehicle traction control. the block diagram of this architecture shown in Figure 4 is the one used on the John
Deere 644K hybrid loader described later in this article.
2) Single generator/multiple-traction motor: this system
has multiple traction motors, i.e., four separate
wheel drives, to allow for independent wheel traction control and a single generator. this architecture
may require two different sets of machines and
inverters: one for the higher-power generator and
one for the smaller-traction motors. this introduces

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