IEEE Electrification Magazine - June 2014 - 58

Active
Rectifier
Lb
Lc

dc-dc
Converter

Vb
Vc

dc Bus

PWM
Inverter

Turbine

Three-Phase
Generator

Load
PM

Figure 11. A typical dc distribution system with ac and dc loads.

electric Machines
Several types of electric machine technologies have been
investigated for automotive propulsion and for aircraft
power generation. These include induction, PM, switchedreluctance, and axial gap machines. Most of the commercially available hybrid vehicles use either induction or PM
machines for electric propulsion. For both EVs/HEVs and
MEAs, the electric machine requirements include high
power density, wide speed range, wide operating temperature range, low volume, rugged structure, reliability, and ease
of control. Today, the interior PM (IPM) synchronous motor is
widely used in automotive propulsion because of its high
efficiency, high torque, high power density, and relative ease
of field weakening operation. The Toyota Prius, Ford Escape,
and Chevy Volt all use IPM machines for propulsion. The
three-phase wound field synchronous generator has been
traditionally used for power generation in aerospace applications. This is a high-performance machine and is inherently
safe. The field can be de-energized in case of overvoltage or
overspeed. The advantages of a wound field synchronous
machine are that it is a known technology in aerospace
applications, the voltage can be easily controlled by controlling the field, the power electronics and control requirements
are fairly well understood, and it is robust.
The PM machines are not inherently fault tolerant. A
key requirement for many aircraft and automotive generation systems is a degree of fault tolerance, i.e., an ability to
continue operating at or near rated power in the event of a
single point fault in the electric machines or its associated
power converter. One way of achieving fault tolerance is to
isolate the phase windings physically, magnetically, and
electrically. These can be three-phase or higher-phase
machines, where each phase of the machine is fed
separately by a single-phase bridge inverter/converter. Consequently, a fault in one phase will not readily propagate to
the adjacent phases. In addition, each phase winding is
designed to have one per-unit self-reactance to limit the
short-circuit current at fault conditions to its rated value.

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

Furthermore, by employing a multiphase (usually more
than four-phase) design, the machine can continue to provide useful power output even with a fault in one of the
phases. Several studies have reported the possibility of
using five-phase fault-tolerant PM machines for embedded
aircraft generator applications, which could also be used in
automotive applications.
Recently, the concentrated winding PM (CWPM) machine
has begun gaining importance. Some automotive companies and researchers are working to further improve its performance. Honda is already using one form of this type of
machine in its hybrid vehicles. The concentrated winding
offers shortened machine length, increased electric loading
to increase torque density, wide constant-power speed
range, improved machine fault tolerance, and increased
machine modularity. The CWPM machine is more fault
tolerant for interwinding short circuits than the regular PM
machines. These concentrated winding machines with
high power density and shorter diameter (required for the
convenience of mounting) could be applied for aircraft
engine starting and power generation applications.
Although induction motors are used for automotive
electric propulsion, induction generators have not been
used in aircraft power generation. They are robust, have
fault tolerant capability, and can operate at high temperatures. With active rectifier and field orientation control, the
use of an induction machine as an embedded starter/generator needs to be further explored for aircraft applications.
A few companies have developed high-efficiency induction
machines for the U.S. Air Force to provide a compact, efficient generator system for stable and safe operation that
also provides the electric start function. Special cooling
techniques have been incorporated to achieve optimum
operation under various load conditions.
Automotive companies have made several attempts
starting from early 1990s to use the switched-reluctance
motors (SRMs) for EV propulsion. Because of the acoustic
noise and torque pulsations associated with SRMs and the

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