IEEE Electrification Magazine - December 2014 - 14

toward hybrid electric technology. In those cases, the main
propulsion is performed by electric drives. This demonstrates that substantial demand has arisen for improved
electric power-generation performance. Future space vehicles will require electric drives for thrust-vector and flightcontrol actuation, demanding more high-quality electric
power. These systems must be more robust and will offer
greatly reduced operating costs and improved safety compared with Space Shuttle hardware.
These recent aerospace trends have created a significant
increase in EPGS power demand. This leads to increased
operating voltages and reduced system losses, weight, and
volume. A new set of power-quality and electromagnetic
interference (EMI) requirements has been created to satisfy
both quality and performance issues. The result has been a
significant increase in the number of installed EMs, creating challenges for using this equipment on the new platforms. Therefore, overall system performance improvement and power-density increases are necessary for the
new-generation machines. Cost is an additional driver that
must be addressed to make the new
platforms affordable. This article
addresses EMs for high-performance
generators primarily for high-speed,
gearless, oil-free integrations as a part
of auxiliary power units (APUs), power
and thermal management systems
(PTMSs), and emergency power units.
However, some of the conclusions
could be extended to main-engineembedded EPGSs. The literature provides an excellent historical perspective of the integration of various EMs
with engines.
A method for EM selection for
EPGSs is discussed in this article.
Examples of high-speed, high-performance generator applications are
shown. A system approach is used for overall EM selection
and optimization. The presented material is a synopsis of
the extensive engineering experience accumulated at Honeywell International. Other examples from the literature
show that a similar approach has been applied to EMs used
in high-performance electric drives.

with their integration into electrical and mechanical systems. The availability of low-cost power electronics and
advances in nonconventional bearing systems, along with
more efficient cooling methods, has boosted EMs toward
better performance.

Output Equation of an Electric Machine
An optimal machine design process starts from a good
understanding of the machine variables expressed by (1).
This equation represents the output power of a machine
as a function of the basic geometry and electromagnetic
characteristics.
P = C # B L # AC # D 2 # L # N.

(1)

P is the electrical power generated at the EM terminals;
N is the shaft speed; L is the length of the magnetically
active machine; D is the rotor (or air-gap) diameter; AC is the
specific electric loading, which is the armature ampere conductors per unit length of armature periphery at the air gap;
B L is the machine magnetic loading,
which represents the average flux density over the air gap; and C is a
machine constant dependent on various factors such as rotor pole numbers
and stator winding arrangements. The
product D 2 # L represents the machine
air-gap volume and is a measure of the
used active magnetic materials.
It is logical that the total weight and
volume of an EM is proportional to the
air-gap volume. Therefore, provisions
for minimizing this number must be
applied aggressively. The output equation shows that increasing the speed
N is one of the most powerful provisions for weight and volume savings.
However, certain control and mechanical limitations should be considered. Increases of B L and AC
are typically associated with the use of more expensive
materials and/or penalties to the cooling system. Therefore,
these provisions must be carefully applied to achieve the
required performance and cost targets. Also, overall system
efficiency can be affected. Nevertheless, aggressive research
for materials improvement is in progress.

Future space vehicles
will require electric
drives for thrustvector and flightcontrol actuation,
demanding more
high-quality electric
power.

electric machines for electric
Power-Generation Systems

Integration Considerations for Electric Machines

EM history is more than 100 years old. Most EMs used today
were invented in the beginning of the electrification era,
when EM theory was created. Over the years, substantial
improvements have been achieved, primarily in the area of
magnetic materials and bearing systems. Most recently,
permanent-magnet machines (PMMs) have made substantial advancements due to the development of stronger
and less-expensive magnets. The EM improvements as
a part of an EPGS must be considered in conjunction

High-performance machines for aerospace applications are
typically integrated into power-generation systems or used for
driving high-speed compressors, fans, pumps, and actuators.
Operating speeds cover a broad range, from 5,000 to
200,000 r/min. Increasing speed is one of the most powerful
elements for improving performance, as demonstrated in the
previous section. However, increasing speed presents
mechanical integration complications. Among these complications are rotor-dynamic behaviors related to the phenomenon

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

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