IEEE Electrification Magazine - December 2014 - 15

of critical speeds. This applies not only to EMs but also to the
entire rotating group-machine, engine, and loads. The magnitude of the air gap also plays an important role. The transition to foil and magnetic bearings has created excellent opportunities for increasing speed and improving reliability.
The highest-speed applications may reach as high as
250,000 r/min. The conventional metal or ceramic bearings,
however, do not work at these speeds. Foil and magnetic
bearing systems are most optimally used in these applications. The acceleration and deceleration time may not be
critical. Hence, the high torque-to-inertia ratio is not
required in most applications. A ratio of L: D of two is selected as a rule. Of course, extensive rotor-dynamic analysis is
required to verify proper mechanical integration early in the
design process. A two-pole machine is selected for speeds
above 60,000 r/min to have the lowest fundamental frequency at maximum speed for easy
integration with the power electronics.
Speeds within the range of 20,000-
60,000 r/min are well within conventional bearing capabilities. However, for
improved reliability, foil bearings are
highly desirable, although the size of the
foil bearings is inversely proportional to
speed. Also, a higher number of machine
poles can be selected for weight reduction. The fundamental of the machine
current is a product of the mechanical
frequency and number of pole pairs.
However, the machine current fundamental should be low enough frequency
for easy integration with the power electronics. Electrical fundamental frequencies above 1.5 kHz should be avoided to
prevent distorted stator currents, which
result in increased machine losses and
EMI. In this speed range, four-, six-, and
eight-pole machines may be selected.
For speeds below 20,000 r/min, foil
bearings are not feasible due to
increased size. Rotor-dynamic behavior usually does not create problems since the first critical
speed is far above the operating speed range. An increased
number of poles can be used for reduced machine sizes.
The size of the magnetic bearings is not very sensitive
to the operating speed ranges. Implementations at high
speeds are more challenging since higher-frequency bandwidth is required. This is because the critical speeds are
within the operating speed range.
In all integrations, the location of the bearings should
be selected carefully to avoid thermal issues. Cooling systems should be designed to protect temperature-sensitive
components from engine-generated heat as well as heat
resulting from the generator losses.
New EPGSs with electronically controlled power conditioning units (PCUs) can experience metal-bearing failures

early in their projected life cycles. These failures can be
caused by the interruption of the high-frequency current
that flows through the bearings. These currents are also
referred to as common-mode noise. The bearing currents
have been around since the invention of the EM, but such
failures have increased recently because modern PCUs,
with their fast-switching voltages and high frequencies,
generate current pulses through the bearings, whose frequent discharges can gradually damage bearing races. If
the energy of the current pulses is sufficiently high, metal
is transferred from the balls and the races to the lubricant.
This phenomenon is known as electrical discharge machining. The effect of a single pulse is not destructive, but
switching frequencies are very high, and the large number
of pulses causes the erosion to be accumulated quickly. As
a result, bearings require replacement after short in-service
times. The common-mode noise
problem can be mitigated through
different provisions for the design of
EMs and PCUs. The replacement of
metal bearings with ceramic, magnetic, or foil bearings may be the ultimate solution.

High-performance
machines for
aerospace
applications are
typically integrated
into powergeneration systems
or used for driving
high-speed
compressors, fans,
pumps, and
actuators.

Electric Machines Classification

Small family EMs are reviewed in Figure 1, which shows the most popular
representatives used in EPGSs for aerospace applications. Level 2 separates
brushless and brush-type machines.
Not surprisingly, the brush-type
machines did not evolve past level 3.
The mechanical contact for transferring power from the rotor reduces
machine reliability substantially. Frequent service and contamination
resulting from mechanical friction create additional difficulties. The dc brush
PMM is still in use for low-speed applications. There is a trend to replace this
machine with brushless designs.
Level 3 brushless machines contain three main branches-synchronous, asynchronous, and reluctance. The most
popular reluctance representative, the switched-reluctance
machine (SRM), has a simple construction and high-speed
capability. However, control is associated with highly distorted nonsinusoidal fluxes leading to excessive losses both
in the stator and rotor. The rotor cross section is not cylindrical, resulting in high windage losses. Also, there are substantial rotor steel losses, creating cooling challenges. SRM
performance is very sensitive to the magnitude of the air
gap. The SRM rotor is made of laminated material, resulting
in a construction that is insufficiently stiff, which results in
certain mechanical integration issues.
In the asynchronous branch of Figure 1, the most popular
type is the squirrel-cage induction machine (IM). During the
IEEE Electrific ation Magazine / d ec em be r 2 0 1 4

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