IEEE Electrification Magazine - December 2014 - 16

400-Hz constant-frequency power-distribution-bus period,
this machine was the workhorse of electric drives. The ability
to operate directly from the bus makes it the lowest-cost
constant-speed drive. Recently, several successful integrations of IMs for EPGSs have been achieved. In addition to
laminated material, IMs contain short-circuited copper or
aluminum bars, which create an additional speed limitation.
The machines in the synchronous branch of the figure
split into two groups: wound-field machines and PMMs.
The wound-field machine is the workhorse of power-generation systems for main-engine and APU applications. This
machine, however, is speed limited, with a complex construction that is inadequate for use in high-performance
EPGSs. The synchronous PMM, on the other hand, progresses to another two levels defined by two main stator
constructions-tooth-type and toothless-and two rotor
constructions-two-pole and multipole.
The tooth-type stator design is not different from any
typical tooth-type design for synchronous or asynchronous machines. Many winding arrangements are possible.
The rotor of a two-pole machine consists of a single-piece
magnet contained in a metal or a composite sleeve. The
multipole rotor has a more complex construction with a
higher number of magnets. In many cases, a rotor back
iron is required to close the magnetic circuits. As in the
two-pole design, a containment sleeve is used.
The toothless stator design uses a simple ring for a
stator core with stator windings that are sectioned in separate segments. The unique feature is lack of cogging
torque and a very large effective magnetic air gap,

resulting in the possibility for creating a large mechanical
air gap. The toothless EM is complemented with rotor
options similar to the tooth-type machine. From the classification chart in Figure 1, six EMs that have found a
place in advanced EPGS systems will be further evaluated
based on their key characteristics (KCs). These machines
are distinguished in the yellow boxes in Figure 1: squirrelcage IMs, SRMs, tooth-type two-pole machines, toothtype multipole machines, toothless two-pole machines,
and toothless multipole machines.
Synchronous PMMs will be reviewed closely, as they are
considered the strongest candidates for high-performance
EPGSs. All machines will be rated based on the derived KCs
needed to satisfy different major requirements (MRs).

major requirements
EMs can be characterized using their MRs, which are a
select few characteristics of an end-unit product that have
a significant impact on the product performance. Using
this approach, weight, volume, reliability, efficiency, and
cost, MRs can be selected to develop balance among them,
resulting in an optimized design. Brief descriptions of the
selected MRs are as follows:
xx
Weight is one of the most important MRs for aerospace applications. This characteristic directly affects
overall vehicle performance.
xx
Volume has recently become a critical MR, due to substantial increases in electric component installations
and overall power electronics. Those increases result
from recent MEA development.

EMs for Aerospace

Brushless

Synchronous

Asynchronous

Wound
Field

PMM

Tooth-Type
Stator

Tooth-Type
Two Poles

Tooth-Type
Multipole

Figure 1. A classification of EMs.

Brush

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

Synchronous
Slip Ring

Reluctance

Squirrel-Cage
IM

Switched
Reluctance

Toothless
Stator

Toothless
Two Poles

Toothless
Multipole

dc Brush PMM

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