By Thomas Jahns
Getting Rare-Earth
Magnets Out of EV
Traction Machines
A review of the many approaches being
pursued to minimize or eliminate rare-earth
magnets from future EV drivetrains.
here are now more than
1.5 million plug-in hybrid electric vehicles (HEVs) and battery electric vehicles
(BEVs) that have been commercially produced in the world during the past 11 years,
and it is likely that the number produced during 2016 exceeded
700,000, a new record. A large percentage of those electrified vehicles use ac permanent magnet (PM) synchronous machines for the
­traction machines used in their drivetrains. Even more specifically,
nearly all of these PM synchronous machines fall into the class of interior PM (IPM) synchronous machines. The origin of this name becomes
apparent by inspecting the cross section of a typical IPM machine in
Figure 1 that has been simplified to highlight the key features. This
figure shows that the magnets are buried in cavities inside the
rotor, and the cavities are carefully shaped, often in V configurations, to concentrate more of the PM magnetic flux into the
machine’s air gap.
So what accounts for the popularity of the IPM
­synchronous machine in EVs? The major features that make it appealing include
those in the following list.
T
Digital Object Identifier 10.1109/MELE.2016.2644280
Date of publication: 7 March 2017
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xx
Like all types of PM synchronous machines, the
magnets, once magnetized, continually deliver magnetic flux into the machine’s air gap without ex­­
ternal excitation. This fulfills one of the key
prerequisites for torque production in any type of
electric machine by providing a magnetic field that
couples with the stator coils. The fact that this field
is created without the need for field excitation current delivered by an ­external electrical power source
(e.g., batteries) saves energy as well as reduces heat
production inside the machine that would otherwise be produced by resistive power dissipation in
the stator windings. This gives PM machines a
potent built-in advantage that can be used by
machine designers to achieve an adjustable combination of the key desired performance attributes for
EV traction machines: higher efficiency, smaller
mass and volume, and lower rotor inertia.
xx
By burying the magnets inside the rotor, as shown in
Figure 1, the IPM machine provides mechanical protection for the magnets, which is more important
than it might first appear because magnet materials
are typically brittle with tensile strengths that are far
lower than that of the lamination steel that surrounds them.
xx
Less obvious from Figure 1 is the fact that IPM
machines provide highly valuable performance
advantages in the hands of a skilled machine designer
that make it much easier to shape the machine’s
torque versus speed characteristics to match the
requirements of modern EV traction drives. In addition, IPM machines provide valuable opportunities
for reducing the amount of required magnet material, which will be discussed in more detail later in
this article.
One of the key factors that gives rise to the last of
these advantages is that there is nearly an endless
number of IPM machine designs. These IPM machine
(a)
(b)
A–
C+
B+
C–
N
A+
B–
S
S
A+
B–
N
C–
B+
C+
A–
Figure 1. A basic IPM machine cross section.
designs are distinguished by key attributes that include
the orientation of the embedded magnets, the number
of layers of magnet cavities in each rotor pole, and the
placement of the magnets inside the cavities, which are
not necessarily filled completely with magnet material.
Some examples of IPM machine rotor cross sections
used in production EVs during the ten-year period from
2003 to 2013 are provided in Figure 2. In addition to
these variations in the IPM machine rotor design, the
choice of the stator winding configuration has a
­dramatic impact on the machine’s performance characteristics as well.
The Problem with Magnets
In general, the magnitude of the advantages enjoyed by
IPM machines increases monotonically with the strength
(c)
Figure 2. Examples of IPM machine rotors with sintered neo magnets used in production EVs: (a) the 2002 Toyota Prius (courtesy of ORNL
report by Hsu et al.), (b) the 2007 T­ oyota Camry (left) and 2008 Lexus LS 600h (right) (courtesy of ORNL report by Burress et al.), and (c) the
2013 Chevrolet Spark (photo courtesy of General Motors Company).
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7
500
450
PM Machines
Price (US$/kg)
400
Reduced RE
Magnet Content
Nd
350
Non-RE Magnets
(e.g., Ferrites)
300
250
25x
Increase
200
Induction
150
100
SynR
50
4
01
3
Non-PM Machines
.2
SR
Ja
n
.2
01
2
Ja
n
.2
01
1
Ja
n
.2
01
0
Ja
n
01
.2
Ja
n
Ja
n
.2
00
9
0
Date
Figure 3. Nd metal prices 2009–2013, showing dramatic swings.
of the magnets. In fact, one of the key enablers of today’s
IPM machine technology used in EV traction machines
and many other applications is the development of
high-strength neodymium-iron-boron (NdFeB) magnets
(referred to henceforth as neo magnets), which dates back
to the early 1980s. Although neo magnets can be manufactured in a variety of forms that involve tradeoffs
between magnet strength, cost, and manufacturability,
nearly all production PM traction machines for EV applications use the highest-strength category of available neo
magnets, which are sintered.
Nd belongs to the family of rare-earth (RE) elements.
Actually, the availability of Nd in the earth’s crust is
­relatively high compared to many
other RE elements, but there are other
key factors that have complicated the
availability of high-strength sintered
neo magnets for use in EV traction
machines. First and ­foremost, approximately 50% of the earth’s known Nd
reserves are located in China. China,
being well aware of its natural advantage in Nd reserves, has encouraged
the development of its neo magnet
­processing industry, which has gained
control of more than 90% of the world’s
production of high-strength sintered
neo magnets during the last ten years.
In 2011, a potent combination of factors, including rapidly increasing ­international demand for
neo magnets, mandated consolidation of the RE mining
industry in China, and a bubble of intense commodity market speculation, ­conspired to send the price of Nd metal
rocketing upward to values that, at their peak in mid-2011,
were more than 20 times higher than Nd’s much ­stabler
base price two years earlier in 2009 (Figure 3). This dramatic
price increase caused great distress among customers who
were major users of neo magnets, including many electric
Figure 4. A tree diagram showing major approaches adopted by
machine designers to significantly reduce or eliminate RE magnets.
traction machine manufacturers. F
­ ortunately, the price of
Nd subsequently dropped after mid-2011 nearly as fast as it
had risen ­during the first half of 2011.
Auto Industry Response
Despite the fact that the price of sintered neo magnets
has fallen to levels that, when adjusted for inflation,
approach the prebubble prices in 2009, many of the major
users of neo magnets around the world decided that they
could not afford the risk of a repeat of the ­costly 2011 neo
price bubble. This, in turn, gave rise to a major surge of
new research and development projects in industry and
academia to find ways to build future traction machines
that could match or exceed the performance of state-of-the-art production PM machines ­w ithout using
sintered neo magnets. Unfortunately,
this worthy objective has turned out
to be a difficult one, resulting in a
variety of alternative machine types
and configurations that are at varying
stages of development and deployment today.
Figure 4 provides a tree diagram
identifying the major paths that
electric machine researchers in
academia and industry around the
world have pursued in their wideranging search to find the most pro­­
mising commercially viable electric traction machine
configurations that escape the heavy dependence on
high-performance sintered neo magnets associated
with classic IPM machines that have dominated HEV
and BEV production applications since the late 1990s.
As indicated in the figure, one of the approaches has
consisted of retaining the use of PM machines but
modifying their topologies and design details to use
either 1) significantly less sintered neo magnet material
One of the key
enablers of today’s
interior permanent
magnet machine
technology is the
development of highstrength neodymiumiron-boron magnets.
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In those applications
that require the
highest possible
torque or power
density, efforts
to replace magnet
torque with
reluctance torque
almost invariably
lead to increases
in the machine
mass and volume.
or 2) alternative non-RE magnets,
such as ferrite magnets. The other
major branch in the Figure 4 tree
diagram is to abandon PM machin­­
es in favor of ­alternative electric
ma­chine types that include, most
importantly, 1) induction machines,
2) synchronous reluctance (SynR)
machines, and 3) switched reluctance (SR) machines.
The objective of this article is to
survey the variety of different ma­­
chine types and configurations re­­
flected in Figure 4 that are receiving
the most serious attention as alternatives to the baseline PM machines
that use high-strength sintered neo
magnets. Rather than attempting to
provide comprehensive quantified
comparisons, the focus will be on discussions of the underlying advantages
and limitations of these alternative
machine types that ­highlight their corresponding levels of
suitability for demanding electric traction applications.
components, referred to here as magnet torque, is developed as a result of
the interaction between the magnetic
flux delivered by the magnets and the
ac currents that flow in the stator
windings. This magnet torque is proportional to the amplitude of the magnet’s flux density, so stronger magnets
(such as sintered neo magnets) in an
IPM machine rotor are capable of delivering more magnet torque than weaker magnets in the same machine rotor.
What sets IPM machines apart from
simpler surface PM (SPM) machines
with the magnets mounted on the rotor
surface is the presence of a second
torque component known as reluctance
torque that does not depend on the
presence of the magnets at all. Reluctance torque is attributable to the same
physical force mechanism that causes a
steel needle to align itself with a magnetic field in which it is immersed. As a result, reluctance
torque is produced in an IPM machine even if all of the rotor
magnets are either physically removed or completely
demagnetized. Recognizing this hybrid nature of torque production in IPM machines leads rather naturally to efforts to
design new IPM machines that are purposely biased to
develop a larger fraction of their total torque from reluctance
torque and correspondingly less from magnet torque.
So how does a designer modify an IPM machine de­­
sign to generate more reluctance torque? One of the
most direct and effective ways to accomplish this objective is to design the rotor with higher numbers of magnet cavity layers in each rotor pole to provide more
directionality to the rotor’s magnetic flux flow paths (see
Figure 5). That is, magnetic flux faces far less resistance
PM Machine-Based Alternatives
As noted above, one of the two major approaches to reducing the dependence on such large numbers of highstrength sintered neo magnets in future traction machines
has been to develop PM machines that do not require as
much magnet flux to achieve the desired performance
requirements. One approach to achieving this objective is to
take advantage of the multiple degrees of design flexibility
that IPM machines make available to their designers. To
appreciate the source of this flexibility, it is first important
to recognize that the torque developed in IPM machines
can be attributed to two distinct sources. One of these two
S
N
S
N
N
S
S
N
(a)
(b)
(c)
Figure 5. The IPM machine rotors with increasing numbers of rotor magnet cavity layers per pole to increase saliency ratio: (a) two barriers/pole,
(b) three barriers, and (c) four barriers.
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20
Inductance Ratio, ξ = Lq /Ld
18
16
SynR Machines
Along Ψm = 0 Axis
Finite Maximum
Speed Region
14
12
10
8
Optimal Constant-Power
Design Curve
6 Unlimited
Maximum
SPM Machines
Speed
4
Along ξ = 1 Axis
Region
2
P∞ <1
1
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Normalized Magnet Flux, Ψm (p.u.)
1
Figure 6. IPM machine inductance saliency ratio versus magnet flux
linkage showing the locus line of machine designs with optimal
­constant-power characteristics.
(known as electromagnetic reluctance) when it is flowing
parallel to the cavity faces in the lamination steel than
when it attempts to flow in perpendicular paths crossing multiple cavities that behave as large air gaps. One
of the most important manifestations of this magnetic
anisotropy is the change in the inductance measured in
any of the machine’s phase windings as the rotor is
rotated to first align the phase winding’s magnetic flux
to flow parallel to the cavities (maximum inductance)
and then perpendicular to the cavities (minimum inductance). The ratio of the maximum to minimum inductance is referred to as the machine’s saliency ratio, which,
in turn, is proportional to the machine’s reluctance
torque production.
For a given total torque required from the machine,
there is an infinite number of combinations of magnet
torque and reluctance torque that sum to that same total
torque value, each combination corresponding to a different IPM machine design. A helpful means of visualizing
this torque segregation into its two components is to use a
two-dimensional (2-D) plot (see Figure 6) in which the
Torque
High-Speed
Regime
ωo
Rotor Speed
Power
Constant Power Range
Terated
rated
P rated = Te
ωo
ωmax = Fωo
Figure 7. The traction machine torque and power versus speed
c­ haracteristics identifying constant torque and constant-power
­capability envelopes.
10
horizontal x axis represents the magnet flux linkage (a
surrogate for the magnet torque) and the vertical y axis
represents the inductance saliency ratio (a surrogate for
the reluctance torque). Note that the saliency ratio axis is
plotted so that the magnetic flux axis corresponds to a
saliency ratio of one (i.e., no saliency, because the inductance is the same for all rotor angles). Each point in the
first quadrant of this 2-D plane corresponds to a different
IPM machine design with its distinct magnet cavity
­configuration and magnet strength that delivers the same
total torque.
Are all of the IPM machine designs that correspond to
all of the points in the first quadrant of Figure 6 equally
good for meeting the machine’s torque versus speed
requirements? Unfortunately, only a small subset of the
machine designs in the first quadrant of Figure 6 are capable of delivering the torque versus speed characteristics
demanded by electric traction applications. The torque
and power versus speed maximum capability envelopes
plotted in Figure 7 illustrate, in idealized form, the key
characteristics that are demanded from high-performance
traction machines. The most important features to note
are that 1) the maximum torque available from the
machine is constrained to a constant value at low speeds
between zero and the corner speed ~0 and 2) the maximum power delivered by the machine is constrained to a
constant value for all speeds above the corner speed up to
a maximum machine speed of F·~0, where F is a dimensionless parameter known as the constant-power speed
ratio (CPSR). Values of the CPSR required in electrified traction vehicles vary over a wide range from three for some
battery-electric passenger vehicles to ten required in some
heavy-duty truck and off-road applications that require
both high torque for hauling at low speeds as well as high
cruising speeds with light loads on the open highway. The
IPM machine designs that can deliver torque versus speed
characteristics matching those in Figure 7 are limited to a
relatively narrow band of designs surrounding the curved
locus line labeled as the optimal constant-power design
curve in Figure 6.
Closer inspection of the optimal constant-power design
curve in Figure 6 reveals the options that are available to a
machine designer who is tasked to design an IPM machine
that falls on or near this locus line. That is, the designer
can choose a machine design that falls at the rightmost
end of the locus line, distinguished by a high magnet flux
linkage and a low saliency ratio of 1. For this particular
case, all of the machine’s torque comes from magnet
torque, which corresponds to an SPM machine in which
the magnets are mounted on the outer surface of a symmetrically round stack of steel l­aminations.
Alternatively, the machine designer can choose to
reduce the magnet flux linkage by using one of several
approaches discussed in more detail in the next paragraph. In order for the machine design to remain on the
optimal constant-power locus line, this magnet flux
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reduction must be accompanied by a corresponding in­­
crease in the machine’s saliency ratio, replacing lost
magnet torque with reluctance torque. This is typically
accomplished by increasing the number of magnet cavity
layers per pole, leading to IPM machine rotor designs that
have three or more layers of cavities in each pole. A significant number of these rotor cavities often contain smaller
magnets that do not fill the cavities or no magnets at all.
The approaches adopted by machine designers to
reduce the rotor magnet flux include removing some of
the high-strength magnets from the rotor cavities or
replacing them with lower-strength magnets. More
­specifically, alternatives include
1) continuing to use high-strength sintered neo magnets
but fewer of them so that the rotor cavities are only
partially filled
2) adopting a lower grade of neo magnet material, such
as injection-molded or compression-bonded magnets
that use less Nd but can be specified to d
­ eliver magnetic flux density values that are a fraction of the typical value of high-strength sintered magnets
3) opting for an entirely different magnet material, such
as ferrite, that can be substantially lower in cost than
sintered magnets in exchange for much lower magnet
flux densities.
Each of these approaches is being seriously evaluated by researchers in industry and academia. One interesting example of a ferrite-magnet-based IPM machine
used in a production automotive application is the
50-kW (peak) generator machine used in the powertrain
of the 2016 Chevrolet Volt, an extended-range HEV. A
view of the rotor cross section for this machine is provided in Figure 8.
A summary of the advantages and disadvantages of
adopting one of these modified PM machine alternative
approaches is provided in Table 1. As indicated by the
entries in Table 1, there are significant engineering tradeoffs that have to be considered before adopting this
approach to reducing or eliminating sintered Nd magnets
from PM machines. In those applications that require the
highest possible torque or power density, efforts to
replace magnet torque with reluctance torque almost
invariably lead to increases in the machine mass and volume. This fact alone goes a long way toward explaining
why automotive manufacturers and their suppliers have
faced such difficult challenges eliminating sintered neo
magnets from the traction machines in their HEV and
BEV products.
Machine Alternatives Without Magnets
SynR Machines
In the preceding section, torque production in IPM
machines was described as a combination of magnet
torque and reluctance torque, with the ratio of these two
torque components determined by the machine designer.
Figure 8. The rotor of a 2016 Chevrolet Volt IPM machine designed
for ferrite magnets (magnets not installed). (Photo courtesy of General Motors Company.)
Table 1. Modified PM machines.
Advantages
Disadvantages
Major reduction or total
­elimination of RE magnets.
Replacing sintered ­magnet
torque with reluctance torque
tends to reduce ­machine
torque/power density.
Can preserve special
­advantages of PM ­machines
including high power ­density
and efficiency, but some
­compromises may be
­necessary.
Reduction or elimination of
sintered magnets tends to
increase magnet resistivity,
lowering rotor losses.
Handling weaker magnets
can simplify manufacturing by requiring less
­expensive ­fixturing and
reduced ­magnetization
­requirements.
Increased inductance
­saliency tends to make
­machine a better candidate
for rotor position self-­sensing
down to zero speed.
Increasing reluctance
torque tends to reduce rotor
mechanical strength due to
more cavity layers as well
as increasing incentives to
reduce air-gap length.
Replacing sintered magnets
often leads to machine
designs that are more
­vulnerable to
demagnetization.
Depending on choice of
magnet material, operating
temperature range may be
reduced on both the high
end and low end.
Complete fault tolerance is
difficult because magnets
cannot be turned off at will.
In the search for promising ways to totally eliminate RE
magnets from traction machines, it is logical to explore
what happens when the magnets in the IPM machine
are eliminated entirely, creating a distinct type of ac machine
commonly known as a SynR machine. Looking back to the
two-axis PM-reluctance machine plane in Figure 6, SynR
machines lie at the extreme left of the plane, along the
vertical saliency ratio axis (i.e., magnet flux = 0). The operating principle of the SynR machine shares much in common with the IPM machine, except for the absence of any
magnet torque. As discussed in the preceding section,
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11
increasing the reluctance torque production depends
heavily on maximizing the machine’s saliency ratio,
which, in turn, favors rotor designs with high numbers
of cavity layers in each pole combined with small airgap lengths.
Although SynR machines have enjoyed limited commercial success in some specialized industrial process
applications in the past, it is only recently that they have
been introduced to the marketplace as a candidate for
general-purpose adjustable-speed motor drive applications (see Figure 9). Although there is evidence that their
performance characteristics are sufficiently attractive to
enable them to compete with well-established induction
machines for these industrial applications, the challenges
that they face to succeed as high-performance traction
machines are significant.
An example of the special challenges faced by SynR
machines for traction applications is the frequent requirement to deliver wide ranges of constant-power operation,
as discussed earlier in conjunction with Figure 7. Unfortunately, the locus of IPM machine designs that are capable
of delivering optimal constant-power operation in F
­ igure 6
never reaches the vertical saliency ratio axis where SynR
machines reside and only approaches the axis asymptotically at high v
­ alues of the saliency ratio. Consistent with
this observation, a rule of thumb often used by designers
of SynR machine drives is that the achievable CPSR is
approximately one half of the machine’s inductance
saliency ratio. Because well-designed SynR machines using
multiple cavity layers per pole typically achieve saliency
ratio values in the range of six to eight, the CPSR limits for
these machines in traction applications is limited to a
maximum of three to four. Unfortunately, this CPSR ceiling
places them at a significant disadvantage compared to
well-designed IPM machines in demanding applications
that require CPSR values of five or higher. A more complete
summary of the advantages and disadvantages of the
SynR machine for EV traction motor applications is provided in Table 2. A review of the entries in this table makes it
clear that current limitations on the achievable saliency
ratio values in practical SynR machine designs severely
restrict their competitiveness for demanding traction
machine applications. However, their competitiveness
would improve substantially if engineers are successful in
developing new ways to significantly increase their saliency ratio values.
One tantalizing approach that has been pursued by
some researchers is to laminate the rotor in the axial
direction using a high number of alternating layers of thin
lamination steel and insulation layers, as illustrated in
­Figure 10. Although it has been demonstrated that this
approach can increase the saliency ratio to values above
ten, the significant manufacturing challenges posed
by this dramatic departure from conventional machine
manufacturing processes have not been successfully overcome to date. As a result, the future of the SynR machine
in demanding traction applications remains in doubt until
some technical breakthrough makes it possible to significantly boost the saliency ratio in commercially manufacturable machines.
Induction Machines
The most popular and widely used ac machine type
for motors in residential, commercial, and industrial
applications is the squirrel-cage induction motor. Its
wide appeal can be attributed to a combination of factors,
Table 2. SynR machines.
Advantages
Disadvantages
Complete elimination of all
rotor magnets, avoiding one
of the highest machine cost
contributors.
Limits on achievable saliency
ratio values reduce machine
torque density compared to
PM machines.
Simplification of rotor
­construction because
there are no magnets or
­embedded conductors that
have to be inserted.
Increasing inductance
­saliency ratio tends to reduce
rotor m
­ echanical strength
due to more c­ avity layers and
increased p
­ ressure to reduce
air-gap length.
Opportunity to achieve low
rotor losses in the absence
of magnets or conductors
in rotor.
Machine’s saliency makes it
an attractive candidate for
rotor position self-sensing
down to standstill.
Figure 9. An ABB SynR machine designed for general-purpose
i­ndustrial applications. (Photo courtesy of ABB under terms of
­website copyright.)
12
Absence of magnets reduces
SynR machine’s vulnerability
to damage caused by short
circuit faults.
Requires very high saliency
ratios to achieve high CPSR
values required in many
­traction applications.
Achievable saliency ratio
values typically result in
low power factor, increasing
inverter power rating.
Generally not a good
­candidate for high-polenumber designs, reducing
its appeal for high-torque,
direct-drive applications.
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Pole
Pieces
Insulation
Lamination
(a)
Shaft
(b)
(c)
Figure 10. The rotor construction details of a prototype axially l­aminated SynR machine. (a) A rotor cross section, (b) a rotor’s components, and
(c) an assembled rotor. (Drawing and photos courtesy of Prof. Wen Soong, University of Adelaide, Australia; used with permission.)
including its line-start capabilities, ruggedness, and
low cost for general-purpose applications. As shown in
­Figure 11, the basic squirrel-cage induction machine
uses a stator with a distributed three-phase ac winding, much like that found in both IPM and SynR
ma­chines. The major distinguishing features are found
in the rotor, where a number of conducting bars made
of aluminum for general-purpose applications are
mounted in slots around the periphery of the rotor and
then short-­circuited to each other using conducting
end rings at both ends of the rotor. Torque is developed due to currents that are induced in the rotor bars
as a result of the rotating magnetic flux wave produced by ac currents flowing in the three stator windings. However, the time-varying magnetic flux in the
rotor requires a difference between the ­rotational
angular frequencies of the stator-applied ­magnetic
field and the rotor mechanical assembly known as the
slip frequency.
The fact that torque is developed in the induction
machines without magnets or galvanic connections to the
rotor windings is a highly desirable feature of induction
machines. However, in the absence of the magnets, the
rotor magnetic field in the induction machine is created
by extra phase current supplied to the stator windings to
induce the necessary rotor currents to create this field.
There are resistive losses associated with generating this
magnetic field in both the stator and rotor that are not
required in IPM machines, putting the induction machine
at a disadvantage in terms of machine efficiency.
Machine designers can take steps to minimize this efficiency penalty by installing low-resistance copper bars in
the rotor squirrel cage in place of conventional aluminum
bars, incurring incrementally higher machine cost in the
process. In addition, unlike IPM machines, the amplitude
of the rotor magnetic field can be reduced by adjusting the
stator currents to minimize losses under light-load operating conditions, further offsetting the impact of the rotor
resistive losses. By combining these design and control
features, induction machines can be designed to challenge
high-performance IPM machines in a number of
demanding electric traction applications that require
extended operating times at high speed with light loads.
The highest-profile automotive EV application in which
induction machines have been chosen for the electric
traction powertrain is undoubtedly the growing family of
Tesla BEVs (see Figure 12), a fitting selection because
Nikola Tesla invented one of the earliest forms of the
induction motor in 1887. These machines are aggressively
designed with copper-cage rotors and advanced cooling to
achieve high-torque density and fast acceleration. However, there are a few other production passenger vehicles
that use induction machines, including the mild hybrid
powertrain in the Buick Lacrosse (see Figure 13), for which
the required induction machine is much smaller than the
traction motor in the Tesla EV.
Changing the focus for a moment to rail vehicles, the
series-hybrid powertrains that are widely used in train
locomotives around the world typically use induction
machines with ratings of 500 kW or higher (Figure 14).
Figure 11. A basic squirrel-cage induction machine cross section.
(Image courtesy of Infolytica.)
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13
(a)
(a)
(b)
Figure 12. Views of a Tesla Roadster 185-kW (peak) induction
machine ­showing (a) a cutaway of an air-cooled stator and (b) a rotor
end view highlighting the copper end ring and rotor bars. [Photo (a)
courtesy of Wikimedia–Creative Commons.]
(b)
Figure 14. (a) An EMD SD70MAC diesel-electric locomotive with
3-MW series-hybrid ­powertrain and (b) one of its six 500-kW induction
traction machines. (Photos courtesy of Electro-Motive Diesel.)
Figure 13. A cutaway view of a 2015 Buick LaCrosse 11.2-kW
(peak) induction machine used for engine starting, acceleration boost,
and accessory power generation. (Photo courtesy of General Motors
Company.)
limits the machine’s achievable CPSR to values that are
typically less than five to one. Raising the machine’s
CPSR generally requires that the machine’s leakage
inductance be reduced, but this can compromise the its
vulnerability to increased pulsewidth modulation (PWM)
current ripple and associated losses. Similarly, the
absence of rotor magnets makes the induction machine
less attractive for high-pole-­number designs that are
often required for high-torque, direct-drive applications.
SR Machines
Nevertheless, despite their success in a variety of electric
traction applications and the fact that they require no
magnets, induction machines are still far outnumbered by
IPM machines in production EV passenger vehicles
around the world.
A summary of the advantages and disadvantages of
the induction machine for electric traction applications
is provided in Table 3. As noted in the table, the fact that
magnets are entirely absent from induction machines
creates engineering tradeoffs, particularly regarding
high-speed operation. Like the case of the SynR machine
discussed earlier, the complete absence of magnets
14
Despite the fact that its name sounds very similar to the
SynR machine discussed earlier, the SR machine is very
different from all three of the other traction machine
types discussed up to this point. While the IPM, SynR,
and induction machines all use balanced three-phase
sinusoidal current excitation to produce a smoothly
rotating magnetic flux wave in the air gap, the SR
machine uses rectangular excitation pulses to excite the
stator phase windings. Unlike the stators for these other
three machines, the stator core of the SR is characterized
by salient poles, and the rotor core has salient poles as
well (see Figure 15).
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One of the most appealing features of the SR machine
is that its rotor requires no magnets or current conductors,
consisting only of a stack of steel laminations with salient
poles. This makes the SR machine rotor even s­ impler and
more rugged than those of nearly all SynR machines. The
stator windings have appealing elements of simplicity as
well, consisting of concentrated solenoidal coils around
individual stator lamination poles, benefiting from lower
winding ­complexity compared to more conventional overlapped distributed windings often found in the other three
ac machine types.
Although there are major differences in the operational
details of SR and SynR machines, the basic nature of their
torque production is the same. More specifically, current
excitation of a winding around one of the stator poles in
the SR machines creates a torque acting on the rotor
salient pole nearest to the pole face of the excited stator
pole, attempting to draw it into alignment even though
the rotor contains no magnets or conductors to generate
its own magnetic field. Provided that the stator and rotor
have different numbers of salient poles, exciting the SR
machine’s multiple machine phases will cause the rotor
to spin at a frequency equal to the excitation frequency
divided by the number of rotor poles.
Similar to trends observed earlier with the IPM and
SynR machines, the reluctance torque in the SR machine is
maximized by increasing the ratio of the inductance values measured in the stator phase winding when a rotor
pole is perfectly aligned with the excited stator pole (maximum inductance) and when it is maximally unaligned
(minimum inductance). That means that, like the SynR
machine, the SR machine’s torque production benefits
from reducing the air-gap length to the smallest ­acceptable
value based on manufacturing and bearing t­ olerances.
Each phase winding is excited with periodic current
pulses so that the machine produces positive motoring
torque while the nearest rotor pole is being pulled into
alignment and negative braking torque if the phase winding is excited while the rotor pole is moving out of alignment. The pulsed nature of the stator winding excitation
tends to produce more torque ripple in SR machines compared to the other three types of ac machines previously
considered, which are all excited with smoothly varying
sinusoidal currents.
In addition, the SR machine’s stator lamination shape,
consisting of stator winding poles interconnected by thinner stator yoke segments, is vulnerable to mechanical
deformation attributable to ovalizing forces caused by
radial forces acting on the excited stator poles. This ovalizing deformation, in turn, frequently results in undesirable
acoustic noise common in many SR machines unless
­special electromagnetic and structural design measures
are taken to minimize the mechanical deformation under
all significant operating conditions.
High-performance control of SR machines depends on
accurate control of the time instants when current
Table 3. Induction machines.
Advantages
Disadvantages
Complete elimination of all
rotor magnets, avoiding one
of the highest machine cost
contributors.
Rotor losses are ­generated
whenever torque is being
produced, even if Fe is
­lossless.
Rotor losses can be
­significantly reduced by
adopting copper squirrel
cage instead of aluminum.
Requires small air gap to
minimize reactive current
and maximize efficiency.
Drive-cycle efficiency of
­well-designed induction
machine can ­challenge that
of an IPM ­machine in some
­applications.
Induction machines are
generally not well suited to
high pole numbers, ­making
them less desirable for
lower-speed, direct-drive
­applications.
Magnetic field strength in
machine can be lowered
via stator current to reduce
­losses under light-load
­condition; valuable for
­extended high-speed cruise.
Constant-power speed range
is more limited than IPM
machines; increasing CPSR
requires reduced ­leakage
reactance, generating
­additional PWM losses.
Absence of magnets
­reduces induction machine’s
­vulnerability to damage
caused by short circuit
faults.
Generally a poor ­candidate
for rotor position self-­
sensing at low speeds
due to ­absence of rotor
saliency.
Figure 15. A basic SR machine cross section. (Image courtesy of
Wikimedia–Creative Commons.)
excitation pulses are applied to each stator phase winding
in relation to the instantaneous angular position of the
nearest rotor pole with respect to the excited stator pole.
By adjusting the advance angle when current is applied
with respect to the time instant of maximum alignment
during high-speed operation, the output power can be
held constant as the speed is increased. This makes it possible to achieve high CPSR values (greater than ten to one
in some cases) that are well suited to traction applications.
Similar to the other magnetless machines that have
been considered in this section, it is difficult for SR
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15
machines to match the peak efficiency values of welldesigned PM machines with sintered neo magnets. However, some of maximum efficiency deficit can be offset
during vehicle drive cycles by reducing the magnetic field
amplitude in the machine during high-speed, light-load
operation, using the same concept discussed earlier for
induction machines.
In recognition of the combination of attractive features
offered by SR machines, including the simplicity and ruggedness of their rotors plus the absence of any magnets,
there have been serious efforts to apply SR traction ma­­
chines in the powertrains of passenger EVs. However, none
of the major automakers has selected them for use in its
production vehicles to date. Despite the acknowledged
strength of their positive features, the difficulty of adequately overcoming their well-known limitations in the
areas of torque ripple and acoustic noise has continued to
challenge engineers in both academia and industry for
more than 35 years.
In contrast, SR traction machines have enjoyed much
more success in off-road construction and mining eq­­
uipment, where products such as large wheel loaders and
bulldozers using multiple SR traction machines in serieshybrid powertrain configurations have been introduced
by multiple manufacturers, achieving some notable market success. For example, the P&H L-2350 wheel loader
(­Figure 16) uses an independently controlled 425-kW SR
traction machine mounted in each of the four 4-m-­diameter
wheels that are all powered by an SR generator rated at
>1.7 kW. For these off-road traction applications, any
acoustic noise associated with the SR machines is overwhelmed by the noise of the diesel engine and, in many
cases, its turbocharger.
A summary of the advantages and disadvantages of
the SR machine for electric traction applications is
­provided in Table 4. The length of the list of advantages
and disadvantages in Table 4 reflects the fact that SR
machines have a number of persuasive features that
both favor and disfavor them, resulting in more polarization among supporters and detractors of SR machines
than is typically encountered for any of the other
machine types. While the ­simplicity and ruggedness
of SR machine rotors are major advantages, their
Table 4. SR machines.
Advantages
Disadvantages
Complete elimination of all
rotor magnets and current
conductors, enabling rugged
rotors capable of high-speed,
high-temperature operation.
Pulsed nature of stator
­winding excitation tends to
result in high-torque ripple
compared to other ­machines.
Simplicity of stator and rotor
construction helps to reduce
SR machine manufacturing
cost.
SR machine rotors ­typically
have low moments of
inertia compared to other
machine types, improving
­acceleration/deceleration
characteristics.
Capable of achieving high
CPSR values, yielding wide
speed ranges of constantpower operation.
(a)
(b)
Figure 16. (a) The P&H L-2350, the world’s largest wheel loader,
and (b) one of four 425-kW SR traction machines used to power the
wheels. (Photos courtesy of OEM Off-Highway Magazine, used with
­permission.)
16
High-frequency radial forces
on stator poles can result in
objectionable acoustic noise
due to excitation of stator
mechanical resonant modes.
Torque density and rated
load efficiency of SR m
­ achine
are lower than that of welldesigned PM machine.
Large amplitude of magnetic
flux variations in rotor poles
can lead to high rotor losses
at high speeds.
Requires small air gap to
maximize torque density.
Due to low magnetic
­coupling between phase
windings, fault tolerance
characteristics of SR
machines are best among
evaluated machines.
SR machine performance
­depends on accurate
­sensing of rotor position
and tight control of current
pulse timing.
SR machine efficiency
­during high-speed, light-load
­operation does not suffer
from need to use stator
current to cancel magnet
flux linkage.
Each SR machine phase
winding requires two external
terminals, doubling number
of excitation cables and
­connector pins compared to
ac machine types.
Number of stator phases in
SR machines can be varied
from two to high numbers
by changing combinations
of stator and rotor pole
numbers.
SR machine inverter does
not use standard full-bridge
inverter widely used by all ac
machines.
Control characteristics are
very nonlinear due to high
magnetic saturation in hightorque-density SR machines.
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Table 5. A feature comparison among alternative traction machine types for passenger EVs.
Legend:
- Comparative Strength
- Intermediate Capability
- Comparative Weakness
Sintered
Neo IPM
PM with
­Alternative
­Magnet
­Materials
SynR
Induction
SR
Machine Features
Torque/power density
Rated operation efficiency
Partial-load high-speed efficiency
High CPSR
Extreme temperature operation and rotor ruggedness
Low cost
Low acoustic noise and torque ripple
Low-speed rotor position self-sensing
Fault tolerance
high-torque ripple and acoustic noise characteristics are
showstoppers for passenger EV traction applications.
For applications that require high fault tolerance, such
as aerospace, the ability to apply a short circuit directly
across one of the SR machine phase windings without
inducing any significant short-circuit current to flow in
the shorted winding is a powerful advantage. However,
their disadvantages in mass, volume, and rated efficiency
compared to well-designed PM machines have limited
their adoption to date in aerospace applications. As a
result of these conflicting pluses and minuses, the SR
machine continues to spark considerable interest and
debate among researchers, but it continues to struggle to
reach its full potential in EV propulsion applications.
Summary and Conclusions
As indicated by the discussion in this article, replacing
high-performance sintered neo IPM machines with alternative machines that significantly reduce or eliminate the
RE magnets poses difficult challenges
for EV powertrain engineers. The fact
that so many production HEVs and
BEVs continue to use sintered neo IPM
machines in their powertrains, more
than five years after the speculative
bubble in Nd prices reached its peak,
is strong testimony to the special
capabilities of these machines to
deliver combinations of high torque/
power density and efficiency that are
difficult to beat for rated power conditions. However, there are at least three
broad conditions that, if any of which
is met, are likely to open the door for
one or more of the alternative machines considered in this
article to be selected as the best candidate for a p
­ owertrain
traction motor application:
1) if powertrain requirements for one of the two special
strengths of sintered neo IPM machines, high torque/
power density or high efficiency, can be relaxed for a
particular powertrain application
2) if efficiency is a key requirement and the powertrain
duty cycle for a particular application includes long
periods of operation at high speeds with light loads
that have a significant impact on the powertrain’s
average efficiency
3) if the powertrain imposes either very harsh environmental conditions (e.g., high temperature or vibration), extremely low cost ceilings, or demanding fault
tolerance requirements on the traction machine
beyond what sintered neo IPM machines can achieve.
If any of these conditions is met, then adoption of
either an induction, SR, SynR, or PM machine using alternative magnet materials can emerge
to be the superior choice for those EVspecific powertrain applications.
Table 5 has been prepared to provide a summary comparison of the
alternative traction machine types that
have been discussed in the article. The
objective is to highlight the relative
strengths and weaknesses of these
machines for a variety of im­­portant
traction machine features, summarizing many of the key points discuss­­
ed in this article for the individual
machine types. There is a real dan­­
ger of unintended overgeneralization
None of the
machines rises
above all of the
others as a clearly
superior choice in
all of the traction
machine feature
categories.
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17
associated with a table of this type because each of the
machine types can be designed in a nearly endless
number of ways that may sacrifice one or more of the
listed features to strengthen the machine’s performance in another feature category. However, this table
is intended to focus on each of the machines types as a
class to highlight those features where the underlying
physics gives the machine type in question a relative
strength or weakness c
­ ompared to the competing traction machine types.
Inspection of the table entries indicates that none of
the machines rises above all of the others as a clearly
superior choice in all of the traction machine feature categories. Each machine type exhibits noteworthy strengths
in two or more categories and comparative weaknesses in
at least one of the other feature categories. The reason
sintered neo IPM machines play such a major role in production passenger EV powertrains is that the specific feature areas where they demonstrate their comparative
strengths align closely with the most important powertrain requirements in many EV ­applications.
In contrast, the lack of success that SR machines have
achieved to date in production passenger EVs despite the
comparative strengths that they display in several feature
categories is that the one machine feature where they
exhibit comparative weakness—acoustic noise and
torque ripple—is a very important requirement for the
majority of the passenger EV applications. In those electrified vehicle applications, such as off-road construction
and mining vehicles, where acoustic noise is not very
important, the SR machine has achieved wide acceptance, as ­discussed previously.
Before closing, it should be noted that space limitations
in this article prevented discussion of other ­interesting
machine types that are also being considered in both academia and industry as candidates for eliminating the use
of sintered neo magnets in HEVs and BEVs. These include
the classic wound-field synchronous machine that has
already been manufactured in limited numbers by Continental for production EVs introduced by Renault (Fluence
and Kangoo) in 2012. Alternatively, variable-flux PM
machines that use either Alnico or samarium-cobalt magnets with reduced magnetic coercivities compared to neo
magnets are also being seriously investigated because of
their ability to actively adjust the magnetization level in
the magnets during operation. Both of these alternative
machines share in common the objective of minimizing
the traction motor’s total energy usage over typical EV
drive cycles by optimizing the rotor field flux amplitude
for every machine torque/speed operating point.
The search for better traction machines for future EVs
is certain to continue during the coming years as EVs
evolve and machine performance characteristics im­­­
prove as well, based on the availability of new materials
that are likely to open the door to innovative machine
topologies. As a result, the competition between the
18
machine types compared in this article and new entries
that have not yet appeared is certain to continue for the
foreseeable future. There is good reason to believe that
the sintered neo IPM machines that have been so successful during the past 15 years will face strong competition to maintain their dominant position in production
passenger vehicles during the next 15 years. Competition
is a powerful motivator, so stay tuned for the next
­chapter in this unfolding story!
For Further Reading
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Prius motor design and manufacturing assessment,” Oak
Ridge Nat. Lab., Oak Ridge, TN, ORNL/TM-2004/137, July 2004.
T. Burress, C. L. Coomer, S. L. Campbell, A. A. Wereszczak, J. P.
Cunningham, L. D. Marlino, L. E. Seiber, and H. T. Lin, “Evaluation
of the 2008 Lexus LS 600H hybrid synergy drive system,” Oak
Ridge Nat. Lab., Oak Ridge, TN, ORNL/TM-2008/185, Jan. 2009.
I. Boldea, L. N. Tutelea, L. Parsa, and D. Dorrell, “Automotive
electric propulsion systems with reduced or no permanent
magnets: An overview,” IEEE Trans. Ind. Electron., vol. 61, no. 10,
pp. 5696–5711, Oct. 2014.
B. Sarlioglu, C. T. Morris, D. Han, and S. Li, “Driving toward
accessibility: A review of technological improvements for
electric machines, power electronics, and batteries for electric and hybrid vehicles,” IEEE Ind. Appl. Mag., vol. 23, no. 1, pp.
14–25, Jan.–Feb. 2017.
Z. Q. Zhu and C. C. Chan, “Electrical machine topologies
and technologies for electric, hybrid, and fuel cell vehicles,” in
Proc. 2008 IEEE Vehicle Power and Propulsion Conf., Harbin, China,
2008, pp. 1–6.
S. Jurkovic, K. Rahman, B. Bae, N. Patel, and P. Savagian,
“Next generation Chevy Volt electric machines: Design, optimization and control for performance and rare-earth mitigation,” in Proc. 2015 IEEE Energy Conversion Congr. Exposition
(ECCE), Montréal, Québec, 2015, pp. 5219–5226.
G. Pellegrino, T. M. Jahns, N. Bianchi, W. Soong, and F. Cupertino, The Rediscovery of Synchronous Reluctance and Ferrite Permanent
Magnet Motors: Tutorial Course Notes (SpringerBriefs in Electrical
and Computer Engineering). New York: Springer -Verlag, 2016.
C. Oprea, A. Dziechciarz, and C. Martis, “Comparative
analysis of different synchronous reluctance motor topologies,” in Proc. 2015 IEEE 15th Int. Conf. Environment and Electrical
Engineering (EEEIC), Rome, Italy, 2015, pp. 1904–1909.
S. Jurkovic, K. M. Rahman, J. C. Morgante, and P. J. Savagian,
“Induction machine design and analysis for General Motors
e-assist electrification technology,” IEEE Trans. Ind. Appl., vol.
51, no. 1, pp. 631–639, Jan.–Feb. 2015.
A. Chiba and K. Kiyota, “Review of research and development of switched reluctance motor for hybrid electrical vehicle,” in Proc. 2015 IEEE Workshop on Electrical Machines Design,
Control, and Diagnosis (WEMDCD), Turin, Italy, 2015, pp. 127–131.
T. Raminosoa, A. El-Refaie, D. Pan, K.-K. Huh, J. Alexander,
K. Grace, S. Grubic, S. Galioto, P. Reddy, and X. Shen, “Reduced
rare-earth flux-switching machines for traction applications,” IEEE Trans. Ind. Appl., vol. 51, no. 4, pp. 2959–2971, July–
Aug. 2015.
Biography
Thomas Jahns ( jahns@engr.wisc.edu) is with the Wisconsin Electric Machines and Power Electronics Consortium,
University of Wisconsin-Madison.
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