IEEE Electrification Magazine - March 2017 - 11

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 signifi-
cant 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 deliver mag-
netic flux density values that are a fraction of the typi-
cal 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 evaluat-
ed by researchers in industry and academia. One inter-
esting 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 pro-
vided 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 trad-
eoffs 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 vol-
ume. 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 oper-
ating principle of the SynR machine shares much in com-
mon with the IPM machine, except for the absence of any
magnet torque. As discussed in the preceding section,
IEEE Electrific ation Magazine / march 2 0 1 7

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2017

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