IEEE Electrification Magazine - March 2017 - 9

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 reluc-
tance (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 alterna-
tives 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 dis-
cussions 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 pro-
portional to the amplitude of the mag-
net's flux density, so stronger magnets
(such as sintered neo magnets) in an
IPM machine rotor are capable of deliv-
ering more magnet torque than weak-
er 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. Reluc-
tance torque is attributable to the same
physical force mechanism that causes a
steel needle to align itself with a mag-
netic 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 pro-
duction 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 objec-
tive is to design the rotor with higher numbers of mag-
net 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 reduc-
ing the dependence on such large numbers of high-
strength 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.

IEEE Electrific ation Magazine / march 2 0 1 7

9



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

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