IEEE Electrification Magazine - March 2017 - 17

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 alter-
native 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 condi-
tions. 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 powertrain
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 environ-
mental conditions (e.g., high temperature or vibra-
tion), 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 alter-
native magnet materials can emerge
to be the superior choice for those EV-
specific powertrain applications.
Table 5 has been prepared to pro-
vide 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, summariz-
ing 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.

IEEE Electrific ation Magazine / march 2 0 1 7

17



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