IEEE Electrification Magazine - March 2017 - 77

motors
Interior permanent magnet (IPM)
machines and IMs are used in EVs
today. IPM machines dominate HEV
use and most models of EVs. They
are prized for their highest peak efficiency, good speed capability, and
most compact size. Their drawbacks
include the use of magnets and possible faults resulting in demagnetization or uncontrolled generation. IM
efficiency can be as good or better

Loss Comparison
120
100
80
Loss (W)

efficiency. High-voltage silicon carbide (SiC) field-effect transistors
(FETs) show a great improvement
over even the best of the now-mature
silicon (Si) insulated-gate bipolar
transistors (IGBTs). SiC FETS are
resistive devices and have as little as
half the losses of IGBTs in the automotive inverter application. SiC
power devices, like junction gate
FETs and metal oxide-semiconductor FETs (MOSFETs), have been used
in selected nonautomotive applications, like photovoltaic converters, for
the past few years (Figure 9).
New technologies, like SiC-based
automotive inverters, are always more
costly than the incumbent conventional technology in the initial stages.
SiC material and processing challenges remain. There are a wide variety of
SiC FET types and features, each with
unique strengths and weaknesses.
There are inverter application development challenges, and innovation
must continue there. However, overall
inverter efficiency, physical size reduction, and higher temperature tolerance give us reasons to be optimistic
about SiC. Ultimately, these should
lead to greater electric drive integration and an overall reduction in EV
propulsion system costs. Most leading
power semiconductor makers are
sampling developmental SiC products
now and projecting initial automotive
production in the next product cycle.
Some device makers are gaining valuable production knowledge for their
devices by putting them in racing
applications today.

Turn-On
Switching
Loss

60
40

Turn-Off
Switching
Loss

(at 30 kHz)

Turn-On Switching Loss
Turn-Off Switching Loss
Conduction Loss

Compared to
IGBTs Loss
Reduced by 73%

20
0

Conduction
Loss
Si IGBT

SiC MOSFET

Figure 9. A comparison of Si IGBT losses versus SiC MOSFET losses. (Courtesy of Rohm.)

than IPM at light loads and high
speeds. They make sense only when
the peak power is very high relative
to the average power level and the
vehicle space affords a larger motor
size. The Tesla powertrain features an
IM, notable with a copper rotor to
further mitigate efficiency disadvantage. Others are said to be developing
IMs for EV use. Both motor technologies will see continued development.
IPM and IM designs featuring rectangular-wire cross sections represent
an innovation that has taken hold of
the EV motor business to support higher duty in today's full-performance
EVs. As the name suggests, these
motors have rectangular section conductors to wind the stator instead of
conventional round section wire. The
technology has many names, such as
bar wound, form wound, hairpin, and
segmented conductor, but it is the
rectangular-wire section and simplified
wave style winding that distinguishes
it from the more conventional stranded, round section wire constructions,
having a variety of winding styles.
A major advantage with motors
having rectangular conductors is that
the wire sections fill the slightly trapezoidal volume of the stator tooth openings much better than round wires.
Basically, with rectangular wire, we can
fit more copper into the motor for

great effect. This is huge for pushing
more current thought the same size
machine to deliver more peak power.
This also creates a great reduction in
copper losses materially, improving
driving efficiency at nominal loads.
The rectangular-wire design reduces
resistance by more than 30% at low
speeds compared with conventional
round-wire machines (Figure 10).
Another benefit of rectangular wire
is heat rejection. The very regularly
formed rectangular-wire armature has
a much greater exposed surface in the
end-turn area when compared to the
tight bundles of stranded round wire.
For today's more advanced EV motor
designs, these wires double as motor
coolers and are directly wetted with
oil. Heat rejection is directly a function
of the exposed surfaces; rectangular
wires create 50-100% more heat
rejection area.
These advantages are not without
issue. Some will argue that the round
wire actually has lower ac resistance
at higher motor speeds because of
proximity effects in the larger cross
sections of the rectangular wire. But a
deeper investigation will reveal that
the speed range where round wire is
advantageous is very high and beyond
the realm of most driving energy. A
second concern is that the costs of a
rectangular-wire machine are higher
IEEE Electrific ation Magazine / march 2 0 1 7

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