IEEE Power Electronics Magazine Compendium - March 2018 - 16

Electric Motor Requirements
for Powertrain Applications

Electrical steels are soft magnetic materials with narrow
hysteresis loops. The saturation flux density can be achieved
with a low coercive force, enabling higher power density. Steel
The current electric traction motors are manufactured in
is basically an iron-carbon alloy. Iron has excellent magnetic
lower volume than conventional ICE vehicles and are relaproperties and would have the hightively expensive. The U.S. Department
est intrinsic saturation magnetic flux
of Energy has defined the aggressive
density. However, the carbon content
cost and power density targets for
Due
to
the
low
should be eliminated to provide easier
electric traction motor drives to
domain wall movement.
reduce the cost of the system and
efficiency of ICE
Iron also has a finite electrical conencourage their large-scale use. In
vehicles and
ductivity. When exposed to alternat2020, electric traction systems are
ing magnetic fields, eddy currents are
expected to cost less than US$8/kW
dependency on fossil
generated, which cause power losses.
with a specific power greater than 1.4
fuels, our conventional
Electrical steel with lower resistivkW/kg and 4 kW/L. In addition, the
ity has naturally lower eddy-current
peak efficiency is expected to be
transportation system
losses. For this purpose, silicon is
greater than 94% under aggressive
is not sustainable.
added to the lamination material durcooling specifications.
ing the rolling process. For electric
Achieving these targets requires
traction motors, the silicon content
delivering the required torque and
is usually limited (up to 3.5%). A higher silicon content
power in a lighter-weight package and within a much
increases resistivity, but, at the same time, reduces the
smaller envelope. This not only affects the electromagnetic
magnetic flux density of the steel. In addition, a higher silidesign but also the dissipation of generated heat and the
con content introduces brittleness and makes the rolling
structural performance of the machine. Considering all of
process harder. For iron-silicon alloys, impurities, such as
these aspects, electric traction motor design will require a
carbon, sulfur, nitrogen, and oxygen, should be minimized
multidisciplinary approach, including expertise in electroby the annealing process to improve the magnetic characmagnetic, mechanical, thermal, and structural systems.
teristics and reduce the effects of magnetic aging.
The materials and manufacturing techniques used in tracTo reduce the eddy-current losses, magnetic cores are
tion motors also affect the performance and cost. In the
made of laminated electrical steel sheets. This reduces the
following sections, the multidisciplinary nature of electric
length of the eddy-current path, leading to lower resistance
traction motor design and development will be investigated
and, hence, lower losses. When selecting electrical steel for
in different machine types, and the related design issues
an electric traction motor, the operational requirements
will be addressed.
should carefully be defined. Selecting the correct lamination material with the proper grade and thickness provides
An Electromagnetic Aspect: Torque
an optimum solution between low enough iron loss content
Generation and Current Density
and high enough saturation flux density.
In electric machines, the electromagnetic force generated
Due to the variance in the direction of the flux in electric
on the rotor creates torque. The electromagnetic force is a
machines,
nonoriented electrical steel is usually used. Nonfunction of the cross product of the current density and
oriented
electrical
steel shows similar magnetic properties
flux density. Therefore, higher current and airgap flux denboth
in
the
rolling
direction
and transverse directions. This
sities would enable achieving higher torque for the same
is
usually
desired
in
electric
machines due to the variation
volume. However, higher current density increases the staof
flux
at
different
rotor
positions
and operating conditions.
tor copper losses. This results in more heat dissipation
Nonoriented
steel
is
generally
produced
by a cold-rolling proand, hence, higher temperature rise in the machine. The
cess. Grain orientation and higher silicon content alloys can
proper design of the windings and the stator structure can
be achieved with a hot-rolling process. High silicon content
enable lower losses by reducing the winding resistance. A
steels are also used in other magnetic components in electrihigher fill factor and improved insulation also help in
fied powertrain applications, such as power inductors.
achieving higher current densities, provided that the coolMetallic alloys are made up of grains, which are microing system is well designed to enable the dissipation of the
scopic areas or crystallites in a certain direction. In nonorigenerated heat.
ented iron-silicon alloys, grain growth by applying annealing process at a higher temperature improves the magnetic
A Material Aspect: Electrical Steels
characteristics and reduces losses. To increase the torque
The torque output in an electric motor can be improved by
output, the saturation magnetic flux density of the steel can
increasing the magnetic flux density. In this case, the electribe increased by texture control, stress-relief annealing, or
cal steel should also be capable of handling high flux dencontrolling the silicon content. Throughout the manufactursity levels, so the required torque can be achieved with a
ing of the motor, the laminations are exposed to additional
lower current.

IEEE PowEr ElEctronIcs MagazInE

z June 2014

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018