IEEE Power Electronics Magazine - June 2014 - 15

stresses from mechanical processes, such as laser cutting
(Figure 4), punching, welding, interlocking, and wire insertion. These processes cause an increase in the iron loss
content. The stress on the cut edges and, hence, iron losses
can be reduced by applying a special heat treatment process
called stress-relief annealing.
In electric traction motors, the lamination thickness is
usually between 0.2 and 0.35 mm. When thinner-gage materials are used to reduce the eddy-current losses, special
attention should be given to the manufacturing process. No
significant deformation should be created on the laminations that might increase the iron loss content of the steel.
Electrical steels are usually selected in terms of their
electrical characteristics. However, especially when operating at high speeds, their mechanical characteristics
also become important since the lamination materials are
exposed to large forces. In the case of interior PM machines,
the yield strength of the steel should be high enough that
the rib area between the airgap and the PM can handle
the stress and prevent failure. When the motor is designed
with a high yield strength material, the rib thickness can be
small. In this case, the magnet would be closer to the airgap,
maintaining higher efficiency.

fig 4 The laser cutting process of laminations. (Photo courtesy
of Polaris Laser Laminations, LLC, Illinois.)

starter generators, the ambient temperature can go up
to 140 °C.
Today, a separate cooling system is generally used in
electric traction motors that usually has a lower temperature than the conventional cooling system of the engine. To
reduce the cost associated with the
cooling system, in some cases, it may
A Thermal Aspect: Thermal
be desirable to share the same cooling
Management System
Significant
powertrain
system between the electric traction
A higher torque and power density in
motor and the engine. In this case,
an electric machine requires higher
improvements are
however, the coolant temperature
current density, flux density, and freneeded, and
might be higher, which will add addiquency. All lead to higher heat dissipational challenges in the motor design
tion, but the increase in losses can be
electrification is a
process. The thermal management
reduced by optimizing the machine's
central focus of many
system has to be fine-tuned to operate
design. Higher efficiency provides
at the optimum temperature for the
lower energy consumption, lower temnew efficient
motor and the engine since the motor
perature rise for the same volume and,
powertrain designs.
losses are related to the coolant temhence, longer operational life. Less
perature. The design challenges might
temperature rise will increase the life
be more significant for PM machines
span of the machine components,
since the coercivity of rare-earth magnets is highly depenincluding the windings and insulation. It helps achieve the
dent on the temperature.
machine life targets for traction motors (typically 15 years
or 150,000 mi).
To achieve the power density targets, an integrated
A Material Aspect: Rare-Earth PMs
thermal management system and better heat transfer techDue to the high-energy product of their rare-earth magnets,
nologies are necessary. In traction motors, adequate heat
PM traction motors provide high torque density and high effidissipation is critical to meeting performance, reliability,
ciency. However, in PM machines, extra caution is usually
and lifetime requirements. The heat dissipation capabilnecessary in the design process since the coercive force of
ity is related to the efficiency or total losses generated
the PMs reduces with an increase in temperature. Coercive
by the motor. The temperature rise can be calculated by
force shows the resistance of the magnet toward demagnetiidentifying the thermal resistance in various parts of the
zation when an external magnetic field is applied. The drop
machine and the heat transfer mechanism involved. The
in the coercive force will be higher with a higher temperaambient temperature should be evaluated carefully when
ture. Therefore, the reverse field capability of the magnetic
designing the thermal management system of an electric
circuit should be carefully evaluated under high-temperature
traction motor. Especially in hybrid electric vehicles, if
conditions, especially at high-speed operation.
the traction motor is located near the engine, the ambiTo increase the intrinsic coercivity and, hence, to
ent temperature can be around 120 °C. If the motor is
improve the resistance of the magnet toward demagnetizalocated on the engine, such as in the case of belt-driven
tion at elevated temperatures, heavy rare-earth elements,
June 2014

z IEEE PowEr ElEctronIcs MagazInE

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - June 2014