IEEE Power Electronics Magazine Compendium - March 2018 - 18

The melting temperature in copper
such as dysprosium, are added. In a
die casting (1,080 °C) is higher than
typical traction motor application,
The
biggest
advantage
that of aluminum (660 °C). In copneodymium makes up ~31% of the
of
the
SRM
is
its
low
per die casting, the dies should be
total magnet mass, whereas the dyspreheated, and, during the casting
prosium content makes up ~8.7%.
cost and rugged
process, the temperature should be
According to the statistics provided
construction,
which
is
controlled. The density of copper is
by the U.S. Department of Energy, due
higher, and this requires specialized
to the increase in the demand, the
attractive for
tooling and higher tonnage presses.
price of rare-earth elements increased
automotive
Considering all of these aspects,
significantly from 2003 to 2008. There
with the current manufacturing
was a much steeper increase between
applications.
technologies, copper die-cast induc2009 and 2011, mainly because of the
tion motors are more expensive than
rare-earth supply quota limitations
the conventional induction motors,
and taxes imposed by China. China
but they have higher efficiency, lower temperature rise
has more than 95% of the rare-earth element production
and, hence, reduced size.
and ~70% of the total rare-earth magnet production in the
world. With the increase in the future demand for electrified
powertrains, the demand for neodymium and dysprosium
A Structural Aspect: Noise and Vibration in SRMs
is expected to increase significantly. Considering the penThe SRM does not have PMs or conductors on the rotor.
etration of two-wheelers and wind turbines in the short and
The rotor and stator are made of laminated steel, and the
long term, neodymium and dysprosium are already defined
stator poles have concentrated winding. It has a low-cost
as critical materials by the U.S. Department of Energy. As a
and robust structure, and it can operate in harsh conditions
result, there is currently a significant effort in the industry
and at elevated temperatures. These features of the SRM are
and in academia toward reducing the dysprosium content
all very attractive for automotive applications.
in the magnets and also designing motors using less or no
Due to the double saliency, the airgap length changes
magnet volume. In addition, with the emergence of new
for the relative positions of the rotor pole. This results in a
companies in the rare-earth supply chain, China's dominatvarying inductance and torque profile. For this reason, the
ing trend in the rare-earth market might tend to fall in the
SRM operates with high-torque pulses, which, if not dealt
mid and long term.
with, might affect the lifetime of the machine and the powertrain components. Torque ripples can still be controlled
by modifying the geometry and, hence, the inductance proA Manufacturing Aspect: Copper Die-Cast IMs
file of the SRM. To reduce the commutation-based torque
IMs do not have PMs on the rotor. However, they have conripples, special control techniques can be applied (such as
ducting bars. The inherent rotor copper losses could be a sigtorque sharing functions), especially at lower speeds when
nificant challenge in IMs. Especially during high-torque operthe phase current is controlled. At high speeds, the rate
ation, this causes difficulties in extracting the heat out of the
of change of the flux linkage can become so high that the
motor and, hence, reduces the torque density. For traction
maximum available voltage would not be enough to control
applications, it is important to design an IM so that the rotor
the current. In this case, single-pulse operation is usually
structure is optimized to deliver the required torque at a
applied, where only the conduction angle is controlled. In
lower slip with the lowest possible rotor copper losses.
single-pulse operation, the current waveform and the peak
Currently, Tesla is using 310-kW, 600-Nm, three-phase IM
current are dependent on the conduction angle, inductance
with copper rotor bars in its Model S electric vehicle. Copper
profile, and the winding resistance. This might lead to a high
has 60% higher conductivity as compared with aluminum.
crest factor, resulting in higher torque ripples.
This reduces the resistance of the rotor conductors and,
Noise and vibration is one of the major challenges of SRM
therefore, for the same current, copper losses are lower. With
architecture. Due to the double salient structure, the radial
the copper bars, the operating temperature can be reduced
component of the airgap force is significant. For radial flux
and the size of the motor can be improved.
machines, tangential force creates the motion. The strong
In conventional IMs, aluminum rotor bars are genradial forces excite vibrations and create acoustic noise.
erally used, which are die casted in the rotor slots. Die
The noise and vibration level in SRMs can be reduced by
casting is a metal casting process where molten metal
modifying the geometry of the core and the frame (Figure 5).
is injected into the die at high pressure. It is a quick and
For this purpose, the natural frequencies for different core
reliable process and is commonly applied in the mass
and frame geometries should be analyzed. When the natural
manufacturing of IMs. The die casting of copper is also
frequency of the frame and the stator match the excitation
applied in the industry; however, due to some inherent
frequency of the electromagnetic force, the vibration level is
challenges compared with the aluminum die casting, the
the highest. It is undesirable to have the natural frequency of
manufacturing cost of the copper bar induction motor is
the frame/stator below the maximum excitation frequency.
higher that of the motor with the aluminum conductors.

IEEE PowEr ElEctronIcs MagazInE

z June 2014

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