IEEE Power Electronics Magazine Compendium - March 2018 - 15

advantage of the SRM is its low cost and rugged constructhe rotor currents create the rotor magnetic field. Torque
tion, which is attractive for automotive applications. Unlike
generation in IM technology is based on the interaction of
the PM synchronous and induction machines, the phases
the stator and rotor magnetic fields.
are mechanically and electrically isolated from each other,
If the rotor speed reaches the speed of the stator magwhich helps to provide fault tolerance.
netic field or the synchronous speed, the stator field linkAn SRM has a single source of excitation, and the torque
ing rotor bars will not change with time. In this case, no
production is based on the difference between the aligned
voltage is induced, no current flows through the conducand unaligned inductances or saliency. The phase current
tors, and the electromagnetic torque will be zero. Then,
is responsible both for producing torque and building up the
the rotor slows down with a rate dependent on the friction
magnetic field. Similar to the IM, a small airgap is required
coefficient of the shaft, and the rotor speed is eventually
in the SRM. When working in the linbalanced at a speed lower than the
ear region of the magnetization curve,
synchronous speed, where the motor
SRM technology has a lower power
torque equals to the load torque of
IPMSMs, IMs, and
factor and less than half of the total
the motor.
magnetic energy is converted into
Compared with the PM machines,
SRMs are the most
mechanical work. The rest is stored
a conventional IM typically has a
frequently considered
in the magnetic circuit and supplied
lower efficiency and power factor.
back to the source at the end of the
Since there is no direct excitation on
electric machine
stroke. This increases the converter
the rotor, the excitation current is
types for traction
size. In practice, SRM technology
drawn from the stator. This reduces
operates in the nonlinear region of
the power factor, and IMs are usuapplications.
its magnetization curve. In this case,
ally designed with a small airgap to
for the same rotor displacement, the
minimize the reactive power requiresaturation limits the flux linkage and,
ments. During high-speed operation,
hence, the induced voltage. As a result, the co-energy ratio
the lack of rotor excitation can provide higher efficiencies.
increases and higher torque can be achieved with the same
An induction motor operates based on the induced
current. This enables a higher power factor and also better
rotor voltages and currents. Therefore, there will always be
utilization of the converter.
rotor copper losses, which are dependent on the difference
The SRM challenges include acoustic noise and vibrabetween the rotor speed and synchronous speed or the sotion. Due to the single source of excitation and salient pole
called slip. The higher the slip, the higher the rotor frequency,
structure, vibration tends to be a limiting factor in SRMs
the higher the rotor voltage, and, hence, the higher the rotor
to achieve higher power densities. The high torque ripple
copper losses. High-efficiency IMs are designed to operate
content is another concern since it affects the noise, vibrawith less than 3% slip, and the rotor frequency is much lower
tion, and harshness characteristics of the powertrain and
than the stator frequency. For this reason, the rotor core
the fatigue of the mechanical components.
losses are smaller than the stator core losses. However, the
resistance of the rotor conductors has a significant impact
in determining the rotor copper losses, which is one of the
factors limiting the performance of the induction motors.

SRMs-Operating Principles
As compared with the PM and induction motors, the SRM
architecture has the simplest, lowest-cost, and most robust
structure. The SRM stator is made of a salient pole laminated core with concentrated windings, while the rotor
also has a salient pole structure, but without windings or
PMs (Figure 3). This enables reliable operation at high
speeds and elevated temperatures.
The torque produced in SRMs is reluctance torque. Due
to the salient pole structure, the airgap length varies according to the relative position of the rotor and stator poles.
This results in varying inductance values in different rotor
positions. When a coil is energized, the flux closes its path
through the closest rotor pole and tends to move it toward
the stator pole to reduce the reluctance of the magnetic path.
The salient pole stator and rotor structure brings some
advantages and also challenges in SRM drives. The biggest

fig 3 A typical SRM. (Photo courtesy of Nidec SR Drives Ltd,
North Yorkshire, United Kingdom.)

June 2014

z IEEE PowEr ElEctronIcs MagazInE

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