IEEE Electrification Magazine - March 2017 - 15

One of the most appealing features of the SR machine
is that its rotor requires no magnets or current conductors,
consisting only of a stack of steel laminations with salient
poles. This makes the SR machine rotor even simpler and
more rugged than those of nearly all SynR machines. The
stator windings have appealing elements of simplicity as
well, consisting of concentrated solenoidal coils around
individual stator lamination poles, benefiting from lower
winding complexity compared to more conventional over-
lapped distributed windings often found in the other three
ac machine types.
Although there are major differences in the operational
details of SR and SynR machines, the basic nature of their
torque production is the same. More specifically, current
excitation of a winding around one of the stator poles in
the SR machines creates a torque acting on the rotor
salient pole nearest to the pole face of the excited stator
pole, attempting to draw it into alignment even though
the rotor contains no magnets or conductors to generate
its own magnetic field. Provided that the stator and rotor
have different numbers of salient poles, exciting the SR
machine's multiple machine phases will cause the rotor
to spin at a frequency equal to the excitation frequency
divided by the number of rotor poles.
Similar to trends observed earlier with the IPM and
SynR machines, the reluctance torque in the SR machine is
maximized by increasing the ratio of the inductance val-
ues measured in the stator phase winding when a rotor
pole is perfectly aligned with the excited stator pole (maxi-
mum inductance) and when it is maximally unaligned
(minimum inductance). That means that, like the SynR
machine, the SR machine's torque production benefits
from reducing the air-gap length to the smallest acceptable
value based on manufacturing and bearing tolerances.
Each phase winding is excited with periodic current
pulses so that the machine produces positive motoring
torque while the nearest rotor pole is being pulled into
alignment and negative braking torque if the phase wind-
ing is excited while the rotor pole is moving out of align-
ment. The pulsed nature of the stator winding excitation
tends to produce more torque ripple in SR machines com-
pared to the other three types of ac machines previously
considered, which are all excited with smoothly varying
sinusoidal currents.
In addition, the SR machine's stator lamination shape,
consisting of stator winding poles interconnected by thin-
ner stator yoke segments, is vulnerable to mechanical
deformation attributable to ovalizing forces caused by
radial forces acting on the excited stator poles. This ovaliz-
ing deformation, in turn, frequently results in undesirable
acoustic noise common in many SR machines unless
special electromagnetic and structural design measures
are taken to minimize the mechanical deformation under
all significant operating conditions.
High-performance control of SR machines depends on
accurate control of the time instants when current

TaBle 3. Induction machines.
Advantages

Disadvantages

Complete elimination of all
rotor magnets, avoiding one
of the highest machine cost
contributors.

Rotor losses are generated
whenever torque is being
produced, even if Fe is
lossless.

Rotor losses can be
significantly reduced by
adopting copper squirrel
cage instead of aluminum.

Requires small air gap to
minimize reactive current
and maximize efficiency.

Drive-cycle efficiency of
well-designed induction
machine can challenge that
of an IPM machine in some
applications.

Induction machines are
generally not well suited to
high pole numbers, making
them less desirable for
lower-speed, direct-drive
applications.

Magnetic field strength in
machine can be lowered
via stator current to reduce
losses under light-load
condition; valuable for
extended high-speed cruise.

Constant-power speed range
is more limited than IPM
machines; increasing CPSR
requires reduced leakage
reactance, generating
additional PWM losses.

Absence of magnets
reduces induction machine's
vulnerability to damage
caused by short circuit
faults.

Generally a poor candidate
for rotor position selfsensing at low speeds
due to absence of rotor
saliency.

Figure 15. A basic SR machine cross section. (Image courtesy of
Wikimedia-Creative Commons.)

excitation pulses are applied to each stator phase winding
in relation to the instantaneous angular position of the
nearest rotor pole with respect to the excited stator pole.
By adjusting the advance angle when current is applied
with respect to the time instant of maximum alignment
during high-speed operation, the output power can be
held constant as the speed is increased. This makes it pos-
sible to achieve high CPSR values (greater than ten to one
in some cases) that are well suited to traction applications.
Similar to the other magnetless machines that have
been considered in this section, it is difficult for SR
IEEE Electrific ation Magazine / march 2 0 1 7

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

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