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FIELD OF THE INVENTION
The present invention relates generally to fault-tolerance in motor drives
and generating systems. More particularly, this invention relates to
switched reluctance machines which have the capability to continue
operating with minimum performance degradation in spite of machine or
inverter faults.
BACKGROUND OF THE INVENTION
AC machines are not inherently fault-tolerant. The primary reason is that
the windings of AC machines are closely coupled magnetically, so that a
short circuit in one winding has serious effects on adjacent phases. The
problem is exacerbated in AC machines having permanent magnets because
rotating magnets excite potentially dangerous high currents in any short
circuit path. Approaches to enhancing the reliability of AC motor drives
and generator systems generally involve the use of two or more AC
machines. For example, a common approach is to connect two or more
machines on a single shaft. Alternatively, gearing is used to couple the
machines together. However, there are weight, volume and cost
disadvantages associated with the use of additional machines, thus making
such approaches undesirable or even impractical for many applications.
Another approach, which is described in U.S. Pat. No. 4,434,389, issued to
Langley et al., is to utilize redundant sets of distributed windings,
i.e., windings spread over a number of slots around the air gap periphery.
This approach, for machines energized through an inverter, involves
dividing a permanent magnet motor into sections, each section comprising
one set of magnetically-coupled distributed windings. Each set of windings
is energized by a separate commutation circuit, so that the total torque
produced is the sum of the torques generated by each set of distributed
windings. For each motor section, a command unit detects failures and
removes the entire failed motor section from service. Disadvantageously,
the close magnetic coupling of the distributed windings makes it necessary
to disable the entire set of section windings, even though the fault has
developed in only one of these windings. Thus, torque production is
reduced by the amount contributed by the entire motor section rather than
by the smaller portion delivered by a single winding.
In contrast to AC machines, a switched reluctance (SR) machine is wound
using concentrated windings, i.e., windings concentrated on projecting
motor poles. As a result, the phase windings of a SR machine are
essentially free of any magnetic coupling so that high currents in one
winding will not magnetically induce high currents in adjacent phase
windings. The present invention utilizes this characteristic magnetic
independence of switched reluctance machine phases as the basis for a
compact, fault-tolerant motor drive or generator system. Such a
fault-tolerant drive can be particularly useful in aerospace applications
for which highly reliable drives are necessary.
Switched reluctance machines conventionally have multiple poles on both the
stator and the rotor; that is, they are doubly salient. There is a
concentrated winding on each of the stator poles, but no windings or
magnets on the rotor. Each pair of diametrically opposite stator pole
windings is connected in series or parallel to form an independent machine
phase winding of the multiphase SR machine. Motoring torque is produced by
switching current in each machine phase winding in a predetermined
sequence that is synchronized with angular position of the rotor, so that
a magnetic force of attraction results between the rotor poles and stator
poles that are approaching each other. The current is switched off in each
phase before the rotor poles nearest the stator poles of that phase rotate
past the aligned position; otherwise, the magnetic force of attraction
would produce a negative or braking torque. The torque developed is
independent of the direction of current flow, so that unidirectional
current pulses synchronized with rotor movement can be applied to the
stator pole windings by an inverter using unidirectional current switching
elements, such as transistors or thyristors. For use as a generator, the
current pulses in each machine phase winding are simply shifted so that
current flows when the rotor poles are moving past alignment towards the
unaligned position.
A SR motor drive or generator system operates by switching the machine
phase currents on and off in synchronism with rotor position. That is, by
properly positioning the firing pulses relative to rotor angle, forward or
reverse operation and motoring or generating operation can be obtained.
Usually, the desired phase current commutation is achieved by feeding back
a rotor position signal to a controller from a shaft angle transducer,
e.g. an encoder or a resolver. However, in order to reduce size, weight
and cost in SR motor drives and generating systems, techniques for
indirect rotor position sensing have been developed, thus eliminating the
need for a shaft angle transducer. One such technique is disclosed in
commonly assigned U.S. Pat. No. 4,772,839, which issued on Sept. 20, 1988
to S. R. MacMinn and P. B. Roemer.
Current regulators are typically employed for controlling phase current
amplitudes in a SR machine. There are several types of current regulators.
For example, individual low-resistance current shunts may be coupled to
each machine phase winding to detect the current level in each phase. The
output of each current shunt is connected to a separate voltage
comparator. Each comparator is also connected to a separate potentiometer
for setting the current limit. Another type of current regulator, which
eliminates the need for discrete current sensors, is disclosed in U.S.
Pat. No. 4,595,865, issued to T. M. Jahns on June 17, 1986 and assigned to
the instant assignee.
Commonly assigned copending U.S. patent application Ser. No. 304,159, filed
on Jan. 31, 1989 by G. B. Kliman, S. R. MacMinn and C. M. Stephens,
discloses a system for detecting and isolating faults in a SR motor drive,
whereby faulted motor phases are deactivated and motor operation is
continued. More specifically, this patent application, which is hereby
incorporated by reference, describes a SR machine fault management system
which detects faults through phase current differential sensing and phase
flux differential sensing. In addition, a method is provided for starting
the motor when stopped in a "torque dead zone" created by a faulted phase.
As used herein, the term "torque dead zone" is a rotor angular position
region in which positive motoring torque cannot be produced by any of the
intact non-faulted phases. By way of contrast, in a SR generator system, a
"voltage output dead zone" is the counterpart to a torque dead zone in a
SR motor drive. As used herein the term "voltage output dead zone" is a
rotor angular position region in which no voltage output can be generated
by any of the intact non-faulted phases.
Although the hereinabove cited patent application advantageously provides a
system for isolating and detecting SR machine phase faults, it is
desirable to enhance the characteristic independence of SR machine phase
windings even further in order to optimize SR machine fault-tolerant
performance. In accordance therewith, it is desirable to simplify the
fault-tolerant SR machine drive and to prevent the development of "torque
dead zones" in motors and "voltage output dead zones" in generators.
OBJECTS OF THE INVENTION
It is, therefore, an object of the present invention to provide a new and
improved switched reluctance motor drive or generator system.
Another object of this invention is to provide a SR motor drive or
generator system which optimizes SR machine fault-tolerant performance by
taking advantage of the characteristic independence of SR machine phase
windings.
Another object of the present invention is to provide a fault-tolerant SR
motor drive or generator system which can continue operating with minimal
performance degradation despite the existence of a fault in the machine or
in its associated power electronics.
Another object of the present invention is to provide a fault-tolerant SR
motor drive or generator system for which the rotor does not experience an
unbalanced magnetic force in spite of the existence of a fault causing
excitation to be removed from a respective stator phase.
Still another object of the present invention is to provide a
fault-tolerant SR motor drive having no "torque dead zones" created by
faulted phases that prevent the intact phases from producing torque in
some rotor positions.
Yet another object of this invention is to provide a fault-tolerant SR
generator system having no "voltage output dead zones" created by faulted
phases that prevent the intact phases from generating output power in some
rotor positions.
SUMMARY OF THE INVENTION
In accordance with the present invention, a new and improved switched
reluctance motor drive or generator system is provided with capability to
continue operating with minimal performance degradation in spite of the
existence of machine or inverter faults. To this end, the present
invention utilizes the characteristic independence of the concentrated
phase windings of a SR machine.
In one embodiment of a SR motor drive according to the present invention,
each stator pole winding is excited by a separate respective inverter
phase leg. For a SR motor having N phases and K stator pole windings per
phase (with K greater than or equal to 2), this drive embodiment uses K
independent inverters, with N phase legs in each inverter. These inverters
can be excited by the same DC source or, preferably, by separate DC
sources to achieve even a higher level of fault tolerance. Loss of
excitation to one stator pole winding does not affect excitation of the
remaining (K-1) pole windings in the same phase, or excitation of any of
the pole windings in the other phases; therefore, average torque
production by the motor remains at approximately (NK-1)/NK of its normal,
pre-fault value. Moreover, no "torque dead zones" are created by faulted
phases in this new SR motor drive; that is, there are no rotor positions
for which the remaining intact phases cannot produce torque. Hence, if the
rotor is brought to a standstill condition following a fault, no special
controls are needed to restart the machine.
In an alternative embodiment of a SR motor drive according to the present
invention, each motor phase comprises at least two pairs of diametrically
opposite stator poles. A stator pole winding is wound on each pole, and
the pole windings on diametrically opposite poles are grouped together
into pairs and connected either in series or in parallel. For a SR motor
having N phases and J stator pole pairs per phase (for a total of 2NJ pole
windings, with J greater than or equal to 2,) this drive embodiment uses J
independent inverters, with N phase legs in each inverter. These inverters
can be excited by the same DC source or, preferably, by separate DC
sources to achieve even a higher level of fault tolerance. Loss of
excitation to one pair of diametrically opposite pole windings does not
substantially affect excitation of the remaining (J-1) pole winding pairs
in the same phase, or excitation of any of the pole winding pairs in the
other phases. Therefore, torque production continues at approximately
(NJ-1)/NJ of its pre-fault value, and no torque dead zone is created.
Advantageously, for this alternative SR motor drive configuration, a fault
in one inverter leg which results in loss of excitation of one pair of
stator pole windings will not produce unbalanced magnetic pull on the
rotor.
Further, according to the present invention, the machine configurations
described herein to realize fault-tolerant switched reluctance motor (SRM)
drives for delivering mechanical power to a load also constitute
fault-tolerant switched reluctance generator (SRG) systems for converting
mechanical power into electrical power. Only the timing of the gating
signals shifts with respect to rotor position in order to convert a motor
drive into a generating system. Moreover, in a SRG system, voltage output
dead zones, which are the counterparts to torque dead zones in a SRM
drive, are eliminated by employing the fault-tolerant configurations of
the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become apparent
from the following detailed description of the invention when read with
the accompanying drawings in which:
FIG. 1 is a schematic illustration of a conventional SRM drive;
FIG. 2 is a cross-sectional view of a SRM illustrating the direction of
current in an exemplary motor phase winding and further illustrating the
direction of magnetic flux resulting therefrom;
FIG. 3 is a graphical illustration of the instantaneous torque waveform for
the SRM drive configuration of FIG. 1 following loss of excitation of a
faulted phase;
FIG. 4A is a cross-sectional view of a SRM constructed in accordance with
the present invention;
FIGS. 4B and 4C are schematic illustrations of the inverters employed to
drive the SRM of FIG. 4A;
FIG. 5 is a graphical representation of the instantaneous torque waveform
for the SRM drive configuration of FIG. 4;
FIG. 6A is a cross-sectional view of an alternative embodiment of a SRM
constructed in accordance with the present invention;
FIGS. 6B and 6C are schematic illustrations of a set of inverters employed
to drive the SRM of FIG. 6A; and
FIGS. 7A-7D are schematic illustrations of an alternative set of inverters
employed to drive the SRM of FIG. 6A.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows a typical switched reluctance motor (SRM) drive configuration.
It is to be understood that the following description of a switched
reluctance motor drive is provided by way of example only and that the
principles of the invention apply equally to switched reluctance generator
systems. Therefore, as used herein and in the claims, the term "machine"
includes motors and generators.
By way of example, SRM 10 is illustrated as a three-phase machine with its
associated power inverter 12. As shown, SRM 10 includes a rotor 14
rotatable in either a forward or reverse direction within a stationary
stator 16. Rotor 14 has two pairs of diametrically opposite rotor poles
18a-18b and 20a-20b. Stator 16 has three pairs of diametrically opposite
stator poles 22a-22b, 24a-24b and 26a-26b. Stator pole windings 28a-28b,
30a-30b and 32a-32b, respectively, are wound on stator pole pairs 22a-22b,
24a-24b and 26a-26b, respectively. Conventionally, the stator pole
windings on each pair of opposing or companion stator pole pairs are
connected in series to form a motor phase winding so that the current I in
each phase produces a net magnetic flux linkage generating flux in the
directions indicated by arrows 52 and 53 in FIG. 2. For example, as shown
in FIG. 2, windings 28a and 28b are connected in series so that the
current flows in the direction indicated. As illustrated in FIG. 1, the
stator pole windings comprising each companion pair 28a-28b, 30a-30b and
32a-32b, respectively, are connected in series with each other and with an
upper current switching device 33, 34 and 35, respectively, and with a
lower current switching device 36, 37 and 38, respectively. The upper and
lower switching devices each comprise a field-effect transistor (FET), but
other suitable current switching devices may be used, such as bipolar
junction transistors (BJTs), gate turn-off thyristors (GTOs) and
insulated-gate bipolar transistors (IGBTs). Each motor phase winding is
further coupled to a DC power supply by flyback or return diodes 45 and
42, 46 and 43 and 47 and 44, respectively. At the end of each conduction
interval of each phase, stored magnetic energy in the respective motor
phase winding is returned to the DC source through the respective pair of
these diodes connected thereto. Each series combination of a motor phase
winding with two corresponding switching devices and two flyback diodes
comprises one phase leg of inverter 12. The inverter phase legs are
connected in parallel to each other and are driven by a DC source, such as
a battery or a rectified AC source, which impresses a DC voltage +V.sub.S
across the parallel inverter phase legs. Capacitance 40 is provided for
filtering transient voltages from the DC source.
Typically, a shaft angle transducer 48 is coupled to rotor 14 for providing
rotor angle feedback signals to a motor control means 50. However, as
hereinabove discussed, techniques are available for eliminating the shaft
angle transducer. Phase current feedback signals are supplied to control
means 50 from a current regulator (not shown), also hereinabove discussed,
which receives phase current feedback signals from current sensors (not
shown). An operator command, such as a torque command, is also generally
inputted to control means 50. In well known fashion, such as described in
U.S. Pat. No. 4,739,270, issued Apr. 19, 1988 to S. R. MacMinn and P. M.
Szczesny and assigned to the instant assignee, the control means provides
firing signals to inverter 12 for energizing the motor phase windings in a
predetermined sequence.
In operation, if a fault occurs in a machine phase or an inverter phase of
a conventional SRM drive such that excitation is lost to two opposing or
companion stator pole windings, a "torque dead zone" is created by the
faulted phase. Although rotor inertia can carry the rotor through this
torque dead zone once it is rotating, special inverter controls are needed
to restart the SRM if it stops in this dead zone created by the faulted
phase. Once rotating, the torque dead zone cannot be eliminated by
overexciting the remaining intact phases.
FIG. 3 is a graphical illustration of the instantaneous torque waveform (T)
for the SRM drive configuration of FIG. 1 following loss of a faulted
motor phase. The lost torque contribution due to the faulted phase is
indicated by dashed lines 56. As illustrated, the average torque
production T.sub.AVE is approximately two-thirds of its initial pre-fault
value T.sub.0.
A fault-tolerant three-phase SRM drive according to the present invention
is shown in FIG. 4A. In the following description, all stator pole
windings which share the same magnetic relationship with the rotor, such
as companion windings 32a and 32b, are considered part of the same machine
phase regardless of whether they are directly interconnected. Unlike the
conventional SRM drive of FIG. 1, the stator pole windings wound on
opposing or companion stator pole pairs are not connected in series.
Instead, each stator pole winding is excited by a separate respective
inverter phase leg. In the preferred embodiment, two independent inverters
60 and 62 are utilized, each comprising three phase legs. Preferably, each
inverter 60 and 62 is driven by a separate DC source to achieve a higher
level of fault tolerance than if one power source were used.
Alternatively, however, both inverters can be driven by the same DC
source. As shown, each respective phase leg of each inverter excites one
stator pole winding respectively. Thus a first phase leg of each of
inverters 60 and 62 excites stator pole windings 28a and 28b,
respectively; a second phase leg of each of inverters 60 and 62 excites
stator pole windings 30a and 30b, respectively; and a third phase leg of
each of inverters 60 and 62 excites stator pole windings 32a and 32b,
respectively. Thus each phase leg, respectively, of each inverter
corresponds to one of the three motor phases, respectively, of SRM 10.
During normal, non-faulted operation, each stator pole winding comprising a
companion pair conducts simultaneously during a predetermined conduction
interval. That is, they are excited coincidentally for torque production
throughout a common time interval. Moreover, the polarities of the
companion stator pole winding pairs are arranged so that the magnetic flux
patterns are identical to those of the conventional SRM, as illustrated in
FIG. 2. In this way, under non-faulted conditions, the new SRM drive
operates in the same manner as the conventional SRM drive of FIG. 1.
However, unlike the conventional SRM drive, if a fault occurs in an
inverter phase or a machine phase of the SRM drive of FIG. 4, then no dead
zone in torque production is created. For example, even if excitation is
lost to stator pole winding 28a due to a fault, uninterrupted excitation
to the opposing or companion stator pole winding 28b ensures that there
nevertheless is some torque production during the conduction interval of
the corresponding motor phase.
FIG. 5 is a graphical illustration of the instantaneous torque waveform (T)
for the SRM drive configuration of FIG. 4 following loss of excitation to
a stator pole winding of a faulted motor phase. The torque contribution
from the companion stator pole winding of the faulted phase is shown by
dashed lines 63. As illustrated, because the companion stator pole winding
of the faulted phase still produces torque during the respective
conduction interval, there is no dead zone and the average torque
production T.sub.AVE is approximately 5/6 of the initial pre-fault value
T.sub.0, averaged over a complete rotation. Moreover, using this
configuration, the post-fault average torque may be increased to the
pre-fault value T.sub.0 if there is sufficient current capacity to
overexcite the remaining intact stator pole windings. Advantageously, in
the absence of a torque dead zone, no special controls are required to
restart the motor if the rotor stops following a fault.
Under normal, non-faulted operating conditions, the excitation of two
opposing or companion stator pole windings with equal currents ensures
that the radial pull forces from the two corresponding poles cancel, while
their torque contributions add. However, when excitation is removed from
only one stator pole winding of a companion pair, there is a net radial
pull force on the rotor in addition to the desired tangential force or
torque. Therefore, it may be necessary to reinforce the motor bearings to
withstand the resulting unbalanced magnetic pull on the rotor.
In an alternative embodiment of the present invention, generation of the
hereinabove described unbalanced magnetic force is prevented. By way of
example, FIG. 6 shows a three-phase SRM 70. As illustrated, SRM 70
includes a rotor 72 within a stationary stator 74. Rotor 72 has four pairs
of diametrically opposite rotor poles 74a-74b, 76a-76b, 78a-78b and
80a-80b. Stator 74 has six pairs of diametrically opposite or companion
stator poles 82a-82b, 84a-84b, 86a-86b, 88a-88b, 90a-90b and 92a-92b,
respectively, fitted with companion stator pole winding pairs 96a-96b,
98a-98b, 100a-100b, 102a-102b, 104a-104b and 106a-106b, respectively. In
this example, each motor phase comprises two pairs of diametrically
opposing or companion stator pole windings; i.e., two companion stator
pole winding pairs. For example, the two stator pole winding pairs 96a-96b
and 102a-102b comprise one of the three motor phases of SRM 70.
Preferably, two independent power inverters 105 and 107 are employed to
drive SRM 70. Each respective inverter phase leg corresponds to a separate
respective motor phase and comprises two semiconductor switches and two
flyback diodes which excite opposite or companion stator pole windings
connected in series with each other. Alternatively, the two stator pole
windings comprising each companion pair, such as 96a-96b, can be connected
in parallel. The four stator pole windings corresponding to each
respective motor phase are excited for torque production during the same
time interval; i.e., they share an entire conduction interval in common.
When a fault occurs in a motor phase of SRM 70 such that excitation is
removed from one pair of companion stator pole windings corresponding to a
respective motor phase, excitation is not interrupted to the other
companion stator pole winding pair. Advantageously, therefore, in this
embodiment of the SRM drive, the fault does not create an unbalanced
magnetic pull on the rotor or its bearings since both diametrically
opposed windings in the faulted phase are unexcited. Moreover, if
excitation is lost to stator pole winding pair 96a-96b, for example,
uninterrupted excitation to the companion pair 102a-102b of that faulted
phase ensures that symmetrical excitation continues. Further, the average
torque production is reduced only by approximately 1/6 of its pre-fault
value for the same current, and no torque dead zone is created by the
fault.
Still another alternative embodiment of the inverter configuration used to
drive SRM 70 is shown in FIGS. 7A-7D. In this embodiment, post-fault
average torque is increased even further. As shown, four independent
three-phase inverters 110, 112, 114 and 116 are employed. Each phase leg
of each inverter corresponds to one respective motor phase and excites one
stator pole winding of a companion pair corresponding thereto. Loss of one
inverter phase leg due to a fault removes excitation from only one stator
pole winding, resulting in loss of only approximately 1/12 of the
pre-fault average torque.
It is to be understood that the present invention is not limited to
three-phase SRM drives and SRG systems, but may be extended to SR machines
having any number of phases. Moreover, the present invention is not
limited to the numbers of stator poles and rotor poles hereinabove
described. For example, for a four-phase SR machine having eight stator
poles and six rotor poles, each of four inverter phases can be used to
excite two companion stator pole windings corresponding to a respective
machine phase. Alternatively, each of the eight stator pole windings can
be excited by a separate inverter phase, the excitation of the four stator
pole winding pairs being synchronized during normal operation.
While the preferred embodiments of the present invention have been shown
and described herein, it will be obvious that such embodiments are
provided by way of example only. Numerous variations, changes and
substitutions will occur to those of ordinary skill in the art without
departing from the invention herein. Accordingly, it is intended that the
invention be limited only the spirit and scope of the appended claims.
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