A switched reluctance d.c. motor having a plurality stators has been disclosed. Each stator comprises phase windings for energizing a set of stator poles. The stator poles are distributed about the circumference of the stator yoke. Two phases activated in succession are associated with distinct ones of the plurality of stators. The motor of the present invention operates under a wide range of rotational speeds while reducing the switching losses and audible noise in the rotor and stator iron.

FIELD OF THE INVENTION 
This invention generally relates to brushless direct current (d.c.) motors, 
and more particularly, to switched reluctance or variable reluctance 
motors that have their phase windings commutated electronically without 
the use of mechanical brushes or commutators. Even more specifically, the 
present invention related to types of brushless d.c. motors producing 
continuous rotational torque without the use of permanent magnets. The 
timing for the phase commutations can be facilitated by sensing the 
position of the rotor. The position sensor may be an encoder a resolver, 
or Hall switch sensor mounted to the shaft of the motor. 
BACKGROUND OF THE INVENTION 
Because of recent developments in power semiconductor devices such as power 
MOSFETS and IGBTS, the proliferation and usage of brushless d.c. motors 
has intensified in recent years. These developments have enhanced the 
spectrum of practical uses for such motors. The applications of such 
motors are centered around either variable/adjustable speed or servo 
positioning systems. Furthermore, switched reluctance motors, for a number 
of reasons are particularly well suited for applications which require 
operation over a wide speed variation such as traction motors for electric 
vehicles such as automobiles, buses and trains without the use of 
transmissions. 
The availability of high energy permanent magnets such as samarium cobalt 
or neodymium boron iron has also contributed to the current interest in 
brushless d.c. motors. Due to the high cost of these high energy magnets 
and mechanical difficulties of retaining them in mountings, however, there 
has also been a keen interest in the class of brushless d.c. motors that 
do not use permanent magnets or windings in connection with the rotating 
member--or rotor. This class of brushless d.c. motors is commonly called 
switched reluctance motors or simply SR motors. The design, operation, 
construction and use of this class of electric motors is documented in 
Switched Reluctance Motors And Their Control, by T. J. E. Miller, Magna 
Physics Publishing, 1993 ISBN1-881855-02-3. 
SR motors have been used extensively as stepping motors known as variable 
reluctance motors. When used as stepping motors, the operation of the 
motor is controlled by a series of clock pulses in an open loop manner. As 
such, the commutation frequency and phase of the motor is driven without 
regard to the angular position of the rotor. 
In stepping motor systems, the motor has typically been referred to as a 
variable reluctance (VR) motor. Many of these so-called VR stepping motors 
are either three-phase or four-phase machines with laminated designs 
having many teeth on each rotor and stator magnetic pole. The many teeth 
facilitate the progression of the rotor at small step angles (e.g., U.S. 
Pat. No. 3,866,104 to Heine). It is known to separate the plurality of 
phases associated with a VR stepper motor so that each stator of a 
plurality of stators for the motor has associated with it a single phase. 
Furthermore, SR motors have been developed wherein the rotor is axially 
displaced from the stators. A pie-shaped rotor comprises alternating 
magnetically permeable/non-permeable slices separated from stator poles by 
axial gaps. Axial gap motors have lower power density and lower torque to 
inertia ratios than radial air gap motors, and are therefore not suitable 
for the high torque applications. 
The present invention concerns a closed-loop continuously-rotating type 
radial gap SR motor rather than a stepping type motor which is controlled 
in an open-loop manner. The type of SR motor to which the present 
invention is directed is designed to convert electrical energy into a 
continuous mechanical rotation instead of bursts of torque which are 
difficult to control as is provided by stepping motors. The SR motor 
produces continuous torque (i.e., minimal torque ripple) at a desired, 
preset or controllable speed of motor rotation. 
SR motors of the type described herein usually have stators wound with 
either three or four phases. Each phase is associated with a separately 
controlled electromagnetic winding. Each phase is energized or connected 
to a d.c. power source and commutated or switched at the optimum rotor 
position in order to produce a desired output torque characteristic having 
minimized torque variation as the motor rotates under the influence of the 
energized phases. Torque variation or torque ripple is minimized for a 
particular motor design at a given rotational speed by careful commutation 
of the phases so as to result in a constant torque vector. 
In known motor control schemes, the commutation controllers energize the 
phases in a manner such that the duty cycles of adjacent phases overlap. 
The summed torque provided by the overlapping energized phases maintains 
the torque at a level near the peak torque for a conventional SR motor 
commutated with unipolar d.c. voltage. 
SR motors having magnetically permeable rotors are very robust motors, have 
a very simple rotor construction and an extremely compactly wound stator. 
Such structural characteristics yield the lowest potential manufacturing 
cost of any known motor. Furthermore, due to their simple construction SR 
motors are well suited for heavy duty use in the most severe environments 
and can operate in temperature extremes, for example, between -100.degree. 
and +500.degree. C. 
In SR motors there is no need for bi-directional current to energize each 
phase in order to produce torque since the stator poles magnetically 
attract soft iron rotor poles rather than north or south magnetized 
permanent magnets. Therefore, the direction of the current energizing the 
stator poles remains unchanged and the rotor poles change in accordance 
with the polarity of the energized stator poles. 
Because polarity of the current is not important in SR motors, the stator 
phase windings are connected in series with switching transistors thereby 
eliminating the possibility of shoot-through faults. This possibility 
cannot be eliminated in induction motors and permanent magnet brushless 
motors where the phase windings are connected in a "Y" or Delta 
configuration. 
While increasing the number of phases may reduce torque ripple, one 
disadvantage of increasing the number of phases and the number of poles in 
a motor is the increase in switching or commutation frequency. When a 
phase of a motor is energized or de-energized the change in magnetic flux 
resulting from the change in current flowing through the phase winding 
causes eddy current losses in the lamination iron of the stator and rotor, 
which in turn causes heating. As the rotation speed of the rotor 
increases, the commutation frequency increases for the phase windings. The 
increased commutation frequency increases losses which in turn causes 
heating in the iron core of the stator and rotor. 
Another loss resulting from magnetic field flux reversals is known as 
hysteresis loss. These flux reversals also cause a heating loss in the 
iron cores of the stator and rotor and the heating effect increases with 
the number of phases, the number of poles and the rotational speed of the 
motor. A full magnetic flux reversal from a positive flux value to a 
negative value of flux causes a "full loop" energy loss. If the flux field 
only increases from zero to some maximum value and then decreases back to 
approximately zero when the phase winding is commutated, then a "minor" 
hysteresis loop is produced in the iron core. 
Induction motors and permanent magnet motors require bi-directional current 
switching resulting in a full magnetic flux reversal in the iron core. 
Thus, the magnetic iron experiences heating due to full magnetic flux 
reversal hysteresis loops. However, SR motors, having rotors comprising 
magnetically permeable material and operating under uni-directional 
current passing through energized stator phase windings, only experience 
heat losses produced by minor hysteresis loops resulting from the flux 
linkage cycling between a near null value to a peak value and then 
decreasing back to the near null value. Therefore, the SR motors generally 
operate with less iron losses per commutation cycle than induction motors 
and permanent magnet motors. 
SR motors having rotors comprising magnetically permeable materials are 
indeed desirable for their relatively lower heat losses. Nevertheless, it 
is desirable to further reduce the heat losses in an SR motor. 
In my U.S. Pat. No. 4,883,999, an SR motor is described that significantly 
reduces the losses experienced in back iron of the motor. In that patent, 
stator windings for the same phase are located adjacently on the stator. 
In my U.S. Pat. No. 5,111,095, an SR motor is described that reduces the 
energy loss experienced in the back iron of the motor and provides for a 
structure wherein two phases are simultaneously energized to provide 
enhanced torque. 
SUMMARY OF THE INVENTION 
It is a general object of the present invention to provide an SR motor 
capable of operating over a very wide rotations-per-minute range with a 
very high, low-speed starting torque as well as a very high speed 
capability. 
It is a further object of the invention to achieve the above wide high 
performance operating speed range with high efficiency. 
It is yet another object of the invention to achieve the above wide high 
performance operating speed range with low torque ripple. 
It is yet another object of the invention to achieve the above wide high 
performance operating speed range with low internal heating losses. 
It is yet another object to provide an SR motor having reduced audible 
noise. 
Another object of the present invention is to provide a lower cost SR 
motor. 
The above and other objects are met in a polyphase split-phase switched 
reluctance (SR) motor comprising a plurality of rotors and a plurality of 
stators. The stators are separated by a radial gap from the rotors. The 
windings of the stators are collectively energized by a set of phases 
provided by a power converter. However, each stator in the set of stators 
is associated with a plurality of energizing phases comprising less than 
the set of phases provided by the power converter. 
In accordance with one aspect of the present invention, an even number of 
total phases are distributed evenly between a first stator assembly and a 
second stator assembly. Phase windings associated are connected to a 
driver circuit such that excitation of phase windings occurs in an 
alternating manner between the first and the second stator assemblies. 
By distributing the phases between a plurality of stators, the effective 
mass subject to each commutation is reduced. Furthermore, the preferred 
embodiment provides desirable torque characteristics while reducing the 
total number of commutations executed by the motor per revolution. As a 
result, switching losses caused by the creation and breaking down of 
magnetic fields is decreased substantially in comparison to comparable 
known motors. 
Furthermore, since the duty cycle of each stator is reduced by the 
distributing of the phases among a plurality of stators, each stator has a 
significant rest period. The rest period permits cooling, and therefore 
the potential for overheating is decreased. 
Various energization schemes may be employed to power an SR motor according 
to the invention. Using a polyphase source, the SR motor may operate with 
only one phase substantially on at a time. However, in practical 
operation, the energizing of the phases is overlapped in order to 
compensate for delays in turning on and off the phases to achieve very low 
torque ripple.

DETAILED DESCRIPTION OF THE DRAWINGS 
Turning to the drawings and referring first to FIG. 1A, a perspective 
illustration is provided of an SR motor embodying the present invention. A 
first stator 2 is preferably formed from a stack of laminations made of 
magnetically permeable steel alloy. The first stator 2 includes eight (8) 
stator poles labelled A or C in accordance with an associated one of two 
phase windings for the first stator 2. As illustrated in FIG. 1A, the 
stator poles extend radially inwardly from an annular yoke 4 and are 
evenly spaced about the inner circumference of the yoke from adjacent 
rotor poles by 45 degrees. Furthermore, the width of each stator pole on 
the end proximate the rotor poles is equal to the gap between the ends 
proximate the rotor poles of adjacent stator poles. 
Continuing with the description of FIG. 1A, a first rotor 6 is matched with 
the first stator 2 to produce torque on an axle 8 illustrated in FIG. 1B. 
The axle 8 is formed from steel or other known suitable materials. The 
first rotor 6 comprises a stack of laminations made of a magnetically 
permeable iron alloy. As depicted in FIG. 1A, the first rotor 6 comprises 
four (4) rotor poles. The four (4) rotor poles extend radially outwardly 
from the axis of rotor rotation. The four (4) rotor poles of the first 
rotor 6 are evenly spaced 90.degree. from adjacent rotor poles. The length 
of a rotor pole should be at least 1.5 times the width of the rotor pole 
at the end of the rotor pole proximate the stator poles in order to 
achieve a high inductance ratio (between a maximum inductance and minimum 
inductance). Furthermore, the width of the rotor poles should be slightly 
greater than the width of the stator poles. 
In the illustrative embodiment of the present invention, the SR motor 
includes a second stator 10 having the same construction as the first 
stator 2. The second stator 10 comprises an annular yoke 12 and eight (8) 
evenly spaced stator poles labelled B or D in accordance with an 
associated one of two phase windings for the second stator 10. The actual 
phase windings have been omitted from FIG. 1A in order to simplify the 
drawing. A second rotor 14 of same construction as the first rotor 6 is 
mounted on the axle 8 as illustrated in FIG. 1B and matched with the 
second stator 10. An axial gap between the first stator 2 and the second 
stator 10 having a distance d provides a high reluctance path between the 
first stator 2 and the second stator 10. The distance d is determined by 
the space required to accommodate the end windings of the stator poles. As 
shown in FIG. 1B, the first rotor 6 and the second rotor 14 are also 
separated by a substantially same spacing along the axle 8 as the distance 
d between the stator 2 and stator 10. 
In the illustrative embodiment of the present invention, the stator poles 
of the second stator 10 are rotationally skewed in relation to the stator 
poles of the first stator 2 by 22.5.degree.. As explicitly shown in FIG. 
1B, the rotor poles of the second rotor 14 are radially aligned with the 
rotor poles of the first rotor 6. Therefore, when the four rotor poles of 
the first rotor 6 are in maximum alignment with four stator poles for one 
of the two phases of the first stator 2 (phase A in FIG. 1A), the four 
rotor poles of the second rotor are out of maximum alignment with the 
stator poles of each of the two phases of the second stator 10 by 
22.5.degree.. As will be apparent from FIG. 1A, when the axle 8 is rotated 
22.5.degree. thus rotating each of the rotors by 22.5.degree., the first 
rotor is positioned 22.5.degree. out of maximum alignment with each of the 
two phases of the first stator 2, and the second rotor 14 is placed in 
maximum alignment with four stator poles for one of the two phases of the 
second stator 10. 
As will be appreciated by those skilled in the art of brushless motor 
design, the inductance and torque characteristics of the motor are 
influenced by a number of rotor and stator physical design factors 
including the dimensions of the poles of the rotors and stators, the 
amount of overlap between rotor poles and stators, and the radial gap 
between aligned rotor and stator poles. Though the rotor and stator poles 
in the illustrative embodiment of the invention have been shown to have 
aligned side edges when in maximum alignment and have completely unaligned 
edges when in maximum disalignment, this is not a requirement for 
practicing the present invention. 
As previously explained, the illustrative embodiment of the SR motor 
includes four phases for energizing the stator poles. The four phases, 
identified by the letters A, B, C and D are each associated with four (4) 
stator poles identified in the drawings by a one of the phase 
identification letters. Turning now to FIG. 2A, in the illustrative 
embodiment of the present invention the polarity of the phase windings 
(indicated in FIG. 2A by cross-hatching) for a same phase are reversed for 
adjacent same phase stator poles while opposite stator poles (top/bottom 
and left/right) are the same polarity thereby creating four (4) flux paths 
when a one of the phases is energized in a stator. As a consequence, the 
rotors are pulled from four directions by an energized phase. The pulling 
from four directions reduces the effect of forces tending to reshape the 
stator yoke from its intended circular shape. This reshaping of the stator 
yoke is a significant source of audible noise during the operation of 
conventional switched reluctance motors. 
Taking for example phase A for the first stator 2 in FIG. 2A, when the 
rotor poles of the first rotor 6 are in maximum alignment with the stator 
poles of an energized phase A of the first stator 2 flux paths P.sub.1 
P.sub.2 P.sub.3 and P.sub.4 are formed in the first stator 2. The flux 
paths P.sub.1 P.sub.2 P.sub.3 and P.sub.4 are in a state of maximum 
inductance. It will be appreciated by those skilled in the art that no 
torque is exerted when the rotor poles are aligned with energized stator 
poles. 
Turning now to FIG. 2B, the flux paths P.sub.5 P.sub.6 P.sub.7 and P.sub.8 
are illustrated for an energized phase B in the second stator 10 when the 
rotor poles are unaligned with the stator poles associated with the 
energized phase B. When in the unaligned position, the flux paths P.sub.5 
P.sub.6 P.sub.7 and P.sub.8 are characterized by a relatively low 
inductance in comparison to the inductance value for the flux paths 
arising from aligned rotor poles and stator poles (as illustrated in FIG. 
2A). As will be appreciated by those skilled in the art of switched 
reluctance motors, the energization of phase B while the second rotor 14 
is in the illustrated position in FIG. 2B will create a rotational force 
causing the rotor to turn in a clockwise direction as the rotor and 
energized stator poles seek a position of maximum inductance. Maximum 
inductance is attained when the rotor poles and the energized phase B 
stator poles are aligned. 
Turning now to FIG. 3, illustrative inductance and torque curves are shown 
for a switched reluctance (SR) motor of the type depicted in FIG. 1A. The 
inductance and torque curves graphically illustrate the relationship 
between rotor/stator pole alignment and inductance and torque (produced by 
a fully energized phase) as a rotor assembly for an SR motor of the type 
illustrated in FIG. 1A rotates 180.degree. in a clockwise direction. In 
order to carry out the 180.degree. rotation, the SR motor progresses 
through two four-phase commutation sequences (of phases A, B, C and D). In 
order to make one full rotation (360.degree.), the four-phase commutation 
sequence is performed four times. 
It is noted that the stator and rotor drawings in FIG. 3 show fragmented 
views of the aligned one of the two (2) rotor and stator pairs at a 
particular rotational position of the illustrative SR motor. However, all 
the stator pole positions are easily determined in view of the illustrated 
rotor and fragmented stator and the overall stator assembly and rotor 
assembly configuration illustratively depicted in FIGS. 1A, 1B, 2A and 2B. 
With respect to the torque versus rotor angle curves in FIG. 3, it is 
assumed that the phases are energized to produce clockwise rotation on the 
rotors. Furthermore, it is assumed that a phase is energized when the 
rotor poles are 45.degree. short of maximum rotational alignment with the 
stator poles for the phase, and an energized phase fully de-energizes when 
the rotor poles are in maximum rotational alignment with the stator poles 
for the phase. It will be understood by those skilled in the art that a 
limitless number of phase energization schemes may be used to drive the 
four phase windings of the four phase motor SR motor illustratively 
depicted in FIG. 1A. Furthermore, the torque and inductance curves are 
shown in broken lines for the B and D phases in order to more easily 
distinguish those curves from the overlapping curves for the A and C 
phases of the illustrative example of an SR motor embodying the present 
invention. 
When the first rotor 6 and second rotor 14 are in a rotational position 
identified in FIG. 3 as 0.degree., the poles of the first rotor 6 are in 
full alignment with the phase C stator poles. Therefore, the inductance is 
at a maximum and the torque is at a minimum with respect to the phase C 
stator poles. Both inductance and torque are minimized with respect to the 
phase A stator poles. Inductance is increasing and torque exerted by the 
energized phase D stator poles is near a maximum value. 
At the 22.5.degree. rotor position, inductance is increasing and torque 
exerted by the energized phase A stator poles is approaching a peak value. 
Also, as illustrated in the inductance and torque curves in FIG. 3, at the 
22.5.degree. rotor position, inductance is maximized and torque is 
minimized for the phase D stator poles. Both inductance and torque are 
minimized with respect to the phase B stator poles. 
As the rotor progresses to the 45.degree. rotor position, inductance is 
maximized and torque is minimized for the phase A stator poles. Also, as 
the rotor passes through the 45.degree. position, inductance is increasing 
and torque exerted by the energized stator poles is approaching a maximum 
value for phase B stator poles. Torque and inductance are minimized for 
the phase C stator poles. The remaining inductance and torque curves for 
the B, C, and D stator poles as the rotors progress through the 
67.5.degree., 90.degree., 112.5.degree., etc. positions will be understood 
by those skilled in the art and therefore are not described further 
herein. 
FIG. 4 provides a schematic diagram for a four phase SR motor 16 and 
control system. The control system is conventional in design and includes 
a rotor position sensor 18 that delivers pulses via line 19 to a 
controller 20 indicating the current rotor position. The signals on line 
19 are also used by the controller 20 to calculate the rotational velocity 
of the SR motor 16. The controller 20 also receives input signals on line 
21 in a known manner for adjusting a set point of the rotational speed of 
the motor. The controller 20 in turn transmits control signals via lines 
23 to phase drivers 22 for the A, B, C and D phases of the SR motor 16. 
The signals transmitted on lines 23 control the turning on and off of 
power transistors in the phase drivers 22. The phase drivers 22, 
illustrated in greater detail in FIG. 5 described below, are powered by a 
DC Power Supply 24. In accordance with the stator winding configuration of 
the illustrative embodiment of the invention in FIG. 1A, the lines 
corresponding to the A and C phase windings are coupled to a Stator #1, 
and the lines corresponding to the B and D phase windings are coupled to a 
Stator #2 in the SR motor 16. 
Feedback signals corresponding to the current passing through each of the 
phases A, B, C and D are transmitted on lines 25 to the controller 20. The 
feedback signals on lines 25 are utilized to control phase drivers in 
order to minimize torque ripple and to prevent excessive current from 
passing through the phase drivers. 
Turning now to FIG. 5, a schematic circuit diagram is illustrated for a 
phase driver circuit for energizing the phase windings A, B, C and D of 
the four phase motor illustratively depicted in FIG. 1A. Two upper power 
transistors 40 and 42 of known construction are connected via line 44 to 
V.sub.s. Power transistor 40 is connected via line 46 to inductors A and 
C, corresponding to the phase A and phase C stator phase windings. Power 
transistor 42 is connected via line 48 to inductors B and D, corresponding 
to the phase B and phase D stator phase windings. The upper transistors 40 
and 42 regulate the current associated with inductors A and C, and B and D 
respectively by regulating the duty cycle of the upper transistors 40 and 
42. If too great a current is detected by the controller 20 through one of 
the phases, then one of the upper transistors 40 or 42 is shut off to 
reduce the current through the phase. If the current level drops below a 
desired level in one of the phases, then the gate voltage of one of the 
upper transistors 40 and 42 is adjusted in order to increase current 
through the phase. 
Inductor A is connected via line 50 to a commutating power transistor 52 of 
known construction. Inductor C is connected via line 54 to a commutating 
power transistor 56. Inductor B is connected via line 8 to power 
transistor 60, and inductor D is connected via line 62 to power transistor 
64. Each of the power transistors 52, 56, 60, and 64 are connected to 
ground via line 66. The gates of each of the power transistors 52, 56, 60 
and 64 are separately connected to a corresponding control line of the 
lines 23 from the controller 20 (in FIG. 4). 
A capacitor 68 is attached in parallel to the commutated inductor circuits 
to protect against transient voltage spikes which could damage the 
semiconductor circuitry. Furthermore, diode 70, inserted between V.sub.s 
on line 44 and one end of the inductor A on line 50, and diode 72, 
inserted between ground on line 66 and the other end of the inductor A on 
line 46, provide a path for dissipating the stored energy in the inductor 
A when the power transistor 52 and/or the upper transistor 40 are switched 
off. Diode 74 is similarly attached to V.sub.s on line 44 and one end of 
the inductor C on line 54 in order to provide an energy dissipation path 
for the inductor C through diodes 72 and 74. Diodes 76, 78, and 80 are 
connected to the portion of the phase driver circuit associated with 
phases B and D (in manner analogous to the connection of diodes 70, 72 and 
74) in order to dissipate the stored energy in the inductors B and D when 
the transistors 42, 60 and 64, associated with phases B and D, are 
switched off. 
Having described the hardware of the illustrative embodiment of the SR 
motor of the present invention, attention is now directed to the control 
of the circuitry to cause the rotor to rotate about the axis defined by 
the axle 8. Turning first to FIG. 6, the commutation sequences are 
summarized for clockwise and counter-clockwise rotation. Turning to the 
column labelled "CW," clockwise rotation is attained in the SR motor 
illustrated in FIG. 1A by energizing the phase A winding, then phase B 
winding, then phase C winding, and then phase D winding. The commutation 
sequence then begins again with the energizing of phase A. Counter 
clockwise rotation is achieved by energizing the phases in a reverse order 
in accordance with the phase progression depicted in the column labelled 
"CCW" (i.e., A, D, C and B). 
FIG. 7 comprises a set of waveforms relating the rotational position of the 
rotor assembly to: (a) the current flowing through energized phase 
windings, and (b) the torque applied to the rotor assembly to provide 
clockwise rotation arising from the energized phase windings. Only two 
iterations of the phase commutation sequence for clockwise motion are 
illustrated by the waveforms in FIG. 7. A single revolution requires four 
iterations of the commutation sequence illustrated in FIG. 7. 
As illustrated by the current waveforms for phase windings I.sub.A, 
I.sub.B, I.sub.C, and I.sub.D, the power transistors 52, 56, 60 and 64 (of 
the driver circuit illustrated in FIG. 5) are commutated so that a 
substantial current is flowing through a stator winding during 
22.5.degree. of rotation of the rotor assembly during each commutation 
sequence. The difference between the maximum and minimum torque applied to 
the rotor assembly as the rotor assembly rotates is referred to as torque 
ripple. One objective when controlling an SR motor is to minimize torque 
ripple which in turn leads to smoother mechanical operation of an SR 
motor. 
In the low RPM operating region of the SR motor embodying the present 
invention, low torque ripple is achieved by initially energizing a stator 
winding by substantial current flow when rotor poles are positioned more 
than 22.5.degree. before maximum rotational alignment with the energizing 
stator poles. The power transistor corresponding to an energized stator 
pole is commutated "off" at a point of rotation of the rotors so that the 
current passing through the energized phase winding is near zero when the 
rotor poles approach maximum alignment with the de-energized stator poles. 
The portion of the torque diagram represented by dashed lines corresponds 
to the portion of the torque potential associated with each phase which is 
not used because the power transistor associated with the phase is 
commutated to the "off" position. As a result, a relatively smooth torque 
curve is produced for the four-phase SR motor embodying the present 
invention when operating at a low rotational velocity under the phase 
energization scheme depicted in the current waveforms of FIG. 7. 
In the illustrated current diagrams, the duty cycle for the adjacent 
energized phases does not overlap and the current is shown to drop off 
very sharply. As will be appreciated by those skilled in the art of SR 
motor phase driver circuit design, the inductors cannot be instantaneously 
turned on and off, and therefore the current waveforms illustrated in FIG. 
7 (having very sharp "on" and "off" points) cannot be achieved at high 
operating rotational velocities. Therefore, during high rotational 
velocity operation, the duty cycles of adjacent phases overlap and the 
total torque is a sum of the torque exerted by each energized overlapping 
phase. 
The rotation angle in which a stator phase winding can be energized to 
provide useful torque is 45.degree.. Thus, in an alternative embodiment of 
the present invention, each of the four phases is energized for a full 
45.degree. of rotation. Energizing a phase begins when the rotor poles are 
45.degree. before maximum alignment with the energizing phase. An 
energized phase is commutated "off" when the stator poles for the 
energized phase are in maximum alignment with the rotor poles. In such a 
scheme, two phases are energized at a time in a staggered fashion. 
Turning now to FIG. 8, the progression of an aligned pair of rotors in an 
SR motor having a pair of stators disaligned by 22.5.degree. (as in the SR 
motor in FIGS. 1A and 1B) is schematically illustrated for one complete 
clockwise revolution of the illustrative SR motor of the present invention 
in a series of 16 snapshot drawings labeled (a) through (p). Each snapshot 
view corresponds to when a one of the rotors 6, 14 is in alignment with 
the stator poles associated with one of the four phase windings of the 
stators 2, 10 (labelled FRONT and REAR respectively). 
Having described the structure and operation of an illustrative embodiment 
of the present invention as well as modifications thereto, attention is 
now directed to FIGS. 9A and 9B illustrating an alternative SR motor 
wherein the stator poles of a first stator 90 are rotationally aligned 
with the stator poles of a second stator 92. Specifically, the phase A 
stator poles are rotationally aligned with the phase B stator poles. The 
phase C stator poles are rotationally aligned with the phase D stator 
poles. However as shown in FIG. 9B, in accordance with an alternative 
embodiment of the present invention, the rotation orientation of the rotor 
poles of a first rotor 94 are skewed 22.5.degree. with respect to the 
rotor poles of a second rotor 96 on axle 98. 
Rotating the rotor assembly in a clockwise direction for the SR motor 
illustrated in FIGS. 9A and 9B is accomplished by commuting the stator 
phases in the sequential order of phase A, phase D, phase C, and phase B. 
The commutation sequence is then repeated starting with phase A. 
Counter-clockwise rotation is accomplished by commuting the phase windings 
associated with the labeled stator poles in the order of phase A, phase B, 
phase C, and phase D. 
Turning now to FIGS. 10A, 10B, 11 and 12, another alternative embodiment of 
the four phase SR motor illustrated in FIG. 1A is provided having an 
enhanced fail-safe architecture. While the orientation of the first stator 
100 and the second stator 102 in relation to the first rotor 104 and the 
second rotor 106 remain unchanged from FIG. 1A, each stator pole is 
provided two, independently controlled, stator phase windings. For 
example, as indicated in FIG. 10A, each of the four stator poles labeled A 
is associated with a stator phase winding A.sub.1 and a stator phase 
winding A.sub.2. Each of the four stator poles labeled C is associated 
with a stator phase winding C.sub.1 and a stator phase winding C.sub.2. 
The sets of four stator poles in FIG. 10b, identified by the labels B and 
D, are each similarly associated with two independently controlled stator 
phase windings. 
Turning now to FIG. 11, an electrical schematic circuit diagram is provided 
of a phase driver circuit in accordance with the alternative embodiment of 
the invention depicted in FIGS. 10A and 10B. Each of the four pairs of 
phase windings is connected in parallel to V.sub.s and ground by an upper 
transistor and a lower, commutating transistor. Though not shown in FIG. 
11, the phase driver circuit includes a capacitor and diodes connected to 
the phase windings (in a manner analogous to the diode connection scheme 
in FIG. 5) in order to de-energize an energized phase winding of the type 
included in FIG. 5. 
Alternative phase driver circuits will of course be known to those of 
ordinary skill in the art. For example, the upper transistors may be 
shared between phase windings in a manner similar to the embodiment 
illustrated in FIG. 5. However, sharing an upper transistor detracts from 
the goal of minimizing the effect of a transistor failure upon the 
operation of the motor since the failure of an upper transistor will 
affect the operation of two (2) phase windings. However, because only half 
of each of the two phases is affected in a phase winding configuration of 
the type illustrated in FIGS. 10A and 10B, the shared upper transistor 
circuit configuration is still very desirable. 
As will be readily apparent from the drawing in FIG. 11, the failure of one 
of the switches associated with, for example, phase winding A.sub.2 will 
result in only a decrease in the torque provided by phase A rather than a 
complete loss of torque from phase A since the phase winding A.sub.1 is 
unaffected by the failure of a switch associated with the phase winding 
A.sub.2. As illustrated in FIG. 12, the torque resulting from the 
energized A.sub.1 phase winding, though considerably less than the torque 
exerted by simultaneously energized stator poles A.sub.1 and A.sub.2, 
reduces the effects of torque ripple and may prevent breakdown of the 
motor in instances where complete failure of the motor would have 
catastrophic consequences. 
Having described various illustrative and alternative embodiments of the 
present invention, the SR motor illustrated in FIG. 1A will be compared to 
the well known 8/6 SR motor illustrated in FIG. 13. The 8/6 SR motor 
includes a stator 108 having eight stator poles. The eight stator poles 
are divided into four pairs of stator poles. The stator poles associated 
with phases A, B, C and D are labeled with an appropriate letter to 
indicate the phase associated with the stator pole. The 8/6 SR motor also 
includes a rotor 110 having six rotor poles. The phases are energized in 
the order A, B, C and D for clockwise rotation and A, D, C, and B for 
counter-clockwise rotation. The commutation sequence is executed a total 
of 6 times for each complete revolution of the 8/6 SR motor. Therefore, 
twenty-four commutations are executed per revolution in the 8/6 SR motor. 
FIG. 14 presents a comparison chart summarizing the structural and 
operational characteristics for the 8/6 SR motor and the SR motor 
illustrated in FIG. 1A (referred to as an 8/8/4 SR motor). While the 
number of rotors and stators is doubled for the 8/8/4 SR motor, advantages 
provided by the SR motor of the present invention effectively overcome 
this apparent shortcoming. 
Both the 8/6 SR motor and the 8/8/4 SR motor contain eight stator poles per 
stator. The 8/6 SR motor includes a rotor having six rotor poles while 
each of the two rotors in the 8/8/4 SR motor includes four rotor poles. As 
previously noted, the 8/6 SR motor has only one stator, while the 8/8/4 SR 
motor has two stators. 
Even though the rotor 110 of the 8/6 SR motor contains six rotor poles, 
only two of the rotor poles are acted upon by two energized stator poles 
during a given stroke. The attractive forces tending to draw the two rotor 
poles to the energized stator poles tend to ovalize the stator yoke in the 
8/6 SR motor since the two attracted stator poles are positioned at 
opposing locations of the stator 108. 
On the other hand, all four rotor poles of one of the two rotors 6, 14 of 
the 8/8/4 SR motor are acted upon by four (4) stator poles associated with 
an energized phase winding. As a result, the attractive forces tending to 
draw the rotor poles to the energized stator poles tend to square the 
stator yoke having the energized stator phase windings in the 8/8/4 SR 
motor. Furthermore, since twice as many rotor poles in the 8/8/4 SR motor 
are acted upon in any stroke, only one-half the torque need be applied to 
each rotor pole in order to achieve the same torque applied to the rotor 
poles of the 8/6 SR motor. Therefore, the length of each stator and rotor 
(in the axial direction) may be reduced to a length one-half the length of 
a rotor and stator in an 8/6 SR motor having comparable operating 
characteristics but attracts only two (2) rotor poles during any given 
stroke. 
In general, the ovalizing forces exerted upon the stator yoke in the 8/6 SR 
motor have been shown to deform the stator yoke to a greater degree than 
the squaring forces exerted upon the stator yoke in the 8/8/4 SR motor by 
an energized stator phase winding. It has been determined through 
experimentation that audible noise arising from the operation of an SR 
motor is caused in substantial part by the deformation of the stator yoke 
during the operation of an SR motor. It naturally follows that reducing 
deformation of the stator yokes in the 8/8/4 SR motor results in reduced 
audible noise during high speed operation of the motor. 
In carefully controlled tests, the audible noise emanating from an 8/8/4 SR 
motor having two 2 inch stacks operating at 3000 rpm has been compared to 
an 8/6 SR motor having a four inch stack operating at 3000 rpm. The 
audible noise (under rated load) from the standard 8/6 SR motor was 72 
decibels while the audible noise (under a same load) from the new 8/8/4 SR 
motor was only 68 decibels. It is believed that this reduction in noise is 
a direct consequence of the reduced deformation of the stator yokes in the 
8/8/4 SR motor. The differences in audible noise will vary according to 
the operating conditions as well as the structure and materials used to 
construct the motors. However, theoretical hypothesis as well as actual 
testing indicate that the more evenly distributed attractive forces 
between the rotors and stators in the 8/8/4 SR motor results in a 
substantial decrease in audible noise in comparison to the 8/6 SR motor. 
While both the 8/6 SR motor and the 8/8/4 SR motor both have four phases, 
the 8/8/4 SR motor splits the phases in an alternating manner between the 
two stators. As a result, each of the two stators in the 8/8/4 SR motor is 
only active for half of the phase cycles. As a result, the stators in the 
8/8/4 are less susceptible to overheating during high speed operation and 
are less likely to require auxiliary cooling mechanisms. 
As is well known in the art, the energy stroke angle (ESA) specifies the 
maximum angle of rotation in which an energized phase may exert a positive 
torque upon the rotor poles. The ESA is calculated for SR motors by the 
following relationship: 
EQU ESA=360.degree. /(2*n.sub.rotor poles) 
where ESA is in degrees and where n.sub.rotor poles equals the number of 
poles on each rotor. The stroke angle for the 8/6 SR motor is 30.degree. 
while the stroke angle for the 8/8/4 SR motor is 45.degree.. 
The commutation angle is the number of degrees of rotation of the rotor 
wherein a selected set of rotor poles are acted upon by an energized 
stator phase winding when the duty cycles of adjacent phases are not 
overlapped. The commutation angle for the 8/6 SR motor is 15.degree., 
while the commutation angle for the 8/8/4 SR motor is 22.5.degree.. 
It is known that the switching losses in an SR motor increase as the number 
of commutations per revolution increase. It is therefore desirable to 
minimize the number of commutations per revolution. As noted in FIG. 14 in 
the row identified as Commutation Cycles/Rev, the number of commutations 
per revolution in the 8/6 SR motor is 24. On the other hand, the number of 
commutations per revolution for the 8/8/4 SR motor is only 16. It is 
further noted that in the 8/8/4 SR motor these commutations are evenly 
divided between each of the two stators. Therefore, while the stator and 
rotor of the 8/6 SR motor experience all 24 commutations per revolution, 
each stator and rotor in the 8/8/4 SR motor experience only 8 commutations 
per revolution. 
Furthermore, the switching losses are generally proportional to the mass of 
the material experiencing the switching. Therefore, not only are losses 
reduced in the 8/8/4 SR motor as a result of the reduced number of total 
commutations per revolution, switching losses are further reduced since 
the loss per commutation in the 8/8/4 SR motor attributed to building and 
breaking down magnetic fields in the rotor and stator iron is roughly one 
half the loss per commutation in the 8/6 SR motor (assuming the total mass 
of each motor is approximately equal). 
The above switching loss comparisons are rough approximations. However, 
they properly represent (albeit through approximation) the comparatively 
lower switching frequency as well as the substantially less mass of the 
rotor and stator iron subject to eddy current losses during the switching 
of the 8/8/4 SR motor. 
An illustrative embodiment and a number of alternative embodiments of the 
present invention have been described. Based upon the illustrative 
embodiments, other variations of the disclosed invention will be apparent 
to those skilled in the art of SR motors. For example, inside-out versions 
of the present invention are contemplated wherein the rotating member is 
disposed outside the fixed member. It is also contemplated to use the 
present invention in a linear motor configuration wherein the rotor and 
stator poles are displaced along a line rather than the circumference of a 
circle wherein, in addition, a four-phase, 16/16/8 split phase SR motor is 
envisioned having two stators and two rotors, and wherein each rotor 
includes eight evenly spaced salient poles and each stator includes 
sixteen poles. Phases A and C are associated with the first stator and 
phases B and D are associated with the second stator. 
In the 16/16/8 SR motor, the poles of a first stator are skewed 11.25 
degrees (approximately the width of one pole) with respect to the poles of 
a second stator. The poles of the first rotor are aligned with the poles 
of the second rotor as in the illustrated embodiment of FIG. 1A. 
Alternatively, the stators may be aligned and the rotors skewed as in the 
alternative embodiment in FIGS. 9A and 9B. Of course, even larger 
four-phase, split phase SR motors (having even more stator poles and rotor 
poles) would be known in view of the disclosed embodiments. 
Yet another alternative embodiment of a split-phase four phase SR motor is 
a 4/4/2 motor wherein each of the two rotors comprises two salient poles 
and each stator comprises four poles. The skew angle in the 4/4/2 split 
phase motor is 45 degrees. 
Yet other embodiments of the present invention will be known in view of the 
disclosed illustrative embodiments. The invention is intended to include 
the invention as well as equivalents thereof falling within the spirit of 
the invention as defined by the claims appended herein below.