The present invention relates to the general group of doubly salient
reluctance machines (DSRMs), including switched reluctance machines (SRMs), also
known as variable reluctance machines, stepping motors and hybrid stepping motors
producing linear or rotary motion.
Doubly salient reluctance motors have received increasing attention
over the past few years, with a large number of publications reviewing their relevant
merits with regard to other machine types. The DSRM has been shown to produce a
high specific output, despite rather poor utilisation of both the electrical and
magnetic circuits, because of the introduction of a magnetic gearing ratio, which
arises from the doubly salient nature of the geometry. The magnetic circuit of
the machine is poorly utilised because each stator tooth can only be excited to
produce positive torque for half of each rotation cycle.
It is to be understood that the term 'saliency', when applied to
reluctance machines, implies magnetic saliency, which may or may not involve actual
A doubly salient reluctance machine has a stator and a rotor, both
of which exhibit saliency. Magnetic saliency is used here as generally understood
in the art, that is, a component of a reluctance machine (either its stator or
rotor) is said to be salient if, in operation, changes in the reluctance of the
magnetic circuit of the machine occur due to the construction of that component
as the relative position of the rotor and the stator changes during operation of
For example a conventional switched reluctance stepping motor is
doubly salient since when a winding is energised and the rotor rotates towards
a new position, the main portion of the cross-sectional areas of the active magnetic
path in the both rotor and the stator increase and the reluctance of the magnetic
circuit decreases due to the construction of both rotor and stator. In operation,
energising different windings selects different active magnetic circuits but the
topography of a selected circuit varies as the rotor rotates.
A fuller description of switched reluctance motors and their principles
and applications can be found in the IEEE Industry Applications Society Tutorial
Course Publication "Switched Reluctance Drives" by J.M. Stephenson, S.R. MacMinn
and J.R. Hendershot, Jr., as presented on October 12, 1990 at the IEEE IAS Conference
in Seattle, Washington. The book "Stepping Motors: a guide to modern theory and
practice" by P.P. Acarnley, published by Peter Peregrinus Ltd. on behalf of the
Institution of Electrical Engineers, provides an equally useful publication on
stepping motors in general.
A related machine which is not a stepping motor is the synchronous
reluctance motor. Such a motor has saliency on the rotor only, the stator being
similar to that of an induction motor. A device of this type is disclosed in US
5010267, which describes a variable speed synchronous reluctance machine with a
multiphase stator and a rotor divided into segments which constitute flux guides.
This machine has a salient rotor, according to the definition of saliency as given
above, but the stator has semi-closed slots and is not salient.
The topography of the active magnetic path is determined by the flux
guides and as the rotor rotates the reluctance of this path changes due to the
construction of the rotor only. The stator of the machine of US 5010267 is fully
pitched, a fairly common winding arrangement for such machines. Further mention
of the significance of fully pitched windings will be made later on in this specification.
The specific design of this machine is intended to reduce any effect of mutual
inductance between phases as much as possible, as it is recognised that in a machine
of this sort mutual inductance will not produce torque which will add to that
resulting from the changing self-inductance of each phase.
A further type of related machine is the hybrid stepping motor. Essentially,
a permanent magnet provides a component of the magnetic flux in this machine, with
currents in at least one stator winding directing the flux along alternative paths.
The interaction of the two magnetic fields, one from the rotor magnet and one
from the stator windings, produces the torque on the rotor. The arrangement of
stator poles and rotor teeth and the selected excitation sequence determine the
motion of the rotor. An introduction to and overview of these machines is given
in the above-mentioned book by P.P. Acarnley on pages 9 to 11.
Like the switched reluctance motor, this type of machine is also
a DSRM. Once again, the stator poles can only be excited to produce torque for
half of each rotation cycle, so the machine cannot be utilised to great efficiency.
Another type of related machine is the so-called Vernier reluctance
motor,described in the Proceedings of the IEE, Volume 121, No. 9, September 1974
"Vernier Reluctance Motor" by K.C. Mukherji and A. Tustin. This machine has three
phase distributed windings, arranged to produce torque due to changing self inductance.
Each phase can contribute to positive torque production for a maximum of one half
of each cycle.
Mention has already been made of fully pitched windings with relation
to the synchronous reluctance motor. The 'pole pitch' of a reluctance machine is
defined as the peripheral distance between corresponding points on two consecutive
simultaneously excited poles of opposite sign, whereas the 'coil pitch' is defined
as the distance between the two active conductors, or coil sides, of a coil. A
fully pitched winding is one in which the ratio of the coil pitch to the pole pitch
is 100%, in other words, the two are equal.
Fully pitched windings may be 'concentrated' or 'distributed'. In
the former, the peripheral distance between each coil side of a coil is equal to
the pole pitch, and there will generally be one winding slot per phase per magnetic
pole. In the latter, each winding is split into a number of regions on each coil
side and the peripheral distance between some of these opposed regions will not
be the same as the pole pitch.
A salient stator in reluctance machines commonly carries a number
of evenly spaced projecting regions, or stator poles, between which the coils are
wound in slots. Furthermore, each stator pole may feature a number of projecting
teeth to act as flux guides at its extremity. The rotor itself may feature radially
projecting portions which in operation define poles and have the effect of making
the rotor 'salient'. Alternatively, as in some synchronous reluctance machines,
the rotor poles may not be readily apparent to the eye. The rotor may have a plurality
of salient teeth around its periphery to act as flux guides. How the poles and
any teeth of the stator and rotor are arranged depends of course on the precise
type and design of machine.
Summary of the invention
It is an object of the present invention to improve the utilisation
of the machine windings by changing the manner in which the machine is wound, so
that more efficient operation is possible.
According to the invention there is provided a reluctance machine
comprising a stator and a rotor, each constructed to cause changes in the reluctance
of the magnetic circuit as the relative position of the rotor and the stator changes
during operation of the machine, the stator carrying conductors arranged and terminated
to allow currents to flow around a plurality of loops each of which has at least
a pair of portions in which current flows in opposite directions with respect
to that direction which is normal to the direction of movement of the rotor to
form magnetic poles and wherein, for each loop, each portion carrying current in
one direction is separated- from each portion carrying current in the opposite
direction by a peripheral distance equal to that separating adjacent magnetic
poles of opposite sign.
In a preferred form of the invention, the conductors are connected
to form stator windings, each of which comprises a group of said loops and forms
a current path such that, in operation, substantial torque developed by the machine
is due to change in mutual inductance between the paths as the rotor rotates.
Torque developed by a machine of the invention may be supplemented
by, or supplemental to, torque due to self inductance of the windings. Thus machines
according to the invention may develop torque partly due to mutual inductance and
partly due to self inductance.
The conductors may be arranged and terminated to allow unidirectional
As a consequence of the construction of the stator and the rotor
each have a number of salient poles, the number of stator poles being other than
an integer multiple of the number of rotor poles.
Doubly salient reluctance machines according to the invention may
be motors or generators.
In operation, the windings are connected to supply means for supplying
a sequence of currents which produce a net unidirectional torque on the rotor.
The supply means usually comprises switching means connecting the windings to a
power supply and controlled to provide the required current sequence.
An advantage of the invention is that a significant increase in torque
and efficiency is produced within a given frame size.
In a preferred embodiment of the present invention, conductors from
a single winding substantially fill the winding region in a slot between adjacent
According to a further aspect of the invention there is provided
a hybrid stepping motor comprising a stator with at least two windings and a rotor,
wherein each winding is fully pitched.
Brief description of the drawings
Certain embodiments of the invention will now be described by way
of example with reference to the accompanying drawings, in which:-
Figure 1 shows the principal components of a switched reluctance drive;
Figure 2 is a cross-section of a prior art switched reluctance motor with six
stator poles and four rotor poles (a 6-4 SRM), showing windings for two stator
Figure 3 shows the resultant magnetic flux pattern from the prior art 6-4 SRM;
Figure 4 is a cross-section of an SRM according to the present invention, showing
two (of three) fully pitched windings;
Figure 5 shows an SRM according to the present invention and illustrates a
fully pitched winding arrangement showing all the windings;
Figures 6 and 7 illustrate examples of possible conduction sequences for the
SRM of Figure 5, to produce anti-clockwise rotation of the rotor;
Figure 8 is a cross-section of an SRM according to the present invention showing
all three fully pitched windings simultaneously excited;
Figure 9 illustrates another possible conduction sequence for the SRM of Figure
5 to produce anti-clockwise rotation of the rotor with simultaneous excitation
of all three phases;
Figure 10 shows the results of torque-angle tests for the conventional machine
and for alternative conduction sequences of the machine according to the invention;
Figure 11 is a long section through the axis of rotation of a conventional
hybrid stepping motor showing the rotor and stator assembly;
Figures 12 and 13 represent transverse cross sections through section X-X and
Y-Y respectively of the assembly of Figure 11;
Figures 14 and 15 illustrate the excitation patterns of the windings of the
assembly of Figure 11 with respectively one and two phases excited;
Figure 16 represents a transverse cross section of a hybrid stepping motor
according to the invention; and
Figures 17 and 18 illustrate the excitation patterns of the windings of the
motor of Figure 13 with respectively one and two phases excited.
The invention will first be described with reference to its application
to a switched reluctance machine.
In Figure 1 the principal components of a switched reluctance drive
are illustrated. In this application the SRM is in operation as a motor.
A d.c. supply is switched in sequence across the windings of a switched
reluctance motor 10 by a switching unit 11 under the control of an electronic control
unit 12. Switching is correctly synchronised to the angle of rotation of the motor
10 by using a rotor position encoder 13 on the motor shaft to supply signals to
the control electronics. In this way each winding of the motor is excited in sequence
for a part of the cycle of rotation. The motor speed can be set at the control
unit 12. Further details of the principles and basic construction of SRMs are given
in pages 4 to 7 of the IEEE IAS Conference Paper mentioned above.
In Figure 2 a typical doubly salient switched reluctance machine
illustrates the prior art, here with six stator poles (S1
and four rotor poles (R1 to R4), that is, a 6-4 SRM. Both
the stator and rotor are laminated and each exciting coil is carried on a single
stator pole, opposite coils being connected to produce a north and south pole-pair.
Only one phase winding, formed by coils 20 and 21, is shown here, to illustrate
the excitation of an opposing pair of stator poles S1 and S4.
In the rotor position shown the coils 20 and 21 when passing the currents indicated
conventionally provide positive reluctance torque on the rotor teeth R1
and R3. This torque is developed by the tendency for the magnetic circuit
to adopt a configuration of minimum reluctance, that is, for the rotor poles to
move into line with the stator poles and to maximise the inductance of the excited
coils. Note that the torque is independent of the direction of current flow so
that unidirectional currents can be used, permitting a simplification of the electronic
switching circuits compared with those required for most other forms of motor.
Figure 3 shows the magnetic flux diagram for the SRM of Figure 2,
again illustrating only one phase winding. In order to produce motoring torque
each winding is switched on at a rotor position corresponding to low self-inductance
and off at a position of high self-inductance. Consequently each winding can only
be utilised for a maximum of half of each rotation cycle, that is, it cannot be
used for the period over which the self-inductance is falling.
Consider now the arrangement illustrated in Figure 4 for a switched
reluctance machine according to an embodiment of the present invention. The windings
are shown by reference numerals 22, 23, 24 and 25 and it is clear that in comparison
to the arrangement of Figure 2 twice the winding area is available to achieve
the same basic excitation pattern.
To achieve this excitation pattern, two windings are used, the windings
each being fully pitched across poles which, in operation, form adjacent excited
poles of opposite sign. The full winding area on either side of each stator pole
is utilised to produce excitation in that pole. One winding is thus formed by
groups of conductors 22 and 23, and another is formed by groups of conductors 24
and 25. With the correct switching sequence the windings can be switched on to
excite the rotor poles in the sequence needed to provide unidirectional torque
on the rotor. Examples of suitable currents are shown by the usual "cross" and
"dot" symbols representing opposite directions of current flow.
In Figure 5 the 6-4 SRM of Figure 4 is shown and all the windings
are represented. Each winding comprises in this case two groups of conductors in
opposite stator slots carrying current in opposite directions. As in a conventionally
wound 6-4 SRM, the power supply from the switching unit 11 is a three-phase supply,
with groups of conductors a+ and a- carrying a single phase A, and so on.
An example of a unipolar switching sequence which produces unidirectional,
anti-clockwise torque on the rotor is given in Figure 6. The angle &thetas; of
the rotor determines when each phase, or winding, is active. In the rotor position
illustrated in Figure 5, corresponding approximately to &thetas; = 45°, phases
A and B are turned on and stator poles S1 and S4 are therefore
excited. Flux linkages with rotor teeth R1 and R3 produce
anti-clockwise torque on the rotor. At &thetas; = 60° the switching unit 11 switches
off phase A and switches on phase C, thus phases B and C only are then active
with stator poles S2 and S5 excited, and the rotor teeth
R2 and R4 are urged into position. The switching sequence
continues according to Figure 6. Each conductor is used to produce excitation
in both neighbouring stator poles but for different periods of the cycle, and thus
unlike the conventional SRM winding arrangement each phase is switched on for two
thirds of each cycle, and throughout this period it is contributing to torque
An alternative sequence is shown in Figure 7 and uses switching circuits
which pass currents in alternate directions through the windings. A "+" in Figure
7 indicates that current is passed in one direction while a "-" indicates current
in the other direction.
In conventional switched reluctance machines the torque produced
is determined directly from the area enclosed by the flux linkage-current locus.
In the new modified winding arrangement the flux linking any one phase winding
is no longer solely a function of position and its own phase current. Coupling
between phases results in the flux linkages also depending upon the other phase
currents. Nevertheless the energy converted to torque each time a phase conducts
remains equal to the enclosed area of the locus for that phase.
The self-inductance of any one phase of a conventional SRM rises
as rotor teeth come into alignment with the excited stator teeth and a torque is
produced according to:-
In general, where more than one phase may conduct at any one instant
this equation may be generalised. For example, in a three phase machine, this gives:-
where ia, ib and ic are the instantaneous phase
Ignoring saturation effects, as the self-inductance of each phase
of a conventional SRM typically rises for a maximum of half of each rotation cycle,
then this means that the phase can only be excited to produce motoring torque for
a maximum of half of each such cycle. The new arrangement produces positive torque
for considerably more time than this, and therefore must use some other mechanism.
There is now considerable coupling between phases, and therefore equation (2) is
By considering the instantaneous voltages for each phase of this
new winding arrangement it can be shown that:-
Comparing equation (3) with equation (2) shows how where there is
coupling between phases then an additional set of terms contribute to the torque,
resulting from the changing mutual inductances between the various phases.
Assuming leakage and fringing fields to be negligible and all iron
paths to be infinitely permeable then in the SRM of Figure 5, the self-inductance
of each phase is directly proportional to the length of overlap between rotor and
stator poles through which flux generated by the phase current passes. As one
rotor pole comes into alignment then another one comes out of alignment, so that
this length of overlap is constant and independent of rotor position. The self-inductance
of each phase therefore does not vary with rotor position and is in fact equal
to the maximum aligned phase inductance of a conventionally wound SRM. Hence the
first three terms on the right hand of equation (3) are zero. Such a conclusion
is very significant in that it is solely these very terms which produce the torque
in a conventional SRM.
Using the same assumptions as above it can be shown that the mutual
inductance MAB between phases A and B is given by the following equation:-
Mab = µo N2 / (2) la / (lag)
number of turns per phase
machine axial length
tooth overlap dimension contributing to mutual inductance. (This dimension
is illustrated by way of example in Figure 4 and in this example the tooth overlap
dimension is the difference between a length x, which is the overlap of the poles
R2 and S3, and a length y, which is the overlap of the poles
R3 and S4.)
magnetic permeability of free space.
The simplified analysis above indicates that, in the 6-4 machine
example the mutual inductance between phases is negative and constant for thirty
degrees of rotation; it then rises to an equal positive value during the next thirty
degrees, only to fall back to the negative value in another thirty degrees - hence
completing the cycle. The magnitude of the maximum positive and negative values
of mutual inductance in this idealised machine are equal to the constant self-inductance
of any one phase, minus the leakage reactance.
As shown above, the idealised machine according to the invention
derives its torque solely from changing mutual inductance. Machines with more than
three phases can be produced according to the invention. Although in such cases
the idealised machine will not derive its torque solely from mutual inductance
terms, a substantial component of the torque will be generated this way.
The instantaneous torque on the rotor when two phases, say a and
b, are conducting is given by:-
T = iaib dMab / (d&thetas;)
Comparison of equation (5) with equation (1) shows that the rate
of change of mutual inductance in equation (5) is twice that of the rate of change
of self-inductance in equation (1). As the two phases are both contributing to
the magnetomotive force (mmf), then for a given instantaneous phase current twice
the mmf is available and therefore four times the torque is produced, ignoring
the effects of saturation, fringing and end winding losses. In a magnetically saturated
machine the gains are substantially reduced, but remain significant.
The switching sequence of Figure 6 uses two positive phase currents
at a time when the mutual inductance between these phases is rising but the falling
portion of mutual inductance is not utilised. When bi-directional-current operation
occurs, as with the switching sequence of Figure 7, then intervals of falling
mutual inductance provide positive torque since one of the phase currents is negative.
An alternative excitation pattern is one in which all three phases
conduct simultaneously, as illustrated in Figure 8, which shows poles S1
and S4 excited. This utilises both the rising and the falling portions
of mutual inductance. A greater MMF is present to produce positive torque on one
pair of rotor teeth, but a small MMF also results in negative torque on the other
pair of rotor teeth. Figure 9 depicts a suitable switching sequence to produce
anti-clockwise rotation of the rotor.
As an alternative to the 6-4 SRM a three phase 12-8 DSRM may be constructed
with six windings, each fully pitched, and machines with any practical integer
multiple of a 6-4 may be made according to the invention. In operation of the 12-8
SRM the windings are excited to provide two active pole-pairs at a time by supplying
the windings in pairs, with the windings of each pair conducting for the same intervals.
Twelve winding regions are provided and a fully pitched winding occupies regions
spaced by 90°, that is, the two portions of each winding are spaced by two regions
occupied by other windings.
Tests were carried out to compare the alternative excitation sequences
described above with a conventionally wound SRM. An existing 7.5 kW commercial
12-8 SRM with a nominal rated torque of 48 Nm, was rewound with fully pitched windings.
This machine was in a D132 frame size, with a core diameter of 210 mm and a stack
length of 194 mm. Torque measurements were made using a commercial torque transducer
with the rotor locked at approximately one degree intervals between the aligned
position (0°) and the unaligned position (22.5°). Apart from the rewinding operation,
no modifications were needed to the machine. The number of series turns per phase
and the winding cross sectional area were kept identical to the conventionally
wound version, but because of the increased end winding length the mass of copper
increased by 44% from 8.34 Kg to 12.0 Kg. There was a corresponding increase in
per phase resistance from 0.797 ohms to 1.147 ohms at 20°C.
Figure 10 illustrates the results, displaying torque against rotor
angle for excitation patterns A (unipolar conduction, two phases conducting at
a time), B (bipolar conduction, two phases conducting at a time, C (bipolar conduction,
all three phases conducting at a time, and D (conventionally wound SRM).
In this test, the peak current in the conventional machine was 15A
and the tests were performed on the basis of equal winding loss in the machine.
It is clear from the results that with the winding arrangement according to the
invention torque is substantially increased. The conventional machine produced
a peak torque of 48.2 Nm, excitation A produced a peak torque of 66.7 Nm (an increase
of 38%), excitation B a peak of 67.8 Nm (an increase of 41%) and excitation C a
peak of 76.8 Nm (an increase of 59%).
The invention has so far been illustrated with reference to a three
phase SRM with 6-4 or 12-8 construction. However, it can be applied to machines
with other numbers of phases and different numbers of stator and rotor poles, depending
on the intended application of the machine. Examples of applications of the present
invention include 8-6 and 12-10 SRMs, and also motors with poles which are divided
into a number of salient regions adjacent to the rotor, but these examples are
in no way limiting.
Other forms of winding giving change in mutual inductance with rotor
position may also be used.
The principle of operation of stepping motors and hybrid stepping
motors is very similar to that of SRMs and thus the present invention also relates
to doubly salient reluctance machines in the form of stepping motors and hybrid
An embodiment of the invention in the form of a hybrid. stepping
motor will now be described.
Figure 11 shows a conventional hybrid stepping motor, although the
figure is equally applicable to a motor according to the invention. A stator 101
carrying stator windings 102 consists of two longitudinally separated sets of toothed
poles 103,104 projecting radially inwardly from a cylindrical back iron 105 mounted
within a cylindrical housing (not shown). A motor shaft 106 carries the rotor 107,
consisting of a cylindrical permanent magnet 108 and two toothed cylindrical end
caps 109,110. Each toothed end cap corresponds in axial position with one of the
sets of toothed stator poles and the two end caps are respectively in magnetic
contact with opposite poles of the permanent magnet. A small air gap 111 remains
between the tips of the stator poles and the tips of the teeth of the rotor end
caps. The stator 101 and the rotor end caps 109,110 are both made from laminated
A magnetic flux path is shown by dotted lines 112 leading from the
north pole of the permanent magnet 108, through and radially outward from rotor
end cap 109, across the air gap 111 and through stator pole 103, axially along
the stator back iron 105, radially inward through stator pole 104, across the air
gap, and back through the rotor end cap 110 to the south pole of the permanent
magnet 108. The magnetic flux therefore has to flow radially outwards from rotor
end cap 109 and radially inwards towards rotor end cap 110.
The cross sections of Figures 12 and 13, which only apply to conventional
hybrid stepping motors, are taken respectively through planes X-X and Y-Y of Figure
11. The motor illustrated has 8 stator poles, numbered sequentially P1 to P8 in
the figure, each pole having 5 uniformally spaced teeth 120 and carrying part
of a winding which extends across the corresponding poles of both sets of rotor
poles. The rotor end caps carry 50 uniformally spaced projecting radial teeth 121,
the position of the teeth being angularly offset between the two rotor end caps
as shown in the figures, such that in axial projection a tooth of one rotor end
cap is located midway between two adjacent teeth of the other rotor end cap. The
angular spacing of the teeth on a single stator pole is substantially equal to
that of the rotor teeth.
Two windings are provided in the assembly, each carrying one phase
of the power supply A or B, and each winding is situated on four of the eight stator
poles, winding A being situated on poles P1, P3, P5, and P7, and winding B being
situated on poles P2, P4, P6 and P8 as shown in the figures. Successive poles of
each phase are wound in the opposite sense, such that, for example, excitation
of winding A results in a magnetic field in one direction in poles P1 and P5 and
in the opposite direction in poles P3 and P7. This winding arrangement is represented
in the individual windings as depicted in Figures 12 and 13, the convention being
used whereby a+ and a-, for example, represent conductors carrying phase A in opposite
directions. As a result of this winding arrangement a magnetic flux may develop
in two adjacent stator poles of the same winding, such as poles P1 and P3, this
flux having the same phase but opposite polarity in the two poles when the stator
winding is energised.
The operation of the conventional hybrid stepping motor will be explained
with reference to Figure 14, which represents section X-X and shows the excitation
of the windings when a single phase is operated. A shaded winding in the figure
represents excitation in the direction indicated by the conventional representation
of 'cross' and 'dot' symbols.
The windings are used to encourage or discourage the flow of magnetic
flux through certain poles according to the rotor position required. With the excitation
as shown in Figure 14 the pole magnetic field will be radially outward in poles
P1 and P5, whilst it will be radially inward in poles P3 and P7. As a result the
rotor will tend to align itself into the position shown in Figure 14, such that
there is rotor teeth alignment with the teeth of stator poles P1 and P5, whilst
at section Y-Y the alignment will be with poles P3 and P7. The torque is therefore
developed due to the changing magnetic flux linkage between stator and rotor teeth.
The phase windings are excited in a sequence to produce rotor motion
as desired. If the excitation of A is removed and B is excited with reverse current
then the stator and rotor teeth will tend to align under poles P4 and P8 of section
X-X and poles P2 and P6 of section Y-Y, thus moving the rotor through one step
in an anti-clockwise direction. To produce continuous rotation in this direction,
the sequence is continued as shown in the following table, which depicts the switching
sequence and the resulting alignments.
Sequence for anti-clockwise rotation
Excitation Pole field at X-X (outwards) Pole field at Y-Y (inwards) Rotor position (Degrees) A+P1,P5P3,P70 B-P4,P8P2,P61.8 A-P3,P7P1,P53.6 B+P2,P6P4,P85.4 A+P1,P5P3,P77.2
The length of each step is 360/4n degrees, where n is the number
of rotor teeth, since a complete excitation cycle of four steps results in alignment
of teeth under the same stator poles. Thus in the illustrated example the motor
has a step length of 360/200, or 1.8 degrees, and it is this small step length
which allows very high resolution in angular positioning of such hybrid motors.
For clockwise rotation the switching sequence will be different,
the excitation of the phases being in the order A+,B+,A-,B-,A+,...
Torque can be improved by exciting more than one phase at a single
instant, and this is represented for the motor of the example in Figure 15, which
shows the excitation at section X-X. The excitation sequence for anti-clockwise
rotation will be as follows:-
This arrangement is however very inefficient, due to the fact that one half of
the slots have no MMF in them since they carry two components of current in opposite
directions (shown unshaded in Figure 15). Hence they still have considerable loss,
but with little benefit in terms of torque production.
Consider now the arrangement illustrated in Figure 16 for a hybrid
stepping motor according to an embodiment of the present invention. Two windings
- phases A and B - are again used. This time the windings are fully pitched across
the stator poles such that each winding spans two stator poles.
The full winding area on either side of each stator pole is utilised
and with the correct switching sequence the windings can be switched on to excite
the rotor poles in the sequence needed to provide the desired motion of the rotor.
The appropriate sequence for anti-clockwise rotation will be B+,A+,B-,A-,B+,...,
and the motor is shown in the position corresponding to the first step of this
sequence in Figure 17, the figure illustrating the situation at section X-X. This
arrangement will generate the same torque as that of the conventional hybrid motor
shown in Figure 15, but will suffer from only one half of the loss in the active
region of the winding (neglecting end losses).
The basic MMF pattern with one phase conducting can be reproduced
with two phases conducting in the machine according to the invention, as shown
in Figure 18. There is now twice the area of copper in which to produce the same
MMF pattern, so that greater torque will result for a given copper loss. Conversely,
for the same torque there will be a much reduced copper loss. The appropriate
switching sequence in this case will be as follows:
It is to be noted that unlike the fully pitched SRM, the machine
does not derive its torque from mutual inductance due to interaction between stator
The invention has been illustrated with reference to a two phase
hybrid stepping motor with 8 stator poles, but this example is in no way restrictive
to the scope of the invention. The invention can be applied to a machine having
stators with other numbers of poles and using more than two phases. Similarly the
number of teeth carried on the rotor and on the stator poles can be varied, depending
on the intended application of the machine.
It is also to be realised that these machines are generally designed
with a number of basic motor units (stacks) along the axial length of the machine,
the number of stacks also being selected as appropriate to the application.
For operation of the hybrid stepping motor, not only the stator and
rotor assembly as described above are required, but also means for providing multi-phase
currents to the windings and the means to accomplish the desired switching. A rotor
position encoder, typically an opto-electronic device operated from the rotor
shaft, may be included to supply signals to control electronics in order to ensure
correct synchronisation of the switching.
The foregoing description concerns applications of DSRMs as motors.
However, DSRMs according to the invention may also be generators. In the absence
of permanent magnetic poles, the windings require excitation from a power supply
by way of switching circuits. The rotor is driven to oppose the toraue developed
by the windings and electrical power flows from the machine into the power supply.
It will be appreciated in this specification and the accompanying
claims that the terms rotor and stator also apply to linear machines, where the
rotor and the stator are the moving and the stationary members, respectively.
While the invention has been described in conjunction with specific
embodiments thereof, it is intended to embrace all other embodiments that fall
within the scope of the appended claims.
Reluktanzmaschine (10) mit einem Ständer und einem Läufer, die jeweils gestaltet
sind, um Änderungen in der Reluktanz des magnetischen Kreises zu bewirken, während
sich die relative Stellung des Läufers und des Ständers im Betrieb der Maschine
ändert, wobei der Ständer Leiter (20, 21; 102) trägt, die angeordnet und begrenzt
sind, um zu ermöglichen, daß Ströme eine Vielzahl von Schleifen durchfließen, von
denen jede mindestens ein Paar Abschnitte aufweist, in denen Ströme in entgegengesetzten
Richtungen bezüglich der Richtung fließen, die zur Bewegungsrichtung des Läufers
senkrecht ist, um magnetische Pole zu bilden, dadurch gekennzeichnet, daß für jede
Schleife jeder einen Strom in einer Richtung tragende Abschnitt von jedem einen
Strom in der entgegengesetzten Richtung tragenden Abschnitt durch einen Umfangsabstand
getrennt ist, der gleich demjenigen ist, der benachbarte magnetische Pole mit
entgegengesetztem Vorzeichen trennt.
Maschine nach Anspruch 1, worin die Leiter (20, 21; 102) verbunden sind, um
Ständerwicklungen zu bilden, wovon jede eine Gruppe der Schleifen umfaßt und einen
Stromweg bildet, so daß im Betrieb ein durch die Maschine entwickeltes beträchtliches
Drehmoment durch eine Änderung der Gegeninduktivität zwischen den Wegen verursacht
wird, während sich der Läufer dreht.
Maschine nach Anspruch 1 oder 2, worin die Leiter angeordnet und begrenzt sind,
um nur Ströme in einer Richtung zu ermöglichen.
Maschine nach einem vorhergehenden Anspruch, worin als eine Folge der Gestaltung
des Ständers und Läufers diese jeweils eine Anzahl ausgeprägter Pole aufweisen,
wobei die Anzahl von Ständerpolen (S1 - S6; P1
- P8) kein ganzzahliges Vielfaches der Anzahl von Läuferpolen (R1
- R4 ; 121) ist.
Maschine nach einem vorhergehenden Anspruch, worin die Leiter in Ständerschlitzen
liegen und jeder Schlitz nur Leiter von einer einzigen Wicklung enthält.
Maschine nach Anspruch 1 oder den Ansprüchen 4 oder 5, soweit sie von Anspruch
1 abhängen, worin der Läufer einen in einer Achsenrichtung polarisierten Permanentmagneten
(108) enthält und so angeordnet ist, daß es für jede von mehreren Läuferstellungen
einen jeweiligen bevorzugten Flußweg zwischen den Läufer- und den Ständerpolen
Maschine nach Anspruch 6, worin der Läufer zwei im allgemeinen kreisförmige
Bauteile mit niedriger Reluktanz (109, 110) enthält, die auf der Läuferachse, eines
an jedem Ende des Permamentmagneten, angeordnet sind, wobei jedes Bauteil eine
Vielzahl um dessen Umfang gleich beabstandeter Zähne (121) aufweist, und der Ständer
eine Vielzahl ausgeprägt geformter Pole (P1 - P8) aufweist, die jeweils eine Vielzahl
dem Läufer benachbarter Zähne (120) tragen.
Maschine nach Anspruch 7 mit zwei Wicklungen, acht Ständerpolen und fünfzig
Maschine nach einem der Ansprüche 1 bis 5 mit drei Wicklungen, sechs Ständerpolen
und vier Läuferpolen oder einem ganzzahligen Vielfachen dieser Zahlen.
Maschine nach einem vorhergehenden Anspruch, in Kombination mit Mitteln (11,
12) zum Zuführen von Mehrphasenströmen zu den Leitern.
Kombination nach Anspruch 10, worin im Betrieb Ströme den Wicklungen in einer
Reihenfolge zugeführt werden, die ein Nettodrehmoment in einer Richtung an dem
A reluctance machine (10) comprising a stator and a rotor, each constructed
to cause changes in the reluctance of the magnetic circuit as the relative position
of the rotor and the stator changes during operation of the machine the stator
carrying conductors (20,21;102) arranged and terminated to allow currents to flow
around a plurality of loops each of which has at least a pair of portions in which
current flows in opposite directions with respect to that direction which is normal
to the direction of movement of the rotor to form magnetic poles, characterised
in that, for each loop, each portion carrying current in one direction is separated
from each portion carrying current in the opposite direction by a peripheral distance
equal to that separating adjacent magnetic poles of opposite sign.
A machine according to claim 1 wherein the conductors (20,21;102) are connected
to form stator windings, each of which comprises a group of said loops and forms
a current path such that, in operation, substantial torque developed by the machine
is due to change in mutual inductance between the paths as the rotor rotates.
A machine according to claim 1 or 2 wherein said conductors are arranged and
terminated to allow unidirectional currents only.
A machine according to any preceding claim, wherein as a consequence of the
construction of the stator and the rotor each have a number of salient poles, the
number of stator poles (S1-S6; P1-P8)
being other than an integer multiple of the number of rotor poles (R1-R4;
A machine according to any preceding claim in which the conductors are located
in stator slots and each slot contains conductors from a single winding only.
A machine according to claim 1 or claims 4 or 5 insofar as dependent on claim
1 wherein the rotor comprises a permanent magnet (108) polarised in an axial direction
and is so arranged that for every one of a plurality of rotor positions there is
a respective preferential flux path between the rotor and the stator poles.
A machine according to claim 6 wherein the rotor comprises two generally circular
low-reluctance members (109,110) arranged on the rotor axis one at each end of
the permanent magnet, each said member having a plurality of teeth (121) equally
spaced around its circumference, any the stator comprises a plurality of saliently
shaped poles (P1-P8) each carrying a plurality of teeth (120) adjacent to the
A machine according to claim 7 comprising two windings, eight stator poles
and fifty rotor teeth.
A machine according to any of claims 1 to 5 comprising three windings, six
stator poles and four rotor poles, or any integer multiple of these numbers.
A machine according to any preceding claim in combination with means (11,12)
for supplying multi-phase currents to the conductors.
A combination according to claim 10 wherein, in operation, currents are supplied
to the windings in a sequence which produces a net unidirectional torque on the
Machine à réluctance (10) comprenant un stator et un rotor, chacun étant construit
pour provoquer des changements dans la réluctance du circuit magnétique lorsque
la position relative du rotor et du stator change pendant le fonctionnement de
la machine, le stator portant des conducteurs (20, 21, 102) disposés et terminés
pour permettre à des courants de s'écouler autour d'une pluralité de boucles, dont
chacune possède au moins une paire de parties dans lesquelles le courant s'écoule
dans des directions opposées par rapport à la direction qui est normale à la direction
du mouvement du rotor, pour former des pôles magnétiques, caractérisée en ce que,
pour chaque boucle, chaque partie transportant le courant dans une direction est
séparée de chaque partie transportant le courant dans la direction opposée par
une distance périphérique égale à celle séparant des pôles magnétiques adjacents
de signes opposés.
Machine selon la revendication 1, dans laquelle les conducteurs (20, 21, 102)
sont connectés pour former des enroulements de stator, chacun d'entre eux comprenant
un groupe desdites boucles et formant un chemin de courant tel que, en fonctionnement,
un couple de rotation substantiel développé par la machine est dû au changement
de l'inductance mutuelle entre les chemins lorsque le rotor tourne.
Machine selon la revendication 1 ou 2, dans laquelle lesdits conducteurs sont
disposés et terminés pour permettre seulement des courants unidirectionnels.
Machine selon l'une quelconque des revendications précédentes, dans laquelle,
comme conséquence de la construction du stator et du rotor, chacun d'eux a un certain
nombre de pôles saillants, le nombre de pôles du stator (S1 -S6
; P1 -P8) étant différent d'un multiple entier du nombre
de pôles du rotor (R1 -R4 ; 121).
Machine selon l'une quelconque des revendications précédentes, dans laquelle
les conducteurs sont logés dans des encoches du stator, et chaque encoche contient
des conducteurs d'un seul enroulement seulement.
Machine selon la revendication 1, ou les revendications 4 ou 5 dans la mesure
où elles dépendent de la revendication 1, dans laquelle le rotor comprend un aimant
permanent (108) polarisé dans une direction axiale et est agencé de telle sorte
que, pour chacune d'une pluralité de positions du rotor, il y a un chemin de flux
préférentiel respectif entre les pôles du rotor et du stator.
Machine selon la revendication 6, dans laquelle le rotor comprend deux éléments
à faible réluctance généralement circulaires (109, 110), disposés sur l'axe du
rotor, un à chaque extrémité de l'aimant permanent, chacun desdits éléments ayant
une pluralité de dents (121) également espacées autour de sa circonférence, et
le stator comprend une pluralité de pôles formés de façon saillante (P1
-P8), chacun portant une pluralité de dents (120) adjacentes au rotor.
Machine selon la revendication 7, comprenant deux enroulements, huit pôles
de stator et cinquante dents de rotor.
Machine selon l'une quelconque des revendications 1 à 5, comprenant trois enroulements,
six pôles de stator et quatre pôles de rotor, ou un multiple entier quelconque
de ces nombres.
Machine selon l'une quelconque des revendications précédentes, en combinaison
avec des moyens (11, 12) pour fournir des courants multi-phasés aux conducteurs.
Combinaison selon la revendication 10, dans laquelle, en fonctionnement, les
courants sont fournis aux enroulements en une séquence qui produit un couple unidirectionnel
net sur le rotor.