Rotor for a reluctance machine

A rotor for a switched reluctance or synchronous reluctance machine includes a central member from each end of which extends a rotor pole. The rotor is made up of continuous laminations which each define a plane parallel to the axis of rotation of the rotor. A reluctance machine is also disclosed in which the rotor is used. The rotor has radially inner pole faces which cooperate with the pole faces of an inner stator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a rotor for a reluctance machine. 
2. Description of Related Art 
Reluctance machines are becoming increasingly widely used because of their 
relative simplicity and the improving switching and control electronics 
that are now available. 
One form of reluctance machine is the switched reluctance machine which can 
be run as either a motor or a generator. The switched reluctance machine 
comprises a stator, defining stator poles, a phase winding for the or each 
phase and a rotor, defining rotor poles which move past the stator poles. 
Switched reluctance machines are described in more detail in the paper 
"The Characteristics, Design and Applications of Switched Reluctance 
Motors and Drives" by Dr. J. M. Stephenson and Dr. R J Blake presented at 
PCIM '93 at Nurnberg, Germany, Jun. 21-24, 1993, which paper is 
incorporated herein by reference. 
Another type of reluctance machine is a synchronous reluctance machine 
which has a rotor similar to that of the switched reluctance machine. The 
stator, however, is similar to that of a conventional, slotted stator, 
alternating current machine, and the windings are wound according to the 
well known principles of such machines. Such stators and their windings 
are described in many textbooks, e.g. "Electric Machines" by Slemon and 
Straughen, Addison-Wesley Publishing Company, 1980, which is incorporated 
herein by reference. 
The rotor is substantially common to both types of machine. Typically, it 
comprises a stack of laminations of a suitable magnetizable material, such 
as Newcor 800-65 manufactured by Orb Electrical Steels of Great Britain. 
The laminations each define the profile of a rotor core and a series of 
angularly arranged rotor poles. The laminations in this form of reluctance 
machine are called radial laminations as they lie in planes which are 
perpendicular to the axis of rotation of the rotor. They are robust 
structures because the radial forces imposed on the rotating rotor are 
resisted by the continuous extent of each lamination. 
An alternative rotor arrangement has axial laminations that extend parallel 
to the axis of rotation of the rotor. In both switched and synchronous 
reluctance machines the axially laminated rotor is generally considered to 
give superior performance because the direction of lamination is such as 
to reduce the minimum inductance of the machine. This improves the 
efficiency of energy conversion. They are only rarely used, however, 
because they are considerably less robust than a radially laminated 
structure. There is no continuity in the direction of the radial forces 
referred to above. The axially laminated rotor has to rely on the strength 
of the bonding holding the laminations together or some other means of 
mechanical retention, eg. as shown in EP-A-621677, which is incorporated 
herein by reference. Thus, to manufacture a reliable axially laminated 
rotor requires a considerably more elaborate procedure than for the 
radially laminated rotor. 
It will be appreciated that a conventional reluctance machine comprises a 
rotor which is intended to rotate within an embracing stator. The rotor 
poles extend radially outwardly and the stator poles radially inwardly. 
However, it is also known to arrange the machine such that an outer rotor 
has radially inwardly extending poles and the inner stator has radially 
outwardly extending poles. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a rotor for a 
reluctance machine that is robust and also simple to manufacture. 
According to embodiments of the invention there is provided a rotor for a 
reluctance machine comprising a stack of laminations, forming at least one 
pair of spaced rotor poles, and a central member defining a flux path 
between the poles, the rotor further comprising journal means, the rotor 
poles extending from one side of the central member parallel to the axis 
of the journal means. 
The flux path of the embodiments of the invention is, thus, in two planes 
because the central member is offset from the rotor poles. 
Preferably the stack of rotor laminations defines arcuate pole faces on the 
rotor poles which are coaxial with the axis of the shaft. These pole faces 
may be on either the inner or outer surfaces of the rotor poles, depending 
on whether the machine has an external or an internal rotor, respectively. 
Preferably, the laminations define planes which extend parallel to the axis 
of the shaft such that the profile of the central member and the rotor 
poles is defined in each continuous lamination. 
It is possible to define a four or a six pole rotor from a single stack of 
laminations by folding the laminations to define pole arms angularly 
interjacent the pole arms defined in the plane of the flat lamination. 
Also according to embodiments of the present invention there is provided a 
reluctance machine comprising a stator having at least one phase winding, 
and a rotor arranged to be rotatable relative to the stator about an axis, 
which rotor comprises at least a pair of rotor poles, each having a pole 
face, and a central member defining a flux path between the poles, wherein 
the central member is axially offset with respect to the poles. 
Preferably, either the radially outer or inner surface of each pole is 
arcuate to define a uniform air gap between the surface and an adjacent 
stator pole, depending on whether the rotor is an internal or external 
rotor. 
The construction and phase winding of the stator may be arranged such that 
the reluctance machine is a synchronous reluctance machine or a switched 
reluctance machine.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show a switched reluctance machine comprising an inner 
radially laminated stator 10 having four equiangularly spaced, and 
radially outwardly extending stator poles 12. The gaps between the stator 
poles hold windings 14A, 14B arranged in two separately energizable phases 
A and B. 
The stator is fixed on a mounting member 16 which comprises a central 
hollow column 18 and a radially outwardly extending apron 20 formed at one 
end of the column 18. A pair of spaced ball races 22 are secured in the 
column 18. A journal shaft 24 of an external rotor 26 is journalled in the 
ball races 22. The journal arrangement by which the rotor is able to 
rotate relative to the shaft can take other forms. The rotor may have a 
female journal member which is arranged to accept a journal member of the 
stator. 
The rotor 26 comprises a stack of continuous U-shaped axial laminations 
which are arranged in planes parallel to the axis of the shaft 24. The 
laminations define a central member 28, which extends radially outwardly 
on opposite sides of the shaft 24, and a rotor pole arm 30 depending from 
each end of the member 28. The pole arms 30 extend parallel to the shaft 
24, spanning the stator, and end in the same plane as that defined by the 
end of the stator poles distal from the central member 28 of the rotor 26. 
It will be seen from FIG. 2 that the pole face 34 of each rotor pole arm 30 
and the directly opposite stator pole faces 36 are arcuate. The radius of 
curvature of the arcuate rotor and stator pole faces are centered on the 
axis of the shaft 24 and, thereby, create a uniform air gap between them. 
The stack of laminations in this embodiment is held together, and the shaft 
24 is secured to the stack of laminations, by a clamp 38 that is riveted 
to the rotor 26 through its spaced clamping jaws 40. 
The manner of controlling the switched reluctance machine of this invention 
is conventional to the art of controlling known two phase, two rotor pole, 
four stator pole switched reluctance machines run as generators or motors. 
No further description is required here in this regard as the control 
techniques will be well known to the skilled person. The alternately timed 
energization of the phase windings 14A and 14B alternately, creates flux 
which flows from one stator pole, across the air gap and around the rotor 
to the opposite stator pole. In this embodiment the flux travels in a 
three-dimensional flux path along the continuous laminations of the rotor, 
firstly in a direction parallel to the axis of the shaft and then radially 
along the central member into the other rotor pole and thence to the 
stator. The continuous laminations present a low reluctance path for the 
flux in the rotor which is beneficial to the efficiency of the machine. 
Other numbers of rotor poles can be implemented according to embodiments 
the invention. FIGS. 5(a) and 5(b) show four and six pole rotors 264, 266 
respectively. Essentially, two or more two-pole rotor stacks are fixedly 
mounted on a common shaft or otherwise secured together. For the special 
case of the four pole rotor, a pair of two-pole stacks are arranged at 
opposite ends of a stator on the same shaft. The pole arms of one stack 
project toward the other stack so that all the rotor poles are within the 
enclosed space defined by the stator. 
The laminations of the rotor could all be made of a suitable magnetizable 
steel, such as Newcor 800-65. However, the stack of laminations could be 
made from alternate layers of magnetizable and non-magnetizable material 
to provide barriers to flux migration between the laminations, further 
enhancing the efficiency of the rotor. 
A reluctance machine incorporating a rotor according to embodiments of the 
invention provides a high specific output and a reduced electronic control 
cost compared with conventional radially laminated rotor designs. This is 
due to the marked reduction in minimum inductance which can be achieved 
with the construction as described by embodiments of the invention. As 
described in the paper by Stephenson and Blake referenced above, the 
minimum inductance of a reluctance machine is determined largely by the 
separation between the rotor poles and the distance from the stator pole 
face to the core or central member of the rotor. In embodiments of the 
invention, the spacing between the rotor poles is very large and the 
distance between the stator pole face and the central member is also very 
large. 
This leads to a low minimum inductance and hence to a high difference 
between the maximum and minimum inductance. This increased difference 
gives a direct increase in torque output for the same current supplied to 
the machine. 
While the externally arranged rotor of embodiments of the invention allows 
a particularly compact machine structure, one limiting factor on size 
could be the need for a transducer to supply information on rotor position 
to the control electronics. However, this can be addressed by dispensing 
with a more conventional rotor position transducer and using a sensorless 
rotor position monitoring technique. For example, EP-A-0573198, which is 
incorporated herein by reference discloses such a technique that could be 
used in the machine of FIGS. 1 and 2. 
FIGS. 3 and 4 illustrate a synchronous reluctance machine which is similar 
in many respects to the machine of FIGS. 1 and 2. Thus, where appropriate, 
like numerals have been used to indicate like parts. 
The synchronous reluctance machine of FIGS. 3 and 4 differs particularly in 
the form of the stator. In this embodiment, a stator 42 comprises six 
stator poles 44, each defining a radially outwardly facing arcuate pole 
face 46. Three pairs of windings 48A, 48B and 48C, constituting three 
phase windings, are each wound in radially opposing gaps between the 
stator poles 44. Again, this is essentially the conventional three-phase 
synchronous reluctance machine winding arrangement with which the skilled 
person will be familiar. It will be apparent that the stator can have 2np 
stator poles, where p is the number of phases and n is greater than or 
equal to 1, in accordance with conventional synchronous reluctance motor 
theory. 
Because of the axial laminations, once the rotor has been assembled and 
clamped together, the pole faces can be accurately machined. In addition, 
the journal shaft of the rotor can be machined accurately as part of the 
same procedure for turning the pole faces to ensure good concentricity. 
A particular form of the invention may be used where it is preferable to 
use standard parts without further machining operations. To this end, 
standard "C-core" laminations, as produced for use in transformers and 
inductive chokes, can be stacked side by side and mounted in the clamp 40. 
This gives a rotor pole face which is flat and tangential to the stator 
pole face. The airgap therefore varies across the face of the rotor pole, 
being in a minimum in the center. Although this somewhat reduces the 
maximum inductance (because the average gap is higher) it gives a very 
economical construction. 
In a particular form of the invention suitable for low speed, low power 
application, the rotor can be completely solid, being machined or formed 
from a solid piece of magnetizable material. While this embodiment can be 
very economical to produce, its performance is not as good as the 
laminated version. 
FIG. 6 shows a layout of laminations to indicate the method by which these 
could be made on a lamination press tool. The U-shaped rotor laminations 
50 are arranged around the lamination for the stator 52. It will be noted 
that the layout is particularly compact and has a very low proportion of 
wastage. This contributes to economic manufacture of the laminations. The 
laminations are cut from a strip 54 of the magnetizable steel which is 
indicated by the broken line. It will be seen that the outer edges of the 
pole arms are defined by the edges of the strip so that waste material at 
the outer edge of the strip is eliminated. Similarly, the outer edges of 
the central member between the pole arms of the rotor laminations can be 
defined without intervening waste material between them. 
Embodiments of the invention provide a compact machine that has the 
advantage of using axial laminations for the rotor which are more 
efficient than radial laminations. The ability to use the axially 
laminated rotor is not at the expense of cost of manufacture. The 
reluctance machine using the rotor of this invention is relatively simple 
to construct. The rotor is also robust because of the continuity of the 
laminations in the axially laminated form. 
It will be apparent that the reluctance machine using the rotor of this 
invention may be packaged in a variety of ways, depending on the 
requirements of the applications. For example, if the machine forms part 
of a drive system which is exposed to the user, a protective shroud or 
guard may be required. If the machine is to operate in a dirty 
environment, a shroud may be formed around the outside of the rotor. Such 
a shroud may have both protecting and strengthening functions. It can be 
made of any suitable material, e.g. steel, aluminium, plastic, etc. 
Further, the shroud could form part of another component, e.g. it could 
also be the hub of a fan, from which the fan blades extend. 
It will be apparent to the skilled person that this form of rotor 
construction does not rely on adhesion between laminations for its 
strength. It therefore follows that a separator could be placed between 
the laminations to reduce the amount of flux flowing across the 
laminations. Such a separator could be made from any non-magnetic 
material, though it would be beneficial to use a material which is also 
non-conducting, e.g. an insulating material such as calendared paper. 
Of course, the offset nature of the rotor poles of the rotor can be 
utilized in an internal rotor having radially outer pole faces and running 
inside an embracing stator. In either case, the offsetting may allow for 
direct connection of a driven member to the core of the rotor rather than 
to the shaft. Again, this may be of advantage in applications where axial 
space is limited. 
While the invention is susceptible to various modifications and alternative 
forms, specific embodiments have been shown by way of example in the 
drawings and have been described in detail. It should be understood, 
however, that this is not intended to limit the invention to the 
particular forms disclosed. On the contrary, the intention is to cover all 
modifications, equivalents and alternatives falling within the scope of 
the invention as defined in the appended claims.