Improvements in electric machines

Abstract: An electric motor comprises a laminated stator core (13) supporting windings (16) to define similar adjacent poles. A rotor (12) is mounted to rotate within the stator. The similar poles in a group define flux paths with adjacent pole groups. Thus the flux paths (20) required are restricted to those between adjacent dissimilar pole groups. In this way the material of the core between the flux paths can be reduced as it is not required to carry flux itself. This redundant portion of the core can be shaped to accommodate neighbouring components or to reduce the material of the core laminations.

Description: This invention relates to electric reluctance machines comprising a primary, having a primary core defining primary poles with pole faces; a secondary having secondary poles with pole faces; and a coil or set of coils. US-A-3047755 discloses a synchronous reluctance motor having a laminated rotor defining rotor poles which coact with corresponding pole faces defined in the stator. The stator is described as having windings in the slots between stator poles. These are indicated as following the conventional pattern of magnetising the poles to alternately opposite polarities. Figure 1 of the drawings illustrates a known construction of single-phase salient pole switched reluctance machine which can be operated as either an electric motor or generator. However, for the sake of convenience the term motor will be used throughout this specification though it is to be understood that the invention is equally applicable to electric generators. The switched reluctance motor of Figure 1 comprises a stator core 1 and a rotor 2 which are both formed with four salient poles 3 and 4, respectively. The stator core is made up of numerous metal lateral laminations in a stack. Each salient stator pole 3 carries an exciting coil 5. The coils 5 are wound such that North (N) and South (S) poles are created alternately around the stator. As illustrated by the broken lines 6, each sector of the assembled laminations of the core 1 carries half the flux created in one of the associated adjacent stator poles. In this kind of motor the laminations are usually stamped from a blank of strip metal. As the general outer shape of each lamination will be circular there will be a significant amount of material left between stampings of laminations which can only be cast aside as scrap metal. Although this scrap can, of course, be recycled, it is clearly desirable to utilise the strip with as little waste as possible. Furthermore, each pole carries its own copper winding. This construction requires a considerable amount of copper which will add both to the weight and cost of the motor. It is an object of the present invention to provide an electric machine which optimises the use of the core material and the copper windings in order to reduce the cost of manufacture. The present invention is characterised in that the or each coil is connected and/or is located with respect to the primary poles so as to define groups of adjacent primary poles, the magnetic polarity of the pole faces in each group of primary poles with respect to the coil or coils being the same and the said magnetic polarity of the pole faces of groups of primary poles being opposite in adjacent groups, and in that a portion of the primary core between adjacent primary poles in a group is relatively radially thinner than that between adjacent groups of primary poles. The terms primary and secondary are used to denote the relatively moving parts of the machine. Clearly, the poles associated with the coil or coils could be the rotor or stator of the machine. In this way the flux through the core between the groups of stator poles is increased by an amount related to the number of stator poles in a group. Also, the flux in the stator core between poles within a group is substantially reduced toward the centre of the group. Hence that portion of the stator core between the stator poles in a group is now less dependent on considerations of the amount of flux in it as it no longer has to serve as a path for all the flux between dissimilar groups of poles. Thus, the thickness of the stator core in this region can be reduced and even shaped and remoulded with less regard to the flux in it. The main remaining criterion lies in providing sufficient mechanical strength for the stator core. However, a slight increase in thickness of the material of the stator core will probably be required between the dissimilar groups due to the increased flux in that region. Preferably, each group of adjacent stator poles is embraced by a single coil. Thus, the amount of material, e.g. copper, needed for the windings in a given machine will be reduced. Preferably, the thinner portion of the stator core defines a flat outer face of the core. Preferably, the core is made up of a plurality of laterally extending laminations. In this case when the flat outer faces are formed of the thinner portions of the core the laminations can be stamped from a strip so as to use less material to form the laminations and have less waste material left over from the stampings. In this latter regard if the thinner or flat portions are stamped adjacent one another in a line from a strip, the overall length of the strip used can also be reduced as well as leaving less scrap. The invention also extends to a machine as described above connected to run as a motor and the same machine connected to run as a generator. The present invention can be put into practice in several ways, some of which will now be described by way of example with reference to the accompanying drawings in which: Figure 2 is a cross section through an electric reluctance motor constructed according to the present invention; and Figure 3. is a cross section through a further electric reluctance motor according to the present invention. Figure 2 illustrates a switched salient pole single-phase reluctance motor comprising a stator 10 within which is mounted a rotor 12. A core 13 of the stator is made up of a stack of lateral metal laminations which also define four angularly equidistant stator poles 14. A copper coil 16 is wound around each of the stator poles 14. The coils are excited simultaneously. The rotor 12 comprises a ferromagnetic steel member defining four angularly equidistant rotor poles 18. It will be apparent that this construction of switched reluctance motor is, in many respects, similar to that in Figure 1. However, in the electric motor of Figure 2 each of the coils 16 is connected so that the stator pole sequence is North/North/South/South (NNSS) around the core 13. By reconnecting the coils 16 in this way, the flux path is limited to that in the back of the core 13 linking adjacent dissimilar groups of pole pairs (NN and SS), as indicated by the broken lines 20. Thus, for a given excitation current in the windings, the amount of flux in this reduced path is doubled. Because of this the essentially redundant connecting portions between the similar poles is of much reduced lateral thickness. It follows that four of the lengths of coil 16 extending axially within the stator 10 are redundant as they play no part in producing the flux in the path between the dissimilar poles pairs (NN an SS) that is used to turn the rotor 12. Figure 3 illustrates a modified switched doubly salient pole reluctance motor with one phase winding for a stack of laminations which exploits the above sequence of orientations of stator poles. In this embodiment a stator core 22, again comprising a series of lateral laminations of metal in a stack, defines four equally angularly spaced stator poles 24. Pairs of stator poles 24 are each connected by a relatively thick portion 26 of the core 22. Adjacent stator poles 24 in each similar pair are joined by a flat relatively thinner portion 28 of the core 22. Copper coils 30 in the core 22 are each wound in a loop to embrace one of the pairs of adjacent similar stator poles 24 joined by the corresponding thick portion 26. As discussed above, by changing the windings to energise the stator poles 24 in an NNSS sequence there is no longer any flux in the sectors of the core 22 between similar poles 24. Thus, the thickness of the portions 28 is no longer dictated by the flux. Hence, their shape and size is reduced to that needed to maintain the overall mechanical strength of the core 22. This allows the thinner portions to be formed with flat outside edges. In other circumstances, the portion 28 could be formed to accommodate an adjacent component in a particularly confined space. In this case the shape would be specifically designed for the particular purpose. However, because the amount of flux in the portions 26 is virtually doubled, it is necessary to increase their thickness over that required in an equivalent conventionally wound motor. However, overall the absence of four coil sides and of the need to carry flux between poles of like polarity leads to a significant reduction in the dimensions of each lamination. As mentioned above the laminations for an electric motor are stamped from a strip of metal. By forming this embodiment of the present invention with flat sides due to the absence of flux in roughly half the core, each lamination can be stamped with considerably less waste material in between it and an adjacent stamping than would be the case in a conventional circular stamping. Thus the amount, and therefore cost, of steel strip required for a given number of laminations is considerably reduced. The effect of constructing each coil to embrace a pair of adjacent poles means that four lengths of coil running the length of a conventional salient pole electric motor are eliminated with little or no diminution in performance. This leads to a consequent reduction in the amount of copper required for the windings as a whole to roughly half that which runs along the length of the motor in addition to a slightly increased amount at the end of each winding. Such an overall reduction in the amount of copper used in the windings will lead to a drop in the winding resistance, as well as a cheaper and lighter core. The performance of the motor can be further improved by adjusting the proportions of the copper and iron core sections to take account of the new winding strategy. For instance, if the two coils now used each have the same number of turns as each of the original four coils connected in series in an equivalent known reluctance motor, the EMF per coil will be doubled in the present embodiment as each coil now embraces two stator poles. Neglecting, for the moment, the end windings, the removal of the four redundant coil sides will halve the resistance of the winding and hence the copper loss and weight of the copper used. However, the amount of copper required for the end windings will need to be somewhat greater than in the previously known equivalent motor to embrace both poles. Clearly, the benefit from the reduction in the amount of copper and the improved efficiency will increase with an increasing length to width ratio of the motor. It will be apparent to those skilled in the art that Figures 2 and 3 represent only two applications of the present invention which serve to illustrate the benefits of the new motor construction. Clearly there are many possible variations in the detailed design of the motor lamination according to the basic principles embodied in Figure 3. For example, pole arcs, air gap diameter relative to overall dimensions, coil layouts and the relative thickness of various parts of the lamination can be changed in particular designs. However, all will benefit from a reduced copper content and a reduction in the amount of lamination material required. Furthermore, the invention could also be applied to motors with increased numbers of poles. For example, the invention could be utilised in an 8/8 pole motor in which 2 coils embrace 4 poles or, alternatively, in which 4 coils each embrace 2 poles. Although it is more usual for the excited member to be the stator, it is, of course, possible for the rotor to be the excited member instead. As well as switched reluctance motors, the invention is also applicable to some forms of stepping motor and synchronous reluctance motor which have salient poles on both stator and rotor. While the invention has been described in relation to a single phase motor, in which the number of rotor and stator poles is usually the same, the invention is equally applicable to combinations of rotor and stator poles in single and multi-phase applications, for example, incorporating multiple stacks of laminations incorporated in a single motor such as a stepping motor. In this case, many combinations of rotor and stator poles are possible. In an alternative embodiment the thinner portions are dispensed with entirely. The remaining portions of the core maintaining the flux path are mounted in a supporting frame or housing which may or may not be a constituent part of the motor. The single-phase machine described can each be used as a single item or as one stack of a multiphase machine.

Claims: A rotary electric reluctance machine comprising a primary (10), having a primary core (13) defining primary poles (14) with pole faces; a secondary (12) having secondary poles (18) with pole faces; and a coil (16) or set of coils; characterised in that the or each coil is connected and/or is located with respect to the primary poles so as to define groups of adjacent primary poles, the magnetic polarity of the pole faces in each group of primary poles with respect to the coil or coils being the same and the said magnetic polarity of the pole faces of groups of primary poles being opposite in adjacent groups, and in that a portion of the primary core between adjacent primary poles in a group is relatively radially thinner than that between adjacent groups of primary poles. A machine as claimed in claim 1, wherein each of the said groups of adjacent primary poles is embraced by a coil. A machine as claimed in claim 1 or 2, wherein the primary core is angularly continuous with respect to the axis of rotation of the secondary relative to the primary. A machine as claimed in claim 1 or 2, wherein the thinner portion of the primary core at least partially defines a substantially flat outer face of the core. A machine as claimed in any of the preceding claims wherein the primary is a stator (10) and the secondary is a rotor (12). A machine as claimed in any of the preceding claims wherein the core is made up of a plurality of laminations extending laterally with respect to the axis of rotation of the secondary relative to the primary. A machine as claimed in any of claims 1 to 5, wherein the current supplied to the coils is switched. A machine as claimed in any of the preceding claims when connected as a motor. A machine as claimed in any of claims 1 to 7, when connected as a generator.

Inventors: STEPHENSON JOHN MICHAEL, STEPHENSON, JOHN MICHAEL
Applicants: SWITCHED RELUCTANCE DRIVES LTD
Application year: 1989
Main classification: H02K19
IPC codes: H02K19