Modular tubular linear switched reluctance machine

A tubular linear switched reluctance machine includes a segmented translator having a non-magnetic material body and ring segments of magnetic material axially separated from each other and provided on the body. The ring segments are distributed along the axial direction of the body. The machine also includes a stator arranged to electromagnetically interact with the segmented translator, wherein the stator has a plurality of coaxially arranged annular modules, and a respective annular non-magnetic spacer arranged between each pair of subsequently arranged modules.

TECHNICAL FIELD

The present disclosure generally relates to electrical machines. In particular it relates to a tubular linear switched reluctance machine which has a modular stator design.

BACKGROUND

The dominant existing linear motor topologies are permanent magnet or induction motors. Switched reluctance linear motors are not widely used, especially of tubular configuration.

Applications in which linear motors are involved require compact, cost-efficient and high performance solutions. Additionally, several applications involve specifications regarding overall weight, reliability and even modularity.

The paper “Identification of some tubular topologies of linear switched reluctance generator for direct drive applications in ocean wave energy conversion”, Mendes et al., Proceedings of the World Congress of Engineering 2014 Vol I, WCE 2014, Jul. 2-3, 2014, London, U.K. discloses a number of structural configurations of a linear switched reluctance generator made with tubular topology. One of these geometries, termed “Model C” in the paper includes a stator with ring-shaped elements of ferromagnetic material, in which are located the respective coil phase. The ferromagnetic material is separated by non-magnetic spacers. Each magnetic pole is formed by two teeth with the same dimensions as the ones that compose the salient profile of the secondary, or translator, which is made of a ferromagnetic material. Another one of these geometries, termed “Model D” includes a translator constituted by non-magnetic material with segments of magnetic material.

Although modular, the “Model C” design is rather heavy, which affects the obtainable thrust force of the secondary. The “Model D” design on the other hand is not modular.

SUMMARY

In view of the above, an object of the present disclosure is to provide tubular linear switched reluctance machine which solve or at least mitigate the above-described problems.

There is hence provided a tubular linear switched reluctance machine comprising a translator having a non-magnetic material body and ring segments of magnetic material axially separated from each other and provided on the body, which ring segments are distributed along the axial direction of the body, and a stator arranged to electromagnetically interact with the translator, wherein the stator has a plurality of coaxially arranged annular modules, and a respective annular non-magnetic spacer arranged between each pair of subsequently arranged modules.

The proposed configuration provides a modular tubular motor configuration, with reduced weight and low cost. The mass/weight of the machine is significantly reduced due to the non-magnetic nature of the translator's body while the segmented design of the stator provides modularity. It involves reduced material, construction and maintenance cost and compactness due to the modular nature of the configuration and the absence of any type of excitation in the translator, i.e. windings or magnets.

The present tubular linear switched reluctance machine exhibits high force density and high peak thrust force, which is significantly higher than the respective of the conventional linear switched reluctance motor. Additionally, the modularity of the design offers the capability to create custom solutions based on fundamental unit cell-modules. For example, in theory an infinite number of modules can be added to the stator, if the spacers are replaced with ones of proper width.

According to one embodiment the body is provided with a plurality of axially equally spaced circumferential recesses, wherein each ring segment is arranged in a respective circumferential recess.

According to one embodiment each module has a stator pole pitch width in the axial direction, and the axial distance between the midpoints of any pair of adjacent ring segments defines a translator pole pitch width, wherein the stator pole pitch width differs from the translator pole pitch width.

Through the proper selection of the ratio of the stator pole pitch width to translator pole pitch width, the thrust force waveform can be appropriately controlled in terms of shape, which thereby increases the force density compared to when the stator pole pitch width and the translator pole pitch width are equal.

According to one embodiment each module has two teeth axially separated by a circumferential channel, wherein the stator pole pitch width is defined by the axial distance between the midpoints of each pair of adjacent teeth of a module.

According to one embodiment the stator pole pitch width τstatoris smaller than the translator pole pitch width τtranslator.

According to one embodiment the ratio ksof the stator pole pitch width and the translator pole pitch width is less than 1.

According to one embodiment a ratio koutbetween the outer axial length of a ring segment and the translator pole pitch width is less than or equal to 1.

According to one embodiment a ratio kinbetween the inner axial length of a ring segment and the translator pole pitch width is less than the ratio kout.

According to one embodiment the ratio ksis greater than 0.6.

According to one embodiment the width of each spacer, in the axial direction, is dependent of the number of modules and the ratio ksof the stator pole pitch width and the translator pole pitch width.

According to one embodiment the width of each spacer is further dependent of the tooth width of the tooth of a module.

According to one embodiment the width of each spacer is determined by the formula

((1-ks)+m-1m)·τtranslator-bs
where m is the number of modules, τtranslatoris the translator pole pitch width, and bsis the tooth width of the tooth of a module.

According to one embodiment each module comprises a tubular magnetic core and a coil.

According to one embodiment the tubular linear switched reluctance machine is a tubular linear switched reluctance motor.

According to one embodiment the external surface of each ring segment is flush with the external surface of the body.

According to one embodiment the stator is arranged to receive the translator, or the translator is arranged to receive the stator.

DETAILED DESCRIPTION

The present disclosure relates to a tubular linear switched reluctance machine (TLSRM), i.e. a tubular linear switch reluctance generator or a tubular linear switched reluctance motor, which comprises a translator that includes a non-magnetic material cylinder and a plurality of ring segments made of magnetic material embedded in circumferential recesses provided in the outer periphery of the body. The presence of the ring segments provides the necessary magnetic reluctance variation with respect to the translator's relative position without the need for a salient geometric profile for the translator. The ring segments are distributed along the translator length, each generally having an equal axial extension.

The proposed configuration provides a structure with significantly reduced weight and mass compared to the conventional TLSRM, mainly due to the non-magnetic nature of the material used for the translator body, which enables the provision of a much lighter mover. For the body of the translator any non-magnetic material that is light but mechanically robust to withstand the high thrust force may be used, e.g. plastic or even a foam material can be used, dramatically reducing both the weight and the cost of the machine, due to the really low value of mass density compared to magnetic steel.

The geometry does not comprise a sequence of teeth separated by air parts, but a sequence of magnetic material ring segments that provide the necessary flux paths, embedded in the non-magnetic cylindrical body of the translator.

The TLSRM also comprises a stator having identical phase sets or modules and non-magnetic spacers between each pair of adjacent modules. Each module is magnetically self-contained and comprises a tubular magnetic core. The modules are hence separated by non-magnetic spacers, a feature that incurs complete independency of the respective magnetic paths.

With reference toFIGS. 1-3, examples of a tubular linear switched reluctance machine will now be described.

FIG. 1shows an example of a tubular linear switched reluctance machine, TLSRM,1including a translator3and a tubular stator5arranged to electromagnetically interact with the translator3. According to the present example, the tubular stator5is arranged to receive the translator3. The translator3is arranged to be axially movable relative to the stator5when arranged inside the stator5. In the assembled state of the TLSRM1, there is an air gap between the stator5and the translator3.

The translator3is segmented and cylindrical and comprises a body3amade of a non-magnetic material and a plurality of axially separated ring segments3bmade of magnetic material. The ring segments3bare axially separated and distributed coaxially around the body3aalong the axial extension thereof. The ring segments3bare hence electromagnetically insulated from each other. All the ring segments3bhave the same dimensions.

According to the present example, the body3ais provided with circumferential recesses7a, or circumferential grooves, extending along the entire circumference of the body3aand distributed along the longitudinal extension of the body3a. Between each circumferential recess7athe body3ahas a circumferential external surface7bwhich distances adjacent circumferential recesses7a. Adjacent ring segments3bare thereby physically separated from each other thereby providing an electromagnetic insulation between adjacent ring segments3b. The circumferential recesses7aare parallel and coaxial around the central axis z of the TLSRM1. Each ring segment3bis arranged in a respective circumferential recess7a. The external surface3cof each ring segment3bis arranged flush with the external surface7bof the body3a. In this manner the translator3obtains its cylindrical non-salient shape.

The tubular stator5comprises a plurality of coaxially arranged annular modules, or stator modules,5aand spacers5b, as shown inFIGS. 1 and 2. Each spacer5bis annular and arranged between respective two adjacent modules5a. Modules5aand spacers5bare hence arranged in an alternating manner. Each module5acomprises a tubular magnetic core5c. Each spacer5bis made of a non-magnetic material. The modules5aare equally spaced apart, i.e. all spacers5bhave the same axial width.

In a typical variation, the number of ring segments3bis not an integer multiple of the number of modules5a.

Each module5afurthermore has two teeth9axially whereby an inner circumferential channel11, or back iron, is formed therebetween. In cross-section, each module hence has a C-type shape. Each module5afurthermore comprises a respective coil5darranged in the space between the two teeth9, i.e. in the circumferential channel11. Each module5amay for example comprise a C-type tubular magnetic core and a spool-type or disk-type coil. Alternatively, the magnetic circuit of each module can be formed of two disks and one ring instead of a C-type tubular magnetic core.

According to one variation, every pair of module5aand coil5ddefines a distinct electric phase.

The tubular stator5is modular in the sense that during manufacturing its length is determined by the number of modules and spacers that are necessary for a particular application. To increase the force density in case the TLSRM1is a tubular linear switch reluctance motor the addition of stator modules can be employed.

The length of the translator3is defined by the specification of the mechanical stroke length.

Independence of magnetic circuits and modularity are the most prominent advantages that the stator configuration offers. In that sense, theoretically an infinite number of independent modules can be added in the stator, and provided that the width of the non-magnetic separators is appropriately adjusted, to produce force waveforms with minimum ripple.

Turning now toFIG. 3a longitudinal section of one half along a symmetry line13of a variation of TLSRM1is shown. In order to obtain higher and a more symmetric thrust force upon operation of the TLSRM1, the stator pole pitch width τstatorof the stator5is designed to differ from the translator pole pitch width τtranslatorof the translator3.

The stator pole pitch width τstatoris defined in the longitudinal direction of the TLSRM1, i.e. in the axial direction z. The translator pole pitch width τtranslatoris also defined in the longitudinal direction of the TLSRM1, i.e. in the axial direction z. In particular, the stator pole pitch width τstatoris defined by the axial distance between the centres, i.e. midpoints, of the two teeth9of a module5a. The translator pole pitch width τtranslatoris defined by the axial distance between the centres, i.e. midpoints, of two adjacent or consecutive ring segments3b, in particular along the external surface of the translator3.

Preferably, the stator pole pitch width τstatoris less than the translator pole pitch width τtranslator. This ensures a higher thrust force and a more symmetric thrust force waveform compared to the situation where the tubular stator5and the translator have identical pole pitch widths. The ratio ks=τstator/τtranslatorof the stator pole pitch width and the translator pole pitch width should hence preferably be less than 1. In general, the exact size of the ratio is dependent of the rating and size of the TLSRM1. For certain designs it has been found by the present inventors that the highest thrust force peak and most symmetric thrust force waveform can be obtained when the ratio ksis below 0.9. It may be particularly advantageous if the ratio ksis less than 0.9, even more preferably less than 0.85, for example 0.8 or 0.78. The ratio ksmay according to one variation be at least 0.6, for example greater than 0.6, such as 0.65. The axial width d, of the spacers5bis determined by the number of utilized modules5aand the value of the ratio ks, and is electromagnetically equivalent to a cycle of the phase inductance variation divided by the number of modules. The introduction of the modules5adecreases the length of the flux paths and can produce high aligned inductance. The inductance is also practically independent of the current value in the fully unaligned position, where the minimum inductance occurs. Additionally, in the fully aligned position, where the maximum inductance occurs, the inductance is strongly affected by the magnitude of the exciting phase current. The maximum thrust force is produced when a stator tooth just starts to overlap with a ring segment3bof the translator3. This overlap provides a really narrow flux path between the teeth9of the module5aand the ring segment that causes the production of a high positive force value, as the stator5attracts the translation towards the direction that the translator3is moving.

For a given number m of modules5athe axial width d of the spacers5bis determined based on the value of the ratio ks. In general, while designing the TLSRM1the width d of the spacers5bis determined based on the value of the ratio ksand on the number m of modules5a. The width d of the spacers5bis furthermore dependent of the tooth width bs, i.e. the axial length of a tooth9. The analytical formula for determining the axial width d of the spacers5bgiven the ratio ksis determined by the following equation.

((1-ks)+m-1m)·τtranslator-bs(1)
where τtranslatoris the translator pole pitch, which is defined by the distance from a first axial end of a ring segment3bto the corresponding first axial end of an adjacent ring segment3bor equivalently, as previously mentioned, the axial distance between the centre of two adjacent or consecutive ring segments3b.

Further parameters that influence the thrust force is the ratio koutbetween the outer axial length W2of a ring segment3band the translator pole pitch width τtranslator, and the ratio kinbetween the inner axial length W1of a ring segment3band the translator pole pitch width τtranslator. According to one variation, the ratio koutbetween the outer axial length W2of a ring segment3band the translator pole pitch width τtranslatoris less than or equal to 1, i.e.

Moreover, according to one variation the ratio kinbetween the inner axial length W1of a ring segment3band the translator pole pitch width τtranslatoris less than the ratio kout, i.e.

The longitudinal sectional shape of each ring segment3bis therefore preferably designed such that the outer axial length W2is longer than the inner axial length W1of a ring segment3b. The ring segments3bmay for example have trapezoidal shaped longitudinal section, as shown in e.g.FIGS. 1and3. Other longitudinal sectional shapes of the ring segments are however also envisaged, for example cyclic quadrilateral shapes or elliptic shape.

The parameters koutand kinprovide additional benefit to the designer of the TLSRM since the designer can work with per unit values. The two lengths W2and W1affect the performance of a motor. Defining them as ratios gives additional degrees of freedom in design.

Turning now toFIG. 4, thrust force waveforms for different ratios ksof the stator pole pitch width and the translator pole pitch width, when only one phase is excited is shown. The waveforms of these curves show overload operation that corresponds to force higher than double the nominal force. The curves C1, C2and C3show the thrust force when the pole pitch widths of the stator and translator are selected such that ksis 0.7, 0.75 and 0.8, respectively, plotted against time corresponding to a translation equal to one translator pole pitch width. The peak value of the thrust force in each curve is obtained when a ring segment starts to overlap with a tooth. When there is a complete alignment, the force is zero. Curve C4shows the thrust force when the ratio ksis equal to 1. It may be understood from these waveforms that the most symmetric thrust force curve and highest peak thrust force can be obtained when the ratio ksis selected to be somewhere between 0.7 and 0.8.

The inventive concept has mainly been described above with reference to a few examples. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended claims. Instead of the internal translator configuration described above, an external translator configuration is also possible, i.e. the translator could be tubular and arranged to receive the tubular stator. In this case, the ring segments would be arranged in circumferential recesses on the inner surface, i.e. along the inner circumference, of the tubular translator.