WO2019142116A1

WO2019142116A1 - Electric machine magnet insertion

Info

Publication number: WO2019142116A1
Application number: PCT/IB2019/050364
Authority: WO
Inventors: James Klassen; Damian SPASOV
Original assignee: Genesis Robotics And Motion Technologies Canada, Ulc
Priority date: 2018-01-18
Filing date: 2019-01-16
Publication date: 2019-07-25

Abstract

Disclosed in an electric machine having a first carrier with an array of electromagnetic elements and a second carrier with an array of posts defining slots between each post. The carriers are arranged to move relative to each other. The carriers are placed in axial alignment and so that an airgap is formed between them. A plurality of permanent magnets is provided, each having a pair of magnetic poles. The plurality of permanent magnets is placed in an alternating polarity arrangement in the slots so that for each post in the array of posts the two permanent magnets placed adjacent to the post are oriented so that the magnetic poles of each permanent magnet face towards the post.

Description

ELECTRIC MACHINE MAGNET INSERTION

FIELD

Electric machines.

BACKGROUND

[0001] In the design of electric machines, it is known to select structural parameters such as slot number depending on the intended application and desired performance characteristics of the machine. However, not all values of the structural parameters are used in practice. There is room for improved performance of electric machines, particularly in robotics.

[0002] Electric machines typically use electrically conductive wire turns wrapped around soft magnetic stator posts (teeth) to generate flux. The manufacturing process for this type of motor construction can be time consuming and expensive. As well, such motors typically have a torque to mass ratio that makes them relatively heavy for mobile actuator applications such as in robotics where the weight of a downstream actuator must be supported and accelerated by an upstream actuator.

[0003] Common permanent magnet direct drive motors can be difficult to assembly because of high permanent magnet forces between the rotor and stator. These high magnetic forces typically require complex fixtures for assembly to avoid damage to parts and injury to personnel as the rotor and stator are brought together.

SUMMARY

[0004] The inventor has proposed an electric machine with a novel range of structural parameters particularly suited for robotics, along with additional novel features of an electric machine.

[0005] In an embodiment there is provided an electric machine having an axial flux configuration. The electric machine comprises a first carrier having an array of electromagnetic elements and a second carrier having an array of posts defining slots between each post, where the posts of the second carrier further comprise a stop. The second carrier is arranged to move relative to the first carrier. The first carrier and the second carrier are placed in axial alignment. The first carrier and second carrier are arranged so that an airgap is formed between the first carrier and the second carrier. A plurality of permanent magnets is provided, each having a pair of magnetic poles.

[0006] The stops may further comprise at least one tab at each of the slots. Placing the plurality of permanent magnets in the slots may further comprise sliding each magnet into each slot so that each permanent magnet is positionally stabilized within the slots by the tabs.

[0007] In another embodiment, there is provided an electric machine having an axial flux configuration. The electric machine comprises a first carrier having an array of electromagnetic elements and a second carrier having an array of posts defining slots between each post. The second carrier is arranged to move relative to the first carrier. The first carrier and the second carrier are placed in axial alignment. The first carrier and second carrier are arranged so that an airgap is formed between the first carrier and the second carrier. A plurality of permanent magnets is provided, each having a pair of magnetic poles. The permanent magnets and the plurality of posts may have cooperating tapered shapes.

[0008] In another embodiment there is provided an electric machine having an axial flux configuration. A first carrier has an array of electromagnetic elements. A second carrier has electromagnetic elements including permanent magnets. The second carrier is arranged to move relative to the first carrier. An airgap lies between the first carrier and the second carrier. The electromagnetic elements of the first carrier comprise posts, with slots between the posts, the slots having one or more electric conductors in each slot. The second carrier comprises a homogenous rigid element and posts, in which the posts comprise homogenous extensions of the rigid element.

[0009] In various embodiments, there may be included one or more of the following or other features. The second carrier may further comprise a plurality of inner flux restrictors on the homogenous rigid element radially inward from the posts in which the plurality of inner flux restrictors further comprises a plurality of holes within the rigid element. The plurality of inner flux restrictors may further comprise a plurality of blind holes. The plurality of inner flux restrictors may further comprise a plurality of through holes. The second carrier may further comprise a plurality of outer flux restrictors on the homogenous rigid element radially outward from the posts, in which the plurality of outer flux restrictors comprises a plurality of holes within the rigid elements. The plurality of outer flux restrictors may further comprise a plurality of blind holes or a plurality of through holes. The second carrier may further comprise a plurality of inner flux restrictors on the homogenous rigid element radially inward from the posts and the plurality of inner flux restrictors may further comprise a plurality of holes within the rigid element. The second carrier may further comprise a plurality of outer flux restrictors on the homogenous rigid element radially outward from the posts and the plurality of outer flux restrictors may further comprise a plurality of holes within the rigid elements. Each of the inner and outer flux restrictors may be radially aligned in an alternating pattern with the posts on the second carrier so that the inner and outer flux restrictors are adjacent to every second post on the second carrier. Each of the inner and outer flux restrictors may be radially aligned with the posts on the second carrier so that the inner and outer flux restrictors are adjacent to each post on the second carrier. The plurality of inner flux restrictors and the plurality of outer flux restrictors may further comprise a plurality of holes having the same geometry. The plurality of holes having the same geometry may further comprise a plurality of holes having a circular shape.

[0010] These and other aspects of the device and method are set out in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Reference will now be made to preferred embodiments of the invention, by way of example only, with reference to the following figures in which:

Fig. 1 is an isometric view of an exemplary actuator;

Fig. 2 is an exploded view of the exemplary actuator of Fig. 1 ;

Fig. 3 is an isometric view of a rotor of the exemplary actuator of Fig. 1 ;

Fig. 4 is an isometric view of a stator of the exemplary actuator of Fig. 1 ;

Fig. 5 is an isometric view of a section of the exemplary actuator of Fig. 1 ;

Fig. 6 is a view of the body of the exemplary actuator along the section A-A in Fig. 1 ;

Fig. 7 is a section view of a rotor and stator including representations of magnetic flux and forces along the section B-B in Fig. 6;

Fig. 8 is a close up view of a rotor during installation and removal of the magnets; Fig. 9 is a partial cross section of a rotor plate section;

Fig. 10A is a partial view of a rotor plate section having flux restricting holes;

Fig. 1 OB is a partial view of a rotor plate section having another arrangement of flux restriction holes;

Fig. 11 is a FEMM simulation result on a rotor plate without flux restricting holes; Fig. 12 is a FEMM simulation result on rotor plate with flux restricting holes;

Fig. 13 is a cross section of a stator plate section with uninterrupted path between ID bearing and OD bearing;

Fig. 14 is an exploded view of an exemplary actuator;

Fig. 15 is a cross section of an embodiment showing an exemplary actuator connected to an upper and lower housing;

Fig. 16 is an exploded isometric view of the exemplary actuator in Fig. 15;

Fig. 17 is an isometric cut away view of the exemplary actuator in Fig. 15;

Fig. 18 is a cross-section through a segment of an axial flux concentrated flux rotor with tapered magnets and flux path restrictions;

Fig. 19 is a close-up section view of a portion of an axial flux concentrated flux rotor with extended length magnets;

DETAILED DESCRIPTION

[0012] Several terms to be used throughout the text will first be defined.

[0013] A carrier, as used here in the context of electric machines, may comprise a stator or a rotor when referring to rotary machines.

[0014] A rotor as used herein may be circular. A rotor may also refer the armature or reaction rail of a linear motor. A stator may be circular. It may also refer to the armature or reaction rail of a linear motor.

[0015] Teeth may be referred to as posts.

[0016] In an electric machine, either a stator or rotor may have a commutated electromagnet array defined by coils wrapped around posts, while the other of the stator or rotor may have magnetic poles defined by permanent magnets or coils or both coils and permanent magnets. An electric machine may be configured as a motor or generator. [0017] Permanent magnets may be used in combinations with electromagnets on the rotor and/or stator to add flux to the system.

[0018] PM means permanent magnet. EM means electromagnet. ID means inner diameter. OD means outer diameter.

[0019] Electromagnetic elements may comprise permanent magnets, posts, slots defined by magnetic posts, which may be soft magnetic posts, and electrical conductors. In any embodiment where one carrier has slots and posts, the other may have permanent magnets for the electromagnetic elements, and for any such embodiment, the term electromagnetic element may be replaced by the term permanent magnet. Magnetic poles in some cases, for example in a concentrated flux rotor embodiment, may be defined by permanent magnets in conjunction with adjacent posts in which a magnetic field is established by the permanent magnets.

[0020] Unless otherwise specified, flux refers to magnetic flux. Soft Magnetic Material is a material that is magnetically susceptible and that can be temporarily magnetised such as but not limited to iron or steel or a cobalt or nickel alloy.

[0021] A fractional slot motor is a motor with a fractional number of slots per pole per phase. If the number of slots is divided by the number of magnets, and divided again by the number of phases and the result is not an integer, then the motor is a fractional slot motor.

[0022] Thrust bearings include any bearing arranged to support a substantial axial thrust, including angular contact bearings and four-point contact bearings as well as pure thrust bearings. A radially locating bearing is a bearing that, in use, prevents relative displacement of the axes of the elements connected by the bearing.

[0023] A bearing can be radial and thrust locating (such as a cross roller bearing) or it can be just radial or just thrust locating.

[0024] A carrier may be supported for motion relative to another carrier by a frame or bearings, and the bearings may be sliding, roller, fluid, air or magnetic bearings.

[0025] An axial electric machine is an electric machine in which magnetic flux linkage occurs across an axial airgap, and the carriers are in the form of discs mounted coaxially side by side. A first carrier can be arranged to move relative to another carrier by either carrier being supported by a frame, housing or other element, while the other carrier moves relative the first carrier.

[0026] A radial electric machine is an electric machine where the airgap is oriented such that magnetic flux is radially oriented, and the carriers are mounted concentrically, one outside the other.

[0027] The airgap diameter for a rotary machine is defined as the diameter perpendicular to the axis of rotation at the centre of the airgap surface. In radial flux motors, all of the airgap resides at the same diameter. If the airgap surface is a disc-shaped slice as in axial flux motors, the average airgap diameter is the average of the inner and outer diameter. For other airgap surfaces such as a diagonal or curved surfaces, the average airgap diameter can be found as the average airgap diameter of the cross-sectional airgap view.

[0028] For a radial flux motor, the airgap diameter refers to the average of the rotor inner diameter and stator outer diameter for an outer rotor radial flux motor or the average of the rotor airgap outer diameter and stator airgap inner diameter for an inner rotor radial flux motor. Analogues of the airgap diameter of a radial flux motor may be used for other types of rotary motors. For an axial flux machine, the airgap diameter is defined as the average of the PM inner diameter and PM outer diameter and EM inner diameter and EM outer diameter.

[0029] The back surface of the stator is defined as the surface on the opposite side of the stator to the surface which is at the magnetically active airgap. In a radial flux motor, this would correspond to either the inner surface of the stator for an outer rotor configuration, or the outer diameter surface of the stator for an inner rotor configuration. In an axial flux motor, the back surface of the stator is the axially outer surface of the stator.

[0030] For distributed windings, the number of slots will be N x the number of poles where N is a multiple of the number of phases. So for a 3 phase machine N could be 3, 6, 9, 12, etc. For concentrated windings, the number of slots can vary but must be a multiple of the number of phases. It does not depend on the number of poles, except that certain combinations of slots and poles will yield higher torque and better noise-reduction or cogging-reduction characteristics. The minimum number of slots for a given number of poles should not be below 50% to obtain adequate torque.

[0031] Conductor volume may be used to refer to the slot area per length of a single stator. The slot area is the area of a cross-section of a slot in the plane which is orthogonal to the teeth but not parallel to the plane of relative motion of the carriers. In an axial motor, this plane would be perpendicular to a radius passing through the slot. The slot area effectively defines the maximum conductor volume that can be incorporated into a stator design, and it is usually a goal of motor designers to have as high a fill factor as possible to utilize all the available space for conductors.

[0032] Since maximum conductor volume in a stator is defined in terms of slot area, any stator referred to as having a maximum conductor volume or slot area must have slots and teeth to define the slots. This parameter is defined for rotary motors as:

N A

Slot area per length slot density A_s

pϋ AG

where As is the cross-sectional area of a single slot, or the average area of a single slot for stator designs that have varying slot areas.

[0033] As a relatively accurate approximation, As may be calculated as the height of the tooth, h_t, multiplied by the average width of the slot, w_s, such that the equation above becomes:

N_sh_t ^w _s

Slot area per length = - = slot density h_tw_s

[0034] Slot depth or post height may also be used as a proxy for the conductor volume. The post height, also known as the tooth height or slot depth, is a proxy for the amount of cross- sectional area in a slot available for conductors to occupy. Although the slots may have a variety of shapes such as curved or tapered profiles, the slot height is based upon the closest rectangular approximation which best represents the total area of the slot which may be occupied by conductors. This dimension does not include features such as pole shoes which add to the height of the tooth without adding substantially to the slot area. For transverse flux motors, the post height is defined as the portion of the post which is directly adjacent to the conductor coil, perpendicular to the direction of the coil windings.

[0035] A concentrated winding comprises individually wound posts or any winding configuration that results in the alternating polarity of adjacent posts when energized. It is understood that not all posts will be the opposite polarity of both adjacent posts at all times. However, a concentrated winding configuration will result in the majority of the posts being the opposite polarity to one or both adjacent posts for the majority of the time when the motor is energized. A concentrated winding is a form of fractional slot winding where the ratio of slots per poles per phase is less than one.

[0036] The terms one-piece, unitary, homogenous, solid, isotropic and monolithic are used interchangeably when referencing a stator or rotor herein. Each of the terms excludes laminates and powdered materials that include significant electrical insulative materials. However, small insulating particles may be present that do not significantly interfere with the electrically conducting properties of the material, for example where the bulk isotropic resistivity of the material does not exceed 200 microohm-cm. A one-piece, unitary, homogenous, solid, isotropic or monolithic material may comprise iron, including ductile iron, metal alloys including steel, and may comprise metal alloys formed of electrically conducting atoms in solid solution, either single phase or multi-phase, or alloys formed of mixtures of metals with other materials that improve the strength or conductivity of the material, for example where the bulk isotropic resistivity of the material does not exceed 200 microohm- cm.

[0037] Embodiments of the present device use an integrated bearing race that is preferably machined into the stator and/or rotor where the bearing races and at least the axial surfaces of the stator and rotor posts can be machined in the same set-up. This can provide for very high tolerance manufacturing of the critical geometry relationship between the bearing race axial and radial positions relative to the stator and rotor posts. Consistency of these geometric relationships is important for consistent cogging and other performance characteristics of the device.

[0038] Embodiments of the present device can allow for streamlined manufacturing with a rotor configuration that allows the permanent magnets to be installed into the rotor individually after the stator and rotor have been assembled.

[0039] Embodiments of the device can provide high torque density, ease of manufacturability, ease of assembly and serviceability due to a very simple assembly with a minimal number of components, and excellent operational safety as a result of high torque-to-inertia which allows very fast emergency stopping.

[0040] As shown in Fig. 1, a non-limiting exemplary embodiment of an axial flux motor 110 is housed in an upper arm member 100 and a lower arm member 200. The upper and lower arm members 100, 200 rotate around a rotational axis 300.

[0041] A non-limiting exemplary embodiment of the device in a robotic arm assembly is shown in Figure 2. The upper arm member 100 includes a support housing 101. The lower arm member 200 includes an arm housing 201. The support housing 101 and the arm housing

201 are preferably made of a light weight material such as, but not limited to, aluminum, magnesium or carbon fiber composite.

[0042] As shown in Figure 2 to Figure 5 the stator 102 is attached to the upper arm 100 such as with bolts and/or adhesive and/or thermal fit or by being formed integrally with the arm. In Fig. 2, the stator 102 is connected to the upper arm 100 using a press fit with a ring 101 A. An outer bearing 302 and an inner bearing 301 allow relative rotation of the stator 102 and rotor

202 and provide precise relative axial location of the stator 102 and rotor 202 to maintain an airgap between stator posts 105 (Fig. 4) and rotor posts 205 (Fig. 3). As shown in Fig. 3, the rotor may have flux restriction holes 206 and permanent magnets 204. The permanent magnets are seated in slots 208.

[0043] As shown in Figures 3 and 4, the rotor 202 includes a rotor plate 203 (Fig. 3) and the stator 102 includes a stator plate 103 (Fig. 4). The stator plate 103, as shown in Figure 4, and the rotor plate 203, as shown in Figure 3, can be made of ductile iron. The permanent magnets 204 can be Neodymium - N52H. Many other materials can be used for the various components. These materials are given by way of example.

[0044] In the non-limiting exemplary embodiment shown in Figure 3 and Figure 4, there are 96 stator posts (corresponding to 96 slots ) and 92 rotor posts with three phase wiring and each phase on the stator being divided into 4 equally array sections of eight posts each. The number of rotor posts in this example is 92 resulting in four equally arrayed angular positions where the rotor and stator posts are aligned. This, in-turn, results in a peak axial attraction force between the stator and rotor in four positions.

[0045] Note that many other combinations of stator post numbers and rotor post numbers may be used. Other numbers of phases may also be used. The examples here have been found to provide beneficial performance but do not limit the various construction principles to these exemplary geometries. For example features of embodiments of the device such as, but not limited to, the magnetically preloaded bearings or the wiring constructions can be used with rotors and stators with much lower or much higher numbers of poles.

[0046] It has been shown by simulation and experimentation that the total axial preload between the stator and rotor, for embodiments of the device, remains relatively constant such as within 10% in a multiphase wiring configuration such as, but not limited to, a three phase configuration, regardless of the current supplied to the windings and the torque developed by the motor. This is because the electromagnetic forces are reasonably equally in repelling and attraction. But although the total axial force on the stator and rotor remains reasonably constant, the axial attraction force on an individual post on the stator or rotor will vary quite a bit more (such as 14% or more). For this reason, in some embodiments, it is beneficial to distribute the number of phase sections into more than two sections per phase so the peak axial load from the permanent magnets occurs at more than one angular position (for example, at four equally arrayed angular positions). This can be beneficial to provide a more consistent axial preload on the bearings around the circumference of especially the OD bearing so any cantilevered external loads that would pull the stator from the rotor (such as a cantilevered load on a SCARA arm that is pulling the stator and rotor apart primarily at one angular position) are opposed by one or more peak axial force areas at all times, regardless of angular position of the arm. The greater the number of sections per phase, the greater the manufacturing complexity, in some respects, so four peak axial force positions (as a result of four sections per phase) is considered a good balance of manufacturability and peak axial force consistency. Four peak axial force positions can be accomplished with many different numbers of stator and rotor posts with the important characteristics being that there is a four post difference between the number of posts on the rotor and the number of posts on the stator.

[0047] Furthermore, it is beneficial for embodiments of the device that use embodiments of the wiring configuration shown that the number of posts on the stator be a multiple of three sections such as 3, 6, 9, 12, 14, 16 etc with each section having an even number of posts such as 2, 4, 6, 8, 10, 12 etc on the stator.

[0048] Another consideration when deciding how many peak axial force positions to choose in the design of an embodiment of the device, is the number of cogging steps that will result. A high number of cogging steps is beneficial to reduce cogging (because a higher number of steps generally results in a lower force variation between the maximum and minimum torque of each cogging step) so a two post difference (corresponding with two sections per phase) between the stator and rotor would seem to be preferable to reduce cogging because, in a non limiting exemplary embodiment of 96 stator posts and 94 rotor posts, the number of cogging steps is 4512, which is a very high, resulting in a theoretical cogging torque that is very low. However, a two magnet difference between stator and rotor results in only two peak axial attraction force position at any given time resulting in a less stable support of a cantilevered load on the output of the actuator such as in a SCARA arm configuration when lifting a payload. For this reason, a rotor/stator post difference of four is considered to be a good choice in terms of payload lifting stability even though it has a lower number of cogging steps and theoretically higher cogging forces. A 96 stator-post to 92 rotor-post configuration results in only 2208 cogging steps which would be expected to result in about two times greater cogging force variation. A post difference of four, would, therefore, not seem to be beneficial in terms of cogging reduction because the cogging steps would be fewer and, as a result, larger in magnitude. However, there can be another benefit of fewer cogging steps (which results from a larger post number difference between the stator and the rotor - such as, for example, a four post difference as shown in Figure 3 and Figure 4 of four, as opposed to a one or two post difference). This advantage is related to a correlation between the size of the cogging steps and the required accuracy of the stator and rotor axis alignment during manufacturing/assembly and in operation under various loads. Specifically, if the cogging steps are smaller (measured circumferentially at the average airgap diameter) than the radial displacement of the rotor axis relative to the stator axis (due to lack of manufacturing accuracy) the stator and rotor will not be aligned sufficiently to achieve consistent cogging steps. This will result in inconsistent cogging forces during rotation. Any radial displacement of the rotor relative to the stator will have a misaligning effect, in the same radial direction, on posts that are diametrically opposed, resulting in less than ideal cogging cancellation. Some combinations of rotor/stator axis misalignment and relative angular position of the stator with very high cogging step embodiments (such as with a two post difference between stator and rotor) may even result in greater cogging force variation in some conditions than if a larger rotor/stator post difference is used (assuming similar radial misalignment in each exemplary case).

[0049] It is common with many typical three phase motors to have wires from two or three phases in a single slot. Embodiments of the present device use a wiring configuration where two or more adjacent slots in a row contain conductors from only one phase. Many different winding methods may be used with this device but the advantages of a winding configuration 104 as shown in Figs. 4 and 5 includes the ability to use axially aligned (circumferentially layered in each slot) non-overlapping flat wire (overlapping the wire - as is typically done in three phase distributed winding machines, is problematic with flat wire). To take advantage of the simplicity of assembly of this winding configuration and method, it can be beneficial to have as few sections per phase as possible (such as one section per phase EG: 32 slots per phase for a 96 slot stator, or two sections per phase EG: 16 slots per phase for a 96 slot stator). The number of rotor posts for this winding configuration is preferably equal to the number of stator slots plus or minus the number of sections per phase EG 94 or 98 rotor posts for a 96 stator slots having two equally arrayed sections per phase.

[0050] In a non-limiting exemplary embodiment of the present device, the outer diameter of the stator is 200mm and the axial air gap is approximately .010”.

[0051] As illustrated in Figure 7, permanent magnets 204 generate magnetic flux represented by the arrow 401. Meanwhile, an adjacent magnet also generates the same polarity magnetic flux 402 into the pole 205. Both flux 401 and 402 travel through the rotor pole 205, pass through the airgap 400, into stator post 105, and generate magnetic attractive forces 403 on both the stator 102 and the rotor 202. The magnetic forces 403 are so strong that they are able to hold the stator and the rotor together during passive and active operation under usable operating conditions for many applications. The posts are connected to a back iron 106. Assembly and disassembly safety concerns may be reduced with embodiments of the device, and the cost and complexity of assembly fixtures may be reduced.

[0052] The rotor plate shown in Figure 3 has no back iron immediately axially outward from the permanent magnets (corresponding to radially outward from the permanent magnets in a radial flux embodiment of the device etc.). As a result, magnet slots 208 are open on the back face of the rotor so magnets can be assembled into the slots after the stator and rotor are assembled. Figure 8 shows that the magnets 204 can be accessed from the back of the rotor which allows each of those magnets to be removed or installed individually without removing the rotor from the stator.

[0053] The magnets 204 may be installed into the slots as follows. Align the magnet to the slot with the same polarity magnetic flux contacting the rotor post as the adjacent magnet contacting the same post. Every second magnet will be in the same circumferential polarity alignment. Every first magnet will be the opposite of every second magnet so the posts are alternating polarity. Slide the magnet into the slot until it is secured against the tabs (if parallel sided) or, if tapered magnets are used, until the tapered magnet seats into the tapered slot. Repeat the above steps until all the magnets are installed. Apply bonding agent (eg, wax, epoxy, glue) to fill the clearance gap. This step may not be necessary in all cases, such as with a precision tapered magnet in a precision tapered slot.

[0054] To remove the rotor and access the stator coils and ball bearings, the rotor can be easily demagnetized by removing the magnets individually.

[0055] As shown in Figure 7, each of the permanent magnets 204 in the rotor generates the same polarity flux as its immediately adjacent permanent magnet which means every magnet will be repelling the adjacent magnets on both sides of it. This would cause the magnets to repel each other, except it has been shown that certain geometries are able to prevent these repelling forces from causing the magnets to dislodge themselves form the slots. The smaller the airgap, for example, the stronger the force, in many cases, which will cause the magnets to lodge themselves into, instead of out of, the slots. The use of tapered magnets is also beneficial in this sense, because a tapered magnet, with the large dimension of the taper toward the back face of the rotor, will generally be more apt to pull itself axially toward the rotor posts and therefore toward the airgap.

[0056] As shown in Figure 9, a physical stop is used to stop the magnet from moving into the airgap. In one embodiment, the stops are tabs 210 on each side of the slot generate attractive forces as the magnet slides into the slot. Their combined force pull the magnet into the slot. Since the repelling forces partially or completely cancelled out, the combined force from the poles and tabs becomes the resultant force acting on the magnet. The magnets sit on the tabs and the magnetic attractive forces secure the magnets to the poles. When configured correctly, as described in an earlier disclosure, the net force on the magnets can be tailored to use the magnetic forces to magnetically retain the magnets in the slots. Adhesive or mechanical mechanism is not required in this case except to prevent side-to-side movement of a magnet in a slot.

[0057] In another non-limiting exemplary embodiment, it is also possible to provide force to retain the magnetics in the rotor slots using a combination of mechanical and magnetic force. Tapered magnets can provide a structure in which a significant percentage of magnetic flux goes through the airgap while retaining the magnets in the rotor slots.

[0058] Magnets which taper tangentially such that they are thinner toward the air gap, can provide high performance in a concentrated flux rotor configuration. Referring to Figs. 30 to 31, there is shown a rotor 3300 in an axial flux configuration with magnets 3302 having tapered ends 3316 and rotor posts 3304 with tapered ends 3318. The magnets and rotor posts taper in opposite directions to form an interlocking arrangement. Permanent magnets taper in the direction of the stator 3330 while rotor posts 3304 taper away from the stator. In this embodiment two substantially mirrored rotors 3300 can be assembled between a pair of stators, with tapered posts of each rotor meeting back to back and tapered magnets of each rotor meeting back to back. Tapering the magnets 3302 in this way, allows for greater rotor post width at the air gap. It also allows for greater magnet width at the wide end of the magnet taper to provide more flux to the rotor post 3304 away from the air gap, where if the sides were parallel the posts 3304 would tend to be less saturated. In this way, the active permanent magnet 3302 and soft magnetic materials are used more effectively to provide more flux at the airgap. The two rotors parts can be secured together for example by an adhesive, but in some preferred variations a mechanical feature such as bolts (not shown) or a securing ring (not shown) may be used.

[0059] The interlocking arrangement of tapered posts 3304 and magnets 3302 operate as stops that prevent the permanent magnets from dislodging, which reduces the need for magnetic force to retain the magnets in the rotor, and therefore reduces the need for magnetic flux to leak through the end iron 3314.

[0060] Fig. 18 shows an axial flux configuration of a tapered slot rotor, but the tapered slot rotor can be equivalently constructed in a radial flux configuration. Tapered magnets may narrow towards or away from the opposing carrier.

[0061] A second effect of tapering the magnets in this way is to bias a high percentage of the flux from a permanent magnet toward the air gap. This is beneficial in at least two ways. A first is that the tapered permanent magnet will be drawn toward the air gap where they will close the airgap between the permanent and the rotor slot wall for lower reluctance flux linkage and where they will be mechanically prevented from further movement and therefore securely retained by the tapered rotor posts. Secondly, the narrower rotor posts at the back surface results in a greater distance from post to post along the center plane of the rotor. This reduces the amount of leakage through the air from post to post along the center plane of the rotor. By assembling two substantially mirrored rotor halves with tapered posts and tapered magnets back-to-back a large percentage of the flux from the permanent magnets can be forced to link across the air gap.

[0062] In this way, very high flux density can be achieved in the air gap while magnetically and mechanically retaining the magnets. A cost effective way to manufacture a tapered rotor post rotor is to use two symmetrical rotors 3300 back to back. This construction does not allow for the use of a back iron to stiffen the rotor, so a soft magnetic end iron 3314 is used instead. The end iron 3314 has sections that are preferably as thin as possible to create a high reluctance flux path between rotor posts through the end iron, and as thick as necessary to provide the mechanical strength and rigidity to maintain a small and consistent air gap.

[0063] To compensate for the loss of flux from post to adjacent post through the end iron connection, an embodiment uses permanent magnets 3302 that are longer than the soft magnetic stator posts 3332 at the air gap. This is shown in Fig. 19 where the permanent magnet 3302 are longer than rotor posts 3304 which would have the same or nearly the same length as the stator posts 3332.

[0064] A non-limiting exemplary embodiment of the actuator in the previous paragraphs is shown in Figure 10A with optional flux restriction holes 206 placed between magnet slots 208, and along the outside and inside radius of the magnet slots 208 on the rotor to reduce flux leakage between the opposite polarity faces of a magnet and between adjacent rotor poles. Magnetic simulation was done to verify if those holes reduce flux leakage and it has been shown that the flux leakage between rotor poles can be substantially reduced while still maintaining the necessary structural strength and stiffness to achieve a small and consistent airgap.

[0065] In some embodiments an array of flux path restrictions 3328 can be formed in the end iron 3314, for example, as holes in the end iron 3314 at the base of each rotor posts 3304 where they connect with the end iron 3314. These flux path restrictions 3328 reduce the available flux path between rotors posts 3304 and end iron 3314.

[0066] The flux restriction holes can, alternatively, be located between every second post on the OD and between every second post on the ID as shown in Figure 10B. As shown in Fig. 10B, the inner and outer flux restriction holes are staggered so that each post is adjacent to only one of the inner or outer flux restriction holes. This provides an unrestricted flux linkage between only the N posts around the OD and only the S posts around the ID as well as increased structural integrity for every first post around the OD and every second post around the ID. These holes can be thru-holes or blind holes, as long as they provide the necessary structural strength and stiffness as well as the desired flux path reluctance.

[0067] Figure 11 shows the flux path from the magnetic simulations without flux restriction holes and Figure 12 shows the flux path from the magnetic simulations with flux restriction holes. From the figures, it is shown that flux restriction holes reduce flux leakage between adjacent rotor poles. For example, when flux restriction holes are used, the flux density increased at the air gap surfaces of the rotor poles and more flux is directed to pass through the stator. As a result, electromagnetic force increases when the coils are engaged and torque generated by the stator and rotor increases.

[0068] As shown in Fig. 19, there are two flux restrictors 3328 adjacent to each end of each rotor post 3304. The rotor posts 3304 have a larger width at the axial outer end of the rotor. The flux restrictors 3328 are larger adjacent to the outer end of the rotor posts and smaller at the inner end of the rotor posts.

[0069] MagNet simulations on the rotor plate with and without flux restriction holes also led to the same conclusions. More flux is directed from the posts into the airgap.

[0070] Referring to Fig. 14, an exploded view of exemplary rotor and stator is shown that is connected to a pair of robot arms using bolts. A first arm 700 is connected to a rotor housing 702 using bolts 718. The rotor housing 702 is connected to a rotor 708 using bolts 720. A first bearing element 706 connects between the rotor 708 and a stator 712 and is connected by a press fit ring 704. A second bearing element 710 also connects between the rotor 708 and the stator 712 using bolts 722. The stator 712 is connected to a stator housing 714 using bolts 724. A second arm 716 is connected to the stator housing 714 using bolts 726.

[0071] Referring to Figs. 15 to 17, a rotor 606 is made from a ferrous material, such as Ductile Iron, and holds an equi-spaced array of magnets 605 that are polarised in a circumferential direction. The polarity of the magnets 605 is alternated in order to generate alternating north and south poles in the radial webs of the rotor 606. The stator 609 is made from a ferrous material, such as Ductile Iron, and includes an equi-spaced array of axial posts around which a set of stator windings 610 are wrapped. Applying commutated power to the stator windings 610 polarises the posts of the stator 609 such that circumferential attraction and repulsion forces are generated between the posts of the stator 609 and the radial webs of the rotor 606, thereby generating torque. The stator windings 610 are encapsulated by the stator potting compound 611, which serves to prevent movement of the wires and helps to transfer heat from the wires to the stator 609. As shown in Fig. 16, a stator cap 612 may be placed over the stator 609 and hold the wires 610 in place.

[0072] The magnets 605 also cause attraction between the stator 609 and the rotor 606. The bearings 603 and 604 counteract the attraction force between the stator 609 and the rotor 606 via the housings 601, 602, 607 & 608 and act to accurately control the gap between them. The axial attraction force between the stator 609 and the rotor 606 is adequate, in most applications, to prevent the upper housing 601 from separating from the lower housing 602, thereby eliminating the need for additional retention between them. Diametral fits at the interfaces between the housings 601, 602, 607 & 608 and the rotor 606 and the stator 609 carry radial loads between the two assemblies via the inner 4-point contact bearing 604. External moments applied to the assembly are carried primarily through the outer thrust bearing 603.

[0073] The flow of current through the stator windings 610 tends to increase the temperature of the stator 609 relative to the other components. Conduction of the generated heat to the adjacent housings helps to reduce the increase to its temperature. The example shown includes light alloy housings which have a higher coefficient of thermal expansion than the stator 609. To maintain an interference fit at the interface between the outer diameter of the stator 609 and the inner diameter of the lower housing 602 as the temperature increases the primary diametral location occurs at the inner diameter of the locating hook of the stator 609.

[0074] In Fig. 17, removeable caps 614 and 616 sit in the arms which allow the stator and rotor to be inserted, and for the magnets to be inserted last.

[0075] The flux restriction holes described for example in the embodiments disclosed in Fig. 3, Fig. 10A, and Fig. 10B, are designed to meet an acceptable trade-off between power and structural strength. The cross-sectional area above the magnets provides the strength to maintain the airgap and the flux restrictors prevent flux from excessively extending between the magnets. The flux restrictors can be placed with holes adjacent to every second post, rather than adjacent to every post, which will provide for a stronger structure but does not have a significant impact on the flux. The flux restrictors could be blind or through-holes, so long as there is a cross-sectional area reduction in the flux path and the structural load path. In a preferred embodiment, the flux restrictors will lie on either end of the posts, between the array of posts and each set of bearings. The flux restrictors will preferably lie parallel with the length of each post. The flux restrictors can be designed so that there is a greater cross-sectional area in a structural load path than in a magnetic flux path. The flux restrictors could also be used in a radial flux machine in an equivalent manner as those described for the axial and linear flux machines described herein. An embodiment of the machines described herein with flux restrictors may have a solid material made for example with ductile iron which is strong enough to support magnetic forces, but thin enough to be lightweight. The flux restrictors may be placed adjacent to every post on the rotor or stator or adjacent to every second post on the rotor or stator. The flux restrictors will generally be placed on both ends of each post, or each second post. The flux restrictors may be placed adjacent to every post on one end of each post and adjacent to every second post on the other end of each post. The flux restrictors may be placed in an alternating pattern so that each post is adjacent to only one flux restrictor, and for each adjacent post, the corresponding flux restrictor is adjacent to an opposite end of the adjacent posts. The flux restrictors may have different sizes while maintaining the same geometry. The cross-sectional flux path may be consistent between every second post, but the cross-sectional flux path may be selected so that it alternates between adjacent posts so that each post has a different cross-section flux path than the post directly adjacent to it. Where the flux restrictors are placed in an alternating pattern so that each second post is adjacent to flux restrictors, then the cross-section of each post that is adjacent to the flux restrictors may be smaller than the cross-section of each post that is not adjacent to the flux restrictors. In such an embodiment, every second post will have a larger cross-section than each of the adjacent posts that are adjacent to the flux restrictors. Although flux restrictors will generally be more effective to reduce cogging when placed on the rotor, rather than the stator, the flux restrictors can be placed on both rotor and stator, or only on the rotor. As shown in Fig. 19, there may be multiple flux restrictors adjacent to each end of the posts.

[0076] Heat can be extracted from the back surface of the stator though direct contact with a cooling fluid or through conduction to another member such as a housing, or through radiation, for example. Other surfaces of the stator or conductors can also be cooled by various means. Cooling the back surface of the stator is shown to be a cost effective and simple option for many motor types. A sample analysis (not shown here) indicates that geometry in the disclosed range which shows better heat dissipation from the back surface of the stator (as compared to motors outside of the disclosed range) will also generally show improved heat dissipation than motors outside of the disclosed range when other surfaces of the stator or conductors are cooled. The back surface of the stator is, therefore, viewed as a useful cooling surface, as well as an indicator of the effectiveness of each motor in the series to the application of cooling to other surfaces of the stator and conductors. The back surface of the stator has been chosen for the main cooling surface for the motor series analysis which is used to identify the disclosed range.

[0077] Other methods of cooling may be applied to an electric machine with the disclosed range of pole density and conductor volume, but the heat flow path from conductors to the back of the stator will preferably always be used for cooling the motor regardless of what other types of cooling (EG: direct coil cooling) are used.

[0078] Stator back iron may have an axial depth that is 50% of the width (circumferential or tangential width) of the posts. The posts may each have a tangential width and the stator may comprise a backiron portion, the backiron portion having a thickness equal to or less than half of the tangential width of the posts, or may be less than the tangential width of the posts. Thicker back iron adds weight with minimal benefit. Thinner backiron helps with cooling but the effect of back iron thickness on cooling is not very significant. The backiron surface may be in physical contact with the housing to conduct heat physically from the stator to the housing, and/or the back surface of the stator can be exposed to an actively circulated cooling fluid and/or the back surface of the stator can be configured for radiative heat dissipation to the atmosphere or to the housing or other components, and/or the back surface of the stator can be configured for convective or passive cooling through movement of air or liquid over the surface of the stator and or housing. Gas or liquid moving past the back surface of the stator may be contained or not contained. The back surface of the stator may be sealed from the atmosphere or exposed to the atmosphere. The atmosphere may be air or water or other fluid surrounding the actuator. The environment may also be a vacuum, such as is necessary for some manufacturing processes or the vacuum of space. The back surface of the stator may be configured with cooling fins which increase the surface area. These cooling fins may be exposed to a cooling fluid and/or in contact with a heat sink such as the housing or other solid member. The cooling fins on a stator may have a height greater than 50% of the post width in the circumferential direction.

[0079] In addition to heat being dissipated from the back surface of the stator, other heat dissipating surfaces may include the surface of a post which may be exposed to a cooling fluid such as air or liquid which is circulated through a slot such as between a conductor and the post.

[0080] Other methods of cooling the stator and/or the conductors may include cooling channels on or below the surface of the stator and/or on or below the surface of the conductors. These and other forms of cooling are seen as supplementary to the primary thermally conductive cooling from the conductors to the back surface of the stator. In some cases the supplementary cooling methods may even draw more heat away from the stator than the primary conductive cooling effect, but active cooling methods require energy and additional cost and complexity, so the conductive cooling path from the conductors to the back surface of the stator is disclosed here as the primary mode of cooling.

[0081] Although the foregoing description has been made with respect to preferred embodiments of the present invention it will be understood by those skilled in the art that many variations and alterations are possible. Some of these variations have been discussed above and others will be apparent to those skilled in the art.

Claims

CLAIMS:

1. An electric machine having an axial flux configuration, comprising:

a first carrier having an array of electromagnetic elements;

a second carrier having electromagnetic elements comprising permanent magnets, the second carrier being arranged to move relative to the first carrier;

an airgap between the first carrier and the second carrier;

the electromagnetic elements of the first carrier including posts, with slots between the posts, the slots having one or more electric conductors in each slot; and

the second carrier comprising a homogenous rigid element and posts, in which the posts comprise homogenous extensions of the rigid element;

in which the posts of the second carrier further comprise a stop.

2. An electric machine having an axial flux configuration, comprising:

a first carrier having an array of electromagnetic elements;

an airgap between the first carrier and the second carrier;

the second carrier comprising a homogenous rigid element and posts, wherein the posts comprise homogenous extensions of the rigid element; and

further wherein the posts are each tapered to narrow in a direction away from the first carrier and the permanent magnets are tapered to narrow in a direction toward the first carrier.

3. The electric machine of claims 1 or 2 in which the posts of the second carrier prevent the electromagnetic elements of the second carrier from moving in a direction towards the first carrier.

4. The electric machine of claim 1 in which stop further comprises a tab on each post adjacent to the airgap.

5. The electric machine of claims 1 or 2 in which the permanent magnets are held in place in the corresponding slots by magnetic force when the first and second carriers are in the operational position.

6. The electric machine of claim 5 in which the permanent magnets are not held in place in the corresponding slots by the magnetic force when the second carrier is separated from the first carrier.

7. The electric machine of any one of claims -1-6 in which the second carrier further comprises a plurality of inner flux restrictors on the homogenous rigid element radially inward from the posts in which the plurality of inner flux restrictors further comprises a plurality of holes within the rigid element.

8. The electric machine of claim 7 in which the plurality of inner flux restrictors further comprises a plurality of blind holes.

9. The electric machine of claim 7 in which the plurality of inner flux restrictors further comprises a plurality of through holes.

10. The electric machine of any one of claims 1-7 in which the second carrier further comprises a plurality of outer flux restrictors on the homogenous rigid element radially outward from the posts, in which the plurality of outer flux restrictors comprises a plurality of holes within the rigid elements.

11. The electric machine of claim 10 in which the plurality of outer flux restrictors further comprises a plurality of blind holes.

12. The electric machine of claim 10 in which the plurality of outer flux restrictors further comprises a plurality of through holes.

13. The electric machine of claim 1 in which the second carrier further comprises a plurality of inner flux restrictors on the homogenous rigid element radially inward from the posts in which the plurality of inner flux restrictors further comprises a plurality of holes within the rigid element and in which the second carrier further comprises a plurality of outer flux restrictors on the homogenous rigid element radially outward from the posts, in which the plurality of outer flux restrictors comprises a plurality of holes within the rigid elements.

14. The electric machine of claim 13 in which the each of the inner and outer flux restrictors are radially aligned in an alternating pattern with the posts on the second carrier, and the inner and outer flux restrictors are adjacent to every second post on the second carrier.

15. The electric machine of any one of claims 13-14 in which each of the inner and outer flux restrictors are radially aligned with the posts on the second carrier, and the inner and outer flux restrictors are adjacent to each post on the second carrier.

16. The electric machine of any one of claims 13-15 in which the plurality of inner flux restrictors and the plurality of outer flux restrictors each further comprise a plurality of holes having the same geometry.

17. The electric machine of claim 26 in which the plurality of holes having the same geometry further comprise a plurality of holes having a circular shape.