GB2508971A

GB2508971A - Permanent magnet machine with offset pole spacing

Info

Publication number: GB2508971A
Application number: GB1318857.8A
Authority: GB
Inventors: Franco Leonardi; Michael W Degner; Feng Liang
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2008-11-11
Filing date: 2009-10-09
Publication date: 2014-06-18
Anticipated expiration: 2029-10-09
Also published as: GB2508971B; GB201318857D0

Abstract

A permanent magnet machine comprises a stator and rotor on a common axis. The rotor comprises plural axially-stacked sections, each having asymmetrically placed magnet openings at its periphery and comprising plural laminations with a key and slot 146 driving arrangement on a shaft. The laminations for a first section have a common geometry with those of a second section, permitting manufacture by a single tool, and are flipped about a radial axis 136 with respect to those of the second section upon assembly so that the openings of the first section are angularly displaced relative to those of the second. There can be more than two sections with laminations of a third section being flipped about a second axis 150 that are displaced by an angle. There can be two slot and key driving features per section that are spaced 180 degrees apart and displaced by 90 degrees from those of another section. Third and fourth sections can have laminates flipped about the 90 degree displaced features. Permanent magnets are placed asymmetrically and angularly offset in the lamination openings to attenuate oscillations in torque caused by harmonic components of magnetic flux.

Description

PERMANENT MAGNET MACHINE WITH OFFSET POLE SPACING

The invention relates to a permanent magnet machine, in particular but not exclusively an inner permanent magnet machine having a rotor with multiple laminations in axially stacked relationship.

An inner permanent magnet machine typically includes a stator with stator coil windings and a rotor with clrcumferentially spaced permanent magnets on the rotor periphery that cooperate with circumferentially spaced stator poles separated from the periphery of the rotor with a calibrated air gap. When the machine is acting as a motor, the coils are energized by an electrical current to effect rotation of the rotor. The current has an aternating waveform, typically of sinusoidal shape, which creates an electromotive rotor torque. The interaction of an electromagnetic flux flow path created by the stator windings with the flux flow path created by the permanent magnets typically is accompanied by harmonic waveform components that induce motor torque fluctuations. This is manifested by a motor torgue ripple, or torque oscillation, accompanied by vibration and noise. Further, operating efficiency of the motor is affected adversely.

A conventional way to reduce motor torque ripple comprises skewing axially placed sections of the rotor, one section with respect to the other. The rotor typically is connected drivably to a rotor shaft using a keyway and slot driving connection. In order to offset or skew a rotor section with respect to an adjacent section, the sections are relatively rotated, usually about one-half of the stator slot pitch. If it is assumed that the rotor is divided into a given number of axial sections (k), the sections are rotated with respect to adjacent sections by an angle equal to: skew angle (k) = 360/(k x NJ in mechanical degrees, where N3 is the number of slots.

The maximum rotation between any two axial sections of the rotor is: max relative skew angle (k) = (k-i) x 360/(k x NJ in mechanical degrees.

For example, in the case of a two section, 48 slot stator, a typical value of the skew angle is 3.75°. The skewing of the rotor is intended to produce a smoother mechanical torque than would otherwise be achieved using a straight rotor. This will eliminate certain undesirable oscillations or ripple of the torque caused by harmonics present in the air gap flux and in the air gap permeance.

According to the present invention there is provided a permanent magnet machine in accordance with Claim 1.

An objective of the invention is to minimize a so-called torque ripple with minimal reduction in average torque. This differs from the invention of copending application Serial No. 11/839,928, filed August 16, 2007 entitled "Permanent Magnet Machine," which is assigned to the assignee of the invention, in which an objective is to improve motor efficiency during operation in a motoring mode by using asymmetry in rotor design features of the motor while allowing an acceptable decrease in regenerative energy recovery during operation in a generating mode.

The present invention may break the symmetry of the rotor laminations, so that at a given instant the torque contributions of the multiple sections will be altered to reduce torque ripple.

Torque ripple may be attenuated by using radial skewing. This may be done by offsetting the magnetic axis of a rotor magnetio pole with respeot to the axis of the adjacent rotor magnetic pole.

Rotor magnets may be arranged in a "V'T configuration.

The shape of the torgue ripple is a function of the shape of the "VI' configuration. By using at least two different, properly designed "V'1 configurations in the laminations, the total machine ripple can be reduced in amplitude.

According to the present invention there is provided a permanent magnet machine comprising a stator with eectrcmagnetic windings for stator poles and a rotor located on a rotor axis that is common to an axis for the stator with an air gap between the stator and the rotor; the rotor comprising a plurality of axially-stacked sections, each section comprising a plurality of stacked laminations on a rotor shaft with a key and slot driving connection therebetween; each section having a plurality of magnets located on its own periphery, the magnets creating a magnetic flux pattern that interacts with a magnetic flux pattern created by the magnetic windings of the stator poles to develop rotor torque; the laminations in one section having asymmetrically placed first magnet openings for receiving rotor magnets; the laminations of a second section having asymmetrically placed second magnet opening s for receiving rotor magnets; the magnet openings for the laminations of eth first section having a common geometry with respect to the magnet openings for the laminations of the second section to permit manufacture with a common machine tool; the laminations for the second section being flipped about a radial axis relative to the laminations for the first section following assembly of the rotor, the rotor openings for the laminations of the seoond seotions being angularly displaced relative to the rotor openings for the laminations of the first sections, whereby rotor torgue fluctuations are attenuated.

The laminations in a multiple section rotor may be arranged in at least three rotor sections, which are relatively rotated in small increments, one section with respect to the other. This can be done by using at least two pairs of key slot positions. In this way, the axis of a magnetic pole of one section may be placed angularly with respect to the pole axis of the adjaoent section.

Embodiments of the invention will now be described, by way of example only, in which: Figure la is a plan view of a rotor lamination; Figure lb is a side view of the rotor lamination; for the motor shown in Figure la; Figure 2a is a diagrammatic view of a motor with a rotor comprised of multiple sections, each section being comprised of multiple laminations wherein flux lines are generated solely by the permanent magnet; Figure 2b is a view similar to the view of Figure 2a wherein the stator has energized windings with electrical current, but wherein the magnets are not included, the flux lines being generated solely by the stator windings; Figure 3 is a schematic representation of a prior art two-section rotor in which the sections are skewed, one with respect to the other, according to a known skewing technique; Figure 4 shows a symmetric lamination for a prior art eight pole rotor design for use in the electric motor rotor seen in Figure 3; Figure 5 shows a rotor lamination with radial skewing; Figure 6 is a plot of rotor rotation angle in mechanical degrees versus motor torque in Newton meters showing the effect on motor instantaneous torque using the magnet arrangement of Figure 5; Figure 7 is a view of a continuous skewing arrangement that may be used rather than the skewing arrangement of Figure 5; Figure 8 is an illustration of a magnet distribution that has a more uniform separation between the rotor poles oompared to the rotor separation of Figure 7; Figure 9 is a plot of motor torque versus rotation angle in mechanical degrees for the rotor design illustrated in Figure 8; Figure 10 shows a prior art rotor configuration of a permanent magnet motor together with some of the variables that can be used to manipulate the harmonic content of the motor torque; Figure 11 is a view of a portion of a laminated rotor with two angular positions of the magnets for adjacent rotor sections; Figure 12 shows a view of a laminated rotor in which adjacent magnets are arranged with a different angle theta at alternate rotor locations; Figure 13 is a schematic representation in three-dimensional form showing the axial alignment of the magnetic poles in a four pole structure;

Figure 14 shows a prior art skewing of rotor

laminations of a permanent magnet rotor of the type shown in Figure 3; Figure 15 shows the effect of flipping a second section of a rotor with respect to a first section;

Figure 16 is an illustration of a prior art final

skewing technigue after adjacent sections of the rotor have been aligned along a key slot for the seotions shown in Figures 14 and 15; Figure 17 is an illustration of an arrangement wherein two key slots are placed relative to each other at approximately 9Q0 to allow oonstruotion of a four-section rotor; Figure 18 is a view similar to Figure 17, but which illustrates the first two sections of a four section rotor; Figure 19 is a combined view of the rotor sections of Figures 17 and 18; Figure 20 is an illustration of the final assembly of the four sections of Figures 17-19; Figure 21 is an enlarged view of the final assembly of the four sections seen in Figure 20; Figure 22 is a view similar to the views of Figures 18- 21 with key slots to improve balancing; Figure 23 is a final assembly view of the rotor laminations shown in Figure 22; Figure 24 is a view of a rotor design seen in Figure 23, but which is provided with built-in keys instead of key slots; Figure 25 is a plot showing a reduction in torque ripple for a conventional skewed design for the present invention and for a rotor that is unskewed; and Figures 26, 27 and 28 show examples of hybrid electric vehicle powertrain architectures capable of using the motor of the present invention.

For the purpose of describing typical operating environments for the permanent magnet machine of the invention, reference first will be made to Figures 26, 27 and 28, which respectively illustrate a power-split hybrid electric vehicle powertrain, a detailed power-split hybrid electric vehicle powertrain corresponding to the powertrain of Figure 26 and a series hybrid electric vehicle powertrain. In the case of the powertrain schematically illustrated in Figure 28, an engine 10 is mechanically connected to a generator 12, which in turn is electrically coupled to an electric motor 14. Typically, the electrical coupling includes a DC link comprising an Ac/Dc inverter 16 and a Dc/Ac inverter 16' . A high-voltage traction battery 18 is coupled to the DC link through a Dc/Dc converter 20.

The motor is mechanically coupled to a geared transmission mechanism 22, which may have multiple-ratio gearing or single-ratio gearing.

Traction wheels 24 are driven by torque output elements of the transmission mechanism. All of the mechanical energy of the engine, except for power losses, is transferred to the generator, which converts mechanical energy to electrical energy for driving the motor 14. Any electrical energy not required to drive the motor is used to charge the battery 18. When the vehicle is braking, all or part of the vehicle mechanical kinetic energy transferred from the transmission to the motor 14, except for losses, is used to charge the battery as the motor 14 acts as a generator.

In contrast to the series arrangement of Figure 28, the series-parallel arrangement of Figure 26 includes a direct mechanical connection between the engine and the transmission, as shown at 26. The series-parallel gearing of the hybrid powertrain of Figure 26 is shown in more detail in Figure 27. Components that are counterparts for components in the series arrangement of Figure 28 have been indicated by common reference numerals, although prime notations are added to the numerals in Figures 26 and 27.

The mechanical connection between the transmission 22' and the engine 10' includes a planetary gear system 26. The planetary gear system, seen in Figure 27, includes a ring gear 28, which acts as a power output member for driving a power input element of the transmission mechanism 22' . A sun gear 30 is mechanically connected to generator 12' . The carrier for the planetary gear unit 26, shown at 32, is connected to the power output shaft or crankshaft of the engine 10' . As the engine delivers torque through the planetary gear unit 26, to the transmission. The sun gear acts as a reaction element since it is mechanically connected to the generator. The load on the generator thus will determine the speed of the engine. During forward drive, torque of motor 14' complements engine torque and provides a second power input to the transmission. During reverse drive, the torque direction of the motor 14' is changed so that it will operate in a reverse direction. The engine is inactive at this time.

When the vehicle is in a braking mode, regenerative energy is delivered from the wheels through the transmission to the motor. The motor at this time acts as a generator to charge the battery. A portion of the regenerative energy is distributed through the transmission to the engine in a mechanical torque flow path, shown in part at 26' in Figure 26. Tn this respect, the regenerative energy flow path of the pcwertrain of Figure 26 differs from the energy flow path for the powertrain of Figure 28, where no mechanical energy during regenerative braking is distributed to the engine.

The rotor and the statcr for the disclosed embodiments of the invention may be comprised of ferrous alloy laminations. A rotor and statcr construction of this type is shown in the partial radial cross-sectional view of Figure lb. A stator lamination is shown at 36 in Figures 2a and 2b, and a rotor lamination is shown at 38. A small air gap 40, seen in Figures lc and 2, is located between the inner periphery of the stator laminations 36 and the outer periphery of the rotor laminations 38. Radially extending openings 37 are formed in the stator laminations and symmetrically positioned magnet openings 42 are formed near the outer periphery of each rotor lamination 38. Each magnet opening receives a magnet 44. Any number of laminations in a given design may be used, depending on design choice. The laminations are arranged in a stack.

Multiple stacks (e.g., one, two or three) may be used.

Figure la and Figure lb illustrate a rotor section construction with multiple laminations arranged in stacked relationship. The magnet openings are shown in Figure la, but this figure omits an illustration of the magnets.

The center of the rotor laminations has a circular central opening 60 for accommodating a driveshaft with a keyway that may receive a drive key 62.

The openings 42 are symmetrically disposed with respect to adjacent pairs of magnet openings 42, one of the axes of symmetry being shown in Figure ía.

Figure 2a is a partial view of a rotor lamination 38.

The stator 36 has stator windings in the openings 37, but they are not illustrated in Figure 2a because it is assumed that in the case of Figure 2a, the stator windings do not carry electrical current. The stator windings with current, however, are shown in Figure 2b.

A magnetic rotor flux flow path is shown at 65 in Figure 2a. A magnetic stator flux flow path is shown at 65 and 66 in Figure 2b. The rotor flux and the stator flux interact, as shown in part at 68, to develop rotor torgue in known fashion.

A known way to reduce motor torque ripple is to skew the sections of the rotor, one with respect to the other, by offsetting one half of the rotor lamination stack with respect to the other half. This is seen in Figure 3, where the X-axis 90 for rotor section 92 is skewed relative to the Y-axis shown at 94 for an adjacent rotor section 96. The amount of rotation of one section relative to the other is usually one half of the stator key or slot pitch. This is expressed as follows: skew angle = l80°/N in mechanical degrees, where N is the number of slots.

Magnet openings in rotor section 92 are shown at 98.

The magnet openings are evenly spaced in the case of the rotor of Figure 3. Magnet openings similar to openings 98 are located in rotor section 96. The rotor spacing about the Z-axis 102 in Figure 3 is uniform. Reference may be made to U.S. Patent 7,170,209 for an illustration of a motor rotor with skewed rotor sections.

Magnet openings in the rotors of the embodiments of the invention that are disclosed need not be shaped as shown in the figures of the drawings. The shape of the magnet openings is a design choice.

Figure 4 shows a plan view of a typical lamination for the sections illustrated in Figure 5. As in the case of Figure 2, rotor sections having laminations of the type shown in Figure 4 may include a key-and-slot connection with a rotor driveshaft, although the key-and-slot connection is not shown in Figure 4.

The rotor design having sections, as illustrated in Figure 4, is divided into a generic number of axial sections K, each section being rotated with respect to an adjacent section by an angle egual to: skew angle(k) = 360/(k*N) in mechanical degrees, where N is the number of slots, -10 -The maximum rotation between any two axial seotions of the rotor is: max relative skew angle(k)=(k_l)*36O/(k*N) in mechanical degrees.

The magnet poles are boated as shown in Figure 4. The angle between the magnetic axes of adjacent poles is 45° for an eight pole design. The angle between the magnetic axes and the interpolar axes is one half of the angle between the magnetic axes of adjacent poles for an eight pole design.

The disclosed embodiments of the invention have eight magnetic poles, but the scope of the invention is not limited to the use of eight magnetic poles. The number of poles used is a matter of design choice.

Figure 5 illustrates where a rotor lamination has poles that are radially skewed. The skewing is realized within each lamination itself by offsetting the magnetic axis of a motor pole with respect to an adjacent pole.

Manufacture of the rotor is simplified by the absence of several steps usually needed to create multiple, axially-stacked rotor sections. This manufacturing method is especially valuable in the case of an integrated starter-generator type motor, where the stacked length of the sections is normally short and the known skewing method described with reference to Figure 3 is not feasible. The embodiment of the invention, however, is not limited to short stack motors and generators, but it can be applied to any permanent magnet machine. It can exceed the performance of an electric machine with known skewing and it may be made using simpler manufacturing processes. The performance improvement is due to a further reduction of the torgue ripple previously described. Further, the embodiment of Figure 5 is not limited by the number of axial segments in -11 -the rotor design. It has as many pole-spacing possibilities as the number of rotor poles.

In the design of Figure 5, the spacing between the axis of symmetry of two adjacent magnets is not constant. It can be either one of two values: i.e., Alphai = 360/poles + skew angle; or Alpha2 = 360/poles -skew angle.

For an eight pole, 48-slot motor and a skew angle of 3.75°, Alphal and Alpha2 are 48.75 and 41.25 mechanical degrees, respectively. Other values of skew angle can be chosen according to design choice. The effect of this magnet arrangement on the motor torque for a typical inner permanent magnet machine is shown in Figure 6. A typical rotor torque ripple plot for a non-skewed rotor is shown at in Figure 6 and a corresponding rotor torgue ripple plot for a skewed rotor, according to the invention, is shown at 112. The amplitude of the ripple of plot 112 is significantly lower than the amplitude of plot 110.

This rotor design is also suitable for other arrangements for the rotor poles, such as the one shown in Figure 7, where poles 1-8 are separated by an angle alpha = + skew angle/7, and poles 8 and 1 are separated by angle beta = 45 -skew angle. In contrast, for the design shown in Figure 5, the skew angle is arbitrarily set to be equal to 3.5°, alpha = 45.5° and beta = 41.5°.

An effect on torque ripple, similar to the effect on torque ripple for the design of Figure 5, can be obtained by the distribution pattern for the magnets seen in Figure 8.

In Figure 8, for any given pole, the offset with respect to the original magnetic axis remains the same as the one shown in Figure 7 (i.e., pole number 2 has a magnetic axis that is displaced 22.00° from one interpolar axis and 23° from the -12 -adjacent interpolar axis), but pole number 3 has taken the place of pole number 8, and pole number 4 has been moved to the location of pole number 3, etc. This distribution has a more uniform spacing between the poles than in the case of the design of Figure 7.

A plot of the motor torque versus rotation angle for the design of Figure 8 is seen in Figure 9. The torque ripple seen in Figure 9 is identified by numeral 106. For purposes of comparison, the torque ripple for a rotor having sections using the known design with no skew is shown at 108.

A plot of motor torque versus rotation angle for the design of Figure 5, as previously mentioned, is seen in Figure 6 where a conventional design with no skew is plotted at 110 and the plot corresponding to the design of Figure 5 is shown at 112. The amplitude of the ripple seen at 106 in Figure 9 has lower amplitude than the amplitude seen at 112 in Figure 6 for the design of Figure 5.

The embodiment of Figures 7-10 is not limited to flat magnets. It may have "V'1 shape magnets or other shapes.

Figure 10 shows a magnet configuration in which the rotor magnets, seen at 114 and 116, are arranged in a "V" shape. In the case of the design of Figure 10, the shape and the amplitude of the torque ripple is a function of the shape and amplitude of the angle theta between the magnets 114 and 116. Parameters that affect this shape and the magnitude of each are identified in Figure 10, where the width of each magnet may be 19.25 mm and the distance between a point of engagement of the magnets 114 and 116 and the air gap may be 10.75 mm. The specific parameters, of course, can be different than those illustrated in Figure 10.

-13 -Figure 11 shows how the angle theta is adjusted to obtain smoother torque production. Although the average values for the torgue will not be greatly affected, the harmonic components of the torque can be manipulated by properly designing the different "V'1 shapes. For purposes of illustration, magnets 118 and 120 for laminations of one section are shown overlapped with respect to magnets for laminations of an adjacent section. Magnets 118 and 120 for one section are separated by an angle theta-, whereas the angle for an adjacent section is theta2.

In addition to the implementation illustrated in Figure 11, the multiple magnetic poles on the rotor can be designed with at least two different arrangements. For example, the eight pole rotor of Figure 12 may have poles 1, 3, 5, and 7 of laminations of one section arranged according to the design of Figure 11, in which the angle is theta, and the other four poles may have a design in which the angle is theta2. Further, to avoid low frequency torque oscillations, the rotor can be divided into two axial segments for the design shown in Figure 11, which are rotated with respect to each other, so that poles 1, 3, 5 and 7 of one section of the rotor are aligned with poles 2, 4, 6 and 8 of an adjacent section. This arrangement is shown in Figure 13, where the magnetic axis of a set of poles A for one section is aligned with the axis of a set of magnetic poles B for an adjacent section.

Ihe concept illustrated in Figure 13 can be extended to include rotor configurations that are different from the "V'1 shaped configuration seen in Figures 11 and 12. For example, pole type A in Figure 13 could include "V'T shaped magnets and pole type B could be flat or surface mounted magnets, as in the case of Figures 5, 7 and 8. The torque harmonics can be manipulated in this fashion to create an -14 -attenuated total torque ripple. Further, more than two types of magnet configurations can be used, and variations can be made in the prcximity of the magnets tc the air gap to manipulate torque harmonics.

An embodiment of the invention makes it possible to form the laminaticns of multiple axial sections in a manufacturing process using a single rotor lamination stamping die in order tc avoid multiple lamination types in the same rotor.

Figure 14 shows a known skewing arrangement for a permanent magnet motor using a single lamination type where the first section of the rotor is assembled by stacking half of the rotor laminations and inserting the magnets in their magnet openings. The laminations have a key slot 124, where key slot axis 130 is rotated with respect to the nearest pole axis 126 by a certain angle. To create the second section of the rotor, the rest of the laminations are flipped around axis 130, as shown in Figures 14 and 15.

Because of the flip, the pole axis shown at 126 in Figure 14 becomes pole axis 128 in Figure 15 and is rotated by Gamma with respect to axis 130 in the counter-clockwise direction.

The optimum angle would be determined based on the harmonic content of the air gap flux and the air gap permeance. When the two rotor sections are aligned using the key slot as the common aligning device, the pole axes of the two rotor sections are displaced by 2 x Gamma with respect to each other.

Performance of the rotor shown in Figure 16 can be improved to approximate a continuous skewing effect by increasing the number of rotor sections and rotating them in smaller incremental steps. The present invention is aimed at accomplishing this task using a single lamination die.

This is illustrated in Figure 17, which shows laminations -15 -with a first key slot at 132 and a second key slot at 134.

The key axis for slot 132 is shown at 136, and the key axis for slot 134 is shown at 138.

The first section of the rotor is obtained by axial stacking one guarter of the rotor laminations and then aligning them along the first key slot. The seoond section is similarly made by flipping the laminations and stacking them, as in the design of Figures 14-16. This will result in the partial assembly shown in Figure 18, which illustrates the first two stages of a four section rotor.

The angle formed by 136 (Gammal) and the nearest pole axis 142 is different from the angle 138 (Gamma2) and the nearest pole axis 144. The magnetic axes shown at 140 and 142 in Figure 18 in one example of the invention are separated by 2 x Gammal. The key slot axis is shown at 136 for the slot 132.

The included angle created by the intersection of axes 136 and 138 for key slots 132 and 134, respectively, may be referred to as angle Delta, expressed as: Delta = N*360/P+2Gammal, where P is the number of poles, and N is any number in the number set of 1, 2, 3, ... P-i.

In Figure 19, the third section of the rotor assembly for an embodiment of the invention is created from the non-flipped laminations, as in the case of the first section, but it is rotated to the angle Delta clockwise so that it is aligned with the first two sections using the second key slot 134. In Figure 19, numeral 154 designates the key slot axis. The third section in the design of Figure 19 is offset by theta2-thetal from the first section. Figure 20 shows the final assembly of all four sections.

Figure 21 is an enlargement of a portion of the four sections illustrated in Figure 20. It shows the axis of the -16 -magnetic poles of the different sections relative to the key slot axes.

The fourth section of Figures 20 and 21 is created from flipped laminations, which are rotated counter-clockwise by the angle Delta and aligned on the second key slot. The resulting structure shown in Figures 20 and 21 has four sections, which have the following rotations with respect to the shaft key: The first is shown at 158, which is rotated thetal in a counter-clockwise direction; the second section shown at 160 is rotated thetal in a clockwise direction; the third section shown at 162 is rotated theta2 in a counter-clockwise direction; and the fourth section shown at 164 is rotated theta2 in a clockwise direction. The key slot axis is shown at 166.

It is possible with this embodiment of the invention to arrange the laminations so that the second key slot is aligned with the magnet axis. In this case, the third and fourth rotor sections will have zero rotation and a balanced symmetrical three section rotor thus becomes possible.

Rotor balancing can be improved in the design of an embodiment with the adoption of a second set of key slots placed at 180° from the other two, as illustrated in Figure 22, where key slots 168 and 170 are spaced 180°, respectively, from key slots 172 and 174. Following the procedure described previously with respect to Figure 21, the resulting design is illustrated in Figure 23 where two keys are used to secure the rotor to the rotor shaft to achieve improved rotor balance. This is seen at 176 and 178 in Figure 23.

Tt is possible in the case of the configuration according to an embodiment of the invention to use a pole number count other than a pole count of eight. A four pole rotor can be treated in the same way as a rotor with a pole -17 -count of eight poles. Also, the assembly teohnigue can be applied to rotors that do not use a key slot, but rather use a tab cr other alignment device, such as a cleat. Further, built-in keys can be used as illustrated in Figure 24. In the case of the design of Figure 24, the rotor shaft will have two key slots 180 and 182 that are as wide as the keys, and two larger key slots 184 and 186 that accommodate a misalignment of the keys, shown at 188 and 190, respectively.

Figure 25 is a plot of torgue ripple obtained by the embodiments of the invention using a finite element simulation technique. A conventional method of skewing will result in a ripple plot as shown at 192. A plot using three lamination sections according to the present invention is shown at 194. For purposes of comparison, an unskewed rotor plot is shown at 196.

Although embodiments of the invention have been disclosed, it will be apparent to persons skilled in the art that modifications may be made without departing from the scope of the invention.

-18 -

Claims

CLAIMS1. A permanent magnet machine comprising a stator with electromagnetic windings for stator poles and a rotor located on a rotor axis that is common to an axis for the stator with an air gap between the stator and the rotor; the rotor comprising: a plurality of axially-stacked sections, each section comprising a plurality of stacked laminations on a rotor shaft with a key and slot driving connection therebetween; each section having a plurality of magnets located on its own periphery, the magnets creating a magnetic flux pattern that interacts with a magnetic flux pattern created by the magnetic windings of the stator poles to develop rotor torque; the laminations in one section having asymmetrically placed first magnet openings for receiving rotor magnets; the laminations of a second section having asymmetrically placed second magnet openings for receiving rotor magnets; the magnet openings for the laminations of the first section having a common geometry with respect to the magnet openings for the laminations of the second section to permit manufacture with a common machine tool; the laminations for the second section being flipped about a radial axis relative to the laminations for the first section following assembly of the rotor, the rotor openings for the laminations of the second sections being angularly displaced relative to the rotor openings for the laminations of the first sections, whereby rotor torque fluctuations are attenuated.

-19 -
2. A permanent magnet machine acccrding to claim 1, wherein the rotor comprises more than two rotor sections; the laminations for the second section being flipped about a first radial axis relative to the laminations of the first rotor section; and the laminations for a third rotor section being flipped about a second radial axis relative to the laminations of the first rotor section, whereby rotor torgue fluctuations are attenuated.
3. A permanent magnet machine aocording to claim 2, whereby the first and second radial axes are displaced to an angle delta.
4. A permanent magnet machine according to any preceding claim whereby the sections are displaced angularly, one with respect to the other, about a geometric rotary axis of the rotor during assembly of the rotor sections.
5. A permanent magnet machine according to any preceding claim wherein the laminations of one section have driving key and slot features at locations spaced approximately 180 degrees apart about a geometric rotary axis of the rotor.
6. A permanent magnet machine according to any preceding claim, wherein the laminations of the first section have first driving key and slot features at locations spaced approximately 180 degrees apart about a geometric rotary axis of the rotor; wherein the laminations of the second section have second driving key and slot features at locations spaced -20 -approximately 180 degrees apart about the geometric rotary axis of the rotor; and wherein the first driving key and slot features are relatively displaced approximately 90 degrees from the second driving key and slot features.
7. A permanent magnet machine according to claim 6 wherein the permanent magnet machine has third and fourth lamination sections with driving key and slot features relatively displaced to an angle delta from the second driving key and slot features of the laminations of the first and second sections.
8. A permanent magnet machine according to claim 7 wherein the laminations of the third and fourth sections are flipped about radial axes extending through the first and second key and slot features.
9. A permanent magnet machine according to any of claims 6 to 8 wherein the key and slot features are configured to provide axial alignment of the lamination sections in axially stacked relationship.-21 -