Electronically commutated motor and stationary assembly therefor with teeth faces having notches and having a skew between the slots and the permanent magnet field to reduce cogging

Abstract: An electronically commutated motor has substan­ tially constant air gap energy to reduce cogging. A rotat­ able assembly has a plurality of permanent magnet elements which rotate about an axis of rotation as the rotatable assembly rotates. The elements are oriented with respect to said axis of rotation to provide a magnetic field with lines of flux along a plane forming a skew angle of s₁ mechanical degrees with respect to the axis of rotation, wherein s is not equal to zero. The elements may be provided with an unmagnetized portion referred to as a phantom skew. A sta­ tionary assembly is in magnetic coupling relation with the permanent magnet elements of the rotatable assembly and has t spaced teeth with adjacent teeth defining a slot therebe­ tween, wherein t is positive integer. Each tooth has a surface adjacent the rotatable assembly having one or more notches forming a skew angle of s₂ mechanical degrees with the axis of rotation. Each tooth has winding stages adapted for commutation in at least one preselected sequence. The skewed magnetic field, skewed notches and phantom magnetic skews reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates while maintaining a back EMF waveform having a maximized flat top width.

Description: BACKGROUND OF THE INVENTION This invention relates to dynamoelectric machines and in particular to an electronically commutated motor having a rotatable assembly having skewed magnetics for reducing cogging and having a stationary assembly with notched teeth to reduce cogging. While conventional brush-commutated DC motors may have advantageous characteristics, including convenience of changing operational speeds, there may be disadvantages such as cogging, brush wear, electrical loss, noise and radio frequency interference caused by sparking between the brushes and the segmented commutator, which may limit the applicability of such brush-commutated DC motors in some fields such as the furnace blower control field. Electronically commutated motors, such as brushless DC motors and permanent magnet motors with electronic commutation, have now been developed and generally are believed to have advantageous characteristics over brush commutated DC motors without many of the disadvantages thereof while also having other important advantages. Various circuit and motor designs have been utilized in the past to develop various types of brushless DC motors, as exemplified in U.S. Patent No. 4,005,347 issued Jan. 25, 1977, U.S. Patent No. 4,015,182 issued Mar. 29, 1975, and U.S. Patent No. 4,449,079 issued May 15, 1984. Such brushless DC motors have a stator with a plurality of windings therein, a rotor with a plurality of constant magnetic polar regions providing variable air gap energy, and sensors for sensing the relative position of the rotor polar regions with respect to the stator. Signals developed by the position sensors were processed by circuitry for selectively energizing the windings of the motor. JP-A-56-153961 discloses a rotary electric machine, which is designed to reduce the cogging force and to equalize the electromagnetic torque of the machine by forming auxiliary grooves on the main and the auxiliary salient poles of an armature core and providing skews on the respective magnet poles. The machine comprises an annular magnet mounted on a rotor which has N poles, S poles and nonmagnetic poles, each of which has equal angular width. An armature core 3 has main salient poles of 60° effective pitch and auxiliary salient poles of 30° effective pitch, and auxiliary grooves each having a pitch (15° interval) of 1/4 of the effective pitch of the main salient pole are provided at the part which is effectively confronted with the main and the auxiliary salient poles. Skews each having a skew angle corresponding to the pitch of the auxiliary groove are formed at the respective poles of the magnet. Further improvements in electronically commutated motor systems can beneficially contribute to more widespread use of such motors in various applications including air handling and pumping systems. Improvements which reduce cogging and which achieve constant torque and speed while maintaining a back electromotive force (EMF) having a waveform with a maximized flat top width would be desirable. Summary of the Invention According to the present invention, there is provided an electronically commutated motor comprising a rotatable assembly having a plurality of radially polarised permanent magnet elements of alternating polarity which rotate about an axis of rotation as the rotatable assembly rotates,and a stationary assembly in magnetic coupling relation with the permanent magnet element of the rotatable assembly, the permanent magnet elements being orientated with respect to the axis of rotation to provide a magnetic field with lines of flux along a plane forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation, wherein s 1 is not equal to zero and the stationary assembly having t spaced teeth with adjacent teeth defining a slot therebetween, each tooth having winding stages with respect to the teeth adapted for commutation in at least one preselected sequence, wherein t is a positive integer; characterised by each of the permanent magnet elements comprising two circumferentially adjacent permanently magnetized magnetic segments having the same polarization and having a space between segments comprising an axially elongate substantially unmagnetized portion forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation; whereby the skewed magnetic field has a substantially constant air gap energy to reduce cogging between the rotatable assembly and the stationary assembly. For a better understanding of the present invention, reference will now be made by way of example, to the accompanying drawings, in which :- Figure 1 is a sectional view transverse to the axis of rotation of the stationary assembly and rotatable assembly of a distributed winding motor according to the invention including notched tooth magnetics illustrating the permanent magnet elements in cross-section. Figure 2A is a partial schematic plan view of a 180 frame stator lamination of the invention having 36 slots and one (1) notch per tooth. Figure 2B is a partial schematic plan view of a 180 frame stator lamination of the invention having 24 slots and two (2) notches per tooth. Figure 3 is an enlarged, partial schematic perspective view of one tooth of a stator lamination of Figure 2B having two skewed notches therein and having skewed slots. Figure 4 is an enlarged, partial schematic perspective view of one of the notches in the tooth of the stator lamination illustrated in Figure 3. Figure 5 is an enlarged, partial schematic perspective view of one skewed tooth of a stator lamination having skewed slots and two skewed notches therein. Figures 6A, 6B and 6C illustrate in plan view various configurations of the face of a notched tooth. Figure 7 is a table illustrating various parameters of distributed winding motors. Figure 8A is a cross sectional view transverse to the axis of rotation of the stationary assembly and rotatable assembly of a salient pole motor having three teeth and two poles according to the invention including skewed transitions and skewed phantom magnetic spaces. Figures 8B and 8C are plan views taken along lines B-B and C-C, respectively, of Figure 8A. Figure 9A is a schematic linear layout of the magnetic orientation of the windings of the stationary assembly and the magnetic poles of the rotatable assembly of the motor of Figure 8A. Figures 9B-9K are schematic linear layouts of the motor of the invention. Corresponding reference characters indicate corresponding parts throughout the several views of the drawings. Detailed Description of Preferred Embodiments High quality servo motors must operate with precision at zero speed. In order to achieve such precision, a servo motor should have essentially no cogging effects such as experienced with a surface mounted magnet, electronically commutated motor. The essential elimination of the cogging effects must be accomplished without adding complexity to the servo control. Cogging in an electronically commutated motor can be substantially eliminated by skewing either the magnetic field of the rotatable assembly by one slot pitch or by skewing the stator laminations by one slot pitch. This skewing must be accomplished without reducing the width (duration) of the "flat top" of the back electromotive force (EMF) waveform. The effect of a flat top of reduced width is an undesirable torque dip whenever a commutation occurs. Placing a notch in the face of a tooth to "fake" a slot opening reduces the skew angle required to reduce cogging by one-half. This significantly improves the duration of the flat top of the back EMF waveform. Further reduction in the skew angle required to reduce cogging can be achieved by employing two or more notches in the tooth face. If two notches are employed in the tooth face, the skew required is reduced to one-third of the skew required when no notches are employed. In this configuration, the flat top duration will very nearly be at the desired value. Referring to Figure 1, cogging is reduced by employing an electronically commutated motor 10 which has substantially constant air gap energy. The motor 10 includes a rotatable assembly 100 having three permanent magnet elements 102 which rotate about an axis 104 of rotation as the rotatable assembly rotates. The motor also includes a stationary assembly 106 in magnetic coupling relation with the permanent magnet elements 102 of the rotatable assembly 100. Stationary assembly 106 has t spaced teeth 108 with adjacent teeth defining a slot 110 therebetween. The stationary assembly 106 has distributed winding stages 107 on three adjacent teeth 108 adapted for commutation in at least one preselected sequence. As illustrated, three windings may be used, each winding being around three adjacent teeth and being indicated by a square, circle or triangle. Although the motor 10 is shown in a standard configuration with the rotatable assembly 100 within stationary assembly 106, it is contemplated that the invention may be used on an inside-out motor wherein the stationary assembly 106 is within the rotatable assembly. As illustrated in Figure 1, a motor with nine (9) teeth 108 is shown so that the number of teeth ( t ) equals 9. Each tooth 108 is provided with n = two notches so that n equals 2. Each tooth 108 has surfaces referred to by reference character 112,114, 116 adjacent rotatable assembly 100 with n=2 elongate notches 118 therein. As illustrated in Figure 2A, a 180 frame lamination 202 with an inner diameter of 58.72 mm (2.312 inches) and an outer diameter of 95 mm (3.74 inches) and having 36 teeth 204 is shown so that t equals 36. Only a portion of the lamination is illustrated. Adjacent teeth 204 define a slot 206 therebetween. Each tooth 204 has a single notch 208 therein so that n equals 1. Each tooth 204 has surfaces 210,212 which would face the rotatable assembly. As illustrated in Figure 2B, a 180 frame lamination 222 with 24 teeth 224 is shown so that t equals 24. Only a portion of the lamination is illustrated. Adjacent teeth 224 define a slot 226 therebetween. Each tooth 224 has two notches 228 therein so that n equals 2. Each tooth 224 has surfaces 230, 232, 234 which would face the rotatable assembly. In general, the stationary assembly 106 comprises a plurality of winding stages adapted to be electrically energized to generate an electromagnetic field. In the case of a salient pole motor, the stages are coils of wire individually wound around a tooth of the laminated stator core. The core is formed from a stack of laminations as illustrated, for example, in Figures 2A and 2B and may be held together by four retainer clips, one positioned within each corner of the core. Alternatively, the core may be held together by other suitable means, such as for instance welding or adhesively bonding, or merely held together by the windings, all as will be understood by those skilled in the art. While stationary assembly is illustrated for purposes of disclosure, it is contemplated that other stationary assemblies of various other constructions having different shapes and with different number of teeth may be utilized within the scope of the invention so as to meet at least some of the objects thereof. Figure 3 is an enlarged perspective illustration of the teeth 224 of the lamination of Figure 2B. Each notch 302 has a length L measured along the elongate dimension of the notch, a transverse width W measured along an axis transverse and perpendicular to the elongate axis and parallel to surface 312, 314, 316. The notch 302 also has a radial depth D measured along the radius of the axis of axis 104 of rotation (shown in Figure 1). Although the notch is illustrated as substantially rectangular in cross-section, it is recognized that the notch may be any shape or cross-section such as semicircular as shown in Figure 2A. The motor assembly operates in the following manner. When the winding stages are energized in a temporal sequence three sets of magnetic poles are established that will provide a radial magnetic field which moves clockwise or counterclockwise around the core depending on the preselected sequence or order in which the stages are energized. This moving field intersects with the flux field of the magnet poles to cause the rotor to rotate relative to the core in the desired direction to develop a torque which is a direct function of the intensities or strengths of the magnetic fields. The winding stages are commutated without brushes by sensing the rotational position of the rotatable assembly as it rotates within the core and utilizing electrical signals generated as a function of the rotational position of the rotor sequentially to apply a DC voltage to each of the winding stages in different preselected orders or sequences that determine the direction of the rotation of the rotor. Position sensing may be accomplished by a position-detecting circuit responsive to the back electromotive force (EMF) to provide a simulated signal indicative of the rotational position of the rotor to control the timed sequential application of voltage to the winding stages of the motor. Accordingly, motor 10 may also include means (not shown), such as a printed circuit board, for sensing the back electromotive (EM F) force induced in each of the winding stages by permanent magnet elements 102. The printed circuit board commutates the winding stages in response to the sensing of the back emf induced in the winding stages to rotate rotatable assembly 100. The skewed notches 118 reduce cogging between the rotatable assembly and the stationary assembly as the rotatable assembly rotates by distributing the magnetic flux so that a substantially constant air gap energy is provided. Other means for position sensing may also be used. As illustrated in Figures 3 and 4, each tooth 224 may be provided with a laterally transversely extending tip 320 integral with the tooth. Tip 320 forms part of the surface 312, 314, 316 which is adjacent the rotatable assembly. Reference character 322 generally marks the boundary between tooth 224 and tip 320. Notch 302 is positioned to be centered at this boundary 322 of tip 318 and tooth 224 so that a portion 324 of the notch 302 is in the tooth 224 and the remaining portion 326 of the notch 302 in the tip 318. In general, notch 302 has opposite sides 328 and 330 which are parallel to each other and a bottom surface. The portion 324 of the bottom surface which is part of and within tooth 224 is substantially parallel to surfaces 312,314 which face the rotatable assembly. The remaining portion 326 of the bottom surface which is part of and within tip 318 is substantially parallel to the surface of tip 318 opposite surface 312, 314, i.e., base surface 336 of tip 318. By orienting the portion 326 of the bottom surface parallel to the surface 336, the total mass of tip 318 is increased as compared to a notch having a substantially rectangular cross section with a bottom surface which is substantially planar and parallel to surfaces 312, 314. By increasing the mass of the tip, the period during which the tip 318 may be magnetically saturated during operation of the motor is reduced or eliminated depending on the operational range of the motor. Alternatively, the mass of the tip 318 may be increased by altering its shape. For example, the axial length L A of the tip may be increased such as by flairing the edges 515 of the tip axially in order to increase the mass of the tip as shown in Figure 5. All dimensions are approximate and the dimensions referred to herein are the average dimension of the notch in each indicated direction. For example, it is contemplated that the notch may be unsymmetrical. Where a dimension of a notch is not the same throughout the entire notch, the average dimension in each indicated direction would be considered the width, depth and length of the notch. The entire notch 118 may be located within tip 120 (not shown). The bottom surface of the tip has a surface substantially parallel to the surface of the tip which is opposite the surface of the tip adjacent to rotatable assembly 100. As shown in Figure 5, it has been found that substantially constant air gap energy can be maintained and thereby cogging is substantially eliminated. Each notch 502,504 is along a helical path 504 which traverses a skew angle of s 2 mechanical degrees with respect to the axis 104 of rotation wherein s 2 is not equal to zero. In general, this path is helical although it is illustrated as a straight line because surface 512, 514, 516 has a generally cylindrical shape as does the outer surface of elements 102 of the rotatable assembly 100 of Figure 1. Slot 520 is also skewed at a skew angle of s 2 mechanical degrees with respect to the axis of rotation. As a result, the two notches 502 are substantially parallel to the slot 520. Figures 6A, 6B and 6C illustrate in plan view various variations of the invention wherein a face has a notched tooth. It has been found that substantially constant air gap energy can be maintained and thereby cogging is substantially eliminated. As shown in Figure 6A, notches 602 and 604 in the surface of the tooth which is adjacent the rotatable assembly define lands 606, 608 and 610. From a plan perspective view, both the notches and the lands have a rectangular shape. Both notches 602 and 604 have the same shape, i.e., width W measured transverse to the axis of rotation 612 and length L measured along the axis of rotation 612. In general, the n notches in the surface of each tooth define n+1 lands. Each land has a surface area substantially equal to the width W times the length L of each notch, i.e., the area of the bottom of each notch is substantially equal to the area of each land. In the Figure 6A neither the notches nor the slots between the teeth and defined by edges 614, 616 are skewed so that the elongate axis of both the notches and the slots are parallel to the axis of rotation represented by line 612. Both the notches and the lands have the same plan shape. The rotor magnetics would be skewed as indicated below. As shown in Figure 6B, notches 622 and 624 in the surface of the tooth which is adjacent the rotatable assembly define lands 626, 628 and 630. From a plan perspective view, both the notches and the lands are in the shape of a parallelogram. Both notches 622 and 624 have the same shape, i.e., width W measured transverse to the axis of rotation 632 and length L measured along the axis of rotation 632. In genaral, the n notches in the surface of each tooth define n+1 lands. Each land has a surface area substantially equal to the width W times the length L of each notch, i.e., the area of the bottom of each notch is substantially equal to the area of each land. In the Figure 6B both the notches and the slots between the teeth and defined by edges 634, 636 are skewed by s 2 mechanical degrees with respect to the axis of rotation so that the elongate axis of both the notches and the slots form an angle s 2 with the axis of rotation represented by line 632. Both the notches and the lands have the same plan shape. The rotor magnetics may be skewed as indicated below. As shown in Figure 6C, notches 652 and 654 in the surface of the tooth which is adjacent the rotatable assembly define lands 656, 658 and 660. From a plan perspective view, the notches are in the shape of a parallelogram, land 658 is in the shape of a parallelogram and lands 656, 660 are in the shape of a trapezoid. Both notches 652 and 654 have the same shape, i.e., width W measured transverse to the axis of rotation 662 and length L measured along the axis of rotation 662. In general, the n notches in the surface of each tooth define n+1 lands. Each land has a surface area substantially equal to the width W times the length L, of each notch, i.e., the area of the bottom of each notch is substantially equal to the area of each land. In the Figure 6C the notches are skewed whereas the slots formed by edges 664, 666 are parallel to the axis of rotation 662 so that the elongate axis of the notches forms an angle s with the axis of rotation represented by line 612 and the lands are parallel to the axis of rotation. The lands and notches have different plan shapes. The rotor magnetics may be skewed as indicated below. In the case that the motor of the invention has a rotatable assembly with p poles and a stationary assembly with 3p teeth and with distributed windings, each winding on three teeth, it has been found that substantially constant air gap energy can be maintained while providing back EMF signal having a maximized flat top width of 120° electrical. In particular, alternatively, the skew angle s 2 substantially equals 360° divided by (n + 1) multiplied by t, i.e., s 2 = 360°/(n + 1)t. This results in a back EMF signal having a substantially flat top width of at least approximately 120 electrical degrees. This example of the invention is generally illustrated in Figure 7 which is a table of the operating parameters of various motors. In general. the larger the skew angle, the smaller the flat top duration. As shown in Figure 7, as the skew angle increases from zero degrees to 30°, the flat top duration decreases. For example, for a 180 frame size with 24 teeth, 15° slot pitch, n=2, 8 poles, magnetic arc of 45°, and coil spread of 9°, a skew of zero degrees yields a flat top having a duration (width) of 144 electrical degrees. Increasing the skew angle to 5° yields a decreased flat top width of 124°. Increasing the skew angle to 7.5° yields a decreased flat top width of 114°. Increasing the skew angle to 15° yields a decreased flat top width of 84°. Therefore, optimum motor performance is achieved by minimizing cogging and maintaining the flat top width at about 120° electrical. To achieve this optimum motor performance, the skew angle must be held to the lowest number of electrical degrees which will provide a flat top width of about 120° electrical. As noted above, it has been found that such optimum motor performance occurs when the skew angle s 2 =360°/(n+1)t. The skew angle which results in this optimum motor performance for each configuration illustrated in Figure 7 is indicated in the column for 360/(n+1)t and corresponds to the flat top duration which has been underlined for each configuration. For example, in the first configuration of Figure 7, optimum motor performance occurs with a 5° skew which provides a flat top duration of 118°. In the last configuration of Figure 7, optimum motor performance occurs with a 10° skew which provides a flat top duration of 124°. The surface of each tooth which is adjacent rotatable assembly 100 has a number of lands which is equal to n + 1 where n is the number of notches within the tooth. For example, the tooth illustrated in Figure 3 has three lands. Cogging is substantially reduced and the duration of the flat top of the back emf is maintained when the area of each of the lands 112, 114. 116 is substantially equal to the area of the bottom surface of each notch, i.e., area of land approximately equals W x L. Preferably, each notch should have a substantially rectangular cross-sectional shape as viewed in a transverse section. In addition, it is preferable that the transverse width W of the notch substantially equals twice the radial depth (W = 2D). As illustrated in Figures 8A, 8B and 9A, the transitions 802, 804 between the north permanent magnet pole 808 and the south permanent magnet pole 810 are oriented obliquely with respect to the axis of rotation 806 to provide a magnetic field with lines of flux along a plane forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation 806, where the skew angle s 1 is not equal to zero. The scale 850 from zero to 360 indicates mechanical agrees about the periphery of the motor of Figure 8A. Stages A,B,C are separated by 10° slots 852. Although Figure 8A shows a salient pole motor, skewed rotor magnetics may also be part of a distributed winding motor. Additionally, as shown in Figures 8A, 8C and 9A, each of permanent magnet elements may be provided with a phantom magnet skew in the form of an axially elongate, substantially unmagnetized portion 812, 814 having a width of about four mechanical degrees (4°). The phantom skew forms a skew angle of s 1 mechanical degrees with respect to axis 806. Alternatively, each of both the north permanent magnet element 808 and the south permanent magnet element 810 may comprise two permanently magnetized segments 816 and 818, 820 and 822, respectively, having the same polarization and having a space 824, 826 therebetween which is oblique to axis 806. Each space 824, 826 is an axially elongate opening forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation and is also referred to as a phantom magnet skew. For the three pole, salient winding motor illustrated in Figure 8A, this phantom magnet skew is at an angle which substantially equals 360° divided by 4(n+1)t. As a result, cogging is substantially eliminated and the back EMF signal has a maximized flat top duration. Phantom magnet skew may also be employed as part of a distributed winding motor. In the event that phantom magnet skews are part of rotatable assembly 100, it is generally unnecessary to also employ skewed notches although it is contemplated that both the notches and the spaces may be skewed. If skewed notches at a skew angle of s 2 mechanical degrees and phantom magnetic skews are employed together in one motor configuration, the total skew s t , which is equal to the sum of s1 and s2 must substantially equal the optimum skew angle as noted above. Table 1 summarizes the optimum total skew required. OPTIMUM TOTAL SKEW WITHOUT PHANTOM SKEW WITH PHANTOM SKEW DISTRIBUTED WINDING 360 / (n+1)t 360 / 2(n+1)t SALIENT POLE 360 / 2(n+1)t 360 / 4(n+1)t Figures 9B, 9C, 9D, 9E, and 9F are in vertical alignment with each other and illustrate the total optimum skew for a six-tooth motor (t=6) without any notches therein. Figures 9G, 9H, 9I, 9J, and 9K are in vertical alignment with each other and illustrate the total optimum skew for a six-tooth motor with each tooth having two notches therein. Only three stages A, B, C are illustrated. Figures 9C, 9E, 91 and 9K correspond to salient pole motors having six teeth and four poles. Only half of such a salient motor is depicted, i.e., three teeth and two poles; the other half would have the same configuration as the illustrated portion. Figures 9D, 9F, 9H and 9J correspond to a distributed wound motor having six teeth and two poles. Only half of such a distributed wound motor is depicted, i.e., three teeth and one pole; the other half would have a configuration symmetrical to the illustration with opposite poles. Table 2 summarizes the features illustrated by each of these figures and the optimum total skew required. FEATURES AND OPTIMUM TOTAL SKEW FOR VARIOUS MOTOR EMBODIMENTS FIGURE 9C 9D 9E 9F NOTCHES SKEWED ROTOR MAGNETICS yes yes yes yes PHANTOM MAGNETIC SKEW yes yes DISTRIBUTED WINDING yes yes SALIENT WINDING yes yes OPTIMUM TOTAL SKEW 360 / 2(n+1)t 360 / (n+1)t 360 / 4(n+1)t 360 / 2(n+1)t FIGURE 9H 9I 9J 9K NOTCHES yes yes yes yes SKEWED ROTOR MAGNETICS yes yes yes yes PHANTOM MAGNETIC SKEW yes yes DISTRIBUTED WINDING yes yes SALIENT WINDING yes yes OPTIMUM TOTAL SKEW 360 / 2(n+1)t 360 / (n+1)t 360 / 4(n+1)t 360 / 2(n+1)t The optimum total skew equals the skew of the notches plus the skew of the rotor magnetics. For motors employing notches with no skewed rotor magnetics, the optimum total skew would equal the skew of the notches as indicated in figures 9C and 9D. In the salient winding motors employing notches, the number of notches per tooth n must be an even number. This is because an odd number of notches per tooth in a salient winding motor will not reduce the amount of skew required to substantially eliminate cogging. Figure 9B illustrates three teeth A,B,C of a six-tooth motor, i.e., t=6. The stationary assembly has no notches therein and is a linear layout along 6 scale 900 from zero to 180 mechanical degrees. Figure 9C illustrates a linear layout of a portion of a rotatable assembly having two permanent magnet poles 902, 904 and having skewed rotor magnetics without phantom skews for the motor in which the stages A,B,C are salient windings. In this motor, the optimum total skew according to the invention is s t =360/2(n+1)t mechanical degrees. Fort=6 and no notches, n=0 so that s t =360/2(1)6=30°. Figure 9C confirms that transition 906 would cause cogging at 30° displacements and transition 908 would cause cogging at 60° displacements, which is an integer multiple of 30° displacements. Therefore, Figure 9C confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a salient pole motor having no phantom magnetic skew. Figure 9D illustrates a linear layout of a portion of a rotatable assembly having one permanent magnet pole 912 and having skewed rotor magnetics without phantom skews for the motor in which the stages A,B,C are distributed windings. In this motor, the optimum total skew according to the invention is s t =360/(n+1)t mechanical degrees. For t=6 and no notches, n=0 so that s t =360/(1)6=60°. Figure 9D confirms that transition 914 would cause cogging at 60° displacements. Therefore, Figure 9D confirms that 360/(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a distributed pole motor having no phantom magnetic skew. Figure 9E illustrates a linear layout of a portion of a rotatable assembly having two permanent magnet poles 922, 924, each having skewed rotor magnetics with phantom skews 926, 930, respectively, for the motor in which the stages A,B,C are salient windings. In this motor, the optimum total skew according to the invention is s t =360/4(n+1)t mechanical degrees. For t=6 and no notches, n=0 so that s t =360/4(1)6=15°. Figure 9E confirms that phantom skew 926 would cause cogging at 15° displacements, transition 928 would cause cogging at 30° displacements, phantom skew 930 would cause cogging at 45° and transition 932 would cause cogging at 60° displacements, each of which is an integer multiple of 15° displacements. Therefore, Figure 9E confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a salient pole motor having phantom magnetic skews. Figure 9F illustrates a linear layout of a portion of a rotatable assembly having one permanent magnet pole 942 having skewed rotor magnetics with phantom skew 944 for the motor in which the stages A,B,C are distributed windings. In this motor, the optimum total skew according to the invention is s t =360/2(n+1)t mechanical degrees. For t=6 and no notches, n=0 so that s t =360/2(1)6=30°. Figure 9F confirms that phantom skew 944 will cause cogging at 30° displacements and transition 946 would cause cogging at 60° displacements, which is an integer multiple of 30° displacements. Therefore, Figure 9F confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a distributed pole motor having phantom magnetic skews. Figure 9G illustrates the three teeth A,B,C of six-tooth motor, i.e., t=6. The stationary assembly has two notches 947, 948 in each tooth and is a linear layout along a scale 900 from zero to 180 mechanical degrees. Figure 9H illustrates a linear layout of a portion of a rotatable assembly having two permanent magnet poles 952, 954 and having skewed rotor magnetics without phantom skews for the motor in which the stages A,B,C are salient windings. In this motor, the optimum total skew according to the invention is s t =38012(n+1)t mechanical degrees. For t=6 and two notches, n=2 so that s t =360/2(3)6=10°. Figure 9H confirms that transition 956 would cause cogging at 10° and transition 958 would cause cogging at 20° displacements, which is an integer multiple of 20° displacements. Therefore, Figure 9H confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a salient pole motor having no phantom magnetic skew. Figure 9I illustrates a linear layout of a portion of a rotatable assembly having one permanent magnet pole 962 and having skewed rotor magnetics without phantom skews for the motor in which the stages A,B,C are distributed windings. In this motor, the optimum total skew according to the invention is s t =360/(n+1)t mechanical degrees. For t=6 and two notches, n=2 so that s t =360/(3)6=20°. Figure 91 confirms that transition 964 would cause cogging at 20° displacements. Therefore, Figure 91 confirms that 360/(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a distributed pole motor having no phantom magnetic skew. Figure 9J illustrates a linear layout of a portion of a rotatable assembly having two permanent magnet poles 972, 974, each having skewed rotor magnetics with phantom skews 976, 980, respectively, for the motor in which the stages A,B,C are salient windings. In this motor, the optimum total skew according to the invention is s t =360/4(n+1)t mechanical degrees. For t=6 and two notches, n=2 so that s t =360/4(3)6=5°. Figure 9J confirms that phantom skew 980 would cause cogging at 5° displacements, transition 978 would cause cogging at 10° displacements, phantom skew 976 would cause cogging at 15° and transition 982 would cause cogging at 20° displacements, each of which is an integer multiple of 5° displacements. Therefore, Figure 9J confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a salient pole motor having phantom magnetic skews. Figure 9K illustrates a linear layout of a portion of a rotatable assembly having one permanent magnet pole 992 having skewed rotor magnetics with phantom skew 994 for the motor in which the stages A,B,C are distributed windings. In this motor, the optimum total skew according to the invention is s t =360/2(n+1)t mechanical degrees. For t=6 and two notches, n=2 so that s t =360/2(3)6=10°. Figure 9K confirms that phantom skew 994 will cause cogging at 10° displacements and transition 996 would cause cogging at 20° displacements, which is an integer multiple of 10° displacements. Therefore, Figure 9K confirms that 360/2(n+1)t specifies that the minimum angle for skewed rotor magnetics required to substantially eliminate cogging and to provide the maximum back EMF flat top width in a distributed pole motor having phantom magnetic skews. In view of the above, it will be seen that the several objects of the invention are achieved and other advantageous results attained.

Claims: An electronically commutated motor (10) comprising: a rotatable assembly (100) having a plurality of radially polarised permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) of alternating polarity which rotate about an axis (104,612,63,662,806) of rotation as the rotatable assembly rotates, and a stationary assembly (106) in magnetic coupling relation with the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) of the rotatable assembly (100), the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) being orientated with respect to the axis of rotation (104,612,63,662,806) to provide a magnetic field with lines of flux along a plane forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation (104,612,63,662,806), wherein s 1 is not equal to zero; and the stationary assembly (106) having t spaced teeth (108; 204,224) with adjacent teeth defining a slot (110,520) therebetween, each tooth (108; 204,224) having winding stages (107) with respect to the teeth (108; 204,224) adapted for commutation in at least one preselected sequence, wherein t is a positive integer; characterised by each of the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) comprising two circumferentially adjacent permanently magnetized magnetic segments (816,818,820,822) having the same polarization and having a space (824,826) between segments comprising an axially elongate substantially unmagnetized portion (812,814) forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation (104,612,63,662,806); whereby the skewed magnetic field has a substantially constant air gap energy to reduce cogging between the rotatable assembly (100) and the stationary assembly (106). The motor of claim 1 characterized in that each tooth (108; 204,224) has a surface (312,314,316; 512,514,516) adjacent the rotatable assembly with n elongate notches (302,502) therein of length L, transverse width W and radial depth D, each notch (302,502) being along a helical path (504) which traverses the skew angle, wherein n is a positive integer. The motor of claims 1 or 2 characterized by means (20) for sensing the back electromotive force (back EMF) induced in each of the winding stages (107) by the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) and for commutating the winding stages (107) in response thereto to rotate the rotatable assembly (100). The motor of claim 3 characterized in that skew angle is substantially 360/2( n +1) t mechanical degrees, where n is a positive even integer. The motor of claim 2 characterized in that at least one of the teeth (224) has a laterally transversely extending tip (318,320) which forms part of the surface adjacent the rotatable assembly; wherein a portion (324) of one of the notches (302) is in the one tooth (224) and a remaining portion (326) of the notch is in the tip (318); wherein the portion (324) in the tooth of the one notch (302) has a first surface (324) substantially parallel to the surface of the one tooth adjacent the rotatable assembly; and wherein the remaining portion (326) of the one notch (302) in the tip (318,320) has a second surface (324) substantially parallel to a surface (336) of the tip (318,320) opposite the surface of the tip (318,320) adjacent the rotatable assembly (100), the first and second surfaces intersecting to form an angle. The motor of claim 2 characterized in that at least one of the teeth (224) has a laterally transversely extending tip (318,320) which forms part of the surface adjacent the rotatable assembly; wherein a portion (326) of one of the notches (302) in the one tooth (224) is in the tip (318,320) with the remaining portion (324) of the notch in the one tooth (224); wherein at least one of the teeth (224) has a laterally transversely extending tip (318,320) which forms part of the surface adjacent the rotatable assembly (100); wherein at least part (326) of one of the notches (302) in the one tooth (224) is in the tip (318,320); wherein the tip (318,320) has a second radially inwardly tapering surface (336) opposite the surface of the tip (318,320) adjacent the rotatable assembly (100); and wherein the notch (302) has a surface parallel to the second surface (336). The motor of claim 3 or 4 characterized in that back EMF signal has a substantially flat top width of at least approximately 120 electrical degrees. The motor of claim 2 characterized in that the n notches (302,502,602,604,622,624,652,654) in the surface of each tooth (224) define n +1 lands, each having a surface area substantially equal to W times L. The motor of claim 8 characterized in that each notch (302) has a substantially rectangular shape when viewed in a transverse cross-section. The motor of Claim 9 characterized in that W = 2D. The motor of Claim 1 characterized in that n = 1 and t = 36. The motor of Claim 1 characterized in that n = 2 and t = 24. The motor of Claim 1 characterized in that n = 1 and t = 24. The motor of Claim 1 characterized in that n = 2 and t = 12. The motor of claim 1 characterized in that each tooth (108; 204,224) forms a skew angle of s 2 mechanical degrees, and a total skew angle s t equals the sum of s 1 and s 2 and substantially equals 360/4( n + 1 ) t . The motor of Claim 1 characterized in that each tooth (108; 204, 224) forms a skew angle of s 2 mechanical degrees, and the total skew angle s t equals the sum of s 1 and s 2 and substantially equals 360/2( n +1) t thereby resulting in a back EMF signal having a substantially flat top of at least approximately 120 electrical degrees. The motor of Claim 1 or 16 characterized in that the skew angle s 1 substantially equals 360/2( n +1) t thereby resulting in a back EMF signal having a maximized flat top width. The motor of claim 1 characterized in that the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) are oriented with respect to the axis of rotation to provide the magnetic field with lines of flux along a plane forming an angle of s 1 mechanical degrees with the axis of rotation, wherein s 1 is not equal to zero. The motor of Claim 2 characterized in that the entire notch is located within the tip. The motor of claim 1 characterized in that each of the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954; 962; 972,974; 992) comprises two permanently magnetized segments having the same polarization and having a space (824,826) between the segments comprising an axially elongated opening forming a skew angle of s 1 mechanical degrees with respect to the axis of rotation. The motor of Claim 1 characterized in that the skew angle s 1 substantially equals 360/( n +1) t thereby resulting in a back EMF signal having a substantially flat top width of at least approximately 120 electrical degrees. The motor of Claim 2 characterized in that the slots (520) are parallel to the notches (502). The motor of claim 1 characterized in that each of the permanent magnet elements (102; 808,810; 902,904; 922,924; 952,954,962; 972,974; 992) comprises two circumferentially adjacent permanently magnetized segments having the same radial polarization. The motor of any one of the preceding claims characterized in that the rotatable assembly (100) rotates within the stationary assembly (106). The motor of any one of Claims 1 to 23 characterized in that the stationary assembly rotates within the rotatable assembly. The motor of any one of the preceding claims characterized in that the winding stages (107) comprise the salient windings. The motor of any one of claims 1 to 25 characterized in that the winding stages (107) comprise distributed windings.

Inventors: HARMS HAROLD BARNARD, KREFTA RONALD JOHN, OLDENKAMP JOHN LEONARD, HARMS, HAROLD BARNARD, KREFTA, RONALD JOHN, OLDENKAMP, JOHN LEONARD
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1989
Main classification: H02K29
IPC codes: H02K29, H02K1, H02K21