Electric machine for a vehicle powertrain

Abstract

An electric machine (10) comprising: a rotor assembly (14, 214) having a rotor core (18, 218) configured to support permanent magnets (20A, 20B, 20C) spaced apart around the rotor core (18, 218) to define a number of rotor poles (40); wherein the rotor core (18, 218) has several rotor slots (22, 24, 26, 28; 220, 124, 126, 128) arranged at each of the rotor poles (40) as several barrier layers (22, 24, 26, 28; 220, 124, 126, 128); wherein the rotor core (18, 218) has a support (33) for a central shaft and several spokes (84, 284) comprising, which extend radially outwards from the support (33) of the central shaft; wherein the multiple spokes (84, 284) are radially aligned towards the rotor poles (40); wherein the barrier layers (22, 24, 26, 28; 220, 124, 126, 128) are positioned adjacent to each other between an inner circumference (23) of the rotor core (18, 218) and an outer circumference (25) of the rotor core (18, 218);wherein the barrier layers (22, 24, 26, 28; 220, 124, 126, 128) comprise a first barrier layer (22; 220) with several segments (60, 62A, 62B; 260A, 262A, 262B) which is closest to the inner circumference (23); and permanent magnets (20A, 20B, 20C) are accommodated in at least some of the segments (60, 62A, 62B; 260, 262A, 262B) of the first barrier layer (22; 220); and wherein several cavities (82) are arranged radially within the first barrier layer (22; 220).

Description

CROSS REFERENCE TO RELATED APPLICATIONThis application claims priority to Indian provisional application No. 457 / KOL / 2014, filed Apr. 12, 2014, which is incorporated herein by reference in its entirety.TECHNICAL FIELDThe present teachings generally include an electric machine for a vehicle powertrain, and more particularly, an internal permanent magnet engine.BACKGROUNDAn electric motor uses potential electrical energy to generate mechanical torque through the interaction of magnetic fields and current carrying electrical conductors. Some electric motors may also function as generators by using torque to generate electrical energy. An internal permanent magnet electric machine has a rotor assembly that includes a rotor core with magnets of alternating polarity spaced around the rotor core.The document FR 1 533 121 A discloses a rotor with inner permanent magnets which are arranged in a curved manner in a plurality of segments.In the publication DE 689 11 570 T2 a rotor is shown with inner permanent magnets which are arranged in an arc shape or consist of straight segments which are arranged in an arc shape.The publication DE 199 41 107 A1 discloses a rotor with permanent magnets located inside it, which are arranged in an arc shape or as rectangular magnets in a V shape.The publication DE 34 29 813 A1 discloses a synchronous motor with permanent excitation, in which a squirrel cage and rod-shaped permanent magnets are arranged in slots of the rotor.DE 21 33 668A discloses an alternator screwed to an engine having one or more cylinders.In the document DE 14 89 184 A a magnet system is disclosed which serves as a rotor or stator, wherein all magnets of one polarity have been replaced by magnets of the other polarity.The document DE 696 29 419 T2 discloses a motor with permanent magnets installed in the rotor, which are arranged in an arc shape.In the publication CN 2 02 444 342 U, a rotor is disclosed with rotor with arc-shaped arranged permanent magnets in several layers, wherein the permanent magnets are omitted in the radially outermost layer.SUMMARYAn electric machine is provided that includes a rotor assembly having a rotor core configured to support permanent magnets spaced around the rotor core to define a number of rotor poles. The rotor core has a plurality of rotor grooves arranged as a plurality of barrier layers on each of the rotor poles. One or more of the barrier layers on each rotor pole may house permanent magnets. The rotor core is configured with an optimal geometry to satisfy predetermined operating parameters. The electric machine can be configured with a polyphase stator arrangement and a rotor arrangement with synchronous reluctance supported by internal permanent magnets. Specifically, the electric machine is designed to achieve predetermined operating parameters including high efficiency, high power density and / or high torque density, relatively large peak power range, maximum speed, relatively low cost, relatively low mass and inertia, and three-phase short-circuit currents less than a rated current, and has the property of fitting into a relatively small packaging space. The rotor core includes a center shaft holder and a plurality of spokes extending radially outward from the center shaft holder. In one embodiment, the electric machine is coupled to an engine by belt drive and provides engine cranking modes (i.e., starting), regeneration modes, and torque assist modes.The above features and advantages and other features and advantages of the present teachings will be readily apparent from the following detailed description of the best modes for carrying out the present teachings when read in connection with the accompanying drawings.BRIEF DESCRIPTION OF THE DRAWINGSFIG. 1 is a schematic side view illustration of a first embodiment of an electric machine in accordance with the present teachings. FIG. 2 is a schematic side view illustration of a rotor assembly of the electric machine of FIG. 1. FIG. 3 is a schematic side view illustration of a stator assembly of the electric machine of FIG. 1. FIG. 4 is a schematic side view illustration of a second embodiment of a rotor assembly for use in the electric machine of FIG. 1 in accordance with the present teachings. FIG. 5 is a schematic side view illustration of a third embodiment of a rotor assembly for use in the electric machine of FIG. 1 in accordance with the present teachings. FIG. 6 is a schematic illustration of a powertrain including the electric machine of FIG. 1. FIG. 7 is a plot of torque in base torque units (pu) and power in base power units (pu) versus speed (revolutions per minute) of the electric machine of FIG. 1. FIG. 8 is an efficiency map at various powers in basic power units (pu) and speeds (revolutions per minute) during a generator mode of the electric machine of FIG. 1. FIG. 9 is an efficiency map at different powers in basic power units (pu) and speeds (revolutions per minute) during a motor mode of the electric machine of FIG. 1. FIG. 10 is a plot of rotor speed (revolutions per minute), power in base power units (pu), and phase A current in base power units (pu) versus time (seconds) during a three-phase short-circuit event.DETAILED DESCRIPTIONReferring now to the drawings, wherein like reference numerals designate like components throughout the several views, FIG. 1 shows an electric machine 10 having a stator assembly 12 and a rotor assembly 14. Specifically, the electric machine 10 is designed to achieve a high efficiency of about 80 percent over a predetermined range of output power and speed, to have a high power density and / or a high torque density, to have a relatively large peak power range, a maximum speed of at least 18,000 revolutions per minute (rpm), relatively low cost by minimizing the required number of permanent magnets and having a relatively low mass and inertia, and to fit within a relatively small amount of packaging space. Alternative rotor assemblies 114, 214 that may be used in place of the rotor assembly 14 are shown in FIGS. 4 and 5 and also have optimal designs and geometries to meet the same predetermined operating parameters. The electric machine 10 with any of the rotor assemblies 14, 114, 214 may be used in a powertrain 300 shown in FIG. 6 in a belt-driven engine assembly to provide engine cranking modes, regeneration modes, and torque assist modes.Referring to FIGS. 1-3, the stator assembly 12 radially surrounds the rotor assembly 14 with an air gap 16 defined therebetween. The electric machine 10 is configured such that the air gap 16 may be not less than 0.2 mm and not more than 0.7 mm, for example only and without limitation, to maximize performance and minimize the number of magnets 20A, 20B, 20C. As another example without limitation, the air gap 16 may be not less than 0.3 mm and not greater than 0.5 mm. Both the stator assembly 12 and the rotor assembly 14 have a generally annular shape and are concentric about a central longitudinal axis A of the electric machine 10 (best shown in FIG. 6 ). The stator assembly 12 includes a stator core 30, and the rotor assembly 14 includes a rotor core 18. Each of the stator core 30 and the rotor core 18 may be assembled from a plurality of laminations axially stacked along the axis A. For example, FIG. 6 shows stacks of stator laminations 19. For purposes of illustration, the housing is not shown in Figure 6, so that the sheets 19 will be visible.The rotor assembly 14 includes a rotor core 18 configured to support a plurality of permanent magnets 20A, 20B, 20C spaced around the rotor core 18. Specifically, the rotor core 18 includes a plurality of rotor grooves 22, 24, 26, 28, also referred to herein as barriers or barrier layers, arranged as a plurality of barrier layers and including a first barrier layer 22, a second barrier layer 24, a third barrier layer 26, and a fourth barrier layer 28. The first barrier layer 22 is closest to an inner periphery 23 of the rotor core 18. The second barrier layer 24 is positioned between the first barrier layer 22 and the third barrier layer 26. The third barrier layer 26 is positioned between the second barrier layer 24 and the fourth barrier layer 28. The fourth barrier layer 28 is farther from the inner periphery 23 than the barrier layers 22, 24, and 26. the fourth barrier layer 28 is closer to an outer periphery 25 of the rotor core 18 than the first barrier layer 22 and at least portions of the second and third barrier layers 24, 26. The other barrier layers 24, 26, 28 act as air barriers. In other embodiments, one or more of the barrier layers 24, 26, 28 may also be filled with permanent magnets.The rotor assembly 14 is configured to be rotatable about the axis A that extends longitudinally through the center of the electric machine 10. The rotor core 18 is rigidly connected to and rotates with a motor shaft 29 (shown only in FIG. 6 ) that extends through a shaft opening 31 in the rotor core 18. The material of the rotor core 18 around the shaft hole 31 functions as the center shaft holder 33.The stator assembly 12 includes a stator core 30 having a plurality of circumferentially spaced stator slots 32. The stator slots 32 extend longitudinally along the axis A. The stator slots 32 are configured to house polyphase stator windings 34. The stator windings 34 may be grouped into different sets, each of which carries an identical number of phases of an electric current, such as three phases, as will be understood by those skilled in the art. The stator windings 34 may extend axially beyond the first and second axial ends 36, 38 of the stator core 30 shown in FIG. 6. The axial length AL of the stacks of sheets 19 (i.e., the distance along the axis A between the axial ends 36, 38) that does not include extending portions of the windings 34 is also referred to herein as the active length of the electric machine 10. A ratio of an outer diameter OD of the laminations 19 of the stator assembly 12 to the active length AL may be, by way of example only and not limitation, not less than 1.5 and not greater than 4, and by way of example only and not limitation, with an active length AL not exceeding 65 mm and an outer diameter OD not exceeding 145 mm to meet predetermined packaging space requirements for a particular application of the electric machine 10, such as in a vehicle powertrain.Referring to FIG. 2, the rotor has eight poles 40 that are made at least in part by the placement of the permanent magnets 20A, 20B, 20C in the first barrier layer 22 that are generally circumferentially disposed within the rotor core 18 and by the selected polarity of the magnets 20A, 20B, 20C. Although eight poles 40 are shown, the electric machine 10 may be configured to have a different number of poles 40. By way of non-limiting example, the number of poles 40 may be between 6 and 12 to satisfy predetermined torque, power, and packaging parameters while meeting predetermined noise limits. Each pole 40 includes a set of the plurality of barrier layers 22, 24, 26, 28. The poles 40 are shown separated from each other by pole boundaries 42 that extend radially through the rotor core 30. Each pole 40 comprises the entire material of the rotor core 30 that is bounded by the respective pole boundaries 42 of the pole 40. Only one pole axis 44 of one of the poles 40 is shown, although each pole 40 has a similar pole axis 44 extending radially through the center of the pole 40. The rotor core 18 is a steel material selected to sustain a high speed rotational load within predetermined limits. By way of non-limiting example, a computerized rotational stress analysis of the rotor assembly 14 indicates that the distalmost portions 45 of a center segment 60 of the first barrier layer 22 are experiencing the greatest rotational stress, and that when the electric machine 10 is operated at 20,000 revolutions per minute and at 150 degrees Celsius in a motor mode, the stress at the distal portions 45 will remain less than a predetermined maximum allowable rotational stress based on material characteristics.Referring to FIGS. 1 and 3, in an exemplary embodiment, the stator core 30 has sixty stator slots 32 circumferentially arranged around the stator core 30 and opening toward the air gap 16 at an inner periphery 50 of the stator core 30. Stator teeth 52 separate all of the stator slots 32 and are formed with ends 54 which retain the stator windings 34. A largest common divider (GGT) of the number of stator slots 32 and the number of poles 40 of the rotor core 18 is the largest positive integer that divides the number of stator slots 32 and the number of poles 40 without a remainder. In the illustrated embodiment, since the stator core 30 has 60 stator slots 32 and the rotor core 18 has eight poles 40, the GGT is 4.A smallest common multiple (KGV) of the number of stator slots 32 and the number of poles 40 is the smallest positive integer divisible by both the number of stator slots 32 and the number of poles 40. In the illustrated embodiment, since the stator core 30 has sixty stator slots 32 and the rotor core 18 has eight poles 40, the KGV 120 is. In other embodiments, the KGV may be a different number and is preferably greater than 60 to minimize slot cogging torque due to the interaction of the permanent magnets 20A, 20B, 20C and the teeth 52 of the stator core 30.Referring now to FIG. 2, it is understood that the first barrier layer 22 includes a plurality of discrete segments that are physically separated from each other by the material of the rotor core 18. Specifically, the segments include a center segment 60 that houses the magnet 20A. The first barrier layer 22 also includes first and second wing segments 62A, 62B positioned generally near the opposing ends 64A, 64B of the center segment 60 toward the outer periphery 25 and angled away from each other. The center segment 60 is positioned such that the magnet 20A housed therein is generally perpendicular to a radius of the rotor core 18, the radius being shown as and represented by the pole axis 44.For cost saving, it is desirable that all the permanent magnets 20A, 20B, and 20C have identical rectangular shapes. This can be accomplished by configuring the center segment 60 and the first and second wing segments 62A, 62B to have identical thicknesses T 1, T 2, T 3. In one non-limiting example, the thicknesses T 1, T 2, and T 3 are between about 1 mm and 3.0 mm, and in a more specific example, between about 2.0 and 2.5 mm, which allows magnets 20A, 20B, 20C having an appropriate width to be fitted therein. It is noted that although the permanent magnets 20A, 20B, 20C have a rectangular shape, the center segment 60 and the vane segments 62A, 62B have a more complex shape, with a generally rectangular center portion matching and firmly holding the magnets 20A, 20B, 20C and air pockets 66 extending at one or both ends. The lengths of the center segments 60 and the vane segments 62A, 62B of the rotor laminations stacked in the direction of the axis A may be the same. The lengths of the center segments 60 and the vane segments 62A, 62B of the rotor lamination stacked in the direction of the axis A may be the same. Thereby, the permanent magnets 20A, 20B, and 20C can have identical rectangular shapes. In each of the aligned segments 20A, 20B, 20C, a plurality of magnets may be positioned in the direction of the length of the axis A.The center segment 60 and the vane segments 62A, 62B of the first barrier layer 22 are separated from each other by material of the rotor core 18. In other words, the center segment 60 and the vane segments 62A, 62B are discrete from each other and are not continuous because the rotor core 18 defines a center bridge 68 between the center segment 60 and the first vane segment 62A and between the center segment 60 and the second vane segment 62B. As an example, which is not intended to be limiting, the rotor core 18 may be configured such that a minimum width WMof each center bridge 68 is not less than 0.7 mm and not more than 2 mm. The minimum width WMis defined as the minimum distance between the center segment 60 and the first vane segment 62A or the second vane segment 62B. A center bridge 68 configured in this manner helps meet the requirement of the predetermined rotational load of the electric machine 10 and minimizes the magnetic material necessary to potentially reduce manufacturing costs.The material of the rotor core 18 also forms a first outer bridge 70 between each of the first and second vane segments 62A, 62B and the outer periphery 25 of the rotor core 18.In addition, the material of the rotor core 18 forms a second outer bridge 72 that extends between both the second barrier layer 24 and the third barrier layer 26 and the fourth barrier layer 28 and the outer periphery 25. In other words, the second outer bridge 72 is that portion of each rotor pole 40 that is located between first and second vane segments 62A, 62B of the rotor pole 40 and the outer periphery 25. To illustrate, FIG. 2 illustrates the circumferential angular extent 78 (i.e., a segment of the circumference of the rotor core 18) of one of the second outer bridges 72. As an example without limitation, a minimum width WT 2 of each second outer bridge 72 is not less than 1 mm and not more than 3 mm. The magnets 20A, 20B, 20C generate the torque generation flux in the electric machine 10 and also serve to saturate the outer bridges 70 to minimize a flux shunt effect.For mass saving, the rotor core 18 has voids 80 between adjacent blade segments 62A, 62B of adjacent sets of first barrier layers 22 of adjacent poles 40. Additional voids 82 are positioned radially inward of the first barrier layers 22 and radially outward of the inner periphery 23. The cavities 80, 82 are regions of relatively low magnetic flux density of the rotor core 18 to reduce the weight and inertia of the rotor core 18. This allows for rapid dynamic response of the electric machine 10, such as when a vehicle operator changes operating requirements, thereby potentially increasing vehicle fuel economy.The cavities 82 are positioned such that spokes 84 are defined by the rotor core 18 between adjacent ones of the cavities 82 and are centered within each rotor pole 40. That is, the spokes 84 are centered below the center segments 60 and the center magnets 20A. By positioning the spokes 84 to lie directly below the center segments 60, the spokes 84 are radially aligned with the poles 40 such that the pole central axis 42 of each pole 40 passes through the radial center of the respective spoke 84 below the center magnet 20A. Thus, the magnetic flux through the rotor core material of the spokes 84 helps to magnetize the magnets 20A, 20B, 20C. In the embodiment shown, the spokes 84 have a non-linear shape because they are partially defined by the circular cavities 82. The spokes 84 extend generally radially between the portion of the rotor core 18 that functions as the center shaft support 33 and the center segments 60.FIG. 4 shows an alternative rotor assembly 114 that may be used in place of the rotor assembly 14 with the stator assembly 12 in the electric machine 10. The rotor assembly 114 includes a rotor core 118 that differs from the rotor core 18 only in that barrier layers 122, 124, 126, and 128 corresponding to the first, second, third, and fourth barrier layers 22, 24, 26, and 28, respectively, have corners that are more rounded, cavities 180 corresponding to the cavities 80 have rounded corners, and cavities 182 have a different shape and arrangement than the cavities 82, resulting in linear spokes 184 extending radially along the pole boundaries 42 rather than along the pole axes 44 (labeled with respect to a pole 40 in FIG. 4 ). Thus, spokes 184 are centered between poles 40 rather than poles 40. Otherwise, the rotor assembly 114 is similar in all aspects to the rotor assembly 14 and functions and behaves as described with respect to the rotor assembly 14 to satisfy the same predetermined operating parameters. Computational rotational load analysis of the rotor assembly 114 indicates that the distalmost portion 145 of the center segment 60A experiences the greatest rotational load, and that when the electric machine 10 is operated in the motor mode at 20,000 revolutions per minute and 150 degrees Celsius, the rotational load at the distal portion 145 is less than the maximum allowable rotational load operating parameter predetermined due to material characteristics. In an exemplary embodiment with the stator assembly 12 and the rotor assembly 114, the electric machine 10 may have a total weight less than 8 kilograms (kg), wherein the stacked stator core 30 weighs less than 3 kg, the stacked rotor core 18 weighs less than 2 kg, the stator windings 34 weighs less than 2.5 kg, and all magnets 20A, 20B, 20C weigh less than 0.5 kg.FIG. 5 shows an alternative rotor assembly 214 that may be used in place of the rotor assembly 14 with the stator assembly 12 in the electric machine 10. The rotor assembly 214 is similar to the rotor assembly 114 in all aspects except that the rotor core 218 is configured such that the first barrier layers 122 are replaced with first barrier layers 220 having center segments 260A that are contiguous with and not separated from the vane segments 262A and 262B. In other words, there are no center bridges separating the center segment 260A from the wing segments 262A and 262B in each first barrier layer 220, and the wing segments 262A, 262B extend from opposite ends of the center segment 260A. In addition, the rotor core 218 is configured such that rotor spokes 284 extend along a pole axis 44 of each rotor pole 40 and cavities 282 between the rotor spokes 284 are centered along the pole boundaries 42. A computational rotational load analysis of the rotor assembly 214 indicates that no portion will experience a rotational load greater than the maximum allowable rotational load predetermined due to material characteristics when the electric machine 10 is operated at 20,000 revolutions per minute and at 150 degrees Celsius in the motor mode.The electric motor 10 may be used in many applications, such as a vehicle. An exemplary use without limitation of the electric motor 10 is shown in FIG. 6. The electric motor 10 is included in the powertrain 300 of a vehicle 302. The powertrain 300 also includes an engine 304 with a crankshaft 306. A belt drive 308 operatively connects the electric machine 10 to the crankshaft 306 when a selectively engageable clutch 322A is engaged. The powertrain 300 is a hybrid powertrain, and more specifically a fossil fuel hybrid electric powertrain, because, in addition to the engine 14 being a first power source operated with fossil fuel, such as gasoline or diesel fuel, the electric machine 10 being operated by stored electric energy is available as a second power source. The electric machine 10 may be controlled to function as a motor or a generator, and may be operatively connected to the crankshaft 306 of the engine 304 via the belt drive 308 when the selectively engageable clutch 322A is engaged. The belt drive 308 includes a belt 310 that engages a pulley 312. The pulley 312 is connected to and rotates with the motor shaft 29 of the electric motor 10 only when the selectively engageable clutch 322A is engaged. The belt 310 also engages a pulley 314 that can be connected to rotate with the crankshaft 306. When the pulley 312 is connected to rotate with the electric machine 10 and the pulley 314 is connected to rotate with the crankshaft 306, the belt drive 308 drivingly connects the electric machine 10 and the crankshaft 306. In this arrangement, the electric machine 10 may be referred to as a belt-driven generator / motor generator starter. Alternatively, the belt drive 308 may include a chain instead of the belt 310 and sprockets instead of the pulleys 312, 314. These two embodiments of the belt drive 308 are referred to herein as "belt drive.".A motor controller inverter module (MPIM) 316 is operatively connected to the stator assembly 12. As shown, the MPIM 316 is directly mounted to the electric machine 10. A battery 318 is operatively connected to the stator assembly 12 through the MPIM 316 and through one or more additional controllers 320, which are also operatively connected to the engine 304, a transmission 321, and clutches 322A, 322B, and 324. The operative connections to the engine 304, transmission 321, and clutches 322A, 322B, and 324 are not shown in the drawings for clarity. Connections to transmission 321 and clutches 322A, 322B, and 324 may be electronic, hydraulic, or otherwise.When clutch 324 is engaged and assuming internal clutches in transmission 321 are controlled to establish a drive connection between transmission input member 334 and transmission output member 336, torque transfer between crankshaft 306 and vehicle wheels 330 may occur through transmission 321 and through differential 332.Under predetermined operating conditions, the controller 320 may cause the clutch 322B to be engaged and the MPIM 316 may control the electric machine 10 to function as a motor. The electric machine 10 may then drive the crankshaft 306 via intermeshing gears 340, 342 to start the engine 14. The gear 340 is mounted on and rotates with a shaft 346, which rotates with the motor shaft 29 when the clutch 322B is engaged. The gear 342 is mounted to and rotates with the crankshaft 306. When cranking the engine 14, the clutch 322A is not engaged.The MPIM 316 is configured to, when the engine 14 is on and when predetermined operating conditions are met, control the stator assembly 12 to achieve a motor mode where the electric machine 10 adds torque to the crankshaft 306 using stored electrical power from the battery 318. The battery 318 has a nominal voltage of 12 volts in the embodiment shown. The electric motor 10 adds torque through the belt drive 308 when the clutch 322A is engaged and the clutch 322B is not engaged. The MPIM 316 is configured to, when the engine 14 is on and other predetermined operating conditions are met, control a flow of power in the stator assembly 12 to achieve a generator mode in which the electric machine 10 converts torque from the crankshaft 306 into electrical power stored in the battery 318 when the clutch 322A is engaged and the clutch 322B is not engaged. Operation of the electric machine 10 as a generator decelerates the crankshaft 306.In the application shown in FIG. 6, or in other vehicle powertrain applications, the electric machine 10 is configured to achieve an efficiency of at least 80% over a predefined range of output power and speed, as shown in FIG. 8. The predefined output power range ranges from 1500 to 5000 watts and the predefined speed range ranges from 2500 to 8000 revolutions per minute and is intended to have a maximum speed of at least 18,000 revolutions per minute. Referring to FIG. 7, a plot 400 on the left side vertical axis 402 shows torque of the electric machine 10 in base torque units (pu). The power of the electric machine 10 in basic power units (pu) is shown on the right side vertical axis 404. The rotational speed of the rotor assembly 14 in revolutions per minute (rpm) is shown on the horizontal axis 406. Some of the predetermined operating parameters that the geometry of the electric machine 10 has been specially designed to satisfy include a peak engine torque request 408, an engine power request 410, and a power generation request 412. A motoring torque 414 that is theoretically achievable by the electric machine 10 exceeds the motoring peak torque request 408. An engine-powered power 416 that can be theoretically achieved by the electric machine 10 exceeds the engine-powered power request 410. The amplitude of the generation of power 418, which can be theoretically achieved by the electric machine 10, exceeds the request 412 for generating power. A generator torque 420 is also shown and extends at least up to a speed of the electric machine 10 of 18,000 revolutions per minute.FIG. 8 shows a map 500 of the efficiency of the electric machine 10 when operated in a 14 volt generator mode. The power of the electric machine 10 in basic power units (pu) is shown on the vertical axis 502. The speed of the electric machine 10 in revolutions per minute is shown on the horizontal axis 504. Regions having different operating efficiencies of the electric machine 10 are shown limited by dashed lines and include: a zone 506 having an operating efficiency of 94%; a zone 508 having an operating efficiency of 92%; a zone 510 having an operating efficiency of 90%; a zone 512 having an operating efficiency of 88%; a zone 514 having an operating efficiency of 85%; a zone 516 having an operating efficiency of 80%; a zone 518 having an operating efficiency of about 75%; a zone 520 having an operating efficiency of about 65%; a zone 522 having an operating efficiency of about 55%; a zone 524 having an operating efficiency of about 45%; a zone 526 having an operating efficiency of about 35%; a zone 528 having an operating efficiency of about 25%; and a zone 530 having an operating efficiency of about 15%.FIG. 9 shows a map 600 of the efficiency of the electric machine 10 when operated in a 12 volt motor mode. The power of the electric machine 10 in basic power units (pu) is shown on the vertical axis 602. The speed of the electric machine 10 in revolutions per minute is shown on the horizontal axis 604. Regions having different operating efficiencies of the electric machine 10 are shown limited by dashed lines and include: a zone 606 having an operating efficiency of 92%; a zone 608 having an operating efficiency of 90%; a zone 610 having an operating efficiency of 88%; a zone 612 having an operating efficiency of 85%; a zone 614 having an operating efficiency of 80%; a zone 616 having an operating efficiency of about 75%; a zone 618 having an operating efficiency of about 70%; a zone 620 having an operating efficiency of about 65%; a zone 622 having an operating efficiency of about 60%; a zone 624 having an operating efficiency of about 55%; a zone 626 having an operating efficiency of about 50%; a zone 628 having an operating efficiency of about 45%; and a zone 630 having an operating efficiency of about 40%.FIG. 10 is a plot 700 of the speed in revolutions per minute of the rotor assembly 14 in revolutions per minute on the leftmost vertical axis 702, a phase A current in base current units (pu) in the windings 34 of the stator assembly 12 on the other left side vertical axis 704, the power of the electric machine 10 in base power units (pu) on the right side vertical axis 706, and the time in seconds on the horizontal axis 708 during a three-phase short-circuit event. The short circuit event occurs when the three phases of the terminals 34 are connected together while the rotor assembly 14 is spinning at high speeds, such as greater than 4000 rpm (i.e., without torque on the motor shaft 29). The resulting rotational speed of the rotor assembly 14 is shown by a curve 710. The phase current of phase A is shown by a curve 712. The power loss in the electric machine 10 is shown by a curve 714. In the exemplary embodiment shown in FIG. 10, the actual short circuit current is less than a predetermined value, for example 0.9 multiplied by the rated current of the electric machine 10.Although the best modes for carrying out the many aspects of the present technical teachings have been described in detail, those skilled in the art to which these teachings pertains will recognize various alternative aspects for practicing the present teachings that are within the scope of the appended claims.

Claims

An electric machine (10) comprising: a rotor assembly (14, 214) having a rotor core (18, 218) configured to support permanent magnets (20A, 20B, 20C) spaced around the rotor core (18, 218) to define a number of rotor poles (40); wherein the rotor core (18, 218) has a plurality of rotor grooves (22, 24, 26, 28; 220, 124, 126, 128) disposed on each of the rotor poles (40) as a plurality of barrier layers (22, 24, 26, 28; 220, 124, 126, 128); wherein the rotor core (18, 218) comprises a center shaft support (33) and a plurality of spokes (84, 284) extending radially outward from the center shaft support (33); wherein the plurality of spokes (84, 284) are radially aligned with the rotor poles (40); wherein the barrier layers (22, 24, 26, 28; 220, 124, 126, 128) are positioned adjacent to each other between an inner periphery (23) of the rotor core (18, 218) and an outer periphery (25) of the rotor core (18, 218); wherein the barrier layers (22, 24, 26, 28; 220, 124, 126, 128) comprise a first barrier layer (22; 220) having a plurality of segments (60, 62A, 62B; 260A, 262A, 262B) closest to the inner periphery (23); and permanent magnets (20A, 20B, 20C) are housed in at least some of the segments (60, 62A, 62B; 260, 262A, 262B) of the first barrier layer (22; 220); and wherein a plurality of cavities (82) are arranged radially within the first barrier layer (22; 220).The electric machine (10) of claim 1, wherein the segments (60, 62A, 62B; 260A, 262A, 262B) of the first barrier layer (22; 220) comprise a center segment (60; 260A) and first and second vane segments (62A, 62B; 262A, 262B) extending from opposite ends of the center segment (60; 260A) toward and away from the outer periphery (25).The electric machine (10) of claim 2, wherein each of the first and second vane segments (62A, 62B; 262A, 262B) and the center segment (60; 260A) have a thickness (T1, T2, T3); and wherein the thickness (T1, T2, T3) of each of the vane segments (62A, 62B; 262A, 262B) and the center segment (60; 260A) is substantially the same.The electric machine (10) of claim 1, further comprising: a stator assembly (12) surrounding the rotor assembly (14, 214) with a gap (16) therebetween; wherein the stator assembly (12) has a number of stator slots (32) circumferentially spaced around the stator assembly (12) and configured to support stator windings (34); and wherein a smallest common multiple of the number of stator slots (32) and the number of rotor poles (40) is greater than 60.The electric machine (10) of claim 1, further comprising: a stator assembly (12) surrounding the rotor assembly (14, 214) with a gap (16) therebetween; wherein the stator assembly (12) has a number of stator slots (32) circumferentially spaced around the stator assembly (12) and configured to support stator windings (34); and wherein a largest common divider of the number of stator slots (32) and the number of rotor poles (40) is at least 4.An electric machine (10) comprising: a rotor assembly (14, 214) having a rotor core (18, 218) configured to support permanent magnets (20A, 20B, 20C) spaced around the rotor core (18, 218) to define a number of rotor poles (40); wherein the rotor core (18, 218) has a plurality of rotor grooves (22, 24, 26, 28; 220, 124, 126, 128) disposed on each of the rotor poles (40) as a plurality of barrier layers (22, 24, 26, 28; 220, 124, 126, 128) between an inner periphery (23) of the rotor core (18, 218) and an outer periphery (25) of the rotor core (18, 218); wherein the barrier layers (22, 24, 26, 28; 220, 124, 126, 128) comprise a first barrier layer (22;220) closest to the inner periphery (23) and having a plurality of segments (60, 62A, 62B; 260A, 262A, 262B) comprising a central segment (60; 260A) and first and second vane segments (62A, 62B; 262A, 262B) extending from opposite ends of the central segment (60; 260A) toward and away from each other toward the outer periphery (25); wherein the thickness of both the vane segments (62A, 62B; 262A, 262B) and the central segments (60; 260A) is between 2.0 mm and 2.5 mm; a stator assembly (12) surrounding the rotor assembly (14, 214) with a gap (16) therebetween; wherein the gap (16) is not less than 0.3 mm and not greater than 0.5 mm; wherein the rotor core (18, 218) comprises a center shaft support (33) and a plurality of spokes (84, 284) extending radially outward from the center shaft support (33); wherein the plurality of spokes (84, 284) are radially aligned with the rotor poles (40); and wherein a plurality of cavities (82) are disposed radially inward of the first barrier layer (22; 220).The electric machine (10) of claim 6, wherein the stator assembly (12) comprises a plurality of axially stacked stator laminations (19).The electric machine (10) according to claim 7, wherein a ratio of an outer diameter (OD) of the stator laminations (19) to an axial length (AL) of the stator laminations (19) is not less than 1.5 and not greater than 4.

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