Patent Application
20240120814
The present disclosure relates generally to electric motors. More specifically, aspects of this disclosure relate to apparatuses for reducing Electric Discharge Machining (EDM) currents in electric motors in vehicles and other machines.
Electric vehicles are becoming increasingly popular, and consequently increasingly prevalent, in the inventory of modern automobile manufacturers. Current production electric vehicles, including both full electric and hybrid vehicles, may be equipped with a powertrain that operates to propel the vehicle via these electric motors. Different types of electric motors have been developed for use in such vehicles. Existing electric motors commonly employ a bearing system used to maintain stable rotation of the motor shaft in an AC induction motor and other motor types, which in turn can be used to rotate a wheel or intermediary structure for powering the vehicle.
Existing electric vehicles and their hybrid counterparts have shortcomings. The bearings used in most types of electric motors are subject to damage due to electrical arcs created by virtue of parasitic charge buildup on the rotor within the drive unit case in which the motor is housed. The charge buildup can be due to the parasitic capacitance between the stator and rotor core, with the gap therebetween acting as a dielectric. The accumulation of charge eventually can cause leakage current to discharge through the motor shaft and the race, which leads to electrical arcs through the bearings. These arcs can create pits and other permanent damage in the bearings.
To ameliorate this problem, manufacturers conventionally couple a conductive spring-loaded brush to the motor shaft. During operation of the motor, the spring presses into the inner drive unit case or other system ground to discharge the parasitic voltage. The continued radial force of the spring on the motor shaft tends to wear down the shaft surface and brush, often causing visible wear lines across the motor shaft's diameter. Permanent damage can thereafter result.
Presented herein are electric motors for use in electric vehicles and other machinery that obviate the above-referenced problems. As noted, conventional solutions common to numerous types of electric motors involve the manufacturer appending a spring onto a radial portion of the motor shaft. The spring is geometrical-shaped to press against a vehicle or system ground during rotation of the motor shaft, such as an interior of the drive unit case, to provide a low resistance current path to ground, and thereby avoid the damaging electric arcs that may pass through the lubricant of the bearings. The collective electrical resistance of the brush, spring and motor shaft is configured to provide a lower resistance path to ground than a path through the race, ball bearings and lubricate film.
One unintended consequence of this solution may occur quickly over time as the brush continues to rotate at the speed of the motor shaft. In so doing, a large centripetal force is asserted inward which increases with the spring rotational speed, which translates to a heavy load on the motor shaft particularly when the spring is spinning at a fast rate. This heavy load may invariably result in premature wear to, and ultimate failure of, the electric motor resulting from damage to the motor shaft. In short, the radial load induced on the rotating shaft by virtue of the spring tends to quickly wear the motor shaft down. The wear may be initially evidenced by wear lines in the shaft's diameter due to contact force abrasion from the brush's radial load. This wear may quickly escalate to radial cracks, generate debris, and ultimately permanent damage to the shaft and bearing system. By the same token, removing the spring tends to redirect the subject parasitic current discharges once again through the bearings, at which point the electrical arcs may quickly result in irreparable damage to the bearing system.
Attempts at fixing this problem have been made without apparent success. The inventor is aware of one electric motor implementation in which a conductive needle is placed at the center of the motor shaft to redirect the current to a spinning spring to ground. However, in addition to acting as a substantial bottleneck to the minimum possible electrical resistance that may in practice be obtained using the narrow needle—which may result in electrical arcs across the bearings despite the needle and spring—the needle is narrow and necessarily brittle to not bend. The needle is consequently easily breakable, especially at the speed of many modern electric motors, which may destroy the motor. For this reason, it is not surprising that most other manufacturers that rely on a spinning spring as an attempted solution connect the spring via a structure that radially loads the motor shaft to adequately support the spring while enabling a low resistance path. This proposed solution, however, in turn results in quick wear and inevitable damage to the shaft for the reasons above.
Aspects of this disclosure are accordingly directed to electric motor configurations for providing a low resistance current path to avoid electrical arcing across the bearings while minimizing or altogether eliminating the above-referenced wear on the motor shaft. In an example, an apparatus is presented for preventing electric charge buildup on the rotor core due to the parasitic capacitance between the stator and rotor core. The electric motor apparatus includes a case, a stator within a first inner surface of the case, and a rotor within the stator and configured to rotate relative to the stator. The electric motor apparatus includes a motor shaft protruding longitudinally through the rotor to include an axial end. A grounding brush has an outer surface opposing an at least partly elliptical surface. Also, a spring may include a first portion to maintain slidable contact with a second inner surface of the case and a second portion coupled to an outer surface of the brush. The at least partly elliptical surface of the brush is attached to the axial end of the motor shaft for causing a current flow via the brush to the second inner surface of the case for discharging an electric charge buildup on the stator.
Because the load on the motor shaft is axial in the subject matter herein, rather than predominantly radial as in conventional solutions, the incidents of premature damage to the motor shaft may be significantly reduced if not altogether eliminated. Meanwhile, the low-resistance current path created selectively from the rotor core via the motor shaft and grounding brush to the second inner surface of the case protects against electrical arcs from compromising the bearing system as the low-resistance path maintains the first and second inner surface of the case at the same ground potential.
In another aspect of the disclosure, an electric motor for a vehicle is presented. The electric motor includes a drive unit case, and a stator arranged within the drive unit case. A rotor is within the stator. A motor shaft extends longitudinally through the rotor core. The electric motor further includes an anti-arc system. The anti-arc system includes a conducting grounding brush and a spring. The spring has a first portion configured to maintain contact with a system or vehicle ground, and a second portion connected to the grounding brush. The grounding brush further includes an elliptical-shaped surface coupled to an axial end surface 423 of the motor shaft to discharge an electric charge buildup on the rotor core.
In still another aspect of the disclosure, an electric motor includes a case, and a stator arranged within the drive unit case. A rotor is at least partly within the stator. A motor shaft extends longitudinally through the rotor. An anti-arc system includes a conducting grounding brush and a spring. The spring has a first portion configured to maintain contact with an interior of the case during motor operation. The spring also has a second portion connected to the grounding brush. The grounding brush includes an elliptical-shaped surface having a center portion coupled outwardly to an axial end surface of the motor shaft.
Additional aspects of this disclosure are directed to electric motors using dedicated low resistance paths for leakage current flow and equalization of voltage across the entire drive unit case or other system ground without an appreciable radial load placed on the motor shaft. As used herein unless otherwise evident from the context, the terms “vehicle” and “electric vehicle” may be used interchangeably and synonymously to include any relevant electric or hybrid vehicle platform, such as passenger vehicles (REV, FEV, fuel cell, fully and partially autonomous, etc.), commercial vehicles, industrial vehicles, tracked vehicles, off-road and all-terrain vehicles (ATV), motorcycles, farm equipment, watercraft, aircraft, etc. In an example, a motor vehicle includes a vehicle body with a passenger compartment, multiple drive wheels rotatably mounted to the vehicle body (e.g., via corner modules coupled to a unibody or body-on-frame chassis), and other standard original equipment. For electric-drive vehicle applications, one or more electric traction motors operate alone (e.g., for FEV powertrains) or in conjunction with an internal combustion engine assembly (e.g., for HEV powertrains) to selectively drive one or more of the road wheels to propel the vehicle. Where an induction motor or synchronous reluctance motor is used or an alternating current is otherwise required for engine operation, a rechargeable traction battery pack, for example, may be mounted onto the vehicle body and may be operable to power the traction motor(s) using a traction power inverter module (TPIM).
The above Summary is not intended to represent every embodiment or every aspect of the present disclosure. Rather, the foregoing summary merely provides an exemplification of some of the novel concepts and features set forth herein. The above features and advantages, and other features and attendant advantages of this disclosure, will be readily apparent from the following detailed description of illustrated examples and representative modes for carrying out the present disclosure when taken in connection with the accompanying drawings and the appended claims. Moreover, this disclosure expressly includes any and all combinations and sub combinations of the elements and features presented above and below.
The present disclosure is amenable to various modifications and alternative forms, and some representative embodiments are shown by way of example in the drawings and will be described in detail herein. It should be understood, however, that the novel aspects of this disclosure are not limited to the forms illustrated in the above-enumerated drawings. Rather, the disclosure is to cover all modifications, equivalents, combinations, sub combinations, permutations, groupings, and alternatives falling within the scope of this disclosure as encompassed, for example, by the appended claims.
This disclosure is susceptible of embodiment in many different forms. Representative embodiments of the disclosure are shown in the drawings and will herein be described in detail with the understanding that these embodiments are provided as an exemplification of the disclosed principles, not limitations of the broad aspects of the disclosure. To that extent, elements and limitations that are described, for example, in the Abstract, Introduction, Summary, and Detailed Description sections, but not explicitly set forth in the claims, should not be incorporated into the claims, singly or collectively, by implication, inference or otherwise.
For purposes of the present detailed description, unless specifically disclaimed: the singular includes the plural and vice versa; the words “and” and “or” shall be both conjunctive and disjunctive; the words “any” and “all” shall both mean “any and all”; and the words “including,” “containing,” “comprising,” “having,” and the like, shall each mean “including without limitation.” Moreover, words of approximation, such as “about,” “almost,” “substantially,” “generally,” “approximately,” and the like, may each be used herein in the sense of “at, near, or nearly at,” or “within 0-5% of,” or “within acceptable manufacturing tolerances,” or any logical combination thereof, for example. Lastly, directional adjectives and adverbs, such as fore, aft, inboard, outboard, starboard, port, vertical, horizontal, upward, downward, front, back, left, right, etc., may be with respect to a motor vehicle, such as a forward driving direction of a motor vehicle when the vehicle is operatively oriented on a horizontal driving surface.
Referring to the drawings, wherein like reference numbers refer to like features throughout the several views,
Referring still to
The motor vehicle 11 shown in
In certain implementations where other types of motors are used (such as a DC brushless motor), the inverter functionality of PIM 28 may not be needed, and the DC current can be supplied directly or after being adjusted as suitable to the wires associated with the ME 26. Otherwise, as appreciated in the art, the PIM 28 includes a DC side 280 and an alternating current (AC) side 380, with the latter being connected to individual phase windings (not shown) of the rotary electric machine 26 when the rotary electric machine 26 is configured as a polyphase rotary electric machine in the form of a propulsion or traction motor as shown.
The battery pack 12 of
Electrical components of the electrified powertrain system 10 may also include an accessory power module (APM) 29 and an auxiliary battery (BAUX) 30. The APM 29 is configured as a DC-DC converter that is connected to the DC bus 27, as appreciated in the art. In operation, the APM 29 is capable, via internal switching and voltage transformation, of reducing a voltage level on the DC bus 27 to a lower level suitable for charging the auxiliary battery 30 and/or supplying low-voltage power to one or more accessories (not shown) such as lights, displays, etc. Thus, “high-voltage” refers to voltage levels well in excess of typical 12-15V low/auxiliary voltage levels, with 400V or more being an exemplary high-voltage level in some embodiments of the battery pack 12.
In some configurations, the electrified powertrain system 10 of
Referring again to
To that end, the ECU 34 may be equipped with one or more processors (P), e.g., logic circuits, combinational logic circuit(s), Application Specific Integrated Circuit(s) (ASIC), electronic circuit(s), central processing unit(s), semiconductor IC devices, etc., as well as input/output (I/O) circuit(s), appropriate signal conditioning and buffer circuitry, and other components such as a high-speed clock to provide the described functionality. The ECU 34 also includes an associated computer-readable storage medium, i.e., memory (M) inclusive of read only, programmable read only, random access, a hard drive, etc., whether resident, remote or a combination of both. Control routines are executed by the processor to monitor relevant inputs from sensing devices and other networked control modules (not shown), and to execute control and diagnostic routines to govern operation of the electrified powertrain system 10.
Depending on the motor type and geometry, the rotor may include windings and/or poles which, for simplicity, are not explicitly detailed and generally may occupy portions of region within the stator 210. Depending on the motor architecture, the stator 210 may additionally or alternatively include a plurality of outcropping salient poles, each of which may include a plurality of windings of insulated coil (not shown). The stator 210 remains stationary as the rotor portion of the motor rotates. The electric motor 200 may be an AC induction, synchronous reluctance or switched reluctance motor, or other viable type of electric motor.
The motor includes a spring 202. The spring includes a tip end 205, and an edge 204, which is jutting out of the picture, generally orthogonal to a plane of the figure. The edge 204 of the spring 202 contacts a radial portion of a motor shaft. The spring 202 further includes a portion 218, which is the part of the spring that is mounted to the back casting, or other inner surface 279 of the spring. Together, the inner surface 269 along with the other inner surface 279 form the inner portion of the case 270.
The electric motor 300 of
Part or all of the drive unit case 370 may be conductive in order to provide the electric motor 300 with a path to ground as described further herein. In other implementations, the electric motor 300 may rely on another system or vehicle ground. Surrounding a periphery of the electric motor is a stator 310, of which a portion is shown in the cut-off view of
Depending on the style of electric motor 300, either the stator 310 or the rotor core 312 may include a plurality of protrusions or indentions that incorporate windings for passing current. In an AC induction motor or a synchronous reluctance synchronous motor, for example, the stator 310 may have a plurality of protrusions (not shown) acting as salient poles. The poles may form a corresponding plurality of slots around the circumference of an inner portion of the stator 310. The protrusions may incorporate respective windings arranged using a dedicated number of phases and magnetic poles to generate a rotating magnetic field (RMF). The rotor core 312 may have a corresponding unique geometry (omitted for simplicity as noted) for enabling, in the case of an example AC induction motor, a corresponding variable current to be generated in the rotor core 312. This corresponding variable current may in turn induce its own magnetic field which opposes the RMF of the stator. The rotor core 312 can rotate as a result of the opposing torque caused by the respective fields.
In other configurations involving synchronous reluctance motors, the rotor core 312 need not generate a current, but may instead have a predetermined number of salient poles that tend to cause the rotor core 312 to align with corresponding poles on the stator 310 and thereby minimize reluctance. This operating principle can in turn be used to maintain a synchronous rotation of the rotor core 312. In other implementations, the rotor core 312 may include a permanent magnet or ferromagnetic material for facilitating the interaction of the respective magnetic fields or currents depending again on the operating principle of the electric motor 300. For purposes of this disclosure, as noted above, this motor may be another suitable type used in electric vehicle (EV) or similar applications.
The motor 300 may further include a motor shaft 314, which may be generally cylindrical in nature. The motor shaft 314 can extend longitudinally through a center of the rotor core 312. The motor shaft 314 may rotate with the rotor core 312 to provide the vehicle or other apparatus with mechanical power. Coupled to the motor shaft 314 is a cross sectional view of a bearing system 321. The bearing system 321 in this configuration includes an inner race 317a and 317b. The inner race 317a-b may be coupled to the motor shaft. Thus, the inner race 317a-b may rotate with the inner shaft. The bearing system 321 may further include bearings 308. Although two bearings 308 are illustrated in
The bearing system 321 is shown as a cross-section of a circular surface. The bearing system 321 may further include an outer race 316a and 316b. The outer race 316a-b may be coupled to a first inner surface 369 of the drive unit case 370. In other implementations, the outer race 316a-b may be coupled indirectly to the drive unit case 370. In still other implementations, the bearing system 321 may be connected to another rigid or sturdy surface relative to the motor 300. In general, the outer race 316a-b may in practice be part of a single, integrated member that is circumferentially disposed around the case 370 to align with the bearings 308, and in turn the inner race 317a-b. The bearings 308 may include ball bearings that further includes gaps 320 between the inner race 317a-b and further gaps 320 between outer race 316a-b. A film of an oil or lubricant may be present or otherwise suspended in the gaps 320 for facilitating low-friction rotation of the bearings relative to the inner race 317a-b and outer race 316a-b. The bearings 308 may sit within the film of oil or lubricant. The oil or lubricant facilitates the movement of the bearings 308. One function of the oil in gap 320 is to prevent the bearings 308 from direct contact with the inner race 317a-b or the outer race 316a-b. This configuration can help prevent the bearings 308 from wearing down prematurely. The bearing system 321 is designed to allow the motor shaft 314 to smoothly rotate to provide mechanical power to a transmission, or directly to the wheel of an electric vehicle.
In one example, the inner race 317a-b, being coupled to the motor shaft, can rotate at very high speeds depending on criteria like the frequency of AC current supplied to the motor windings. The outer race 316a-b may, as noted, be stationary around the periphery of the first inner surface 369 of the drive unit case 370. The outer race 316a-b and the gaps 320 filled with oil may enable the bearings 308 to spin at high speeds in a stable manner. The drive unit case 370 may have a circular surface, hexagonal, or similarly shaped first inner surface 369 extending circumferentially around the motor. In some embodiments this first inner surface 369 may only represent an inner portion of a larger case, or the geometry of the inner surface 369 may be different. All or a dedicated portion of this larger case or other geometry may function as a ground for various electronic components in the motor 300.
A variety of bearing types and geometries may exist, each of which are intended to be included within the scope of the present disclosure. For example, roller bearings may be used.
Referring still to
This well-understood EDM current is undesirable, because the resulting electrostatic discharge (ESD) cause arcs 327 across the bearings and can create pits and other geometrical anomalies in the bearings 308 and causes damage, resulting in premature wear on the bearings 308, which can result in failure of the electric motor. The higher the buildup of electric charge 364, the more damage that can result on the bearings 308 as metal elements from the bearings 308 can splatter across the surrounding portions of the inner and outer race surfaces.
In one attempt to obviate this problem as shown in
The motor shaft 314 and rotor core 312 spin at a potentially very high speed because of the PIM 28. A significant problem with the above-described solution is that, instead of causing damage to the bearing system 321 by inducing electrical arcs 327 via conductive path 390, the spring 302 instead is designed to pass the current through the motor shaft via the spring 302 to the second inner surface 379 of the case 370. The unwanted electrical arcs 327 across the bearings can be avoided. A problem of potentially greater magnitude, however, is unintentionally created in the process. The radial load on the motor shaft caused by the constant spinning action of spring 302 may cause the motor shaft 314 to deteriorate. That is, the continued rotational force of the spring 302 asserts an undesirable radial load on the motor shaft 314. The inventors have observed this problem by identifying wear in the motor shaft 314 of various existing motors, such as that shown by the wear line 306 in
As with the example of
Accordingly, in various aspects of the disclosure to mitigate or altogether obviate the above shortcomings, a grounding brush having an elliptical surface is configured to couple to a generally central axial portion of the motor shaft at one end. The grounding brush has another generally opposing surface configured to connect to a spring at another end. The spring has a first portion coupled to the opposing surface of the grounding brush and a second portion designed to apply a force to maintain a firm contact with an adjacent inner conducting surface of the case (or with a similar vehicle or system ground). The grounding brush in this implementation beneficially significantly reduces, and in some cases avoids altogether, placing an undesirable radial load on the motor shaft as in prior attempted solutions. At the same time, the grounding brush can be tuned geometrically and using appropriate material s/alloys to provide the requisite low resistance path to system ground whenever a buildup of electrical charge occurs on the rotor core. The grounding brush is part of an anti-arc system along with the spring that applies a force against the grounded case. This configuration in turn provides the suitable current flow to maintain the grounded case at an equal potential across the ground.
As in
Thus, the grounding brush 428 in various embodiments can be connected at the elliptical or partly elliptical surface 445 to the axial portion 422 at the center of the motor shaft's end face 423 at axial portion 422, as shown, or as close to a “center” that minimizes a radial load on the motor shaft 414 if the geometry of the motor shaft is not quite cylindrical, or not concentric at its end face 423. The elliptical surface 445 of the grounding brush 428 can oppose an outer surface 466, the latter of which can be connected to a first portion 440 of the spring 402. As generally shown by directional arrow 406, the spring 402 can be extended and curved so that a second portion 430 of the spring 402 can align generally flush with a second inner surface 479 of the drive unit case 470 to thereby apply a force via the second portion 430 of the spring 402 to the second inner surface 479 of the drive unit case 470 using a screw or other fastener (not shown). The combination of the grounding brush 428 and its respective surface, the spring 402 and its two portions, and the second inner surface 479 of the drive unit case 470 can be implemented as an effective anti-arc system 444. The combined current path from the rotor core 412 through the anti-arc system 444 to the ground of the second inner surface 479 (or in other embodiments, to another system ground) can be designed to have a lower electrical resistance across the range of motor speeds than a current flow leading to an electrical arc across the bearings 408. That is, the resistance can be chosen in a manner that ensures that accumulated electric charge 464 can be effectively discharged via the current path 406, and not through the inner race 416 (and therefore across the bearings, as also shown in
In the embodiment shown, the grounding brush 428 has an elliptical surface 445, most easily seen in the cutaway view of the motor as a “half-ellipse” or “half-sphere” that can be symmetrically arranged at the axial portion 422 of the motor shaft 414. The point 422 of contact between the axial portion 422 and the grounding brush 428 can be large enough to maintain electrical contact along with the low electrical resistance, but small enough to minimize, if not substantially eliminate, an appreciable radial load on the motor shaft 414 or its orthogonal end face 423 such as shown in
To the right of the second portion 430 of the spring 402, a face-on, top-down view 432 of key elements of the anti-arc system 444 is shown, with the elliptical portion 428a of the grounding brush 428 and the end face 423 of the motor shaft 414 omitted for clarity. In this embodiment, the first portion 440 of the spring 402 (transparent for illustrative purposes) is adhered, welded, or otherwise connected to the outer surface 466 of the grounding brush 428. In this top-down view 432, the first portion 440 wraps around to show a top-down view 432 of the spring (in which second portion 430a corresponds to the second portion 430 of the spring 402) that is shown transparently through the corresponding second portion 430a of the spring. The second portion 430a of the spring is pressed (using a force proportional to the spring constant k) against, or otherwise adhered to, the second inner surface 479 of the case. In sum, the anti-arc system 444, along with a subset 440a of its constituent parts, provides an effective, electrically conductive ground path without damaging or bending the motor shaft 414.
The grounding brush 428 may use different compositions. In various exemplary embodiments, for high wear resistance applications, the grounding brush may be composed of alloys such as, for example and without limitation, phosphor bronze, silicon bronze, brass, copper-nickel alloys, and other alloys or metals with similar properties. In some configurations, various Aluminum (Al) and steel alloys might be applicable as well.
In various embodiments, the grounding brush 428 may benefit from a high hardness material to meet a Rockwell Hardness of about 60 HRC for the shaft material. Additionally, the grounding brush may include a material that is more conductive than the steel used in the motor shaft and bearings (which may be about 6-7% IACS). One such example alloy is a C17200 alloy, which is a Be Cu alloy with both a high hardness and an electrical conductivity of 22-28% IACS.
While spring 402 is shown, practitioners in the art upon review of the present disclosure can appreciate that different embodiments and configurations of the spring 402 and other portions of the anti-arc system 444 are possible. For example, a different type of spring 402 can be used, or an element without a spring can be used, to provide a current path flow to any system ground that may be shared with stator 410 to ensure that the electric charge buildup due to C2 is discharged. Other implementations of such a spring or other conductive element to the case or another system ground is deemed to fall within the scope of the present disclosure where the predominant load on the motor shaft is axial in nature.
As another example, it will be appreciated that, while the axial end face or end face 423 of the motor shaft 414 can be circular, in some embodiments the axial end may be just partly circular, often depending on the remaining construction of the motor shaft 414. The axial end may be hexagonal or another partially symmetric shape, for example. The grounding brush 428 may be elliptical, or another shape. In the example shown, the grounding brush can be shaped as a portion of a sphere or 3-D ellipse, such as a hemisphere. In other embodiments, the elliptical surface may just be partly elliptical. For example, the elliptical surface may include a flat lip around its perimeter, e.g., for providing additional surface area when being adhered to the spring 402. Further, where the motor shaft 414 is uneven, the grounding brush 428 can be shaped differently to maintain an axial load on the motor shaft 414 while the spring 402 is rotating. Numerous other configurations can be contemplated that remain within the scope of the disclosure.
The detailed description and the drawings or figures are supportive and descriptive of the present teachings, but the scope of the present teachings is defined solely by the claims. While some of the best modes and other embodiments for carrying out the present teachings have been described in detail, various alternative designs and embodiments exist for practicing the present teachings defined in the appended claims. Moreover, this disclosure expressly includes combinations and sub-combinations of the elements and features presented above and below.
