DEVICE FOR GROUNDING AN ELECTRIC MOTOR SHAFT

Abstract

An electric motor comprising a motor housing, a shaft with an axis of rotation, a first end and a second end, a first main bearing for rotatable support of the shaft at the first end within the motor housing, a second main bearing for rotatable support of the shaft at the second end within the motor housing, and an earthing bearing for conducting a bearing current between the shaft and the motor housing.

Description

INTRODUCTION

[0001] The present disclosure relates generally to electric motor shaft grounding systems, and more particularly to a method and apparatus for employing conductive bearings in an electric motor at one or more motor shaft grounding locations to improve grounding and reduce bearing current.

[0002] Bearing current is a high-frequency current that flows through the bearings of an electric motor. Bearing current is typically caused by a current and / or voltage on the motor shaft induced by the motor windings or an inverter, driving a discharge current through the bearings. Bearing currents and gear currents are a challenging problem for electric machine systems with motors and generators, including battery, hybrid, and fuel cell electric vehicles, trains, ships, and wind turbines. These currents can cause damage to motor and gearbox bearings, gears and their lubricants, NVH problems, gear and bearing failures, and motor and gearbox failures, reducing motor life and reliability and potentially leading to expensive warranty claims.

[0003] Bearing current can damage bearings in several ways. The current flowing through the bearings generates heat, which can cause the bearings to overheat and fail. This heat can break down the lubricant in the bearings, leading to increased friction and wear. The heat can also cause the bearing materials to expand, which puts stress on the bearing and can lead to premature failure. The current flowing through the bearings can cause erosion of the bearing surfaces. This flow of electrons between the bearing ball and the bearing carrier through the lubricant layer creates a discharge current that can erode the bearing surfaces.

[0004] The contact between the bearing ball and carrier is nano- or micro-sized, resulting in a very high current density. Furthermore, the buildup of shaft voltage causes an excessive electric field that disrupts the oil film between the bearing ball and carrier, resulting in an electrical discharge machining (EDM) effect and creating nano- or micro-sized craters on the surface. Bearing current can also lead to scoring on the bearing raceways, which can cause the bearings to vibrate. The vibrations are caused by the current flowing through the bearings, which creates a magnetic field that can accelerate bearing wear and lead to premature motor failure. In addition to the direct damage caused by bearing currents, bearing currents can also damage the motor indirectly by causing the motor to overheat, which can damage the motor windings and other components.

[0005] It is desirable to overcome the aforementioned problems by providing systems for bearing current reduction in electric motors, transmissions, and drive units. Other desirable features and characteristics of the present invention will become apparent from the following detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. DESCRIPTION

[0006] The present invention encompasses methods and systems for electric drives and associated electrical systems for providing motor systems, methods for manufacturing and operating such systems, as well as motor vehicles and other devices such as aircraft, ships, wind turbines, and other electric vehicles equipped with on-board propulsion systems. By way of example and not limitation, various embodiments of systems for mitigating electric motor bearing damage and wear resulting from bearing currents induced by electric motors and electric motor power systems are presented.

[0007] According to one aspect of the present disclosure, an electric motor includes a bearing current mitigation system to reduce bearing wear of the electric motor. According to one aspect of the present disclosure, the electric motor includes a motor housing, a shaft having a rotational axis, a first end, and a second end, a first main bearing for rotatably supporting the shaft at the first end within the motor housing, a second main bearing for rotatably supporting the shaft at the second end within the motor housing, and a grounding device, i.e., a bearing, a slide clamp, a brush, or a rotating device filled with conductive fluid, for conducting a bearing current between the shaft and the motor housing.

[0008] According to another aspect of the present disclosure, the grounding bearing further comprises a spring preload for applying a force between a bearing outer ring and a bearing inner ring along the rotation axis.

[0009] According to another aspect of the present disclosure, wherein the grounding bearing has fewer rolling elements than the second main bearing.

[0010] According to another aspect of the present disclosure, wherein the ground bearing comprises a conductive lubricant and the second main bearing comprises a dielectric lubricant.

[0011] According to another aspect of the present disclosure, wherein the grounding device is at least one of a thrust bearing, a sliding clamp, or a brush.

[0012] According to another aspect of the present disclosure, wherein an electrical impedance of the second main bearing is greater than an electrical impedance of the ground bearing.

[0013] According to another aspect of the present disclosure, wherein the grounding bearing is installed adjacent to the second main bearing having a bearing inner race conductively connected to the shaft and a bearing outer race conductively connected to the motor housing.

[0014] According to another aspect of the present disclosure, wherein the shaft has a cooling channel extending along the rotation axis within the shaft, and wherein at least one of the grounding bearing, a grounding brush, and a grounding spring is installed within the cooling channel, wherein a bearing outer ring is conductively coupled to the shaft and a bearing inner ring is conductively coupled to a cooling tube.

[0015] According to another aspect of the present disclosure, wherein the grounding bearing comprises a bearing inner race conductively coupled to the shaft and a bearing outer race coupled to the motor housing via at least one of a stranded wire and a conductive spring.

[0016] According to a further aspect of the present disclosure, an apparatus comprises an electric motor having a motor shaft rotatably supported within a motor housing by a first main bearing and a second main bearing, a gearbox having a gearbox housing and a load shaft, the gearbox being coupled to the electric motor via the motor shaft, the motor shaft being rotatably supported within the gearbox housing by a first gearbox bearing and the load shaft being rotatably supported within the gearbox housing by a second gearbox bearing, and a ground bearing for conducting an alternating current between the load shaft and the gearbox housing.

[0017] According to another aspect of the present disclosure, wherein the load shaft is conductively coupled to the motor shaft via a plurality of electrically conductive gears.

[0018] According to another aspect of the present disclosure, wherein the electric motor is a single-phase electric motor or a multi-phase electric motor.

[0019] According to another aspect of the present disclosure, wherein the grounding bearing is disposed adjacent to the second transmission bearing, and the grounding bearing has a smaller radius of rotation than the second transmission bearing.

[0020] According to another aspect of the present disclosure, wherein the load shaft is configured to drive a load, wherein the load is a wheel assembly.

[0021] According to another aspect of the present disclosure, wherein the transmission housing is conductively coupled to the motor housing.

[0022] According to another aspect of the present disclosure, wherein the grounding bearing has fewer rolling elements than the second transmission bearing.

[0023] According to another aspect of the present disclosure, wherein the grounding bearing has a larger total contact area than the motor bearing or the second transmission bearing.

[0024] According to another aspect of the present disclosure, wherein the load shaft has a lower speed than the motor shaft.

[0025] According to another aspect of the present disclosure, wherein the grounding device is a thrust bearing, a sliding clamp, or a brush.

[0026] According to another aspect of the present disclosure, a vehicle drive system includes a battery for providing a direct current, an inverter for converting the direct current into a three-phase alternating current, an electric motor for rotating a motor shaft in response to the three-phase alternating current, the electric motor further including a motor housing, a first main bearing for rotatably supporting the motor shaft at a load end within the motor housing, a second main bearing for rotatably supporting the motor shaft at a non-load end within the motor housing, a ground bearing installed adjacent to the second main bearing for conducting a bearing current between the motor shaft and the motor housing, and a gearbox mechanically coupled to the motor shaft.

[0027] In accordance with another aspect of the present disclosure, a brush or grounding bearing is installed adjacent to each of the main bearings to bypass the circulating bearing current and ground the shaft voltage to eliminate the EDM bearing current.

[0028] According to another aspect of the present disclosure, wherein the grounding bearing includes a conductive lubricant and fewer rolling elements than the second main bearing, and wherein the grounding bearing has a smaller radius of rotation than the second main bearing. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] The exemplary embodiments are described below in conjunction with the following drawing figures, wherein like numerals indicate like elements, and wherein: Fig. 1 shows an exemplary three-phase asynchronous motor with associated motor shaft grounding according to various embodiments; Fig. 2 shows an exemplary configuration for electric motor bearing current reduction using a gearbox and load shaft grounding according to various embodiments; Fig. 3a shows an exemplary configuration for electric motor bearing current reduction using a preloaded grounding bearing in accordance with various embodiments; Fig. 3b shows a cross-section of a preloaded grounding bearing for use in electromotive bearing current reduction in accordance with various embodiments; Fig. 4 shows an exemplary configuration for electric motor bearing current reduction using a thrust bearing or a thrust brush as a grounding bearing according to various embodiments; Fig. 5 shows an exemplary arrangement for electric motor bearing current reduction using a grounding bearing. Stranded wire and grounding spring in accordance with various embodiments; and Fig. 6 shows an exemplary configuration for electric motor bearing current reduction using a cooling tube grounding bearing configuration in accordance with various embodiments. DETAILED DESCRIPTION

[0030] The following detailed description is merely exemplary and is not intended to limit the application and uses. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, summary, or the following detailed description. As used herein, the term module refers to any hardware, software, firmware, electronic control component, processing logic, and / or processor device, alone or in any combination, including, but not limited to: application-specific integrated circuits (ASICs), an electronic circuit, a processor (collectively, dedicated, or as a group), and memory executing one or more software or firmware programs, combinational logic circuitry, and / or other suitable components that provide the described functionality.

[0031] Bearing currents can be generated by magnetic fields created during normal operation of an electric motor and can be coupled to the motor shaft via capacitive or inductive coupling. The motor bearings that support the motor shaft can then act as capacitors, and the voltage across the motor shaft can induce a current in the bearings. Capacitive coupling is the transfer of energy between two conductors separated by an insulator. Capacitive coupling works by creating an electric field between two conductors. This electric field then induces a current in the other conductor. The magnitude of the induced current is proportional to the strength of the electric field and the capacitance between the two conductors.There are a number of factors that can affect the efficiency of capacitive coupling, including the distance between the conductors, the area of ​​the conductors, and the dielectric constant of the insulator between the conductors.

[0032] Inductive coupling is the transfer of energy between two or more conductors through a magnetic field. The windings of an electric motor can generate a magnetic field, and this field can induce a current in the bearings. Inductive coupling works by creating a magnetic field around a conductor. This magnetic field then induces a voltage in another conductor located within the field. The magnitude of the induced voltage is proportional to the strength of the magnetic field and the distance between the two conductors. Several factors can affect the efficiency of inductive coupling, including the size and shape of the conductors, the spacing between the conductors, and the permeability of the material between the conductors.

[0033] In Fig. 1 illustrates an exemplary three-phase AC induction motor 100 with associated motor shaft grounding according to an exemplary embodiment. The exemplary motor 100 includes a motor housing 101, a stator 103, a rotor 105, a motor shaft 110, a fan 115, a main bearing 120, a ground bearing 130, an inverter 140, a three-phase electrical power cable 145, a DC power supply 150, and a DC electrical cable 155.

[0034] A three-phase induction motor 100 is an electric motor that operates on three-phase alternating current (AC). Three-phase AC induction motors are used in a variety of applications, such as electric vehicles, aircraft, ships, pumps, wind turbines, fans, and conveyor belts. A three-phase induction motor 100 consists of two main parts: the stator 103 and the rotor 105. The stator 103 is the stationary part of the motor and is mechanically connected to the motor housing 101. The rotor 105 is the rotating part of the motor 100 and is mechanically coupled to the motor shaft 110. In some example embodiments, the stator 103 has three coils arranged in a triangular shape. The coils are connected to a three-phase alternating current supplied by the three separate conductors of the three-phase power cable 145. When the three-phase alternating current is applied to the coils, it creates a rotating magnetic field.

[0035] The rotor 105 may consist of a series of metal bars connected by short-circuited rings. The rotating magnetic field of the stator 103 induces a current in the rotor bars. This current creates a magnetic field in the rotor 105 that opposes the magnetic field generated by the stator 103. The interaction between the magnetic field generated by the stator 103 and the magnetic field of the rotor 105 causes the rotor 105 to rotate. The speed of the rotor 105 is determined by the frequency of the three-phase current and the number of poles of the stator 103.

[0036] The exemplary electric motor 100 is supplied with DC power by a DC power supply 150. In some exemplary embodiments, the DC power supply 150 may be a lithium-ion battery pack of an electric vehicle that supplies a DC voltage to the inverter 140 via the DC cable 155. The inverter 140 serves to convert the DC voltage 155 into a three-phase AC current.

[0037] A three-phase inverter 140 is a power electronic device that converts a DC input voltage into a three-phase AC output voltage. It is used in a wide variety of applications, including motor drives, power supplies, and welding equipment. The basic operating principle of a three-phase inverter 140 is to use a series of switches to controllably switch the DC input voltage on and off. The switches can be arranged in a three-phase bridge configuration, with each switch connected to one of the three phases of the AC output voltage. The switching of the switches is typically controlled by a microcontroller or other digital control device. The switches in the inverter are typically semiconductor devices, such as transistors or thyristors.The controller uses a feedback signal from the AC output voltage to determine when to turn each switch on and off. This ensures that the AC output voltage has the desired frequency, amplitude, and phase angle. In some example embodiments, inverter 140 may be an H-bridge inverter or a cascaded inverter.

[0038] One problem with a three-phase inverter 140 is that switching high DC voltages can result in common-mode voltage transients being introduced into all conductors of the three-phase power cable. These common-mode voltage transients can have a higher frequency, high voltage, and high switching speed. These common-mode voltage transients can generate a common-mode current in the motor, which is coupled between the stator 130 and the rotor 105, into the motor shaft 110, and discharged through the motor bearings to the motor housing 101 and ultimately to ground. This electrical discharge through the motor bearings can cause the bearings to overheat and wear prematurely.

[0039] To solve this problem, a motor bearing configuration is used that incorporates conductive devices—i.e., bearings filled with conductive fluids, brushes, grounding springs, sliding brackets, rotating devices—which, in addition to the main bearings 120, include at least one grounding bearing 130 to achieve better performance and reliability at lower costs and losses. The grounding bearing 130 can be positioned adjacent to the main bearing 120 to better attenuate the bearing current to protect the motor bearings, gears, and gearbox bearings.The grounding bearing 130 may have a lower load rating than the main bearing 120 and may be installed on the non-drive side or the drive side of the motor shaft 110, on either side of the motor shaft 110, or on a gear shaft, reducing the speed of the conductive bearing to reduce its electrical impedance, in or on the shaft, or directly on the cooling tubes to reduce costs and improve cooling.The proposed configurations minimize the overall electrical impedance of the bearing to better mitigate EDM bearing currents and circulating bearing currents by exploiting the skin effect, installing the conductive bearing on the outer surface of the shaft and forming a shorter current path, using conductive lubricant and less viscous lubricant and / or bearing preload to reduce oil film thickness, and increasing the contact area within the bearing to reduce electrical impedance. The skin effect is a tendency for alternating current to flow primarily near the outer surface of an electrical conductor, such as a metal wire or a shaft. The effect becomes increasingly pronounced with increasing frequency.

[0040] Various configurations of the main bearing 120 and grounding bearing 130, as well as electric motor configurations, are disclosed so that the electrical impedance of the grounding bearing is lower than that of the main bearing. This impedance difference can be achieved by using conductive or less viscous oil, adding preload, reducing the oil film thickness, and increasing the contact area. A smaller AC reactor can be attached to the device to directly filter the bearing current, which is more effective than using a reactor on the motor cable to filter the larger common-mode current flowing there. The carbon pin and brush are effective for grounding but can wear out relatively quickly. The new device provides a more reliable ground with lower electrical impedance (Ze) and friction to better attenuate the EDM bearing current and bypass the circulating bearing current.

[0041] The ground bearing 130 can be designed to have fewer rolling elements than the main bearing 120, resulting in lower resistance at the rolling element location. More rolling elements result in a more even distribution of force on the bearing and a more even distribution of lubricant, better balancing resistance across the bearing. Fewer rolling elements result in an uneven distribution of forces and an uneven distribution of lubricant, creating at least one path with lower resistance than the others. This path of least resistance would be the preferred path for an electrical discharge, increasing wear and / or damage to the ground bearing 130 and reducing damage to the motor bearings 130.

[0042] The ground bearing 130 may use a conductive or dielectric lubricant to reduce impedance, and the main bearing 120 may use an insulating lubricant to increase impedance. To reduce electrical impedance, the ground bearing may use a thinner or less viscous lubricant to reduce the bearing's oil film thickness and thus the impedance of the ground bearing lubricant. Furthermore, the ground bearing may use rolling elements with a smaller rotation radius than the rolling elements used by the main bearings or by making the ground bearing inner ring smaller. A less viscous lubricant may be used for the ground bearing 130 to reduce the clearance between the rolling element and the bearing rings, thus reducing the impedance of the ground bearing 130.In some exemplary embodiments, an oil-wiping tip / ring / spring may be added in front of the tip or bearing roller to minimize oil thickness and thus reduce preload to optimize wear and conductivity. The contact area of ​​the bearing elements may be increased to reduce impedance by increasing the number of rollers or using bearings with larger contact area, such as roller bearings, needle bearings, or thrust bearings. Reducing the bearing radius of the ground bearing 130 relative to the main bearing 120 may reduce the rotational speed of the ground bearing relative to that of the main bearing 120, thereby allowing for a less viscous or thinner lubricant and / or fewer rollers, thereby reducing the impedance of the ground bearing 130.In addition, an AC reactor may be added to the grounding bearing 130, as opposed to adding the AC reactor to the three-phase power cable 145, to filter the bearing current, since the bearing current at the grounding bearing 130 may be up to 20 times smaller than the total common-mode current on the three-phase power cable 145.

[0043] In Fig. Figure 2 illustrates an exemplary configuration 200 for bearing current reduction of an electric motor with a gearcase and load shaft grounding according to an exemplary embodiment. The exemplary configuration 200 includes a three-phase AC induction motor 210 with a motor case 205, a main bearing 245, a motor shaft 240, a gearcase 250, a ground bearing 255, a load 260, and a load shaft 265.

[0044] In some example configurations, the load shaft 265 may rotate at a lower speed than the motor shaft 240. The lower speed may result from a gear ratio of the gears in the transmission housing 250. This reduction in speed may result in less bearing wear, particularly when low-wear lubrication is used in the bearing. In some example embodiments, the ground bearing 255 may be disposed on a load shaft 265 that has a lower bearing speed to reduce its oil film thickness, thereby creating a smaller ground path for motor shaft currents. The load shaft 265 drives a load 260, such as a wheel, differential, transmission, transfer case, or other load.

[0045] In some example embodiments, electrical currents may be induced at the motor shaft 240 through inductive or common-mode coupling of electrical noise / leakage currents generated during operation of the three-phase induction motor 210. To avoid damage to the main bearing 245 of the three-phase induction motor 210 due to bearing currents, the example configuration 200 may be arranged so that the induced leakage current is coupled from the motor shaft 240 to the load shaft 265 via conductive gears within the gearbox 250. A motor-side main bearing 252 is arranged on the motor side of the gearbox 250 to support the motor shaft 240. The ground bearing 255 is arranged along with a load-side main bearing 254 to support the load shaft 265 on the load side of the gearbox 250. Since the speed of the load shaft 265 is lower than that of the motor shaft 240, the wear of the grounding bearing 255 is reduced.This reduction in excessive wear and heat may allow for the use of a thinner lubricant, a smaller number of rolling elements, and / or preloading the ground bearing 255 to reduce the impedance of the ground bearing. Furthermore, the radius at the junction between the inner portion of the ground bearing 255 and the outer portion of the ground bearing 255 may be selected to further reduce the angular velocity at the junction, thereby reducing wear and heat generation at the ground bearing 255. In some exemplary embodiments, an RF choke 252 or other RF impedance may be connected in series with a ground point 250 of the motor housing 205 such that the RF impedance to the ground point 250 via the gearbox housing 250 is lower than the RF impedance between the motor housing 205 and the ground point 250.This can improve the effectiveness of the grounding bearing 255, which can be electrically coupled to the gear housing 250, and reduce damage to the main bearing 245 of the three-phase asynchronous motor 210.

[0046] In Fig. Figure 3a shows an exemplary configuration for bearing current reduction of the electric motor using a preloaded grounding bearing according to an exemplary embodiment. Bearing preload refers to the application of a force to the bearing that is not caused by an external load. This force can be applied either in the axial direction, along the axis of the shaft, or in the radial direction, perpendicular to the axis of the shaft. The bearing can be preloaded with a spring preload, in which a spring is placed between the bearing and the housing or shaft, or with a fixed preload, in which a spacer or other fixed device is used to compress the bearing.

[0047] In some exemplary embodiments, an electric motor 301 is configured with a main motor shaft bearing 302 and a preloaded ground bearing 305. Preloading the ground bearing 305 allows a thin and optimal oil film to be maintained to compensate for changes in bearing clearance due to loading, shock, vibration, or displacement. In some exemplary embodiments, preloading the ground bearing 305 is achieved using a spring preload 310 to apply a bearing preload in the opposite direction to a load / lubricant zone of the main bearing 302 to compensate for changes in oil film thickness due to loading or road shock. Advantageously, the reduced lubricant thickness and constant rolling element contact and pressure reduce the impedance between the ground bearing housing and the shaft, thereby improving bearing current discharge through the ground bearing 305. Fig. 3b shows a cross section through a preloaded grounding bearing 320 with a bearing housing 322, a bearing shaft 330 and a spring preload 324. The spring preload 324 is designed to maintain an overpressure on the rolling element 325 through the bearing housing 322 and the bearing shaft 330.

[0048] In Fig. 4 illustrates an exemplary configuration 200 for electric motor bearing current reduction using a thrust bearing or thrust brush as a grounding configuration according to an exemplary embodiment. A thrust bearing 420 is a type of bearing that supports axial loads or loads parallel to a shaft, such as a motor shaft 430 in an electric motor 401. The thrust bearing 420 may include a bearing shaft 421 that is mechanically attached to and rotates with the motor shaft 430, rolling elements in a cage 423, and a housing that is mechanically attached to the electric motor housing 404. The thrust bearing 420 may be either preloaded or unpreloaded, lubricated or unlubricated.Because the thrust bearing 420 does not support the motor shaft 430, the thrust bearing 420 can be subjected to less mechanical stress and therefore a thinner or less viscous lubricant, or no lubricant, can be used to improve the conductivity of the thrust bearing 420 to bearing currents between the motor shaft 430 and the motor housing 404. The thrust bearing 420 can be installed externally to the motor housing 404 or internally to the motor housing 404. In some example embodiments, the thrust bearing 420 can be installed on the load side of the motor shaft 430 or on the non-load side of the motor shaft 430. The thrust bearing is configured as a low-resistance electrical ground path between the motor shaft 430 and the motor housing 404.

[0049] In Fig. Figure 5 illustrates an exemplary configuration for electric motor bearing current reduction using a grounding bearing 530 according to an exemplary embodiment. The exemplary configuration utilizes a grounding bearing 530, a stranded wire 550, a grounding hook 560, a main bearing 540, a motor shaft 520, and an electric motor 501 with a motor housing 502. In some exemplary embodiments, the grounding hook 560 may be a grounding spring, such as a copper spring finger or the like.

[0050] The stranded wire 550 may be a stranded wire or cable used to transmit alternating current at radio frequencies. The stranded wire 550 is formed to reduce the RF impedance caused by skin effect and proximity effect losses in electrical conductors. Furthermore, one or more grounding hooks 560 may be used to compensate for displacement / load variations between the motor shaft 520 and the motor housing 502 and ensure low impedance. The grounding hook 560 may include one or more semi-rigid conductive elements, such as beryllium copper fingers, metal brushes, or the like, to maintain positive pressure between the grounding bearing 530 and the motor housing 502.In some example embodiments, the grounding hook 560 may be comprised of one or more metal brushes with wear-resistant tips to electrically connect the grounding bearing 530 to the motor housing 502 with or without the grounding bearing 530.

[0051] In Fig. Figure 6 illustrates an exemplary configuration for electric motor bearing current reduction with a cooling tube grounding bearing 650 configuration according to an exemplary embodiment. The exemplary configuration may include an electric motor 601, a main bearing 620, a cooling tube 630, an electrical grounding point 640, a cooling tube grounding bearing 650, and a motor shaft 610.

[0052] Cooling tubes are used in electric motors to dissipate the heat generated by the rotor, windings, and bearings of the electric motor. The tubes are typically made of copper or aluminum and can be filled with a coolant such as water, oil, or a pressurized gas such as air. The coolant circulates through the tubes with the help of a pump, absorbing the heat from the motor. The heated cooling liquid can then be cooled by a radiator or heat exchanger and passed back through the electric motor 601. One way to cool the rotor and motor shaft 610 of an electric motor is to provide a hollow motor shaft 610 and to pass the coolant through the hollow part of the motor shaft 610. The coolant tube 630 must then have a smaller diameter than the hollow part of the motor shaft 610.To provide a grounding path for unwanted bearing currents, a cooling tube grounding bearing 650 may be installed between the outer surface of the cooling tube 630 and the inner surface of the hollow portion of the motor shaft 610. This cooling tube grounding bearing 650 would have a small radius and thus a lower angular speed than the outer surface of the motor shaft 610. Furthermore, the cooling tube grounding bearing 650 would bear little or no load from the electric motor and therefore experience less load wear. The cooling tube grounding bearing 650 may utilize a conductive lubricant and a conductive roller to create a low-resistance electrical connection between the interior of the motor shaft 610 and the cooling tube 630. In some exemplary embodiments, the cooling tube may be electrically coupled to a grounding point 640 to connect electrical current to a vehicle chassis or other grounding point.Additionally, an electrical choke 645 may be attached to a portion of the cooling tube 630 after the grounding point 640 to provide increased impedance to any remaining electrical current, such that the remaining electrical current is connected to the grounding point 640.

[0053] Although at least one exemplary embodiment has been presented in the foregoing detailed description, it should be understood that numerous variations exist. It should also be noted that the exemplary embodiment or exemplary embodiments are merely examples and are not intended to limit the scope, applicability, or configuration of the disclosure in any way. Rather, the foregoing detailed description is intended to provide those skilled in the art with a convenient guide for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes in the function and arrangement of elements may be made without departing from the scope of the disclosure as set forth in the appended claims and the legal equivalents thereof.

Claims

[1] An electric motor comprising: an engine housing; a shaft having an axis of rotation, a first end and a second end, a first main bearing for rotatably supporting the shaft at the first end within the motor housing; a second main bearing for rotatably supporting the shaft at the second end within the motor housing; and a ground bearing for conducting a bearing current between the shaft and the motor housing. [2] The electric motor of claim 1, wherein the grounding bearing further comprises a spring preload for applying a force between a bearing outer ring and a bearing inner ring along the rotation axis. [3] The electric motor of claim 1, wherein the grounding bearing has fewer rolling elements than the second main bearing. [4] The electric motor of claim 1, wherein the ground bearing comprises a conductive lubricant and the second main bearing comprises a dielectric lubricant. [5] The electric motor according to claim 1, wherein the grounding bearing is at least one of a thrust bearing, a grounding spring or a thrust brush. [6] The electric motor according to claim 1, wherein an electrical impedance of the second main bearing is greater than an electrical impedance of the ground bearing. [7] The electric motor according to claim 1, wherein the grounding bearing is installed adjacent to the second main bearing with a bearing inner ring conductively connected to the shaft and a bearing outer ring conductively connected to the motor housing. [8] The electric motor according to claim 1, wherein the shaft has a cooling channel extending along the rotation axis within the shaft, and wherein at least one of the grounding bearing, a grounding brush, and a grounding spring is installed within the cooling channel, wherein a bearing outer ring is conductively coupled to the shaft and a bearing inner ring is conductively coupled to a cooling tube. [9] The electric motor of claim 1, wherein the grounding bearing comprises a bearing inner race conductively coupled to the shaft and a bearing outer race coupled to the motor housing via at least one of a stranded wire and a conductive spring. [10] A device comprising: an electric motor having a motor shaft rotatably supported in a motor housing by a first main bearing and a second main bearing; a transmission having a transmission housing and a load shaft, wherein the transmission is coupled to the electric motor via the motor shaft, wherein the motor shaft is rotatably supported in the transmission housing by a first transmission bearing and wherein the load shaft is rotatably supported in the transmission housing by a second transmission bearing; and a grounding bearing for conducting an alternating current between the load shaft and the gearbox housing.

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