Electric motor shaft grounding device

Abstract

An electric motor includes a motor 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.

Description

Technical Field

[0001] The present disclosure relates generally to electric motor shaft grounding systems and, more particularly, to methods and apparatus for employing conductive bearings at one or more motor shaft grounding locations in an electric motor to improve grounding and mitigate bearing currents. Background Art

[0002] Bearing currents are high frequency currents that flow through the bearings of an electric motor. Bearing currents are typically caused by currents and / or voltages induced on the motor shaft by the motor windings or by a power inverter that drives discharge current through the bearings. Bearing and gear currents are a challenging problem for electromechanical 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 bearings, gear bearings, gears and their lubricants, NVH issues, gear and bearing failures, motor and gearbox failures, reducing motor life and reliability, and potentially generating expensive warranty replacement claims.

[0003] Bearing currents can damage bearings in a number of ways. Current flowing through the bearing generates heat, which can cause bearing overheating and failure. Heat can cause the lubricant in the bearing to break down, resulting in increased friction and wear. Heat can also cause the bearing material to expand, which can stress the bearing and cause premature failure. Current flowing through the bearing can cause corrosion on the bearing surface. The flow of this electrons through the lubricant film between the bearing balls and the bearing housing creates a discharge current that erodes the bearing surface.

[0004] The contact between the bearing balls and the bearing seats is nano- or micro-sized, resulting in very high current densities. In addition, the increase in shaft voltage causes excessive electric fields to destroy the oil film between the bearing balls and the bearing seats, resulting in an electrical discharge machining (EDM) effect, producing nano- or micro-pits on the surface. Bearing currents also cause grooves on the surface of the bearing races, which causes bearing vibrations. The vibrations are caused by the current flowing through the bearings that creates a magnetic field, which accelerates bearing wear and causes premature motor failure. In addition to the direct damage caused by bearing currents, bearing currents can also indirectly damage the motor by causing the motor to overheat, which can damage the motor windings and other components.

[0005] It is desirable to overcome the above problems in order to provide a system for mitigating bearing currents in electric motors, gearboxes and drive units. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background. Summary of the invention

[0006] Disclosed herein are methods and systems for providing electric propulsion of motor systems and related electrical systems, methods of manufacturing such systems, and methods of operating such systems, and motor vehicles and other equipment, such as aircraft, ships, wind turbines, and other electric vehicles equipped with onboard propulsion systems. By way of example and not limitation, various embodiments of systems for mitigating electric motor bearing damage and wear caused by bearing currents induced by electric motors and electric motor power systems are provided.

[0007] According to one aspect of the present disclosure, an electric motor includes a bearing current mitigation system to reduce electric motor bearing wear. According to one aspect of the present disclosure, the electric motor includes a motor housing, a shaft having a rotation 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 for conducting bearing current between the shaft and the motor housing, i.e., a bearing, a sliding clamp, a brush, or a rotating device filled with a conductive fluid.

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

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

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

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

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

[0013] According to another aspect of the present disclosure, wherein the ground bearing is mounted adjacent to the second main bearing, wherein the inner bearing ring is conductively coupled to the shaft and the outer bearing ring is conductively coupled to the motor housing.

[0014] According to another aspect of the present disclosure, wherein the shaft includes a cooling channel extending within the shaft along the rotation axis, and wherein the grounding device is mounted within the cooling channel, wherein the outer bearing ring is conductively coupled to the shaft and the inner bearing ring is conductively coupled to the cooling tube.

[0015] According to another aspect of the present disclosure, the grounded bearing includes an inner bearing ring conductively coupled to the shaft via at least one of a Litz wire and a conductive spring and an outer bearing ring conductively coupled to the motor housing.

[0016] According to another aspect of the present disclosure, an apparatus includes an electric motor having a motor shaft rotatably supported in a motor housing by a first main bearing and a second main bearing; a gearbox having a gearbox body and a load shaft, wherein the gearbox is coupled to the electric motor via the motor shaft, the motor shaft is rotatably supported in the gearbox body by a first gearbox bearing, and wherein the load shaft is rotatably supported in the gearbox body by a second gearbox bearing; and a grounding bearing for conducting alternating current between the load shaft and the gearbox body.

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

[0018] According to another aspect of the present disclosure, 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 positioned adjacent to the second gearbox bearing, and wherein the grounding bearing has a smaller rotation radius than the second gearbox bearing.

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

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

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

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

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

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

[0026] According to another aspect of the present disclosure, a vehicle propulsion system includes: a battery for providing direct current; an inverter for converting the direct current into three-phase alternating current; an electric motor for rotating a motor shaft in response to the three-phase alternating current, the electric motor also 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 grounding bearing installed adjacent to the second main bearing for conducting bearing current between the motor shaft and the motor housing, and a gear box mechanically connected to the motor shaft.

[0027] According to another aspect of the present disclosure, a brush or grounding bearing is mounted adjacent each main bearing to bypass circulating bearing currents and to ground the shaft voltage to eliminate EDM bearing currents.

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

[0029] Exemplary embodiments will be described hereinafter in conjunction with the following drawings, wherein like numerals represent like elements, and wherein:

[0030] Figure 1 shows an exemplary three-phase AC induction motor including associated motor shaft grounding in accordance with various embodiments;

[0031] Figure 2 shows an exemplary configuration for mitigating electric motor bearing currents using gearbox and load shaft grounding in accordance with various embodiments;

[0032] Figure 3a An exemplary configuration for mitigating electric motor bearing currents using a preloaded grounded bearing is shown in accordance with various embodiments;

[0033] Figure 3b shows a cross section of a preloaded grounded bearing for mitigating electric motor bearing currents according to various embodiments;

[0034] Figure 4 An exemplary configuration for mitigating electric motor bearing currents using a thrust bearing or a thrust brush as a ground bearing is shown in accordance with various embodiments;

[0035] Figure 5 shows an exemplary configuration for mitigating electric motor bearing currents using a grounded bearing, Litz wire, and a grounding spring in accordance with various embodiments; and

[0036] Figure 6 An exemplary configuration for mitigating electric motor bearing currents using a cooling tube grounded bearing configuration is shown in accordance with various embodiments. DETAILED DESCRIPTION

[0037] The following detailed description is merely exemplary in nature and is not intended to limit application and use. In addition, there is no intention to be bound by any express or implied theory presented in the aforementioned technical field, background technology, summary of the invention 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), electronic circuits, processors (shared, dedicated or groups) and memories that execute one or more software or firmware programs, combinational logic circuits, and / or other suitable components that provide the described functions.

[0038] Bearing currents can be generated by the magnetic field generated during normal operation of an electric motor and can be coupled to the motor shaft via capacitive or inductive coupling. The motor bearings supporting the motor shaft can then act as capacitors, and the voltage on the motor shaft can induce currents 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 the two conductors. This electric field then induces a current in the other conductor. The amount of current induced is proportional to the strength of the electric field and the capacitance between the two conductors. There are many factors that 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.

[0039] Inductive coupling is the transfer of energy between two or more conductors via 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 one conductor. This field then induces a voltage in another conductor that is within the field. The amount of voltage induced is proportional to the strength of the magnetic field and the distance between the two conductors. There are many factors that affect the efficiency of inductive coupling, including the size and shape of the conductors, the distance between the conductors, and the magnetic permeability of the material between the conductors.

[0040] Now go to Figure 1 , shows an exemplary three-phase AC induction motor 100 including 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 grounding bearing 130, an inverter 140, a three-phase power cable 145, a direct current (DC) power source 150, and a DC power cable 155.

[0041] The three-phase AC induction motor 100 is an electric motor that uses three-phase alternating current (AC) to power it. Three-phase AC induction motors are used in various applications, including electric vehicles, aircraft, ships, pumps, wind turbines, fans, and conveyors. The three-phase AC induction motor 100 has two main components: a stator 103 and a rotor 105. The stator 103 is the fixed component of the motor and is mechanically coupled to the motor housing 101. The rotor 105 is the rotating component of the motor 100 and is mechanically coupled to the motor shaft 110. In some exemplary embodiments, the stator 103 has three coils arranged in a triangular shape. The coils are connected to the three-phase AC provided by three discrete conductors of the three-phase power cable 145. When the three-phase AC is applied to the coils, it generates a rotating magnetic field.

[0042] The rotor 105 may include a series of metal bars connected by short-circuit rings. The rotating magnetic field from the stator 103 induces a current in the rotor bars. This current generates a magnetic field in the rotor 105 that is opposite to 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 AC and the number of poles in the stator 103.

[0043] DC power is provided to the exemplary electric motor 100 by a DC power source 150. In some exemplary embodiments, the DC power source 150 may be a lithium-ion battery pack of an electric vehicle, delivering a DC voltage to the inverter 140 via a DC power cable 155. The inverter 140 is operable to convert the DC voltage 155 to three-phase AC.

[0044] The three-phase AC inverter 140 is a power electronic device that converts a DC input voltage into a three-phase AC output voltage. It is used in various applications, including motor drives, power supplies, and welding equipment. The basic principle of the operation of the three-phase AC inverter 140 is to use a series of switches to connect and disconnect the DC input voltage in a controlled manner. The switches can be arranged in a three-phase bridge configuration, where each switch is connected to one of the three phases of the AC output voltage. The switching of the switches is usually controlled by a microcontroller or other digital controller. The switches in the inverter are usually semiconductor devices, such as transistors or thyristors. The controller uses a feedback signal from the AC output voltage to determine when to connect and disconnect each switch. This ensures that the AC output voltage has a desired frequency, amplitude, and phase angle. In some exemplary embodiments, the inverter 140 can be an H-bridge inverter or a cascade inverter.

[0045] A problem with the three-phase AC inverter 140 is that the switching of high DC voltages may cause common mode voltage transients to be introduced on all lines of the three-phase power cable. Such common mode voltage transients may have higher frequencies, high voltages, and high switching transition rates. Such common mode voltage transients may generate common mode currents in the motor, which may couple to the motor shaft 110 between the stator 130 and the rotor 105, and discharge through the motor bearings to the motor housing 101, and ultimately to ground. Such discharges on the motor bearings may cause bearing overheating and premature wear.

[0046] To address this problem, a motor bearing configuration is used that employs conductive devices, i.e., bearings, brushes, grounding springs, sliding clips, rotating devices filled with conductive fluid, and at least one grounding bearing 130 in addition to the main bearing 120 to achieve better performance and reliability at lower cost and loss. The grounding bearing 130 may be positioned next to the main bearing 120 to better mitigate bearing currents, thereby protecting the motor bearings, gears, and gear bearings. The grounding bearing 130 may have a lower load rating than the main bearing 120 and may be mounted on the non-drive end or drive end side of the motor shaft 110, either end of the motor shaft 110 or the gear shaft, thereby reducing the speed of the conductive bearing to reduce its electrical impedance in or on the shaft, or directly on the cooling pipe to reduce cost and improve cooling. The proposed configuration minimizes the overall bearing electrical impedance by mounting the conductive bearing on the outer surface of the shaft, using a conductive lubricant and a less viscous lubricant and / or bearing preload to reduce the oil film thickness to form a shorter current path, and increasing the contact area in the bearing to reduce the electrical impedance, thereby taking advantage of the skin effect to better mitigate EDM bearing currents and circulating bearing currents. The skin effect is the tendency of alternating current to flow primarily near the outer surface of an electrical conductor (such as a metal wire or shaft). This effect becomes increasingly pronounced as the frequency increases.

[0047] Various configurations of the main bearing 120 and the ground bearing 130 and electric motor configurations are disclosed such that the ground bearing electrical impedance is lower than the main bearing electrical impedance. This impedance difference can be achieved by using a less conductive or less viscous oil, adding preload, reducing the oil film thickness, and increasing the contact area. A smaller AC choke can be added to the device to filter the bearing currents directly, more effectively than using a choke on the motor cable to filter the larger common mode currents flowing. Carbon rods and brushes can be effectively grounded, but may wear relatively quickly. The new device provides a more reliable ground with lower electrical impedance (Ze) and friction to better mitigate EDM bearing currents and bypass circulating bearing currents.

[0048] The ground bearing 130 can be configured to have fewer rolling elements than the main bearing 120, thereby having lower resistance at the points of the rolling elements. More rolling elements result in a more even distribution of forces on the bearing and a more even distribution of lubricant, thereby more effectively evening out the resistance on the bearing. Fewer rolling elements result in an uneven distribution of forces and an uneven distribution of lubricant, thereby causing at least one path to have a lower resistance than the other paths. This path of least resistance will be the preferred path for any discharge, thereby increasing wear and / or damage to the ground bearing 130 and reducing any damage to the motor bearings.

[0049] The ground bearing 130 may use a conductive lubricant or a 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 oil film thickness and thereby reduce the impedance of the ground bearing lubricant. In addition, the ground bearing may employ rolling elements having a smaller radius of rotation than the rolling elements employed by the main bearing, or by making the bearing rings smaller within the ground bearing. A less viscous lubricant may be used for the ground bearing 130 to reduce the distance between the rolling elements and the bearing rings, thereby reducing the impedance of the ground bearing 130. In some exemplary embodiments, a scraper tip / ring / spring may be added to the tip or in front of the bearing rolling element to minimize the oil thickness and thereby reduce the preload to optimize wear and conductivity. The bearing element contact area may be increased to reduce impedance by increasing the number of rolling elements or using bearings with increased contact area, such as roller bearings, needle bearings, or thrust bearings. Reducing the bearing radius of the ground bearing 130 compared to the main bearing 120 can reduce the rotation speed of the ground bearing compared to the rotation speed of the main bearing 120, thereby allowing for a less viscous or thinner lubricant and / or fewer rolling elements, thereby reducing the impedance of the ground bearing 130. In addition, as opposed to adding an AC choke to the three-phase power cable 145 to filter the bearing current, an AC choke can be added to the ground bearing 130 because the bearing current at the ground bearing 130 can be up to 20 times smaller than the total mode current at the three-phase power cable 145.

[0050] Now go to Figure 2 , shows an exemplary configuration 200 for mitigating electric motor bearing currents using gearbox and load shaft grounding according to an exemplary embodiment. The exemplary configuration 200 includes a three-phase AC induction electric motor 210 having a motor housing 205, a main bearing 245, a motor shaft 240, a gearbox 250, a grounded bearing 255, a load 260, and a load shaft 265.

[0051] In some exemplary configurations, the load shaft 265 can rotate at a slower rotational speed than the main motor shaft 240. The reduction in rotational speed can be caused by the gear ratio of the gears within the gear housing 250. The reduction in rotational speed can result in reduced bearing wear, especially when a non-wearing lubricant is used in the bearings. In some exemplary embodiments, the grounding bearing 255 can be positioned on the load shaft 265 at a slower bearing rotational speed to reduce its oil film thickness, thereby providing a smaller Ze grounding path for the motor shaft current. The load shaft 265 is operable to drive a load 260, such as a wheel, a differential, a transmission, a transfer case, or any other load.

[0052] In some exemplary embodiments, current may be induced on the motor shaft 240 due to inductive mode or common mode coupling of electrical noise / leakage current generated during operation of the three-phase AC induction motor 210. To avoid damage to the main bearing 245 of the three-phase AC induction motor 210 due to the bearing current, the exemplary configuration 200 may be arranged such that the induced leakage current is coupled from the motor shaft 240 to the load shaft 265 via the conductive gears within the gear housing 250. The motor-side main bearing 252 is positioned on the motor side of the gear housing 250 to support the motor shaft 240. The ground bearing 255 is positioned with the load-side main bearing 254 to support the load shaft 265 on the load side of the gear housing 250. Since the rotational speed of the load shaft 265 is lower than the rotational speed of the motor shaft 240, the wear on the ground bearing 255 is reduced. This reduction in excessive wear and heat may allow the use of thinner lubricants, a smaller number of rolling elements, and / or preloading of the ground bearing 255 to reduce the ground bearing impedance. Additionally, the radius of 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 of the junction, thereby reducing wear and reducing heat on the ground bearing 255. In some exemplary embodiments, an RF choke 252 or other RF impedance may be placed in series with the ground point 250 of the motor housing 205 such that the RF impedance to the ground point 250 via the gear housing 250 is lower than the RF impedance between the motor housing 205 and the ground point 250. This may increase the effectiveness of the ground bearing 255, which may be electrically coupled to the gear housing 250, and reduce damage to the main bearing 245 of the three-phase AC induction motor 210.

[0053] Now go to Figure 3a, shows an exemplary configuration for mitigating electric motor bearing currents using a preloaded grounded bearing according to an exemplary embodiment. Bearing preloading refers to the process of applying a force to the bearing that is not caused by an external load. The force can be applied axially along the axis of the shaft, or it can also be applied radially perpendicular to the axis of the shaft. The bearing can be preloaded using a spring preload, where the spring is placed between the bearing and the housing or shaft or a solid preload using a shim or other solid device to force the bearing together.

[0054] In some exemplary embodiments, the electric motor 301 is configured with a main motor shaft bearing 302 and a preloaded ground bearing 305. The preload of the ground bearing 305 can maintain a thin and optimal oil film to compensate for changes in bearing spacing due to load, shock, vibration, or displacement. In some exemplary embodiments, the preload of the ground bearing 305 is achieved using a spring preload 310 to apply the bearing preload in the opposite direction of the load / groove area of ​​the main bearing 302 to compensate for changes in oil film thickness due to load 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 the discharge of bearing currents on the ground shaft 305. Figure 3b A cross section of a preloaded ground bearing 320 is shown having a bearing housing 322, a bearing shaft 330, and a spring preload 324. The spring preload 324 is configured to maintain a positive pressure on the rolling element 325 through the bearing housing 322 and the bearing shaft 330.

[0055] Now go to Figure 4 , shows an exemplary configuration for mitigating electric motor bearing currents using a thrust bearing or thrust brush grounding configuration according to an exemplary embodiment. The thrust bearing 420 is a type of bearing that supports an axial load or a load parallel to an axis such as the motor shaft 430 in the electric motor 401. The thrust bearing 420 may include a bearing shaft 421 mechanically fixed to the motor shaft 430 and rotating with the motor shaft 430, rolling elements within a cage 423, and a housing mechanically fixed to the electric motor housing 404. The thrust bearing 420 may be preloaded or unpreloaded, lubricated or unlubricated. Since the thrust bearing 420 does not provide support for the motor shaft 430, the thrust bearing 420 may be subject to less mechanical stress, and therefore a thinner or less viscous lubricant, or no lubricant, may be used to improve the thrust bearing 420's conductivity to the bearing currents between the motor shaft 430 and the motor housing 404. The thrust bearing 420 may be mounted on the outside of the motor housing 404 or inside the motor housing 404. In some exemplary embodiments, the thrust bearing 420 can be mounted on the load end of the motor shaft 430 or on the non-load end of the motor shaft 430. The thrust bearing is configured as a low impedance electrical ground path between the motor shaft 430 and the motor housing 404.

[0056] Now go to Figure 5 , shows an exemplary configuration for mitigating electric motor bearing currents using a grounding bearing 530 according to an exemplary embodiment. The exemplary configuration uses a grounding bearing 530, a Litz wire 550, a grounding hook 560, a main bearing 540, a motor shaft 520, and an electric motor 501 having a motor housing 502. In some exemplary embodiments, the grounding hook 560 can be a grounding spring, such as a copper spring finger, or the like.

[0057] The Litz wire 550 may be a multi-stranded wire or cable for carrying AC current at radio frequencies. The Litz wire 550 is configured to reduce RF impedance caused by skin effect and proximity effect losses in electrical conductors. In addition, one or more ground hooks 560 may be used to compensate for displacement / load changes between the motor shaft 520 and the motor housing 502 to ensure low impedance. The ground hook 560 may include one or more semi-rigid conductive members, such as beryllium copper fingers, metal brushes, etc., to maintain a positive pressure between the ground bearing 530 and the motor housing 502. In some exemplary embodiments, the ground hook 560 may be one or more metal brushes with a wear-resistant tip to electrically couple the ground bearing 530 to the motor housing 502 with or without the ground bearing 530.

[0058] Now go to Figure 6 , shows an exemplary configuration for mitigating electric motor bearing currents using 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 ground point 640, a cooling tube grounding bearing 650, and a motor shaft 610.

[0059] Cooling tubes are used in electric motors to dissipate heat generated by the rotor, windings, and bearings of the electric motor. These tubes are typically made of copper or aluminum, and they can be filled with a coolant, such as water, oil, or compressed gas, such as air. The coolant is circulated in the pipes by a pump, and it absorbs heat from the motor. Then, in the case of a liquid coolant, the heated coolant can be cooled by a radiator or heat exchanger, and it circulates back through the electric motor 601. One way to cool the electric motor rotor and the motor shaft 610 is to provide a hollow motor shaft 610 and flow the coolant through the hollow portion of the motor shaft 610. The cooling tube 630 must then have a smaller diameter than the hollow portion of the motor shaft 610. In order to provide a grounding path for unwanted bearing currents, a cooling tube grounding bearing 650 can be added between the outer surface of the cooling tube 630 and the inner surface of the hollow portion of the motor shaft 610. The cooling tube grounding bearing 650 can have a small radius and therefore have a lower angular rotation speed than the outer portion of the motor shaft 610. In addition, the cooling tube ground bearing 650 will carry little or no electric motor load and will therefore experience less load wear. The cooling tube ground bearing 650 can use a conductive lubricant and conductive rolling elements to create a low impedance electrical connection between the interior of the motor shaft 610 and the cooling tube 630. In some exemplary embodiments, the cooling tube can be electrically coupled to a ground point 640 for coupling current to a vehicle chassis or other ground point. In addition, an electrical choke 645 can be secured to a portion of the cooling tube 630 after the ground point 640 to provide increased impedance to any residual current so that the residual current is coupled to the ground point 640.

[0060] Although at least one exemplary embodiment has been proposed in the foregoing detailed description, it should be understood that there are a large number of variations. It should also be understood that the exemplary embodiment or multiple exemplary embodiments are only examples and are not intended to limit the scope, applicability or configuration of the present disclosure in any way. On the contrary, the foregoing detailed description will provide a convenient roadmap for implementing the exemplary embodiment or multiple exemplary embodiments for those skilled in the art. It should be understood that various changes can be made to the functions and arrangements of the elements without departing from the scope of the present disclosure as set forth in the attached claims and their legal equivalents.

Claims

1. An electric motor comprising: Motor 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; as well as A grounded bearing is provided for conducting bearing currents between the shaft and the motor housing.

2. The electric motor according to claim 1, wherein: The ground bearing further comprises a spring preload for applying a force between the outer bearing ring and the inner bearing ring along the axis of rotation.

3. The electric motor according to claim 1, wherein: The ground bearing has fewer rolling elements than the second main bearing.

4. The electric motor according to claim 1, wherein: The ground bearing includes a conductive lubricant and the second main bearing includes a dielectric lubricant.

5. The electric motor according to claim 1, wherein: The ground bearing is at least one of a thrust bearing, a ground spring, and a thrust brush.

6. The electric motor according to claim 1, wherein: The electrical impedance of the second main bearing is greater than the electrical impedance of the ground bearing.

7. The electric motor according to claim 1, wherein: The ground bearing is mounted adjacent to the second main bearing, wherein an inner bearing ring is conductively coupled to the shaft and an outer bearing ring is conductively coupled to the motor housing.

8. The electric motor according to claim 1, wherein: The shaft includes a cooling channel extending within the shaft along the rotation axis, and wherein at least one of the grounding bearing, the grounding brush and the grounding spring is mounted within the cooling channel, wherein an outer bearing ring is conductively coupled to the shaft and an inner bearing ring is conductively coupled to a cooling tube.

9. The electric motor according to claim 1, wherein: The grounded bearing includes an inner bearing ring conductively coupled to the shaft via at least one of a Litz wire and a conductive spring and an outer bearing ring conductively coupled to the motor housing.

10. An apparatus comprising: an electric motor having a motor shaft rotatably supported within a motor housing by first and second main bearings; a gearbox having a gearbox body and a load shaft, wherein the gearbox is coupled to the electric motor via the motor shaft, the motor shaft is rotatably supported within the gearbox body by a first gearbox bearing, and wherein the load shaft is rotatably supported within the gearbox body by a second gearbox bearing; and A grounding bearing is used to conduct alternating current between the load shaft and the gear housing.

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