Patent Publication Number: US-2023155452-A1

Title: Methods and systems for oil cooled rotor laminations

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a divisional of U.S. Non-Provisional Pat. Application No. 16/870,656, entitled “METHODS AND SYSTEMS FOR OIL COOLED ROTOR LAMINATIONS”, and filed on May 8, 2020. The entire contents of the above-listed application are hereby incorporated by reference for all purposes. 
    
    
     FIELD 
     Embodiments of the subject matter disclosed herein relate to cooling electric motors, more specifically to cooling laminations of the rotor of an electric motor. 
     BACKGROUND 
     In order to achieve the desired levels of performance and reliability in an electric vehicle, the vehicle demands the temperature of the electric motor remain within its specified operating range regardless of ambient conditions or how hard the vehicle is being driven. Thus, the electric motor of the vehicle necessarily demands a cooling process. Since an air cooling system is insufficient for a motor with a power output of 15 to 20 kW or more, a water cooling system or oil cooling system is used. The oil and/or water cooling systems may be employed so that degradation of coil coatings and irreversible demagnetization of a permanent magnet are prevented to thereby increase the output range of the motor. Thus, cooling type and/or cooling efficiency are important factors in the configuration of electric motors. 
     BRIEF DESCRIPTION 
     In one embodiment, a method comprises a system for cooling an electric motor that includes a rotor shaft rotatably mounted inside a motor housing, a lamination stack integrally connected to the rotor shaft, an encoder-end balance plate integrally connected to a first end of the lamination stack and a first end of the rotor shaft, an output-end balance plate integrally connected to a second end of the lamination stack and a second end of the rotor shaft, a coolant supply (e.g., oil, water, glycol solution, etc.) coupled to the output-end balance plate and the rotor shaft. A closed-looped coolant pathway is formed between the transmission, the rotor shaft, the encoder-end balance plate, the lamination stack, and the output-end balance plate. In some examples, cooling may also be used with a standalone motor, in which the system may connect with a coolant jacket of the motor (e.g., a closed-looped coolant pathway may be formed within the coolant jacket). 
     It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below: 
         FIG.  1    schematically shows a vehicle with a hybrid propulsion system; 
         FIG.  2 A  is a cross-sectional view of a non-limiting example of a cooling system of an electric motor in accordance with one or more embodiments of the present disclosure; 
         FIG.  2 B  is a second cross-sectional view of a rotor shaft of the electric motor of  FIG.  2 A ; 
         FIG.  3 A  is a side perspective view of an encoder-end balance plate of the cooling system; 
         FIG.  3 B  is a cross-sectional view of the encoder-end balance plate of  FIG.  3 A ; 
         FIG.  4 A  is a side perspective view of an output-end balance plate of the cooling system; 
         FIG.  4 B  is a cross-sectional view of the output-end balance plate of  FIG.  4 A ; and 
         FIG.  5    is a method for cooling laminations of a rotor of an electric motor according to the embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Electric motors generate heat as a result of electrical, magnetic, and mechanical losses, with excessive heat production often leading to motor performance issues (e.g., decreased torque production, control, efficiency) and enhanced component degradation. In electric motors used in vehicles, losses tend to be high during motor starting or dynamic braking events. Thus, thermal management for electric motors is important as the automotive industry continues to transition to more electrically dominant vehicle propulsion systems. In vehicles, the sizing of the motor depends directly on the how the motor is cooled. As such, with the push to reduce component size, lower costs, and reduce weight without sacrificing performance or reliability, the challenges associated with thermal management for electric motors increase. For example, the motor’s ability to increase running time at higher power levels within electrical operating limits is directly related to the ability to remove heat from certain components. As thermal management improves, there will be a direct tradeoff among motor performance, efficiency, cost, and the sizing of electric motors to operate within the thermal constraints. 
     The heat generated by the electric motor is distributed throughout multiple components within the electric motor. For example, heat may be generated within the stator slot-windings, stator end-windings, stator laminations, rotor laminations, and rotor magnets or conductors. The distribution of the generated heat within the components is dependent on the motor type and the operating condition (torque/speed) of the motor. Thus, the selected cooling approach for the motor impacts the path of heat flow through the motor as well as the temperature distribution amongst the motor components. One current cooling approach involves passive air cooling in which heat generated by the motor may be conducted away from hotter components of the motor to a coupled heat sink (e.g., a mounting surface) and/or fins. Heat may then be transferred from the heat sink and/or fins to the air via convection. However, air cooling systems have been found to be insufficient if the motor has a power output of 15 to 20 kW or more. As such, the additional weight of the fins tends to outweigh the cooling benefits the fins provide. Further, the cooling capacity of air cooling systems may be affected by environmental temperatures. For example, the effectiveness of air cooling systems may be dramatically decreased in hotter climates. 
     Another current approach involves phase change material based cooling systems in which a phase change material absorbs heat energy from the motor by changing from a solid to a liquid or from a liquid to a gas. While changing phase, the material can absorb large amounts of heat with little change in temperature. Thus, phase change material cooling systems can meet the cooling requirements of the motor, however the volume change that occurs during a phase change restricts its application. Further, the phase change material can only absorb heat generated, not transfer the heat away, thus the phase change material may not reduce overall temperature within the vehicle propulsion system (e.g., the phase change material may only smooth the temperature distribution). 
     Other current cooling approaches include cooling via a liquid coolant. Liquid coolants have a higher heat conductivity and heat capacity (e.g., ability to store heat in the form of energy in its bonds) than air, and therefore perform more effectively by comparison. Further, liquid coolants offer the advantage of a compact structure as compared to phase change materials. One current liquid cooling system involves squirting or injecting an oil coolant into through-holes within the rotor shaft. Another system involves pumping the oil coolant into a first end of the rotor shaft where the coolant may be spread via centrifugal force as the rotor shaft rotates. However, these systems may lead to non-uniform or unbalanced cooling of the motor. While the cooling capacity of oil coolant may be sufficient to cool the rotor shaft, the flow properties of the oil as well as the contact resistance between the oil and the inner surface of the rotor shaft may limit the effectiveness of these forms of liquid cooling. 
     As such, there is a demand for an electric motor cooling system that provides uniform and effective temperature control without substantially increasing vehicle weight and manufacturing costs. Thus, according to the embodiments disclosed herein, methods and systems are provided for a system that provides uniform cooling to an electric motor. More specifically, the system cools laminations of a rotor of the electric motor using a dielectric fluid, such as oil, passed through a hollow rotor shaft as a coolant. Balance plates located on the ends of the rotor shaft may contain and direct oil (e.g., coolant) from the rotor shaft to channels within the lamination channels in a continuous cooling loop. Oil may be supplied to and absorb heat within an axial center of the rotor shaft as the oil is directed toward a first balance plate. The first balance plate may contain the oil as it exits the rotor shaft and direct the oil to the channels within the laminations using centrifugal force (e.g., from rotation of the rotor shaft). Heat may then be transferred to the oil as the oils passes over the surface area of the laminations and towards a second balance plate. The second balance plate may then direct the oil to a transmission which pumps the oil back into the rotor shaft thereby creating a continuous loop of uniform cooling within the electric motor. Additionally or alternatively, the system may include a coolant pump to contribute to/maintain oil flow through the formed cooling circuit (e.g., the system may be self-pumping and/or include the coolant pump). By bringing the oil/coolant into direct contact with the laminations, the thermal pathway between the rotor laminations and the cooling fluid is greatly reduced thereby aiding cooling performance. 
       FIG.  1    is an example of a hybrid propulsion system for a vehicle that includes an electric motor.  FIGS.  2 A and  2 B  illustrate cross-sectional views of a non-limiting example of a cooling system, according to the present disclosure, that may be used to cool the electric motor of the propulsion system of  FIG.  1   .  FIGS.  3 A- 4 B  show different views of balance plates that may be employed in the cooling system of  FIGS.  2 A and  2 B .  FIG.  5    is a method for cooling laminations of a rotor of an electric motor according to the embodiments disclosed herein. A set of reference axes  201  are provided for comparison between views shown, indicating a y-axis, a z-axis, and an x-axis. In some examples, the y-axis may be parallel with a direction of gravity, with the x-axis defining the horizontal plane. 
       FIG.  1    illustrates an example vehicle propulsion system  100 . Vehicle propulsion system  100  includes a fuel burning engine  102  and a motor  110 . As a non-limiting example, engine  102  comprises an internal combustion engine and motor  110  comprises an electric motor. Motor  110  may be configured to utilize or consume a different energy source than engine  102 . For example, engine  102  may consume a liquid fuel (e.g., gasoline) to produce an engine output while motor  110  may consume electrical energy to produce a motor output. As such, a vehicle with propulsion system  100  may be referred to as a hybrid electric vehicle (HEV). 
     The engine  102  and motor  110  may be coupled to a transmission  104 . The transmission  104  may be a manual transmission, automatic transmission, or combinations thereof. Further, various additional components may be included, such as a torque converter, and/or other gears such as a final drive unit, etc. Transmission  104  is shown coupled to a drive wheel  106  which is in contact with a road surface  108 . Thus, the electric motor  110  may be drivingly coupled to the engine  102  and the drive wheel  106  via transmission  104 . The depicted connections between the engine  102 , motor  110 , transmission  104 , and drive wheel  106  indicate transmission of mechanical energy from one component to another, whereas the connections between the motor  110  and the energy storage device  114  may indicate the transmission of electrical energy forms. 
     Vehicle propulsion system  100  may utilize a variety of different operational modes depending on operating conditions encountered by the vehicle propulsion system  100 . Some of these modes may enable engine  102  to be maintained in an off state (e.g., set to a deactivated state) where combustion of fuel at the engine is discontinued. For example, under select operating conditions, motor  110  may propel the vehicle via the drive wheel  106  while engine  102  is deactivated. During other operating conditions, engine  102  may be set to a deactivated state (as described above) while motor  110  may be operated to charge an energy storage device  114  (e.g., a battery, capacitor, flywheel, pressure vessel, so on). For example, motor  110  may receive wheel torque from the drive wheel  106  where the motor  110  may convert the kinetic energy of the vehicle to electrical energy for storage at the energy storage device  114 . This operation may be referred to as regenerative braking of the vehicle. Thus, motor  110  can provide a generator function in some embodiments. However, in other embodiments, a generator  120  may instead receive wheel torque from the drive wheel  106 , where the generator  120  may convert the kinetic energy of the vehicle to electrical energy for storage at the energy storage device  114 . 
     During still other operating conditions, engine  102  may be operated by combusting fuel (e.g., gasoline, diesel, alcohol fuels, fuel blends) received from a fuel system. For example, engine  102  may be operated to propel the vehicle via the drive wheel  106  while motor  110  is deactivated. During other operating conditions, both engine  102  and motor  110  may each be operated to propel the vehicle via the drive wheel  106 . A configuration where both the engine  102  and the motor  110  may selectively propel the vehicle may be referred to as a parallel type vehicle propulsion system. Note that in some embodiments, motor  110  may propel the vehicle via a first set of drive wheels and engine  102  may propel the vehicle via a second set of drive wheels. 
     In other embodiments, the vehicle propulsion system  100  may be configured as a series type vehicle propulsion system, whereby the engine  102  does not directly propel the drive wheel  106 . Rather, engine  102  may be operated to power the motor  110 , which may in turn propel the vehicle via drive wheel  106 . For example, during select operating conditions, the engine  102  may drive the generator  120  which may, in turn, supply electrical energy to one or more of the motor  110  or energy storage device  114 . As another example, the engine  102  may be operated to drive motor  110  which may, in turn, provide a generator function to convert the engine output to electrical energy, where the electrical energy may be stored at the energy storage device  114  for later use by the motor  110 . 
     In some embodiments, the energy storage device  114  may be configured to store electrical energy that may be supplied to other electrical loads residing on-board the vehicle (other than the motor), including cabin heating and air conditioning, engine starting, headlights, cabin audio and video systems, etc. As a non-limiting example, energy storage device  114  may include one or more batteries and/or capacitors. 
     A control system  122  may communicate with one or more of the engine  102 , motor  110 , energy storage device  114 , generator  160  and/or additional components of the vehicle propulsion system  100 . For example, the control system  122  may receive sensory feedback information from one or more of the engine  102 , motor  110 , energy storage device  114 , and generator  120 . Further, control system  122  may send control signals to one or more of the engine  102 , motor  110 , energy storage device  114 , and generator  160  responsive to this sensory feedback. Control system  122  may receive an indication of an operator requested output of the vehicle propulsion system from a vehicle operator (e.g., via a pedal position sensor communicatively coupled to an acceleration and/or brake pedal). 
     Energy storage device  114  may periodically receive electrical energy from an external energy source  116  residing external to the vehicle (e.g., not part of the vehicle). As a non-limiting example, the vehicle propulsion system  100  may be configured as a plug-in hybrid electric vehicle (PHEV), whereby electrical energy may be supplied to the energy storage device  114  from the external energy source  116  via an electrical energy transmission cable. While the vehicle propulsion system  100  is operated to propel the vehicle, the external energy source  116  may be disconnected from the energy storage device  114 . The control system  122  may identify and/or control the amount of electrical energy stored at the energy storage device, which may be referred to as the state of charge (SOC). In other embodiments, electrical energy may be received wirelessly at the energy storage device  114  from the external energy source  116 . For example, energy storage device  114  may receive electrical energy from the external energy source  116  via one or more of electromagnetic induction, radio waves, and electromagnetic resonance. As such, it should be appreciated that any suitable approach may be used for recharging the energy storage device  114  from a power source that does not comprise part of the vehicle. In this way, the motor  110  may propel the vehicle by utilizing an energy source other than the fuel utilized by the engine  102 . 
     Thus, it should be understood that the exemplary vehicle propulsion system  100  is capable of various modes of operation. In a full hybrid implementation, for example, the vehicle propulsion system  100  may operate using the motor  110  as the only torque source propelling the vehicle. This “electric only” mode of operation may be employed during braking, low speeds, while stopped at traffic lights, etc. Further, the motor  110  includes a cooling system  112  as heat may be generated as a chief by-product when the motor  110  is in use. While a hybrid implementation of the cooling system  112  is shown, the cooling system  112  for the motor  110  could also be provided in a fully electric vehicle or another suitable electric motor. In some examples, the cooling system  112  may be an air cooling system, a phase change material based cooling system, or a liquid cooling system. In some examples, the cooling system  112  may conduct heat generated by the motor  110  away from hotter components of the motor  110  (e.g., a rotor, rotor laminations) to a coupled heat sink (e.g., a mounting surface) and/or fins. In some examples, the cooling system  112  may include a phase change material (e.g., salt hydrates, metals, alloys, polyalcohols, eutectics, paraffins). The phase change material may absorb heat energy from the motor  110  by changing state from solid to liquid. In some examples, a liquid coolant (e.g., water, an oil coolant) may be squirted onto or passed through the hotter surfaces and/or components of the motor  110 . For example, the motor  110  may include a rotor that is fixed to a hollow rotating rotor shaft and a stator or lamination stack. Liquid coolant may be squirted into a through-hole or multiple through-holes within the rotor shaft to induce cooling within the motor  110  via heat transfer from the rotor shaft to the liquid coolant. In another example, a supply line may pump cooling oil into a first end of the rotating rotor shaft and the cooling oil is moved through the rotor shaft by centrifugal force thereby cooling the motor  110 . 
     However, as previously described, air cooling systems are insufficient for cooling the motor  110  if the motor has a power output of 15 to 20 kW or more. Further, the additional weight of the fins and/or heat sinks tends to outweigh the cooling benefits they provide, particularly in hot climates. Similarly, the volume change that occurs during a phase change restricts its application in vehicles. Thus, liquid cooling systems are commonly employed in vehicles as they overcome the limitations of air and phase change material based cooling systems. However, current liquid cooling systems also have shortcomings. For instance, in the liquid cooling examples described above, cooling within the motor  110  may be unbalanced. For example, by supplying cooling oil only to the first end of the rotating rotor shaft, a second end of the rotating rotor shaft may fall outside of a desired temperature range. Similarly, squirting the liquid coolant into through-hole(s) within the rotating rotor shaft may also lead to non-uniform cooling of the electric motor  110  depending on the location of the through-hole(s), contact resistance, and the centrifugal force acting on the liquid coolant. For example, if the through-hole is inward toward a middle point of the rotating rotor shaft, at least a portion of the rotating rotor shaft may not be cooled via direct contact with the liquid coolant. Unbalanced cooling of the electric motor may lead to electrical overload, low resistance, and/or the enhanced degradation of components of the electric motor  110  (e.g., rotating rotor shaft, coil coatings). Thus, there is a demand for uniform cooling of a high power density electric motor as provided by the cooling system presented herein. 
       FIG.  2 A  is a cross-sectional view  200  of a non-limiting example of a rotor cooling system  202  for an electric motor (e.g., electric motor  110  of  FIG.  1   ). The cooling system  202  may include an oil supply (e.g., a transmission oil reservoir), an oil pump (e.g., a transmission oil pump  244 , as further described below), and a plurality of interconnected passages within the electric motor. The plurality of interconnected passages may form a closed-loop pathway between an output-end balance plate (e.g., output-end balance plate  220 , as further described below), a rotor (e.g., rotor  206 , as further described below), and an encoder-end balance plate (e.g., encoder-end balance plate  224 , as further described below). 
     The cooling system  202  may flow oil from the oil supply through the closed-loop pathway, the closed-loop pathway including an interior passageway of a rotor shaft (e.g., rotor shaft  208 , as further described below), one or more passages and/or recesses of the encoder-end balance plate, one or more weight-reduction channels of a rotor lamination stack (e.g., lamination stack  210 , as further described below), and one or more passages and/or recesses of the output-end balance plate. The oil may be pumped from the oil supply into the interior passageway of the rotor shaft. The oil may then flow from the rotor shaft to the one or more passages and/or recesses of the encoder-end balance plate (via centrifugal force and/or pumping), from the one or more passages and/or recesses of the encoder-end balance plate to one or more weight-reduction channels of the lamination stack, from the one or more weight-reduction channels of the lamination stack to one or more passages and/or recesses of the output-end balance plate, and from the one or more passages and/or recesses of the output-end balance back to the oil supply23. As the oil passes through the closed-loop pathway, heat generated via electric motor use may be transferred to the oil thereby cooling the electric motor. 
     If the cooling system  202  is suitably sized, the system  202  may be self-pumping (e.g., if the outlet of the rotor cooling circuit is radially more outlet to the inlet, centrifugal forces may enforce an oil flow). In some examples, the system  202  may be driven and/or assisted by an external pump. In some examples, the cooling system  202  may include a liquid coolant supply instead of the oil supply, with liquid coolant pumped through the closed-loop pathway to induce cooling of the electric motor (e.g., water glycol solution may be passed through the inner surface of axial holes lined with a dielectric liner). 
     The electric motor may generally include a stator  203 , stator windings  205 , and a rotor  206 , the rotor  206  comprising a hollow rotor shaft  208  and a lamination stack  210 , with the aforementioned components enclosed within a motor housing  212 . The housing  212  may further enclose additional components of the electric motor such as a plurality of magnets, an electromagnetic coil wound around protrusions of a stator, and/or a rotational sensor (not shown for brevity). The rotor shaft  208  may have a columnar shape with a circular cross-section (e.g., along the z-axis, as further shown and described with respect to  FIG.  3   ) and be rotatably supported about its own axis by an output-end bearing  214  and encoder-end bearing  216  provided between both ends thereof and the housing  212 . The rotor shaft  208  may be comprised of a suitable material (e.g., aluminum, SAE 1045 in cold or hot rolled steel, C1045). 
     The lamination stack  210  may be a package of individual electromagnetic plates separated by electrically insulating layers to suppress eddy current losses under magnetic loading. For example, the lamination stack  210  may be comprised of a number of disk shaped steel plates laminated in silicone. The plates comprising the lamination stack  210  may be stacked loosely together, welded together (e.g., plasma welded, laser welded, TIG-resistance welded), or otherwise suitably bonded (e.g., via interlocking, bonding varnish). The lamination stack  210  may be tube shaped and surround a portion of the rotor shaft  208  so that two ends of the rotor shaft  208  (e.g., an output-end  230  and an encoder-end  232 ) are located outside of the lamination stack  210 . 
     As shown in a cross-sectional view  203  of  FIG.  2 B  taken across axis A1 in  FIG.  2 A , the rotor shaft  208  may be inserted through a central opening  260  that runs through the lamination stack  210  along the x-axis. Further, portions of the lamination stack  210  may be removed to reduce the overall weight of the rotor  206 . Thus, the lamination stack  210  may further include a plurality of axial channels  262  that run horizontally (e.g., parallel to the x-axis) through the lamination stack  210  and surround the central opening  260 . The plurality of axial channels  262  may be comprised of a ring of six evenly spaced apart adjacent channels. For example, the plurality of axial channels  262  may include: a first channel  264  adjacent to and spaced away from a second channel  266 ; a third channel  268  adjacent to and spaced away the second channel  266  and a fourth channel  270 ; and so on around the outer perimeter of the central opening  260 . The channels comprising the plurality of axial channels  262  may be tube-shaped. As the plurality of axial channels  262  function as weight-reduction holes, the dimensions, positioning, shape, and/or number of channels comprising the plurality of axial channels  262  may vary. In some examples, the plurality of axial channels  262  may have more or less than six channels. 
     The lamination stack  210  may have the same axial center as the outer peripheral portion of the rotor shaft  208 . The lamination stack  210  may be connected or suitably coupled to the rotor shaft  208  so that both components are integrally rotated within the motor in response to an input of electrical energy to the motor. For example, the plurality of magnets may be arranged within the housing  212  around the outer peripheral portion of the lamination stack  210 . Thus, as current flowing through the electromagnetic coil is appropriately changed (e.g., via output from a coupled energy storage device such as energy storage device  114  of  FIG.  1   ), the magnetic field generated in the protrusions of the stator will change. In turn, the change in the magnetic field of the stator will cause rotation of the lamination stack  210  and rotor shaft  208  (e.g., via the plurality of magnets) which may be output as a mechanical driving force for the vehicle. 
     As shown in  FIG.  2 A , an output-end balance plate  220  may be disposed at a first end  222  of the lamination stack  210  and an encoder-end balance plate  224  may be disposed at a second end  226  (e.g., opposite the first end  222 ) of the lamination stack  210 . The encoder-end balance plate  224  may be substantially disk shaped and include a central aperture surrounded by a recessed region  248 , as further shown and described with respect to  FIGS.  3 A and  3 B . Similarly, the output-end balance plate  220  may include a central aperture surrounded by a recessed region  252 . The output-end balance plate  220  also includes a plurality of inner pathways that connect to the recessed region  252  such as a first pathway as indicated by a series of dashed arrows  254 , as further shown and described with respect to  FIGS.  4 A and  4 B . The balance plates  220 ,  224  may be positioned around the rotor shaft  208  via the central apertures as further described below. Further, the balance plates  220 ,  224  may be fixedly attached to the ends  222 ,  226  of the lamination stack  210  so as to rotate integrally with the lamination stack  210  and the rotor shaft  208 . The balance plates  220 ,  224  may function as weights for adjusting the dynamic balance of the rotor  206  when the rotor shaft  208  rotates. Specifically, a dynamic balance may be obtained by forming holes at appropriate positions on the outer peripheral surfaces of the balance plates  220 ,  224  to prevent vibration when the rotor shaft  208  rotates (as further shown and described with respect to  FIGS.  4 A and  4 B ). 
     Further, the balance plates  220 ,  224  may secure the lamination stack  210  within the rotor  206  via a clamping force applied by a locking nut  228 . The locking nut  228  may be threaded onto the output-end  230  of the rotor shaft  208  and continually tightened until it comes into face-sharing contact with a first end  234  of the output-end balance plate  220 . As the locking nut  228  is tightened, the lamination stack  210  may be clamped between the output-end balance plate  220  and the encoder-end balance plate  224  within the rotor  206 . A second end  236  (e.g., opposite the first end  234 ) of the output-end balance plate  220  may exert lateral (e.g., parallel to x-axis) force against the first end  222  of the lamination stack  210  via tightening of the locking nut  228 . The lateral force may result in the second end  226  of the lamination stack  210  being pressed against a back face  238  of the encoder-end balance plate  224 . In turn, the pressing force of the lamination stack  210  against the back face  238  may cause a front face  240  (e.g., opposite the back face  238 ) of the encoder-end balance plate  224  to mate with a shoulder  242  of the rotor shaft  208 . Thus, the encoder-end balance plate  224  may be held in a fixed position via mating of the front face  240  and the shoulder  242  so that lateral force applied by tightening of the locking nut  228  results in the lamination stack  210  being clamped between the balance plates  220 ,  224 . 
       FIGS.  3 A and  3 B  show a side perspective view  300  and a cross-sectional view  302 , respectively, of the encoder-end balance plate  224 . The encoder-end balance plate  224  may substantially disk shaped and include a central aperture  308  lined by a first groove  306  that houses an O-ring (not shown) or another suitable sealing device. The central aperture  308  may be of a suitable size and dimensions (e.g., circular with a diameter just larger than the outer diameter of the rotor shaft  208 ) so that the rotor shaft  208  may inserted into the central aperture  308  and secured to the encoder-end balance plate  224  via face-sharing contact with the first groove  306  (e.g., the rotor shaft  208  may be press-fit into the central aperture  308  of the encoder-end balance plate  224 ), with the O-ring within the first groove providing a seal between the encoder-end balance plate  224  and the rotor shaft  208 . The central aperture  308  may be located within the recessed region  248  of the encoder-end balance plate  224  and surrounded by a plurality of supports  312 . The plurality of supports  312  may include a series of equally spaced apart substantially square supports that protrude from the back face  238  of the encoder-end balance plate  224  within the recessed region  248 . 
     The plurality of supports  312  may include a first support  314  adjacent to and spaced apart from a second support  316 , a third support  318  adjacent to and spaced apart from the second support  316 , and so on around the outer diameter of the central aperture  308  (e.g., the plurality of supports  312  may surround the central aperture  308 ), with all of the supports being the same dimensions. The plurality of supports  312  may not extend beyond or protrude from the recessed region  248  of the encoder-end balance plate  224  (e.g., the plurality of supports  312  may not extend beyond the planar surface of the back face  238 ). The recessed region  248  may be ring shaped and surrounded by a second groove  310 , with both the recessed region  248  and the second groove  310  concentric to the central aperture  308 . Thus, when the lamination stack  210  is secured between the balance plates  220 ,  224  via tightening of the locking nut  228  as described above, the plurality of supports  312  may interact with the plurality of evenly spaced apart channels of the lamination stack  210 . In this way, the plurality of supports  312  may ensure that the clamping force applied via the locking nut  228  is evenly distributed across the back face  238  of the encoder-end balance plate  224  and the second end  226  of the lamination stack  210 . Further, similar to the first groove  306 , the second groove  310  may house an O-ring that may be used to form a seal between the encoder-end balance plate  224  and the laminated stack  210  when the two components are securely connected. 
     As shown in  FIG.  3 B , the front face  240  of the encoder-end balance plate  224  includes a first flat (e.g., parallel with the y-axis) portion  320  that surrounds and includes the central aperture  308 . The first flat portion  320  may mate with the shoulder  242  of the rotor shaft  208  as previously described. An angled edge  322  may connect the first flat portion  320  to a second flat portion  324  of the front face  240  of the encoder-end balance plate  224 . In addition to holes formed within the balance plates  220 ,  224 , the material comprising the angled edge  322  and the second flat portion  324  of the encoder-end balance plate  224  may be removed as desired to adjust the dynamic balance of the rotor  206 . For example, the angled edge  322  may be made straight (e.g., parallel to the x-axis) and/or the thickness (e.g., with respect to the x-axis) of the second flat portion  324  may be decreased via material removal. Material may also be removed from the outermost section of the second flat portion  324  by drilling or another suitable technique to dynamically balance the rotor  206 . 
       FIGS.  4 A and  4 B  show a side perspective view  400  and a cross-sectional view  402 , respectively, of the output-end balance plate  220 . The output-end balance plate  220  may be T-shaped and include a disk shaped top portion  404  connected to a tubular shaft  406 . A central aperture  408  may run through the length (e.g., parallel to the x-axis) of the top portion  404  and the tubular shaft  406 . The central aperture  408  may be of a suitable size and dimensions (e.g., circular with a diameter just larger than the outer diameter of the rotor shaft  208 ) so that the rotor shaft  208  may inserted into the central aperture  408 . As previously described, the rotor shaft  208  may be supported within the rotor  206  by the bearings  214 ,  216 . The output-end bearing  214  may be supported by the output-end balance plate  220  at the output-end  230  of the rotor shaft  208 . For example, the output-end bearing  214  may surround a portion of the tubular shaft  406  adjacent to the top portion  404  of the output-end balance plate  220 , with the output-end  230  of the rotor shaft  208  being partially housed within the tubular shaft  406  and the top portion  404 . Thus, the outer diameter of the tubular shaft  406  may be of suitable dimensions where output-end bearing  214  may be slid onto the tubular shaft  406  and the two components functionally coupled. 
     The top portion  404  may include the recessed region  252  that surrounds a ring shaped supporting face  412 , with the central aperture  408  passing through the center opening of the supporting face  412 . The supporting face  412 , the recessed region  252 , and a first groove  422  that surrounds the recessed region  252  may all be concentric to the central aperture  408 . Similar to the plurality of supports  312  of the encoder-end balance plate  224 , the supporting face  412  may evenly distribute the clamping force applied via the locking nut  228  across the second end  236  of the output-end balance plate  220  and the first end  222  of the lamination stack  210 . Further, the size and dimensions of the first groove  422  may be suitable for insertion of an O-ring. Thus, the first groove  422  may seal the output-end balance plate  220  against the lamination stack  210  via the inserted O-ring when the lamination stack  210  is clamped between the balance plates  220 ,  224 . 
     The top portion  404  also includes a region of additional material  430  that surrounds the outer diameter of the first groove  422 , with the region of additional material  430  being ring shaped and concentric to the central aperture  408 . The region of additional material  430  may define the outer perimeter of the top portion  404  of the output-end balance plate  220 . The region of additional material  430  or portions thereof may be removed as desired to adjust/enhance the dynamic balance of the rotor  206 . 
     The output-end balance plate  220  further includes a plurality of holes  414  that pass through (e.g., along the x-axis) the supporting face  412  and continue into a shell  428  comprising the tubular shaft  406  as shown in  FIG.  4 B . The plurality of holes  414  may be a series of evenly spaced apart adjacent holes that are drilled or otherwise suitably cut into the output-end balance plate  220 . For example, the plurality of holes  414  may include a first hole  416  that is adjacent to and spaced apart from a second hole  418 , a third hole  420  that is adjacent to and spaced apart from the second hole  418 , and so on around a top surface  421  of the supporting face  412 . 
     Each hole of the plurality of holes  414  may be identical in shape and dimensions, with the shape and dimensions suitable so that the holes may traverse the supporting face  412  and extend into a portion of tubular shaft  406  along the x-axis. For example, the first hole  416  may fully traverse the supporting face  412  and terminate within the tubular shaft  406  before coming into contact with the first end  234  of the output-end balance plate  220 . The plurality of holes  414  may terminate at or just beyond a second groove  426  that surrounds the outer diameter of the tubular shaft  406 . The second groove  426  may partially extend through and terminate within each hole of the plurality of holes  414 . For example, the second groove  426  may terminate halfway through the width (e.g., along the y-axis) of each hole of the plurality of holes  414 . 
     A side surface  423  of the supporting face  412  may further include a third groove  424 . The third groove  424  may surround the outer diameter of the supporting face  412  and partially traverse each hole of the plurality of holes  414  along the y-axis as shown in  FIG.  4 B . For example, the third groove  424  may extend through the side surface  423  and terminate at a midpoint within each of hole of the plurality of holes  414 . Thus, pathways that connect to the recessed region  252  and extend through the output-end balance plate  220  are formed via coupled openings of the second groove  426 , the third groove  424 , and the plurality of holes  414 . Two pathways are illustrated in  FIG.  4 B  the first pathway is indicated by the dashed arrows  254  (as also shown in  FIG.  2 A ) and a second pathway is indicated by a series of dashed arrows  432 . For example, the first pathway indicated by the dashed arrows  254  may begin within the recessed region  252 , go through the side surface  423  of the supporting face  412  and into the first hole  416  via the third groove  424 . The first pathway may then follow the length (e.g., along the x-axis) of the first hole  416  and end outside the shell  428  of the tubular shaft  406  at the second groove  426 . Thus, the output-end balance plate  220  may have multiple pathways, with one formed at each hole of the plurality of holes  414 . The configurations of the output-end balance plate  220  and the encoder-end balance plate  224  may be used to provide uniform cooling the motor as further described below. 
     Returning now to  FIG.  2 A , when the output-end balance plate  220  and encoder-end balance plate  224  are clamped to the lamination stack  210  (e.g., via the lateral force applied by tightening the locking nut  228 ), the pathways within the output-end balance plate  220  may align with and connect to the plurality of axial channels  262  within the lamination stack  210  via the recessed region  252 . Further, the plurality of axial channels  262  within the lamination stack  210  may align with and connect to the recessed region  248  of the encoder-end balance plate  224 . Moreover, the recessed region  248  of the encoder-end balance plate  224  may connect to the hollow portion of the rotor shaft  208  via a plurality of holes  256  that perpendicularly traverse a shell  258  comprising the rotor shaft  208 . The plurality of holes  256  may be located within the portion of the rotor shaft  208  that is housed within the encoder-end balance plate  224 . Thus, a continuous pathway in which an oil or liquid coolant may flow through the rotor  206  is formed between the balance plates  220 ,  224 , the lamination stack  210 , and the rotor shaft  208 . 
     For example, during motor use, a transmission oil pump  244  may pump an oil supply from the output-end  230  through the center of the rotating rotor shaft  208  toward the encoder-end  232 . In some examples, the oil supply may come from a reservoir within the transmission. In some examples, the oil supply may come from any suitable source such as the engine oil gallery. In some examples, the oil supply may be pumped into the rotor shaft  208  by another pump. The oil may then flow from the encoder-end  232  of the rotor shaft  208  into the recessed region  248  of the encoder-end balance plate  224  via the plurality of holes  256 , with the encoder-end balance plate  224  serving to control the oil as it flows radially outward from the center of the rotor shaft  208 . The oil contained within the recessed region  248  of the encoder-end balance plate  224  may then flow into the plurality of axial channels  262  of the lamination stack  210  and back toward the output-end  230  of the rotor shaft  208  (e.g., the flow of the oil may be diverted 180 degrees from the direction it was pumped). The oil may then flow from the plurality of axial channels  262  into the recessed region  252  of the output-end balance plate  220 . The oil may then flow from the recessed region  252  through the third groove  424  of the output-end balance plate  220  and into the plurality of holes  414  where it may exit via the second groove  426  of the tubular shaft  406 . The exiting oil may flow into the transmission oil pump  244  where it may then be pumped back into the encoder-end  232  of the rotor shaft  208 . An example of this pathway through the fourth channel  270  of the lamination stack  210  is indicated by a series of arrows  245 . 
     Thus, the oil may continually and uniformly flow through rotor  206  when the motor is in use. As the oil is circulated through the rotor  206 , generated heat may be transferred to the oil thereby cooling the motor. Further, by passing the oil through the plurality of axial channels  262 , overall resistance between the oil and the lamination stack  210  may be reduced thereby increasing the effectiveness of cooling provided by the oil. Thus, uniform cooling may be provided by the cooling system  202  without adding excessive weight to the motor. In some embodiments, the inner surface of plurality of axial channels  262  may be lined with a liner (e.g., a dielectric liner) so that another type of liquid coolant (e.g., water) may be employed by the cooling system  202 . In some embodiments, a passive catch system rather than the transmission oil pump  244  may be used to maintain the continuous flow of the oil through the rotor  206 . 
       FIG.  5    is a flowchart illustrating a method  500  for cooling laminations of a rotor of an electric motor according to the embodiments of the present disclosure. Method  500  is described with respect to the system and components described above with respect to  FIG.  1   -4B but could also be carried out with other systems/components (e.g., an electric motor of an electric vehicle, electric motors of machines) without departing from the scope of this disclosure. Method  500  may be carried out according to instructions stored in non-transitory memory of a computing device such as a central processing unit (CPU) or control system (e.g., control system  122  of  FIG.  1   ) of a vehicle. 
     At  502 , method  500  may include feeding oil through the center of a rotating rotor shaft of the electric motor via a pump or passive catch system. The pump or passive catch system may be within a transmission (e.g., transmission oil pump  244  of  FIG.  2 B ) to which the rotor shaft is functionally coupled. The oil may be fed from the transmission into a first end of the hollow rotor shaft where the oil flows toward a second end of the rotor shaft. At  504 , the oil may flow outward from the rotor shaft (e.g., via centrifugal force or pumping). The rotor shaft may be comprised of a shell that includes one or more passages adjacent to the second end of the rotor shaft. As the rotor shaft rotates, oil within the rotor shaft may flow out of the passages via centrifugal force. For example, the rotor shaft may include a plurality of perpendicular holes (e.g., plurality of holes  256  of  FIG.  2 A ) that are adjacent to the second end and encompass the outer circumference of the rotor shaft. 
     At  506 , the outward flowing oil may be directed toward axial holes within the rotor. The outward flowing oil may be directed toward the axial holes of the rotor via a balance plate (e.g., encoder-end balance plate  224  of  FIGS.  2 A,  3 A, and  3 B ) integrally coupled to the second end of the rotor shaft, with the balance plate encompassing the region containing the passages within the shell, and a lamination stack. For example, the balance plate may be substantially disk shaped and include a central aperture lined with an O-ring. The balance plate may further include at least one recessed region within a back face, with an O-ring lined groove surrounding the recessed region. The rotor shaft may be inserted into the central aperture at the back face, with a seal being formed between the rotor shaft and the balance plate via the O-ring. Coupling of the rotor shaft to the balance plate may further secure the back face of the balance plate to the lamination stack (e.g., the back face of the balance plate and a first end of the lamination stack may be in face-sharing contact), with the O-ring lined groove surrounding the recessed region forming a seal between the lamination stack and the balance plate. The lamination stack may include a plurality of axial holes that may be used for weight reduction of the rotor. The axial holes may align and connect with the recessed region of the balance plate. Thus, as the oil flows outward from the passages within the shell of the rotor shaft, the oil may be collected within the recessed region of the balance plate where it is directed into the axial holes of the lamination stack via centrifugal force of the rotor (e.g., the axial holes are the only connected openings to the recessed region that may allow for the passive flow of the oil via centrifugal force as the oil is being actively pumped from the connected rotor shaft). 
     At  508 , the oil may flow through the axial holes of the rotor. As the rotor continues to rotate, the oil that has flowed from the recessed region of the balance plate into the lamination stack may flow back toward the first end of the rotor shaft and the transmission. At  510 , the oil may be directed from the axial rotor holes to the pump or passive catch system. The oil may flow outward from the axial holes of the lamination stack and into a second balance plate. The second balance plate may be integrally coupled to the lamination stack and include a recessed region. 
     The recessed region of the second balance plate may be connected to a series of holes that traverse the majority of the length of the balance plate and terminate within a connected groove. When the lamination stack is coupled to the second balance plate, the axial holes may connect with the recessed region so that oil may flow from the axial holes into the recessed region (e.g., transported by the positive pressure gradient resulting from the encoder-end balance plate, the outlet is radially outward with respect to the inlet). Oil within the recessed region may then be directed into the series of holes within the second balance plate and toward the groove. Oil within the groove may then be directed out of the balance plate and into the transmission via centrifugal force. After which, if the motor is still in use, method  500  may return to  502 . If the motor is no longer in use (e.g., the rotor is no longer being rotated), method  500  may end. 
       FIG.  1   -4B are shown approximately to scale, however other relative dimensions may be used. 
       FIG.  1   -4B show example configurations with relative positioning of the various components. If shown directly contacting each other, or directly coupled, then such elements may be referred to as directly contacting or directly coupled, respectively, at least in one example. Similarly, elements shown contiguous or adjacent to one another may be contiguous or adjacent to each other, respectively, at least in one example. As an example, components laying in face-sharing contact with each other may be referred to as in face-sharing contact. As another example, elements positioned apart from each other with only a space there-between and no other components may be referred to as such, in at least one example. As yet another example, elements shown above/below one another, at opposite sides to one another, or to the left/right of one another may be referred to as such, relative to one another. Further, as shown in the figures, a topmost element or point of element may be referred to as a “top” of the component and a bottommost element or point of the element may be referred to as a “bottom” of the component, in at least one example. As used herein, top/bottom, upper/lower, above/below, may be relative to a vertical axis of the figures and used to describe positioning of elements of the figures relative to one another. As such, elements shown above other elements are positioned vertically above the other elements, in one example. As yet another example, shapes of the elements depicted within the figures may be referred to as having those shapes (e.g., such as being circular, straight, planar, curved, rounded, chamfered, angled, or the like). Further, elements shown intersecting one another may be referred to as intersecting elements or intersecting one another, in at least one example. Further still, an element shown within another element or shown outside of another element may be referred as such, in one example. 
     In this way, uniform cooling of laminations of a rotor of an electric motor may be achieved using oil. The oil may be pumped by a transmission oil pump through a rotor shaft as the rotor shaft rotates where the oil is subsequently diverted to the axial holes of the rotor via a first balance plate and centrifugal force. The oil may then flow through the rotor in a direction opposite to which it was pumped. The oil is brought into direct contact with the surface of the rotor laminations, thereby increasing contact area and reducing overall thermal resistance and uniformly cooling the rotor. The oil is then passed through a second balance plate where it is diverted back to the transmission and may be pumped back into the rotor shaft for continuous cooling of the motor. 
     The cooling system may include an oil supply, with the oil supply configured to flow oil through a closed-loop pathway, the closed-loop pathway only including an interior passageway of the rotor shaft, one or more passages and/or recesses of the encoder-end balance plate, one or more weight-reduction channels of the lamination stack, and one or more passages and/or recesses of the output-end balance plate. The oil supply may only flow oil to the interior passageway, the interior passageway may only flow the oil to the one or more passages and/or recesses of the encoder-end balance plate, the one or more passages and/or recesses of the encoder-end balance plate may only flow the oil to the one or more weight-reduction channels of the lamination stack, the one or more weight-reduction channels of the lamination stack may only flow the oil to the one or more passages and/or recesses of the output-end balance plate, and the one or more passages and/or recesses of the output-end balance plate may only flow the oil to back to the oil supply. No other passages or channels may come into the closed-loop pathway. The closed-loop pathway may only receive oil from the oil supply. In some embodiments, the cooling system may include a coolant supply, with the coolant supply configured to flow coolant through the closed-loop pathway as described above. The closed-loop pathway may only receive coolant from the coolant supply. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” of the present invention are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising,” “including,” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. The terms “including” and “in which” are used as the plain-language equivalents of the respective terms “comprising” and “wherein.” Moreover, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements or a particular positional order on their objects. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable a person of ordinary skill in the relevant art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those of ordinary skill in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.