Patent Publication Number: US-2023134353-A1

Title: Oil lubricant quality sensor for a lubricant supported electric motor and an oil cooled inverter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This utility application claims the benefit of U.S. Provisional Application No. 63/273,295 filed Oct. 29, 2021. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to a lubricant supported electric motor and an oil cooled inverter. More specifically, the present disclosure relates to oil quality diagnostics used in association with a lubricant supported electric motor and an oil cooled inverter which share a common lubricating and cooling fluid. 
     BACKGROUND OF THE INVENTION 
     This section provides a general summary of background information and the comments and examples provided in this section are not necessarily prior art to the present disclosure. 
     Various drivelines in automotive, truck and certain off-highway applications take power from a central prime mover and distribute the power to the wheels using mechanical devices such as transmissions, transaxles, propeller shafts, and live axles. These configurations work well when the prime mover can be bulky or heavy, such as, for example, various internal combustion engines (“ICE”). However, more attention is being directed towards alternative arrangements of prime movers that provide improved environmental performance, eliminate mechanical driveline components, and result in a lighter-weight vehicle with more space for passengers and payload. 
     “On wheel”, “in-wheel” or “near-wheel” motor configurations are one alternative arrangement for the traditional ICE prime mover that distributes the prime mover function to each or some of the plurality of wheels via one or more motors disposed on, within, or proximate to the plurality of wheels. For example, in one instance, a traction motor, using a central shaft though a rotor and rolling element bearings to support the rotor, can be utilized as the “on wheel”, “in wheel” or “near wheel” motor configuration. In another instance, a lubricant supported electric motor can be utilized as the “on wheel”, “in wheel” or “near wheel” motor configuration. While each of these motor configurations result in a smaller size and lighter weight arrangement as compared to the prime movers based on the internal combustion engine, they each have certain drawbacks and disadvantages. 
     One aspect of electric drive motors that adds to their cost and complexity is the requirement for a variety of fluids used for different functions of the electric drive motor and associated systems. For example, a wheel end electric system often includes a water-glycol cooling fluid for the electric motor and separate cooling fluids for other components (e.g., the inverter, gearing, bearings, and hydraulic brakes) of the electric drive system. Each of these fluids require separate storage compartments and distribution channels, as well as systems for distributing or cycling the fluids to the desired locations within the systems. In the water-glycol cooled traction motors, the cooling fluid also does not touch the moving motor parts, such as the bearing surfaces, and thus cannot cool these components and is unable to support the rotor relative to the stator, such as is the case with lubricant supported electric motors. In addition, fluid passages defined by the electric motor are not narrow enough to block the flow of the coolant. 
     Additionally, water-based coolants must be separated from hydrocarbon lubricated surfaces and from high voltage and low voltage electronics. A water-glycol based coolant coming into contact with electronics can lead to electrical shorts and substantial damage to the electrical components. Thus, using water-glycol coolants to cool electronics requires the use of heat exchangers, which are themselves costly, bulky and heavy. Accordingly, most inverters require that the electronic components are separated by an aluminum plate from the coolant fluid, so that the fluid is never in contact with the power components. For these reasons, a water-glycol fluid is not a suitable a candidate for a shared lubricant and cooling system which utilizes the same fluid to lubricate and cool the electric motor and electric components, such as the inverter. However, it is recognized that a motor may be cooled using a hydrocarbon lubricant which is then connected to heat exchanger wherein the heat from the hydrocarbon lubricant is rejected to a water-glycol coolant loop, which may or may not also cool the electronics. 
     It is also a requirement of the prior art systems to detect for major contaminants (e.g., metal particles and water) in the lubricants or coolants which if contaminated can reduce the life of the lubricant/coolant and lead to premature failure of the motor and/or the electric components, such as the inverter. Foaming is not a contaminant, but it can starve the pump resulting in reduced fluid flow and possible motor or inverter failure. Thus it is also desirable to detect this foaming condition and take appropriate action to mitigate its presence. 
     Sensors are often used in internal combustion engines (“ICEs”) to detect for the presence of major contaminants in the oil. However, the presence of contaminants in engine oil is mitigated through regular maintenance of the internal combustion engines, namely the changing of engine oil on regular intervals (e.g., every 3,000 to 5,000 miles) which necessarily results in the disposal of any contaminated oil on a regular basis. However, in contradistinction to ICEs, lubricant supported electric motors are preferably designed such that the lubricant is never changed for the life of the electric vehicle. Furthermore, with ICEs, the presence of small quantities of water in the oil is removed through boiling from contract with engine surfaces having temperatures in excess of the boiling point of water. However, these ICE surface temperatures are far in excess of the maximum temperatures experienced in an electric vehicle drive application. Thus, the need to remove major contaminants (oil and water) is a more significant problem in lubricant supported electric motors compared to their ICE counterparts. 
     The limited number of prior art lubricant cooled electric motors often utilize a standard oil filter to separate out metal particles, but do not have any mitigation against water ingress into the lubricant. Furthermore, most elective vehicle drivetrains are water/glycol cooled, and thus the concern with the presence water in the lubricant does not typically arise. Thus, there remains a continuing need for improved diagnostics for detecting the presence of both metal and water in a shared fluid used for both a lubricant supported electric motor as well as to cool electric components, such as an inverter. 
     SUMMARY OF THE INVENTION 
     The present disclosure provides a quality diagnostics system for lubricant/coolant fluid. The system includes a lubricant supported electric motor including a stator and a rotor defining a gap therebetween, with the lubricant/coolant fluid disposed in the gap for supporting the rotor while allowing the rotor to rotate relative to the stator. The system also includes an inverter including a plurality of power switches configured to supply an alternating current (AC) power to the lubricant supported electric motor for driving the rotor to rotate. The system also includes a passageway configured to convey the lubricant/coolant fluid between the lubricant supported electric motor and the inverter. The system also includes a lubricant quality sensor including a set of sensor plates disposed along the passageway, an excitation source configured to apply an AC excitation voltage to a first sensor plate of the set of sensor plates, and an electrical sensor configured to measure a response to the AC excitation voltage. It should be noted that while the operation of a single sensor is described in this disclosure, it may be advantageous to deploy more than one sensor in the system. It is further noted that the configuration of multiple sensors is not necessarily the same, as it may be advantageous to change the separation between the plates or the surface area of the plates, to enhance the ability to detect various contaminants. 
     The present disclosure also provides a quality diagnostics system for lubricant/coolant fluid of a vehicle. The system includes a lubricant supported electric motor including a stator and a rotor defining a gap therebetween, with a lubricant/coolant fluid disposed in the gap for supporting the rotor while allowing the rotor to rotate relative to the stator. The system also includes a passageway configured to convey the lubricant/coolant fluid between the lubricant supported electric motor and one or more other components. The system also includes a lubricant quality sensor including a set of sensor plates disposed along the passageway and an excitation source configured to supply an AC excitation voltage to at least one sensor plate of the set of sensor plates. The system also includes a controller configured to cause the excitation source to apply the AC excitation voltage to the at least one sensor plate. The system also includes an electrical sensor configured to measure an electrical response to the application of the AC excitation voltage. The controller is further configured to determine, based on a sensor signal from the electrical sensor, at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     The present disclosure also provides a method for sensing contaminants in a lubricant/coolant fluid of a lubricant supported electric motor. The method includes: circulating the lubricant/coolant fluid through a passageway and to the lubricant supported electric motor; applying, by an excitation source, an AC excitation voltage to a first sensor plate disposed adjacent to and along the passageway; sensing, by an electrical sensor, a response to the application of the AC excitation voltage; and determining, based on the response to the application of the AC excitation voltage, at least one of metal contamination and water contamination in the lubricant/coolant fluid. It should be noted that the detection of contaminants in the overall lubricant/cooling fluid may occur outside the motor enclosure itself. It may be advantageous to place a circuit board with the necessary plate configurations somewhere in the oil flow path to determine if contaminants exist in the lubricant/coolant fluid. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drawings described herein are for illustrative purposes only of selected embodiments and not all possible implementations, and are not intended to limit the scope of the present disclosure. 
         FIG.  1    shows a schematic view of a system including a lubricant supported electric motor and an inverter having a coolant/lubricant fluid, in accordance with an aspect subject disclosure; 
         FIG.  2    shows a partial cut-away perspective view of the inverter of the present disclosure, illustrating a fluid passage for conveying the coolant/lubricant fluid therethrough; 
         FIG.  3    shows an electrical schematic diagram showing a portion of the inverter in accordance with an aspect of the present disclosure; 
         FIG.  4    shows a schematic block diagram of a monitoring system for detecting contamination in the lubricant/coolant fluid; and 
         FIG.  5    shows a flow chart illustrating steps in a method for sensing contaminants in a lubricant/coolant fluid of a lubricant supported electric motor, in accordance with an aspect of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE ENABLING EMBODIMENTS 
     Example embodiments of a common lubrication, cooling and quality diagnostics system  10  for electric powertrains that include lubricant supported electric motors and inverters will now be more fully described. Each of these example embodiments are provided so that this disclosure is thorough and fully conveys the scope of the inventive concepts, features and advantages to those skilled in the art. To this end, numerous specific details are set forth such as examples of specific components, devices and mechanisms associated with the lubricant supported electric motor to provide a thorough understanding of each of the embodiments associated with the present disclosure. However, as will be apparent to those skilled in the art, not all specific details described herein need to be employed, the example embodiments may be embodied in many different forms, and thus should not be construed or interpreted to limit the scope of the disclosure. 
     The use of a shared lubricant/coolant fluid system including a common lubricant quality sensor improves contamination detection, while also reducing the overall size, weight, and volume of the system relative to a system requiring the use of different lubricant fluids for different applications. The lubricant quality sensor of the present disclosure also improves on prior art contamination detection mechanisms that are only able to reliably detect for the presence of oil or water, but not both. 
     The present disclosure provides a lubricant supported electric motor and an electric component, such as an inverter, having a shared lubricating and cooling system. More specifically, the system uses a common lubricant/coolant fluid that is both disposed within a lubricant supported electric motor to lubricant the electric motor and support the rotor relative to the stator, while also being used to cool electronic modules of the system, such as the inverter. The lubricant is preferably a dielectric oil such that the lubricant can be introduced directly into the inverter such that it is able to make direct contact with its components, while also serving to lubricate and support the rotor of the lubricant supported electric motor. A lubricant quality sensor is disposed in communication with the common lubricant/coolant fluid to detect for both the presence of metal and water contamination within the common lubricant/coolant fluid system. 
       FIG.  1    illustrates a common lubrication, cooling and quality diagnostics system  10  for a lubricant supported electric motor  11  of the disclosure. As best illustrated in  FIG.  1   , the lubricant supported electric motor  11  includes a stator  12  and a rotor  14  extending along an axis A and movably disposed within the stator  12  to define a support chamber  16  or gap G therebetween. Although illustrated and described with the rotor  14  disposed within the stator  12 , the arrangement of these components can also be reversed (i.e., with the rotor  14  disposed in surrounding relationship with the stator  12 ) without departing from the scope of the subject disclosure. A lubricant/coolant fluid  18  is disposed in the support chamber  16  for supporting the rotor  14  within and relative to the stator  12 , allowing the rotor  14  to rotate relative to the stator  12  and lubricating and cooling these components. The lubricant/coolant fluid  18  acts as a buffer (e.g., suspension) between the rotor  14  and stator  12  minimizing or preventing contact therebetween. In other words, the lubricant/coolant fluid  18  prevents direct contact between the stator  12  and rotor  14  and provides a lubricant supported electric motor  11  which is robust to shock and vibration loading due to the presence of the lubricant/coolant fluid  18  in the support chamber  16 . Additionally, and alternatively, a lubricant/coolant fluid that is substantially incompressible may be used in order to minimize the gap between the stator  12  and rotor  14 . 
     As further illustrated in  FIG.  1   , the rotor  14  is interconnected to a drive assembly  20  for coupling the lubricant supported electric motor  11  to one of the plurality of wheels of a vehicle. For example, in one instance, the drive assembly  20  may include a planetary gear system. Alternatively, the drive assembly  20  may include one or more parallel axis gears. In either arrangement, the lubricant supported electric motor  11  is arranged in an “on-wheel”, “near-wheel” or “in-wheel” motor system in which the lubricant supported electric motor  11  is disposed proximate to, on, or within the vehicle wheel. Although not expressly illustrated, accordingly to another aspect, the lubricant supported electric motor  11  can be connected directly to the vehicle wheel, without the use of this drive assembly  20  to establish the “on-wheel”, “near-wheel” or “in-wheel” electric motor arrangement. In any arrangement, the stator  12  and rotor  14  are configured to exert an electromagnetic force therebetween to convert electrical energy into mechanical energy, moving the rotor  14  and ultimately driving the wheel coupled to the lubricant supported electric motor  11 . If present, the drive assembly  20  may provide one or more reduction ratios between the lubricant supported electric motor  11  and the wheel in response to movement of the rotor  14 . 
     As further illustrated  FIG.  1   , the stator  12  defines a motor passageway  22  disposed in fluid communication with the support chamber  16  for introducing the lubricant/coolant fluid  18 . However, the motor passageway  22  could be provided on any other components of the lubricant supported electric motor  11 , without departing from the subject disclosure. An inverter  24  is disposed in electrical communication with the lubricant supported electric motor  11  and defines an inverter passageway  26  disposed in fluid communication with the motor passageway  22  for allowing the lubricant/coolant fluid  18  to also pass through the inverter  24  and over its electronic components. Thus, the lubricant/coolant fluid  18  used to lubricate and cool the lubricant supported electric motor  11  is also used for cooling the inverter  24 . The lubricant/coolant fluid  18  may be a dielectric oil having a composition that acts as an electrical insulator, such that the lubricant/coolant fluid  18  will not conduct electricity, making the lubricant/coolant fluid  18  suitable for direct contact with electric components of the inverter  24 . The dielectric oil also has good heat transfer properties, such that it may act well as a coolant for both the lubricant supported electric motor  11  as well as the inverter  24 . The dielectric oil is also incompressible, making it a good candidate for supporting the rotor  14  relative to the stator  12  in the lubricant supported electric motor  11 . Finally, the dielectric oil also serves as a good lubricant for use within the lubricant supported electric motor  11 . The inverter  24  includes a number of electric components necessary to convert DC current into AC current, such as switches, transistors, semiconductors and the like. 
     As illustrated in  FIG.  1   , the lubricant/coolant fluid  18  is cycled or pumped through both the motor passageway  22  and the inverter passageway  26 , and into their respective components, as one continuous fluid communication line. For example, a pump  36  may be fluidly coupled to a sump or reservoir  38  of the lubricant/coolant fluid  18 , such that the lubricant/coolant fluid  18  is pumped from the reservoir  38 , through the inverter passageways  26  and then into the motor passageways  22  and through the support chamber  16  of the lubricant supported electric motor  11 . Alternatively, the lubricant/coolant fluid  18  may flow first through the lubricant supported electric motor  11  and then through the inverter passageways  26 . In an alternative arrangement, rotation of the rotor  14  relative to the stator  12  could act as a self-pump to pull the lubricant/coolant fluid  18  through the passageways  22 ,  26  and into the support chamber  16 . Although not expressly illustrated, a further enhancement of the system  10  includes that the reservoir  38  is designed with a low point where water present in the lubricant/coolant fluid  18  could collect. A diagnostic message to a driver of the vehicle could be sent indicating that a drain plug of the reservoir  38  needs to be opened to purge the tank of water. 
     With reference to  FIG.  2   , the inverter  24  includes an enclosure  25  that defines a fluid inlet  28  and a fluid outlet  30  such that the inverter passageway  26  flows between the fluid inlet  28  and the fluid outlet  30 , allowing the lubricant/coolant fluid  18  to make direct contact with and flow over these electric components dispersed throughout the inverter  24 . However, metal contaminants in the lubricant/coolant fluid  18  passing through the inverter passageway  26  could cause a short circuit between adjacent conductors, such as positive and negative device terminals of electric components within the inverter  24 . This could result in failure of the inverter  24 , which may require replacement of the inverter  24 . 
     Accordingly, as further illustrated in  FIG.  2   , the inverter  24  includes a lubricant quality sensor  32  for detecting for the presence of both water and metal contamination in the lubricant/coolant fluid  18  that is passing through the inverter  24  as well as the lubricant supported electric motor  11 . As will be appreciated in view of the following more detailed disclosure, the use of a lubricant quality sensor  32  to detect for major contaminants in the lubricant/coolant fluid  18  used by both the inverter  24  as well as the lubricant supported electric motor  11  improves on the prior art systems that require separate detection mechanisms dedicated to each of these components. 
     As further illustrated in  FIG.  2   , the lubricant quality sensor  32  includes a sensor on the power board, which consists of a set of sensor plates  34   a ,  34   b  disposed along and on opposite sides of the inverter passageway  26 . 
       FIG.  3    shows an electrical schematic diagram of a portion of the inverter  24 . The inverter  24  includes a DC positive conductor  40   a  and a DC negative conductor  40   b  having a DC voltage therebetween. The inverter  24  includes an A-phase driver  42   a  configured to generate an alternating current (AC) power on an A-phase motor lead  44   a  for supplying current to a corresponding winding of the lubricant supported electric motor  11 . The inverter  24  also includes a B-phase driver  42   b  configured to generate AC power on a B-phase motor lead  44   b  for supplying current to a corresponding winding of the lubricant supported electric motor  11 . The inverter  24  also includes a C-phase driver  42   c  configured to generate AC power on a C-phase motor lead  44   c  for supplying current to a corresponding winding of the lubricant supported electric motor  11 . Each of the phase drivers  42   a ,  42   b ,  42   c  includes a high-side power switch  46   h  configured to selectively conduct current between the DC positive conductor  40   a  and a corresponding one of the motor leads  44   a ,  44   b ,  44   c . Each of the phase drivers  42   a ,  42   b ,  42   c  also includes a low-side power switch  46   l  configured to selectively conduct current between the DC negative conductor  40   b  and a corresponding one of the motor leads  44   a ,  44   b ,  44   c . Each of the power switches  46   h ,  461  is shown schematically as a single insulated gate bipolar transistor (IGBT). However, other types of power switching devices may be used, such as field-effect transistors (FETs) or other types of junction devices. In some embodiments, one or more of the power switches  46   h ,  461  may include a parallel-connected combination of two or more discrete devices, such as IGBT devices. The inverter  24  is illustratively shown as a three-phase device having three of the phase drivers  42   a ,  42   b ,  42   c . However, the principles of the present disclosure maybe applied to other types of poly-phase systems, such as where the inverter  24  and the lubricant supported electric motor  11  are each six, nine, or twelve-phase devices. 
     Each of the phase drivers  42   a ,  42   b ,  42   c  also includes two DC link capacitors  48  connected between the DC positive conductor  40   a  and the DC negative conductor  40   b  adjacent to the switches  46   h ,  461  to supply relatively large inrush currents to the switches  46   h ,  461  and to reduce electromagnetic interference (EMI). A set of two input capacitors  50  are connected in series between the between the DC positive conductor  40   a  and the DC negative conductor  40   b.    
     An excitation source applies an AC excitation voltage to a first plate  34   a  of the set of sensor plates  34   a ,  34   b . In some embodiments, the inverter  24  function as the excitation source. For example, and as shown in  FIG.  3   , the first plate  34   a  may be connected to the A-phase motor lead  44   a . However, the first plate  34   a  could be connected to any one of the motor leads  44   a ,  44   b ,  44   c . The first plate  34   a  may also be called an excitation plate because of its configuration to have an AC excitation voltage applied thereto. A second plate  34   b  of the set of sensor plates  34   a ,  34   b  is connected to a midpoint node between the two input capacitors  50 . The series combination of the two input capacitors  50  may maintain the second plate  34   b  at a steady DC voltage that is mid-way between the voltages of the DC positive conductor  40   a  and the DC negative conductor  40   b . The second plate  34   b  may also be called a reference plate because of its steady or reference voltage. Alternatively, another power source, such as a dedicated AC source independent of the inverter  24 , may function as the excitation source for applying the AC excitation voltage to the first plate  34   a  of the set of sensor plates  34   a ,  34   b.    
       FIG.  4    shows a schematic block diagram of a monitoring system  100  for detecting contamination in the shared lubricant/coolant fluid. The monitoring system  100  includes a controller  110  for controlling various functions. The controller  110  may control operation of the inverter  24 . For example, the controller  110  may generate a gate control signal for controlling operation of each of the high-side power switches  46   h  and the low-side power switches  46   l  of the inverter  24 . The controller  110  may control operation of the inverter  24  for applying the AC excitation to the first plate  34   a  of the lubricant quality sensor  32 . The lubricant quality sensor  32  also includes an electrical sensor  120  configured to measure an electrical response to the application of the AC excitation. The electrical sensor  120  may include a current sensor configured to measure electrical current in a conductor attached to one of the sensor plates  34   a ,  34   b . Such a current sensor may, thereby, measure current between the sensor plates  34   a ,  34   b . For example, and as shown in  FIG.  4   , the electrical sensor  120  is configured to measure electrical current in a conductor between the midpoint node and the second plate  34   b  of the lubricant quality sensor  32 , i.e. the reference plate. Additionally or alternatively, the electrical sensor  120  may include other types of sensing devices, such as a voltage sensor and/or a sensor configured to detect an induced magnetic field. 
     In some embodiments, the electrical sensor  120  may be disposed on a printed circuit board (PCB) of the inverter  24  on which some or all of the power switches  46   h ,  461  of the inverter  24  are located. 
     According to a further aspect of the disclosure, the lubricant/coolant fluid  18  in the reservoir  38  is heated to a predetermined temperature (such as greater than 90 degrees Celsius) which results in the water present in the lubricant/coolant fluid  18  evaporating off to an environment of the reservoir  38 . This heating process can occur during normal operation if the motor is operated in an inefficient manner, which results in an increase in the shared lubricant/coolant fluid temperature to the desired limit. This heating step would preferably coincide with an opening of a vent on the reservoir  38 , which releases the resultant water vapor to the outside environment, but doesn&#39;t allow contaminants to enter the reservoir  38 . This venting to the atmosphere could also be achieved with a screen and vent which do not need to be actuated, without departing from the scope of the subject disclosure. According to an aspect, the heating of the lubricant/coolant fluid  18  can be achieved by, or augmented by, increased switching frequency of the switches  46   h ,  461  of the inverter  24 . 
     According to a further aspect of the disclosure, and as shown in  FIG.  1   , the lubricant quality sensor  32  could also include thermal spectroscopy sensor  39  configured to detect foaming in the lubricant/coolant fluid  18 . This addition would result in a lubricant quality sensor  32  which could detect not only for the presence of metal and oil contaminants, but also foaming in the lubricant/coolant fluid  18 . 
     The controller  110  may be operably connected to the electrical sensor  120  to receive a sensor signal indicative of the electrical current between the sensor plates  34   a ,  34   b  and in response to the AC excitation applied thereto. In some embodiments, the controller  110  may be operably connected to the thermal spectroscopy sensor  39  for receiving a signal therefrom indicative of foaming in the lubricant/coolant fluid  18 . In some embodiments, the controller  110  may be operably connected to the inverter  24  for controlling operation of the phase drivers  42   a ,  42   b ,  42   c  for supplying AC current to the windings of the lubricant supported electric motor  11 . 
     In some embodiments, the controller  110  may control may control other functions and/or components within the common lubrication, cooling and quality diagnostics system  10 , such by measuring one or more temperatures and/or pressures of the shared lubricant/coolant fluid and/or by controlling operation of other components, such as the pump  36 . The controller  110  includes a processor  112  coupled to a storage memory  114 . The storage memory  114  includes an instruction storage  116  for storing instructions, such as program code for execution by the processor  112 . The storage memory  114  also includes a data storage  118  for holding data for use by the processor  112 . The data storage  118  may record, for example, values of parameters measured by one or more sensors, such as the lubricant quality sensor  32 , and/or the outcome of functions calculated by the processor  112 . 
     The controller  110  is configured to cause the inverter  24  to apply a plurality of AC frequencies to the sensor plates  34   a ,  34   b  at a fixed voltage, such that a current which flows between the sensor plates  34   a ,  34   b  is used to determine a complex impedance of the lubricant/coolant fluid  18  passing through the inverter passageway  26 . From this complex impedance, a presence of water or metal contaminants in the lubricant/coolant fluid  18  can be determined. Accordingly, the lubricant quality sensor  32  utilizes existing high voltage as the excitation to perform a high-frequency sweep of the lubricant/coolant fluid  18  to detect for the presence of metal and water contaminants. This high-frequency sweep can flexibly be performed at vehicle start-up, while the vehicle is running, or after the vehicle has been shut down. The lubricant quality sensor  32  also advantageously makes an additional use of the high voltage which is already present in the electric powertrain to sense for major contaminants in the lubricant/coolant fluid  18 , and thus reduces the number of additional components required to perform diagnostic measurements on the lubricant/coolant fluid  18  as compared to the prior art systems. 
     The controller  110  is configured to compare the amplitude and phase of a current measured by the electrical sensor  120  with the amplitude and phase of the exciting frequency and to thereby determine a complex impedance of the overall structure, with note that the resulting impedance is determined principally by characteristics of the lubricant/coolant fluid  18  and any contaminants therein. 
     In some embodiments, the system  10  is configured to determine the complex frequency by sweeping the excitation source to apply a range of different AC frequencies to the set of sensor plates  34   a ,  34   b . Alternatively or additionally, the system  10  may be configured to determine the impedance of the lubricant/coolant fluid  18  during normal operation of the inverter  24  when the desired excitation frequencies occur without explicit programming. To assist in use of a normal inverter operating mode as a means to excite the lubricant/coolant fluid  18  plus contaminants, at least one additional plate  34   c  may be disposed closer or further away from the first plate  34   a . This may affect sensitivity of the impedance observer algorithm in certain modes where contaminants are more difficult to determine in the lubricant/coolant fluid  18 . This may also be advantageous when the frequency is swept as part of an off-line diagnostic test and the inverter  24  is not driving the lubricant supported electric motor  11 . 
     As will be appreciated in view of the above-mentioned disclosure, the use of impedance measurements on the lubricant/coolant fluid  18  is significantly more advanced than any techniques used today for ICEs. Additionally, the lubricant quality sensor  32  is more sophisticated that prior art mechanisms for detecting for the presence of contaminants, and advantageously addresses the full spectrum of the contamination problem in fluids and oils—not just the problem of metal contaminants. 
       FIG.  5    shows a flow chart illustrating steps in a method  200  for sensing contaminants in a lubricant/coolant fluid of a lubricant supported electric motor. As can be appreciated in light of the disclosure, the order of operation within the method  200  is not limited to the sequential execution as illustrated in  FIG.  5   , but may be performed in one or more varying orders as applicable and in accordance with the present disclosure. 
     The method  200  includes circulating the lubricant/coolant fluid through a passageway and to the lubricant supported electric motor at step  202 . 
     The method  200  also includes applying, by an excitation source, an AC excitation voltage to a first sensor plate disposed adjacent to and along the passageway at step  204 . 
     In some embodiments, step  204  further includes applying a predetermined range of AC frequencies to the first sensor plate to generate an electric field in the lubricant/coolant fluid, and where the predetermined range of AC frequencies is selected to generate a corresponding response to an interaction between the electric field and the at least one of metal contamination and water contamination in the lubricant/coolant fluid. In some embodiments, the predetermined range of AC frequencies are substantially higher than a frequency of the AC power applied to the lubricant supported electric motor for driving the rotor to rotate. 
     The method  200  also includes sensing, by an electrical sensor, a response to the application of the AC excitation voltage at step  206 . For example, the electrical sensor  120  may sense an electrical current having a particular frequency and/or phase as the response to the application of the AC excitation voltage. 
     The method  200  also includes determining, based on the response to the application of the AC excitation voltage, at least one of metal contamination and water contamination in the lubricant/coolant fluid at step  208 . 
     In some embodiments, step  208  includes comparing at least one of an amplitude and a phase of the response measured by the sensor with a corresponding one of an amplitude and a phase of the AC excitation voltage. For example, the processor  112  may execute program instructions to compare the amplitude and/or the phase of the response measured by the sensor with the corresponding amplitude and/or the phase of the AC excitation voltage. 
     The method  200  also includes supplying, by a plurality of power switches of an inverter, an alternating current (AC) power to the lubricant supported electric motor for causing a rotor thereof to rotate at step  210 . In some embodiments, step  204  includes the plurality of power switches of the inverter also functioning as the excitation source to apply the AC excitation voltage to the first sensor plate. 
     A quality diagnostics system for lubricant/coolant fluid includes a lubricant supported electric motor including a stator and a rotor defining a gap therebetween, with the lubricant/coolant fluid disposed in the gap for supporting the rotor while allowing the rotor to rotate relative to the stator. The system also includes an inverter including a plurality of power switches configured to supply an alternating current (AC) power to the lubricant supported electric motor for driving the rotor to rotate. The system also includes a passageway configured to convey the lubricant/coolant fluid between the lubricant supported electric motor and the inverter. The system also includes a lubricant quality sensor including a set of sensor plates disposed along the passageway, an excitation source configured to apply an AC excitation voltage to a first sensor plate of the set of sensor plates, and an electrical sensor configured to measure a response to the AC excitation voltage. 
     In some embodiments, the set of sensor plates includes two sensor plates disposed on opposite sides of the passageway. 
     In some embodiments, the set of sensor plates are disposed within an enclosure of the inverter. 
     In some embodiments, the set of sensor plates includes a reference plate having a reference voltage. 
     In some embodiments, the inverter includes a DC positive conductor and a DC negative conductor and series combination of input capacitors connected therebetween to define a midpoint node, and wherein the reference plate is connected to the midpoint node. 
     In some embodiments, the electrical sensor is configured to measure an electrical current in a conductor attached to one of the sensor plates. 
     In some embodiments, the inverter includes a printed circuit board with the plurality of power switches and the electrical sensor disposed thereupon. 
     In some embodiments, the plurality of power switches in the inverter are further configured to function as the excitation source for applying the AC excitation to the first sensor plate of the set of sensor plates. 
     In some embodiments, the system further includes a controller configured to cause the excitation source to apply the AC excitation voltage to the first sensor plate and to receive a sensor signal from the electrical sensor indicative of the response to the application of the AC excitation voltage. In some embodiments, the controller is further configured to determine, based on the sensor signal, at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     In some embodiments, the causing the excitation source to apply the AC excitation voltage to the first sensor plate includes the controller commanding the excitation source to apply a predetermined range of AC frequencies to the first sensor plate to generate an electric field in the lubricant/coolant fluid between the set of sensor plates, the predetermined range of AC frequencies configured to generate a corresponding response to an interaction between the electric field and the at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     In some embodiments, the predetermined range of AC frequencies are substantially higher than a frequency of the AC power applied to the lubricant supported electric motor for driving the rotor to rotate. 
     In some embodiments, determining the least one of metal contamination and water contamination in the lubricant/coolant fluid further includes the controller being configured to compare at least one of an amplitude and a phase of the sensor signal with a corresponding one of an amplitude and a phase of the AC excitation voltage. 
     In some embodiments, the lubricant quality sensor further includes a thermal spectroscopy sensor configured to detect foaming in the lubricant/coolant fluid. 
     A quality diagnostics system for lubricant/coolant fluid of a vehicle includes a lubricant supported electric motor including a stator and a rotor defining a gap therebetween, with a lubricant/coolant fluid disposed in the gap for supporting the rotor while allowing the rotor to rotate relative to the stator. The system also includes a passageway configured to convey the lubricant/coolant fluid between the lubricant supported electric motor and one or more other components. The system also includes a lubricant quality sensor including a set of sensor plates disposed along the passageway and an excitation source configured to supply an AC excitation voltage to at least one sensor plate of the set of sensor plates. The system also includes a controller configured to cause the excitation source to apply the AC excitation voltage to the at least one sensor plate. The system also includes an electrical sensor configured to measure an electrical response to the application of the AC excitation voltage. The controller is further configured to determine, based on a sensor signal from the electrical sensor, at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     In some embodiments, the controller is configured to apply the AC excitation voltage to the at least one sensor plate during at least one of a vehicle startup and a vehicle shutdown. 
     A method for sensing contaminants in a lubricant/coolant fluid of a lubricant supported electric motor includes: circulating the lubricant/coolant fluid through a passageway and to the lubricant supported electric motor; applying, by an excitation source, an AC excitation voltage to a first sensor plate disposed adjacent to and along the passageway; sensing, by an electrical sensor, a response to the application of the AC excitation voltage; and determining, based on the response to the application of the AC excitation voltage, at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     In some embodiments, the method further includes supplying, by a plurality of power switches of an inverter, an alternating current (AC) power to the lubricant supported electric motor for causing a rotor thereof to rotate. In some embodiments, the plurality of power switches of the inverter also function as the excitation source to apply the AC excitation voltage to the first sensor plate. 
     In some embodiments, applying the AC excitation voltage further includes applying a predetermined range of AC frequencies to the first sensor plate to generate an electric field in the lubricant/coolant fluid, the predetermined range of AC frequencies configured to generate a corresponding response to an interaction between the electric field and the at least one of metal contamination and water contamination in the lubricant/coolant fluid. 
     In some embodiments, the predetermined range of AC frequencies are substantially higher than a frequency of the AC power applied to the lubricant supported electric motor for driving the rotor to rotate. 
     In some embodiments, determining the at least one of metal contamination and water contamination in the lubricant/coolant fluid further includes comparing at least one of an amplitude and a phase of the response measured by the sensor with a corresponding one of an amplitude and a phase of the AC excitation voltage. 
     The foregoing description of the embodiments has been provided for purposes of illustration and description. It is not intended to be exhaustive or to limit the disclosure. Individual elements or features of a particular embodiment are generally not limited to that particular embodiment, but, where applicable, are interchangeable and can be used in a selected embodiment, even if not specifically shown or described. The same may also be varied in many ways. Such variations are not to be regarded as a departure from the disclosure, and all such modifications are intended to be included within the scope of the disclosure.