Patent Publication Number: US-10326336-B2

Title: Coolant flow distribution using coating materials

Description:
TECHNICAL FIELD 
     The present disclosure relates to the use of coating materials for coolant flow distribution, for example, in electric machines and/or inverters. 
     BACKGROUND 
     The power density of electronic devices, such as electric machines and power inverters, is continuously increasing to meet weight and cost reduction targets. To meet these targets, it is generally important to increase the efficiency and performance of the electronic device. The performance and efficiency of electric machines are generally limited by the temperature constraint on the copper windings and/or the maximum current which should overcome the friction losses in the air gap. For electric machine thermal management, it may be important to maintain uniform coolant flow (e.g., water/glycol, oil, or Automatic Transmission Fluid) on the end-windings of the electric machine to remove the localized hot spots. The performance and efficiency of power inverters are generally limited by the thermal constraints in insulated-gate bipolar transistors (IGBTs) and/or diodes because the soldering materials of the components can be damaged by exceeding the temperature limit. For inverter thermal management, a liquid cold plate may be used to cool components, such as IGBTs, diodes, solder layers, etc., inside the inverter. 
     SUMMARY 
     In at least one embodiment, an electric machine is provided. The electric machine may include a stator core defining a cavity, windings disposed within the cavity and including end windings protruding from the cavity, and one or more layers of an oleophobic or hydrophobic coating overlying the end windings and configured to control a flow of coolant over the end windings. 
     The one or more layers may include two spaced apart layers that define a coolant flow path. A layer of an oleophilic or hydrophilic coating may be disposed within the coolant flow path and overlying the end windings. In one embodiment, the coolant flow path is configured to direct coolant to a neutral point of the end windings. The two spaced apart layers may define a primary coolant flow path and one or more additional layers of the oleophobic or hydrophobic coating may be disposed within a portion of the primary coolant flow path to form one or more secondary flow paths. There may be at least two secondary flow paths and each secondary flow path may be configured to direct coolant to a hot spot or a neutral point of the end windings. A spacing between the two spaced apart layers may include an inflection point along a length of the coolant flow path. 
     In one embodiment, a first layer of the one or more layers is an arcuate line on a top half of the end windings and is configured to prolong a contact time of the coolant with the top half of the end windings. The one or more layers may further include a second arcuate line disconnected from the first layer and formed on a bottom half of the end windings. In another embodiment, the one or more layers includes two layers disposed on either side of a gap in the end windings and configured to prevent a flow of coolant through the gap. The oleophobic or hydrophobic coating may be a super oleophobic or hydrophobic coating having a contact angle with an oil-based or water-based coolant of at least 150 degrees, respectively. 
     In at least one embodiment, a method is provided including applying one or more layers of a oleophobic or hydrophobic coating to end windings of an electric machine stator such that the one or more layers control a flow of coolant over the end windings. 
     The one or more layers may include two spaced apart layers that define a coolant flow path. The coolant flow path may be configured to direct the coolant to a hot spot or a neutral point of the end windings. In one embodiment, the two spaced apart layers define a primary coolant flow path and the method further includes applying one or more additional layers of the oleophobic or hydrophobic coating within a portion of the primary coolant flow path to form one or more secondary flow paths. In another embodiment, the method may further include applying one or more layers of an oleophilic or hydrophilic coating in the coolant flow path. A first layer of the one or more layers may be applied as an arcuate line on a top half of the end windings and may prolong a contact time of the coolant with the top half of the end windings. In one embodiment, a layer of the oleophobic or hydrophobic coating is applied on two sides of a gap in the end windings and prevents a flow of coolant through the gap. The one or more layers of the oleophobic or hydrophobic coating may be applied by spraying. 
     In at least one embodiment, a method is provided including applying one or more layers of an oleophobic or hydrophobic coating and one or more layers of an oleophilic or hydrophilic coating to end windings of an electric machine, and applying an coolant to the end windings such that the oleophobic/hydrophobic and oleophilic/hydrophilic layers control a flow of the coolant over the end windings and direct the coolant to a hot spot of the end windings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a schematic diagram illustrating an example of an electrified vehicle; 
         FIG. 1B  is a perspective view of an example of an electric machine; 
         FIG. 2  is a schematic of a droplet of water on a super hydrophobic or super oleophobic coating surface having a contact angle of at least 150 degrees; 
         FIG. 3  is a top view of an example of a coolant channel formed between two repellant coating; 
         FIG. 4  is a schematic illustration of a coolant channel formed by a repellant coating, according to one example; 
         FIGS. 5A, 5B, 5C, 5D, 5E, and 5F  are examples of coolant channel shapes or patterns that may be formed using a repellant coating, including a straight, zig-zag, contracting, expanding, contracting-to-expanding, and expanding-to-contracting, respectively; 
         FIG. 6  is an end view of stator end windings including a line of repellant coating and a resulting effect on coolant flow, according to an embodiment; 
         FIG. 7  is a perspective view of stator end windings having a repellant coating applied thereon to form a forked coolant flow path, according to an embodiment; 
         FIG. 8  is a perspective view of stator end windings having a repellant coating applied thereon to form a contracting-to-expanding (e.g., hourglass) coolant flow path, according to an embodiment; 
         FIG. 9  is a perspective view of stator end windings having a repellant coating applied thereon to form an expanding-to-contracting flow path, according to an embodiment; 
         FIG. 10  is an end view of two adjacent windings each having a repellant coating thereon to prevent coolant from falling through a gap therebetween, according to an embodiment; 
         FIG. 11  is a side view of stator end windings having a repellant coating thereon to prevent coolant from falling through a gap therebetween, according to an embodiment; 
         FIG. 12  is a perspective view of hairpin end windings having a repellant coating thereon to prevent coolant from falling through a gap therebetween, according to an embodiment; 
         FIG. 13  is a schematic partial section view of a cold plate (e.g., for a power inverter) including a plurality of lines of repellant coating to facilitate coolant distribution at the inlet and the outlet, according to an embodiment; 
         FIG. 14A  is a schematic top view of a turn in a channel of a cold plate without a repellant coating showing the formation of recirculation zones, according to an embodiment; 
         FIG. 14B  is a schematic top view of a turn in a channel of a cold plate with a repellant coating in the corners showing the elimination of recirculation zones, according to an embodiment; and 
         FIG. 15  is a schematic partial section view of a cold plate having a repellant coating on a top surface of the channel walls to prevent coolant from flowing between the channels and the cold plate cover, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. The figures are not necessarily to scale; some features may be exaggerated or minimized to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for teaching one skilled in the art to variously employ the present invention. 
       FIG. 1A  depicts a schematic of an example of a PHEV, referred to as a vehicle  12  herein. The vehicle  12  may comprise one or more electric machines  14  mechanically connected to a hybrid transmission  16 . The electric machines  14  may be capable of operating as a motor or a generator. In addition, the hybrid transmission  16  may be mechanically connected to an engine  18 . The hybrid transmission  16  may also be mechanically connected to a drive shaft  20  that is mechanically connected to a set of wheels  22 . The electric machines  14  can provide propulsion and deceleration capability when the engine  18  is turned on or off. The electric machines  14  may also act as generators and may provide fuel economy benefits by recovering energy that would normally be lost as heat in the friction braking system. The electric machines  14  may also provide reduced pollutant emissions since the hybrid-electric vehicle  12  may be operated in electric mode or hybrid mode under certain conditions to reduce overall fuel consumption of the vehicle  12 . 
     A traction battery or battery pack  24  stores and provides energy that may be used by the electric machines  14 . The traction battery  24  may provide a high voltage DC output from one or more battery cell arrays, sometimes referred to as battery cell stacks, within the traction battery  24 . The battery cell arrays may include one or more battery cells. The traction battery  24  may be electrically connected to one or more power electronics modules  26  through one or more contactors (not shown). The one or more contactors isolate the traction battery  24  from other components when opened and connect the traction battery  24  to other components when closed. The power electronics module  26  may also be electrically connected to the electric machines  14  and provides the ability to bi-directionally transfer electrical energy between the traction battery  24  and the electric machines  14 . For example, the traction battery  24  may provide a DC voltage while the electric machines  14  may require a three-phase AC voltage to function. The power electronics module  26  may convert the DC voltage to a three-phase AC voltage as required by the electric machines  14 . In a regenerative mode, the power electronics module  26  may convert the three-phase AC voltage from the electric machines  14  acting as generators to the DC voltage required by the traction battery  24 . Portions of the description herein are equally applicable to a pure electric vehicle. For a pure electric vehicle, the hybrid transmission  16  may be a gear box connected to an electric machine  14  and the engine  18  may not be present. 
     In addition to providing energy for propulsion, the traction battery  24  may provide energy for other vehicle electrical systems. A DC/DC converter module  28  may convert high voltage DC output of the traction battery  24  to a low voltage DC supply that is compatible with other vehicle loads. Other high-voltage loads, such as compressors and electric heaters, may be connected directly to the high-voltage without the use of the DC/DC converter module  28 . The low-voltage systems may be electrically connected to an auxiliary battery  30  (e.g., 12V battery). 
     A battery electrical control module (BECM)  33  may be in communication with the traction battery  24 . The BECM  33  may act as a controller for the traction battery  24  and may also include an electronic monitoring system that manages temperature and charge state of each of the battery cells. The traction battery  24  may have a temperature sensor  31  such as a thermistor or other temperature gauge. The temperature sensor  31  may be in communication with the BECM  33  to provide temperature data regarding the traction battery  24 . The temperature sensor  31  may also be located on or near the battery cells within the traction battery  24 . It is also contemplated that more than one temperature sensor  31  may be used to monitor temperature of the battery cells. 
     The vehicle  12  may be, for example, an electrified vehicle which includes components for a PHEV, a FHEV, a MHEV, or a BEV. The traction battery  24  may be recharged by an external power source  36 . The external power source  36  may be a connection to an electrical outlet. The external power source  36  may be electrically connected to electric vehicle supply equipment (EVSE)  38 . The EVSE  38  may provide circuitry and controls to regulate and manage the transfer of electrical energy between the power source  36  and the vehicle  12 . The external power source  36  may provide DC or AC electric power to the EVSE  38 . The EVSE  38  may have a charge connector  40  for plugging into a charge port  34  of the vehicle  12 . The charge port  34  may be any type of port configured to transfer power from the EVSE  38  to the vehicle  12 . The charge port  34  may be electrically connected to a charger or on-board power conversion module  32 . The power conversion module  32  may condition the power supplied from the EVSE  38  to provide the proper voltage and current levels to the traction battery  24 . The power conversion module  32  may interface with the EVSE  38  to coordinate the delivery of power to the vehicle  12 . The EVSE connector  40  may have pins that mate with corresponding recesses of the charge port  34 . The various components discussed may have one or more associated controllers to control and monitor the operation of the components. The controllers may communicate via a serial bus (e.g., Controller Area Network (CAN)) or via discrete conductors. 
     Current examples of thermal management assemblies for electric machines may introduce oil to portions of the electric machine for cooling purposes. The oil may be dripped or sprayed onto wire end windings of the electric machine. However, this practice may not be very effective in cooling the end windings due to a non-uniformity of coolant flow as applied to the end windings. An air cooled thermal management assembly is another example of an assembly to assist in managing thermal conditions of an electric machine. In this example, a fan or blower may be located adjacent the end windings to push air thereto for cooling purposes. 
       FIG. 1B  shows an example of an electric machine for an electrified vehicle, referred to generally as an electric machine  100  herein. The electric machine may include a stator core  102  and a rotor  106 . Electrified vehicles may include two electric machines. One of the electric machines may function primarily as a motor and the other may function primarily as a generator. The motor may operate to convert electricity to mechanical power and the generator may operate to convert mechanical power to electricity. The stator core  102  may define an inner surface  108  and a cavity  110 . The rotor  106  may be sized for disposal and operation within the cavity  110 . A shaft (not shown) may be operably connected to the rotor  106  to drive rotation thereof. 
     Windings  120  may be disposed within the cavity  110  of the stator core  102 . In an electric machine motor example, current may be fed to the windings  120  to obtain a rotation force on the rotor  106 . In an electric machine generator example, current generated in the windings  120  by a rotation of the rotor  106  may be removed to power vehicle components. Portions of the windings  120 , referred to as end windings  126  herein, may protrude from the cavity  110 . During operation of the electric machine  100 , heat may be generated along the windings  120  and end windings  126 . 
     In at least one embodiment, the present disclosure includes the application of a coating or coatings to affect, alter, and/or control coolant distribution on an electronic device, such as an electric machine or a power inverter. The coating(s) may be a (super) hydrophobic and/or (super) oleophobic coating or a (super) hydrophilic and/or (super) oleophilic coating. Hydrophobic and oleophobic coatings are those that repel or have a very high contact angle with water and oil, respectively. Hydrophilic and oleophilic coatings are those that attract or have a very low contact angle with water and oil, respectively. With reference to  FIG. 2 , a schematic of a droplet of water or oil (e.g., automatic transmission fluid, ATF) is shown on a super hydrophobic/oleophobic coating. As shown, the droplet forms almost a perfect sphere on the coating due to its repellence. Super hydrophobic/oleophobic coatings may cause droplets of water/oil to form contact angles of 150 degrees or higher with the coating. In contrast, on super hydrophilic and oleophilic coatings, droplets spread out and form an almost even layer on top of the coating. Super hydrophilic/oleophilic coatings may cause droplets of water/oil to form contact angles of 25 degrees or lower with the coating. 
     Any suitable hydrophobic, oleophobic, super hydrophobic, or super oleophobic coatings may be used in accordance with the present disclosure. In general, such coatings may have a high contact angle with the water or oils. Hydrophobic/oleophobic materials may generally be those forming a contact angle of at least 90 degrees, such as at least 100, 110, 120, 130, or 140 degrees, while super hydrophobic/oleophobic materials may generally be those forming a contact angle of at least 150 degrees with water/oil. The coatings may form such high contact angles as a result of a nano-scale surface structure. For example, the surface of the coating may be covered in very small projections, making the coating rough on a nano-scale. The gaps between the projections may trap air and make it energetically unfavorable for liquids to wet the surface. Similarly, any suitable hydrophilic, oleophilic, super hydrophilic, or super oleophilic coatings may be used in accordance with the present disclosure. In general, such coatings may have a low contact angle with the water or oils. Hydrophilic/oleophilic materials may generally be those forming a contact angle of at most 50 degrees, such as at most 40 or 30 degrees, while super hydrophilic/oleophilic materials may generally be those forming a contact angle of 25 degrees or less with water/oil. 
     Examples of various (super) hydrophobic/oleophobic and (super) hydrophilic/oleophilic compositions and treatment methodologies are provided in U.S. Patent Publication Nos. 2013/0109261, 2012/0009396, 2010/0314575, 2012/0045954, and 2006/0029808, and also in U.S. Pat. Nos. 8,007,638, 6,103,379, 6,645,569, 6,743,467, 7,985,451, 8,187,707, 8,202,614, 7,998,554, 7,989,619, 5,042,991, 8,361,176, 8,258,206, 6,458,867, 6,503,958 and 6,723,378, and also in International Publication No. WO2013/058843, the disclosures of which are incorporated herein by reference. 
     The (super) hydrophobic/oleophobic coating may be applied to the electronic device using any suitable method, which may depend on the composition of the coating itself. In one embodiment, the coating may be applied by spraying. In another embodiment, the coating may be applied using a form of deposition, such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). In another embodiment, the coating may be physically transferred to the electronic device, such as by rolling or brushing. Regardless of the method of application, masks may be used to only coat certain desired areas or regions. 
     With reference to  FIGS. 3 and 4 , a schematic example and an experimental sample are shown of a hydrophobic/oleophobic (herein after, “repellant”) coating forming a flow path  50  for a liquid, such as a coolant (e.g., water or ATF). The flow path  50  may be formed by one or more lines, strips, layers, or areas  52  of the repellant coating. In the example shown in  FIG. 3 , two spaced apart lines  52  form the flow path  50 . The lines  52  may be evenly spaced to form a flow path  50  having a constant or substantially constant width. However, in other embodiments, the flow path  50  may have a non-constant width. The lines  52  may be configured to form a flow path  50  having any shape or pattern. In the embodiment shown in  FIG. 3 , the lines  52  for a zig-zag flow path  50 , which may also be referred to as a sinuous, serpentine, winding, or oscillating flow path  50 . 
     In at least one embodiment, the line(s)  52  of the repellant coating may form the flow path  50  without any raised walls or sunken/carved channels. That is, the flow path  50  may be formed due solely to the repellence of the liquid (e.g., coolant) from the coating. The use of the lines, strips, etc. of the repellant coating may therefore allow for control or influence of the flow of coolant without the need for relatively large physical barriers, such as channels walls or channels trenches or troughs. Instead, a thin coating may be used to control/influence the flow of coolant. As shown in  FIG. 4 , the coating may be very thin so as to be essentially even or flush with the surface applied thereto relative to the height of the coolant being controlled. For example, the coating may have a thickness of less than 1 mm, such as less than 500 μm, 250 μm, 100 μm, 50 μm, 25 μm, or 15 μm. 
     In at least one embodiment, in addition to the line(s)  52  of the repellant coating, a hydrophilic or oleophilic coating  54  may also be applied to form the flow path  50 . In one embodiment, at least a portion of the area of the flow path  50  may be coated with the hydrophilic or oleophilic coating (herein after “wetting coating”). For example, the entire area of the flow path  50  may be coated with the wetting coating. In another embodiment, lines of the wetting coating may be applied within the flow path  50 . For example, lines may be applied that are parallel to the lines  52  of the repellant coating. Accordingly, the wetting coating may help further control the flow of a coolant in the flow path. The lines  52  of the repellant coating may form an outer boundary of the flow path  50  while the wetting coating encourages the coolant to flow across it in the desired direction. 
     With reference to  FIGS. 5A-5F , examples of different shapes and patterns of flow paths  50  that may be formed using the repellant coating are shown. The patterns shown are merely examples, however, and other patterns may be formed depending on the desired flow path.  FIG. 5A  shows a straight flow path  50  having a constant width, which may be formed by spaced apart parallel lines  52  of repellant coating.  FIG. 5B  shows a zig-zag flow path  50 , which may also be referred to as a sinuous, serpentine, winding, or oscillating flow path  50 . This flow path may be formed by spaced apart lines  52 . Segments of the lines  52  may be parallel, such that the flow path  50  has a constant width, however, this is not required.  FIG. 5C  shows a contracting flow path  50 , in which the width of the flow path becomes smaller in one direction (e.g., from top to bottom, as shown).  FIG. 5D  shows an expanding flow path  50 , in which the width of the flow path becomes larger in one direction (e.g., from top to bottom, as shown).  FIG. 5E  shows a flow path  50  that contracts or narrows over a certain distance and then the contraction stops at an inflection point and then expands or widens. This may be referred to as an hourglass flow path.  FIG. 5F  shows a flow path  50  that expands or widens over a certain distance and then the expansion stops at an inflection point and then contracts or narrows. This may be referred to as a bulging flow path. Similar to above with respect to  FIGS. 3-4 , a wetting coating  54  may also be applied in the flow path  50 , however, it is not required. 
     The examples shown and described with reference to  FIGS. 3-5  relate to flow paths in which there are two spaced apart lines or strips of coating to form one flow path. However, there may be fewer lines (e.g., one) or there may be more lines (e.g., three or more) to control the flow of a liquid, such as coolant. For example, a single line  52  may be applied to a surface in order to prevent or reduce the flow of liquid from passing that line and/or to keep the liquid on one side of the line. In other examples, three or more lines  52  may be applied to a surface to create multiple flow paths  50 . In one embodiment, a single flow path  50  may be split into multiple flow paths, such as two (bifurcated), three (trifurcated), or more flow paths. Of course, multiple sets of lines  52  may also be used to form a plurality of discrete flow paths  50 . Similarly, lines, layers, or strips of the wetting coating  54  may be applied to attract or encourage coolant to flow in a direction or path of the coating  54 . The wetting coating  54  may be used in conjunction with the repellant coating for even greater control of the coolant flow. 
     In at least one embodiment, the repellant coating may be used to form one or more coolant flow paths  50  on an electric machine, such as electric machine  100 . For example, the repellant coating may be used to form flow paths  50  on the windings  120 , such as the end windings  126  of an electric machine. During operation of an electric machine, coolant, such as an oil-based coolant (e.g., ATF), may be introduced onto the end windings  126  to remove heat therefrom. The coolant may be introduced onto the end windings  126  in a variety of ways, depending on the design of the particular electric machine. In some designs, coolant may be pumped or otherwise released onto the end windings  126 . The coolant may be introduced at the top of the end windings or on an upper portion of the end windings and allowed to fall by gravity to cool the lower portion of the end windings. 
     With reference to  FIG. 6 , a schematic example of an end view of an electric machine end windings  126  is shown. In the example shown, coolant may be introduced onto a top portion  128  of the end windings  126  on one side of center, as indicated by the arrow  130 . The coolant may be introduced at an angle, such that the coolant flow has a horizontal and a vertical component when it contacts the end windings  126 . This may cause the coolant to flow around the top of the end windings  126  and onto the opposite side, as well as down the side on which the coolant was introduced, as indicated by arrows  132 . 
     In one embodiment, the repellant coating may be applied to the end windings  126  in order to control or affect the flow of the coolant over the end windings  126 . The repellant coating may be used to form flow paths that provide more uniform coolant flow over the end windings than if no coating were present. The repellant coating may also be used to channel or guide coolant to areas requiring increased cooling or areas that would receive insufficient cooling if the coolant flow was not adjusted. For example, areas that receive insufficient cooling may form areas called “hot spots.” Other areas that may require or benefit from additional coolant flow may include the neutral point. The neutral point may be the connection of all three phase wires for a three phase electric motor. Since the neutral point joins all wires at one point, heat can be concentrated at that point and may form a type of hot spot. 
     In the embodiment shown in  FIG. 6 , lines  52  of the repellant coating are applied to increase the coolant flow in certain portions of the end windings  126 . The lines  52  may be arranged as arcs extending around the end windings. For example, the arcs may be concentric with the end windings (e.g., circular or circular segments sharing a center with the end windings). In the embodiment shown, one line  52 ′ may be positioned in a top portion  128  of the end windings and may be a concentric arc that is at or near the midpoint of the of end windings (e.g., halfway between the inner and outer radius of the end windings). As shown in  FIG. 6 , this line  52 ′ may increase the coolant flow in a region  134  above the line  52 ′. Due to the repellant nature of the coating (e.g., oleophobic), the coolant, such as ATF, may remain above the line  52 ′ on end windings  126  instead of immediately falling or dipping downward due to the force of gravity. Instead, the coolant may flow along the top of the end windings  126  and down the side opposite from where the coolant was introduced (e.g., arrow  130 ). The line  52 ′ of repellant coating may therefore increase the coolant flow and/or the time the end windings in region  134  are in contact with the coolant, resulting in improved heat removal in region  134 . Similar to above, the region  134  may also include a line, layer, strip, etc. of a wetting coating  54  to further encourage the coolant to flow thereon. A portion of all of the region  134  may be coated with the wetting coating  54 . 
     In addition to, or instead of, the line  52 ′, one or more additional lines  52  may also be applied to the end windings  126 . For example, a line  52 ″ may be applied to a bottom portion  136  of the end windings  126 . Similar to line  52 ′, line  52 ″ may be an arc and may be concentric with the end windings. Line  52 ″ may also be at a midpoint of the windings. In the embodiment shown, the line  52 ″ may be located on a side of the end windings opposite the side where coolant is introduced (e.g., arrow  130 ). However, the line  52 ″ may be located on the same side as the coolant or centered, similar to line  52 ′. Positioning the line  52 ″ on the opposite side of the coolant inlet may further facilitate even or uniform coolant flow, particularly if line  52 ′ is present. Line  52 ″ may cause the coolant that stayed above line  52 ′ to continue to move horizontally across the end windings  126  rather than dripping vertically downward. There may be a gap region  138  between line  52 ′ and line  52 ″. The gap region may be located in a middle region of the height of the end windings  126 . When the coolant from region  134  flows down the end windings, some of the coolant will be further guided radially outward to a region  140  by the line  52 ″ while some coolant will be guided radially inward by the line  52 ″ to a region  142 . Similar to the coolant being maintained above line  52 ′, the coolant in region  142  may be maintained above line  52 ″ instead of flowing downward. This may increase the flow of coolant to the end windings in this area and/or increase the contact time of the coolant with the windings in the area. Regions  140  and/or  142  may have a wetting coating  54  applied thereon, similar to region  134 . 
     While two lines  52 ′ and  52 ″ are shown and described in  FIG. 6 , there may be fewer (e.g., one) or more (e.g., three or greater) lines  52 . Based on the present disclosure, one of ordinary skill in the art will understand that the lines  52  may be positioned in order to guide or increase contact time of the coolant with areas of the end windings  126  that may benefit from it, based on the design of the particular electric machine. While the lines  52  are shown as solid lines, they may be dashed or intermittent, such that there are gaps in the lines that allow some coolant to flow through. This may allow for the end windings on the other side of the lines to still receive coolant flow. In addition, while the coating may prevent a certain volume or flow of coolant from crossing the lines, at a certain volume or flow rate, a portion of the coolant may cross the line due by overcoming the repellant force. 
     With reference to  FIGS. 7-9 , additional examples of repellant coatings used to affect coolant flow are shown. The Embodiments shown each include at least two lines  62  of the repellant coating to form at least one flow path  60  on the end windings  126  of an electric machine  100 . In the embodiment of  FIG. 7 , the flow path  60  may include a single flow portion  60 ′ that is split or bifurcated into two secondary flow paths  60 ″. The flow path  60  may include two spaced apart outer lines  62  of repellant coating that may define the outer boundaries of both the single flow portion  60 ′ and the secondary flow paths  60 ″. Two inner lines  62 ′ may be disposed between the outer lines  62  and for define the inner boundaries of the secondary flow paths  60 ″. The inner lines  62 ′ may begin at an intersection  64  and branch out from each other to form the inner boundaries of the secondary flow paths  60 ″. Accordingly, in the embodiment shown in  FIG. 7 , coolant may flow down from the top of the end windings  126  through the single flow portion  60 ′ of the flow path  60  and then may be split into two secondary flow paths  60 ″ at a bottom portion of the end windings  126 . However, it is to be understood that the particular orientation of the flow paths and the location of the split are merely examples and not limiting. For example, the secondary flow paths  60 ″ could be facing the top of the windings, receive the coolant separately, and then funnel it to a single flow portion  60 ′ at a bottom portion of the windings. 
     As shown in  FIG. 7 , the secondary flow paths  60 ″ may be configured to direct coolant such that it contacts and/or flows over a hot spot  66 . As known to those of ordinary skill in the art, hot spots may occur in the windings for a variety of reasons. Hot spots may occur due to factors such as the winding pattern, the density of the windings, the current density, exposure to air or coolants, proximity to the central windings or other heat sinks, or other factors. Based on the design of the electric machine, the hot spot(s)  66  may be in predictable locations. Accordingly, the flow path(s)  60  may be configured such that extra coolant is delivered to the hot spots  66  or so that the hot spots  66  receive similar coolant flow to other areas of the windings. As shown in  FIG. 7 , the secondary flow paths  60 ″ may each be configured to guide coolant to a hot spot  66 . Similar to above with respect to  FIGS. 3-4 , a wetting coating may also be applied within the flow paths  60 ,  60 ′, and/or  60 ″ to further encourage coolant to flow therethrough. 
     With reference to  FIG. 8 , an embodiment is shown in which two spaced apart lines  72  of repellant coating form a flow path  70  on the end windings  126  of an electric machine  100 . In this embodiment, the spacing between the lines  72  changes as the lines extend from a top portion of the end windings towards a bottom portion of the end windings. The line spacing may decrease along a first region  74  of the end windings such that the flow path  70  narrows in the first region  74 . At an inflection point  76 , the line spacing (e.g., flow path width) may be at a (local) minimum and the line spacing may begin to increase to form a second region  78  in which the flow path  70  widens. This line spacing pattern may be referred to as a contracting-to-expanding pattern or an hourglass pattern. Similar to  FIG. 7 , the pattern of the flow path  70  may be configured to guide coolant to a hot spot  66 , shown in  FIG. 8  as located within the second region  78  (e.g., expanding portion). Similar to above with respect to  FIGS. 3-4 , a wetting coating may also be applied within the flow path  70  to further encourage coolant to flow therethrough. 
     With reference to  FIG. 9 , an embodiment is shown in which two spaced apart lines  82  of repellant coating form a flow path  80  on the end windings  126  of an electric machine  100 . In this embodiment, the spacing between the lines  82  changes as the lines extend from a top portion of the end windings towards a bottom portion of the end windings. The line spacing may increase along a first region  84  of the end windings such that the flow path  80  widens in the first region  84 . At an inflection point  86 , the line spacing (e.g., flow path width) may be at a (local) maximum and the line spacing may begin to decrease to form a second region  88  in which the flow path  80  narrows. This line spacing pattern may be referred to as an expanding-to-contracting pattern or a bulging pattern. Similar to  FIGS. 7 and 8 , the pattern of the flow path  80  may be configured to guide coolant to a hot spot  66 , shown in  FIG. 9  as located at or near the inflection point  86  (e.g., bulging portion). Similar to above with respect to  FIGS. 3-4 , a wetting coating may also be applied within the flow paths  60 ,  60 ′, and/or  60 ″ to further encourage coolant to flow therethrough. 
     In the embodiments described above, the end windings (e.g., copper windings) are described as being covered or coated by a repellant coating, however, the coating may also extend over other accessory or ancillary materials or components related to the end windings. For example, the accessory or ancillary materials may include support materials or fasteners, such as straps or ties that hold the metal (e.g., copper) windings in place. These materials may be disposed on an outer surface of the end windings to secure them. Accordingly, the lines of the repellant coating, such as lines  52 ,  62 ,  72 , and  82  may traverse over both the metal windings and the support materials to form the flow paths (e.g.,  50 ,  60 ,  70 ,  80 ). The flow paths may therefore also extend over both the metal windings and the support materials. Therefore, as used herein, a line, strip, etc. of the coating that is described as extending over windings or end windings may also extend over accessory or ancillary materials associated therewith. 
     With reference to  FIGS. 10-12 , the repellant coating may also be used for preventing a liquid (e.g., a coolant, such as water or oil) from falling or passing through gaps or channels between two surfaces. The two surfaces may be surfaces of components of an electric machine, such as electric machine  100 . For example, the surfaces may be on adjacent windings (e.g., end windings) or adjacent metal bars (e.g., copper bars). In the embodiment shown in  FIG. 10 , there are two adjacent surfaces  90  shown in transverse cross-section. The surfaces  90  may be slanted towards each other such that a gap  92  between them is narrower at their bottoms than at their tops. Accordingly, absent any barrier, a liquid (e.g., a coolant) would flow between the two surfaces and fall through the gap  92  at the bottom. However, by applying a layer  94  of the repellant coating at or near the bottom of each surface  90 , the coolant may be prevented from falling through the gap  92  (or a lesser amount may fall through). Applying the layers  94  may therefore allow coolant to flow over gaps between two surfaces, which may increase the flow on top of and/or across the surfaces. 
     With reference to  FIG. 11 , a side perspective view of end windings  126  of an electric machine  100  are shown. An arrow  96  indicates a flow of coolant around the circumference of the end windings  126 . Similar to above, absent a barrier, the coolant may be free to fall through gaps  92  between individual bundles of windings, which may reduce the coolant contact time with the windings and reduce the heat removal from the windings. However, if a layer  94  of the repellant coating is applied on either side of the gap  92 , then the coolant (e.g., ATF) may be prevented from falling through the gap  92 , or a reduced amount of coolant may fall through the gap  92 . While a single pair of layers  94  is shown in  FIG. 11 , there may be a plurality of pairs of layers  94 . For example, each gap  92  may have a layer  94  on either side. Alternatively, certain gaps  92  may have a layer  94  on each side, such as those that tend to have a lot of coolant fall therethrough or in areas where increased coolant contact time is desired. In one embodiment, a plurality of gaps  92  on a top half of the end windings  126  may include a layer  94  on either side to encourage coolant flow around the circumference of the windings instead of immediately falling through them. Similar to above with respect to  FIGS. 3-4 , a wetting coating may also be applied to the end windings  126  to further encourage coolant to flow along the path of arrow  96 . For example, the wetting coating may be applied parallel to the arrow  96  and/or perpendicular to the layers  94  of the repellant coating. 
     With reference to  FIG. 12 , a schematic of a different style of end windings  126  are shown, known as hairpin windings. Hairpin windings are known in the art as a type of winging and will not be described in detail. In general, the copper wires of a conventional winding may be replaced with metal (e.g., copper) strips or bars  98 . Similar to the bundles of windings shown in  FIG. 11 , there may be gaps  92  between adjacent strips  98  through which coolant may fall. Accordingly, layers  94  of the repellant coating may be applied to the strips  98  on either side of the gaps  92  to prevent coolant from falling through, or to reduce the amount that falls through. The layers  94  may be applied to the side edges of the strips (e.g., perpendicular to the top surface), as shown, or they may be applied to the edges of the strips  98  on the top surfaces, or both. Similar to the embodiment of  FIG. 11 , some or all of the gaps may have layers  94  on either side, such as those on the top half of the end windings. 
     Accordingly, (super) hydrophobic and/or oleophobic coating materials and/or (super) hydrophilic or oleophilic coating materials may be utilized in electric machines thermal management. The use of these coating materials may allow for a uniform flow distribution without additional coolant loop(s) and pumping power, which may require additional costs and may result in efficiency losses. In addition, the coating materials may improve the reliability of electric machines by eliminating localized hot spots and decreasing power loss due to the reduction of end-winding temperatures. Moreover, a reduction in electric machine size may be realized due to the super hydrophobic/oleophobic coated surfaces. 
     As described above, the repellant coating may also be applied to a power inverter cooling system, such as in a cooling plate. With reference to  FIGS. 13-15 , embodiments are shown of a cold plate  200  including a repellant coating for controlling or affecting coolant flow therein.  FIG. 13  shows a schematic example of a cold plate  200  having an inlet  202  and an outlet  204 . Between the inlet  202  and the outlet  204  may be the cooling or heat-transfer region or area  206 . Between the inlet  202  and the cooling region  206  there may be a flow spreading region or area  208  and between the cooling region  206  and the outlet  204  there may be a flow collecting region or area  210 . In one embodiment, the inlet  202  and/or the outlet  204  may be narrower (e.g., smaller width) that the cooling region  206 . Therefore, the flow spreading region  208  may increase in width from the inlet  202  to the cooling region  206  and/or the flow collection region  210  may decrease in width from the cooling region  206  to the outlet  204 . The increase or decrease may be continuous, however it is not required. For example, the increase/decrease may be at a constant rate (e.g., a straight line, as shown), or it may be non-constant (e.g., a curve or exponential). 
     Since the inlet  202  and/or the outlet  204  may be narrower than the cooling region  206 , there may be non-uniform flow of coolant out of the inlet and/or into the outlet. To increase the uniformity of the coolant flow distribution, physical barriers may be included in the flow spreading or flow collection regions, such as channel walls or flow steps. However, this may result in increased pressure drops within the cold plate  200 , which may require increased pumping power of the coolant. This increased pressure drop and pumping power may reduce the efficiency of the cooling system. 
     With reference to  FIG. 13 , it has been discovered that lines  212  of the repellant coating described above may be used to improve the uniformity of the coolant flow in the flow spreading and/or the flow collection regions. As shown, one or more lines  212  of the repellant coating may be applied in the flow spreading region  208  and/or the flow collecting region  210 . In the flow spreading region  208 , the line(s) may at least partially redirect the flow from the inlet  202  such that it is spread out when it reaches the cooling region  206 . For example, there may be a plurality of lines  212 , at least some of which fan out from the inlet  202  towards the cooling region  206 . A portion of the lines  212  may extend from an area adjacent to the inlet  202  to an area adjacent to the cooling region  206 . Some of the lines  212  may extend outward in a direction from the inlet  202  to the cooling region  206 , similar to the expanding width of the flow spreading region  208 . The lines  212  may form one or more (e.g., a plurality) of flow channels  214  extending between the inlet  202  and the cooling region  206 . In addition to lines  212  that extend from the inlet  202  to the cooling region  206 , there may be additional, shorter lines  212  that sub-divide a flow channel  214  into secondary or smaller flow channels  216 . 
     In the flow collecting region  210 , the line(s) may at least partially redirect the flow from the cooling region  206  such that it is condensed or funneled towards the outlet  204 . For example, there may be a plurality of lines  212 , at least some of which fan out from the outlet  204  towards the cooling region  206 . Alternatively, the lines  212  may be described as funneling towards the outlet  204 . A portion of the lines  212  may extend from an area adjacent to the outlet  204  to an area adjacent to the cooling region  206 . Some of the lines  212  may extend inward in a direction from the cooling region  206  towards the outlet  204 , similar to the narrowing or contracting width of the flow collecting region  210 . The lines  212  may form one or more (e.g., a plurality) of flow channels  214  extending between the outlet  204  and the cooling region  206 . In addition to lines  212  that extend from the outlet  204  to the cooling region  206 , there may be additional, shorter lines  212  that sub-divide a flow channel  214  into secondary or smaller flow channels  216 . Similar to above with respect to  FIGS. 3-4 , a wetting coating  218  (e.g., similar to wetting coating  54 ) may also be applied within the flow channels  214  and/or  216  to further encourage coolant to flow therethrough. For example, the wetting coating  218  may be applied to a portion or all of the flow channels  214 / 216 , such as in the form of lines or as a continuous layer. The wetting coating(s)  218  may therefore further facilitate the spreading/collection of the coolant within the flow channels. 
     With reference to  FIGS. 14A and 14B , the repellant coating may also be applied to internal coolant channels  220  within the cold plate  200  to improve flow therein. The cooling region  206  shown in  FIG. 13  may include one or more coolant channels  220  through which the coolant flows to exchange heat. The coolant channels  220  may generally form a winding, tortuous, and/or serpentine shape that increases the contact time of the coolant with the cold plate (relative to a straight path from the inlet to the outlet). Any suitable channel shape or path may be used with the disclosed cold plate  200 . In general, the channels  220  will include at least one turn therein having a relatively large angle. In the examples shown in  FIGS. 14A and 14B , the channel walls  222  form a turn having a 180 degree angle (e.g., complete reversal of direction). However, other turn angles are possible and the illustrated turns are merely an example. 
     As shown in  FIG. 14A , it has been found that large turns in the coolant channels  220  may result in regions or zones where the coolant stagnates and/or recirculates (e.g., forms eddies). These zones, referred to herein as recirculation zones  224 , may result due to low or no-slip conditions at the channels walls  222 . These conditions may be causes by the relatively high angle turns in the channels  220 . The recirculation zones  224  may occur in the corner(s)  226  of a high angle turn, as shown in  FIG. 14A . Accordingly, a portion of the coolant may become stuck in the recirculation zones  224  during operation of the cold plate  200  and may reduce the effectiveness and/or efficiency of the cold plate  200 . 
     With reference to  FIG. 14B , it has been discovered that placing one or more layers  228  of the repellant coating in the corners  226  of the channels  220  may change the walls  222  from no/low-slip to a slip condition, thereby reducing or eliminating the recirculation zones  224 . In one embodiment, where two walls  222  meet to form a corner  226 , a layer  228  of the repellant coating may be applied to one or both walls  222  at or near the meeting point of the walls  222 . The term corner does not necessarily mean a 90 degree intersection, but may refer to two walls meeting at an angle (e.g., 45-135 degrees, 60-120 degrees, or 75-105 degrees). In the embodiment shown in  FIG. 14B , there are two 90 degree corners formed in the outer walls  222  of the channel  220 . A layer  228  of repellant coating is applied on the walls  222  on either side of the corner  226 . As shown, the layers  228  of the repellant coating may prevent the formation of recirculation zones  224  and may cause most or all of the coolant to continue flowing around the turn instead of stagnating. In areas where there is relatively little turn in the channels  220 , there may be no coating applied. Accordingly, by applying a liquid repellant coating in areas with sharp turns, coolants in a cold plate cooling region may flow more evenly and cooling may be performed more effectively and/or efficiently. Similar to above with respect to  FIGS. 3-4 , a wetting coating  218  may also be applied within the channels  220  to further encourage coolant to flow therethrough. For example, the wetting coating  218  may be applied to a portion or all of the channel  220 , such as in the form of lines or as a continuous layer. In one embodiment, the wetting coating  218  may be applied in an arcuate direction in the area indicated by the flow arrows. The wetting coating(s)  218  may therefore further facilitate the flow of coolant through the channels  220  so that it does not get stuck in recirculation zones  224 . 
     With reference to  FIG. 15 , an example of a partial cut-away top view is shown of a cooling region of a cold plate. As described with reference to  FIGS. 14A and 14B , the cooling region may include channels  220  formed by walls  222 . The arrows  230  indicate the direction of coolant flow through the channels  220 . To enclose the channels  220 , the cold plate may include a cover  232  that is configured to contact the top surfaces  234  of the walls  222 , thereby sealing off one channel  220  from another. However, it has been found that in practice, small gaps may occur between the top surfaces  234  of the walls  222  and the cover  232 . The cover  232  may be a rigid or semi-rigid plate or sheet. During manufacturing, small variations in the tolerances of the walls  222  and/or the cover  232  may cause the cover  232  to not completely cut off the flow of coolant between the cover  232  and the top surfaces  234  of the walls  222 . If such gaps exist, it may reduce the efficiency and/or effectiveness of the cold plate cooling. For example, gaps may allow relatively cold coolant at or near the inlet to mix with relatively warm coolant at or near the exit. Gaps may also allow coolant to take a more direct path from the inlet to the outlet without completely flowing through the channels  220 , which, as described above, may be configured to be winding, tortuous, etc. to increase the contact time of the coolant. 
     In at least one embodiment, it has been discovered that a repellant coating (e.g., hydrophobic or oleophobic, described above) may be use as a gap filler and/or barrier to prevent cross-flow of coolant. Layers  236  of the repellant coating may be applied to the top surfaces  234  of the walls  222 . Accordingly, even if gaps exist between the top surfaces  234  and the cover  232 , the coolant may be repelled from passing through the gap into another channel  220 . In addition to, or instead of, applying the coating to the top surfaces  234 , the layers  236  may be applied to an underside of the cover  232  in areas corresponding to the location of the top surfaces  234  when the cold plate is assembled. 
     Accordingly, (super) hydrophobic and/or oleophobic coating materials and/or (super) hydrophilic and/or oleophilic coating materials may be utilized for the inverter thermal management. The use of these coating materials may allow for a uniform flow distribution without adding physical channel walls or steps and pumping power. These additions may add costs, increase pressure drop, and add efficiency losses. In addition, the coatings may improve the reliability of the inverter system by eliminating localized hot spots and/or decreasing power losses due to the reduction of component temperature increases caused by the localized hot spots. 
     While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention.