Patent Publication Number: US-2022231625-A1

Title: Electric machines with air gap control systems, and systems and methods of controlling an air gap in an electric machine

Description:
FIELD 
     The present disclosure relates to electric machines with air gap control systems, and systems and methods of controlling an air gap in an electric machine. 
     BACKGROUND 
     Electric machines are used in a wide variety of settings, including industrial, commercial, and consumer applications. A typical electric machine includes a rotor core assembly and a stator core assembly that circumferentially surrounds the rotor core assembly. The space between the inner surface of the stator core and the outer surface of the rotor core assembly is commonly referred to as an “air gap.” An air gap of sufficient length is necessary to allow the rotor core assembly to freely rotate without contacting the stator core. While the length of an air gap in an electric machine may depend on the particular electric machine, typically it is desirable to minimize the length L of the air gap because air has a high magnetic reluctance. Additionally, resistive losses in an electric machine each attributable to eddy currents, windage, and hysteresis typically increase as the length L of the air gap increases. 
     Electric machines with improved energy efficiency are desired. High performance applications typically call for electric machines with a high-power density, meaning that electric machines with increasingly larger power outputs and yet increasingly smaller machine sizes are desired. The energy efficiency of an electric machine may be described as a ratio of its useful power output to its total power input, typically expressed as a percentage. The difference between the useful power output of an electric machine and its total power output are sometimes referred to as resistive losses. Resistive losses attributable to the air gap in an electric machine are a major contributor to an electric machine&#39;s energy efficiency. A smaller air gap length can reduce the magnetizing power requirement of the electric machine and thereby improve its energy efficiency. However, when an air gap becomes too small there is an increased risk of contact between the rotor core and the stator core. Even slight contact between the rotor core and the stator core may cause damage to the electric machine and may also diminish the performance and energy efficiency of an electric machine. More significant contact between the rotor core and the stator core may cause more severe damage to the electric machine including potentially catastrophic failures. 
     Electric machines with an optimized air gap length are desired, as an optimized air gap length can lead to improved performance and energy efficiency. However, there are numerous operating parameters and/or operating conditions that can affect both the actual length of the air gap as well as the optimum air gap when operating an electric machine under a given set of operating parameters and/or operating conditions. Such operating parameters and/or operating conditions may be complexly interrelated and variable depending on how a particular electric machine is actually or uniquely operated. 
     Accordingly, there exists a need for electric machines with air gap control systems, and for systems and methods of controlling an air gap in an electric machine. 
     BRIEF DESCRIPTION 
     Aspects and advantages will be set forth in part in the following description, or may be obvious from the description, or may be learned through practicing the presently disclosed subject matter. 
     In one aspect, the present disclosure embraces methods of controlling a length of an air gap in an electric machine using an air gap controller. An exemplary method may include: determining an air gap length value for an electric machine at least in part using an air gap controller, comparing the determined air gap length value to an air gap target value using the air gap controller, and outputting one or more control commands from the air gap controller to one or more controllable devices associated with an air gap control system when the determined air gap length value differs from the air gap target value by a predefined threshold. 
     The one or more control commands may be configured to impart a change to one or more operating parameters associated with the air gap control system to adjust a length of an air gap between an outer surface of a rotor core and an inner surface of a stator core of the electric machine. The air gap length value may be determined based at least in part on an air gap length model, and the air gap length model may be to utilize one or more model inputs to calculate the air gap length value. The one or more controllable devices and/or the one or more operating parameters may be associated with an electric machine and/or an air gap control system. 
     In another embodiment, an exemplary method of controlling a length of an air gap in an electric machine using an air gap controller may include: receiving at an air gap controller, one or more model inputs comprising data associated with an air gap control system, determining an adjusted air gap target value for an electric machine at least in part using the air gap controller, comparing an air gap length value for the electric machine to the adjusted air gap target value using the air gap controller, and outputting one or more control commands from the air gap controller to one or more controllable devices associated with the air gap control system when the air gap length value differs from the adjusted air gap target value by a predefined threshold. The one or more control commands may be configured to impart a change to one or more operating parameters associated with the air gap control system to adjust a length of an air gap between an outer surface of a rotor core and an inner surface of a stator core of the electric machine. 
     In another aspect, the present disclosure embraces air gap control systems. An exemplary air gap control system may include an electric machine, a coolant circulation system, and an air gap controller. An exemplary electric machine may include a rotor core assembly having a rotor core and a rotor shaft operably coupled to the rotor core, and a stator core assembly having a stator core and a stator housing operably coupled to the stator core, with the stator core circumferentially surrounding the rotor core. An exemplary electric machine may further include an air gap having a length, L, with the air gap located between and defined by an inner surface of the stator core and an outer surface of the rotor core. An exemplary coolant circulation system may include a cooling conduit defining a pathway for circulating coolant through the electric machine and/or an air conduit defining a pathway for supplying cooling air to the electric machine. 
     An exemplary air gap controller may be configured to control the length, L of the air gap at least in part by controlling or more of: a temperature of coolant flowing through the cooling conduit, a flow rate of coolant flowing through the cooling conduit, a temperature of cooling air flowing through the air conduit, a power input to the electric machine, and/or a rotor shaft speed of the electric machine. By way of example, the presently disclosed air gap control systems may be implemented in an aircraft, a marine vessel, or a motor vehicle. 
     These and other features, aspects and advantages will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate exemplary embodiments and, together with the description, serve to explain certain principles of the presently disclosed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended Figures, in which: 
         FIGS. 1A-1C  schematically depict exemplary air gap control systems; 
         FIG. 2  schematically depicts an exemplary air gap controller which may be implemented in an air gap control system; 
         FIG. 3  schematically depicts and exemplary air gap control model which may be utilized by an air gap controller; 
         FIG. 4  is a chart depicting an exemplary correlation between an air gap target value and an operating parameter; 
         FIG. 5  schematically depicts an exemplary training computing device which may be used to train an air gap target model; and 
         FIG. 6  is a flow chart depicting an exemplary method of controlling a length of an air gap in an electric machine. 
       Repeat use of reference characters in the present specification and drawings is intended to represent the same or analogous features or elements of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Reference now will be made in detail to exemplary embodiments of the presently disclosed subject matter, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation and should not be interpreted as limiting the present disclosure. In fact, it will be apparent to those skilled in the art that various modifications and variations may be made in the present disclosure without departing from the scope or spirit of the present disclosure. For instance, features illustrated or described as part of one embodiment may be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present disclosure covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     It is understood that terms “upstream” and “downstream” refer to the relative direction with respect to fluid flow in a fluid pathway. For example, “upstream” refers to the direction from which the fluid flows, and “downstream” refers to the direction to which the fluid flows. It is also understood that terms such as “top”, “bottom”, “outward”, “inward”, and the like are words of convenience and are not to be construed as limiting terms. As used herein, the terms “first”, “second”, and “third” may be used interchangeably to distinguish one component from another and are not intended to signify location or importance of the individual components. The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. 
     Here and throughout the specification and claims, range limitations are combined and interchanged, and such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. For example, all ranges disclosed herein are inclusive of the endpoints, and the endpoints are independently combinable with each other. 
     Approximating language, as used herein throughout the specification and claims, is applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about”, “approximately”, and “substantially”, are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value, or the precision of the methods or machines for constructing or manufacturing the components and/or systems. 
     The present disclosure generally provides electric machines with an air gap control system, and systems and methods of controlling an air gap in an electric machine. The length of an air gap may vary because of changing operating parameters and/or operating conditions associated with an electric machine. Additionally, an optimum air gap length may vary depending on the particular operating parameters and/or operating conditions with which an electric machine happens to be operating at any given time. Numerous factors impact both the actual length of the air gap as well as the optimum air gap length. For example, the length of the air gap may vary with centrifugal force of the rotor core assembly, and/or with thermal expansion and contraction of the rotor core assembly, the stator core assembly, and/or various other components of the electric machine. Additionally, the magnetic field in an electric machine may cause the rotor shaft to flex slightly, particularly at regions of the rotor shaft that are furthest from the journals or bearings, etc. that support the rotor shaft. Likewise, these and other factors may affect the optimum air gap length. 
     Exemplary air gap control systems may improve the performance and energy efficiency of an electric machine. For example, an air gap control system may be configured to control the length of the air gap to optimize for or accommodate different operating parameters and/or operating conditions, or by mitigating or preventing changes to the length of the air gap due to changing operating parameters and/or operating conditions. An air gap control system may be configured to optimize for performance and/or energy efficiency with respect to particular operating parameters and/or operating conditions of the electric machine. For example, an air gap control system may allow an electric machine to operate with a smaller air gap, thereby enhancing the energy efficiency of the electric machine. In some embodiments, an air gap control system may mitigate the risk of the rotor contacting the stator, thereby allowing an electric machine to operate with a smaller air gap that otherwise would not be feasible without such an air gap control system in view of acceptable risk tolerance levels. Exemplary air gap control systems also may reduce resistive losses attributable to eddy currents, windage, and/or hysteresis by controlling the length of the edge gap. Furthermore, in some embodiments an air gap control system may allow an electric machine to operate at higher rotating speeds by opening the air gap to compensate for centrifugal growth. 
     An electric machine may function as an electric motor and/or an electric generator. An electric motor converts electrical energy into mechanical energy. An electric generator converts mechanical energy into electrical energy. Some examples where an electric machine may be utilized include aircraft, marine vessels, motor vehicles, power generation facilities, manufacturing facilities, industrial machinery, and the like. In the context of an aircraft, an electric machine may be used to supply power to a turbomachine engine, such as a turbofan engine in an aircraft. The power from the electric machine may be used to start the turbomachine engine, or to provide propulsive thrust to the aircraft, including commercial, military, or civilian aircraft, as well as unmanned aircraft such as unmanned aerial vehicles, electric rotorcraft, drones, and the like. In the context of a generator, an electric machine may be used to supply electrical power to auxiliary systems, including auxiliary systems in an aircraft. In some embodiments, and electric machine may function as both an electric motor and as a generator during different operating states. For example, an electric machine may function as an electric motor to start an aircraft engine, and as a generator to supply electric power to systems in the aircraft. It will be appreciated that an electric machine may be used in numerus other settings, and it is intended that the presently disclosed air gap control systems may be implemented in an electric machine in any setting without departing from the scope or spirit of the present disclosure. 
     Various embodiments of the present disclosure will now be described in greater detail. Referring to  FIGS. 1A-1C , various embodiments of an exemplary air gap control system  100  are shown. An exemplary air gap control system  100  includes an electric machine  102 , and a coolant circulation system  104 . All or a portion of the coolant circulation system  104  may be formed as an integral part of the electric machine  102  or all or a portion of the coolant circulation system  104  may be separate from but in fluid communication with the electric machine  102 . The electric machine  102  includes a rotor core assembly  106  and a stator core assembly  108 . The rotor core assembly  106  includes a rotor core  110  and a rotor shaft  112  operably coupled to the rotor core  110 . The stator core assembly  108  includes a stator core  114  and a stator housing  116  operably coupled to the stator core  114 . The stator core  114  circumferentially surrounds the rotor core  110 . The electric machine  102  includes an air gap  118  having a length, L located between and defined by the inner surface  120  of the stator core  114  and the outer surface  122  of the rotor core  110 . 
     The coolant circulation system  104  may include a cooling conduit  124 . The cooling conduit  124  defines a pathway for circulating a coolant  126  through the electric machine  102 , including the rotor core assembly  106  and/or the stator core assembly  108 . At least a portion of the cooling conduit  124  may include structures integrally formed within the electric machine  102 . These structures may include one or more of internal or external channels, tubes, pathways, inter-connected or interlaced unit cells, cooling jackets, or the like. The cooling conduit  124  may include a rotor core assembly-cooling conduit  128 , which includes structures that define a pathway for circulating coolant  126  through or around at least a portion of the rotor core assembly  106 , including at least a portion of the rotor core  110  and/or at least a portion of the rotor shaft  112 . Additionally, or in the alternative, the cooling conduit  124  may include a stator core assembly-cooling conduit  130 , which includes structures that define a pathway for circulating coolant  126  through or around at least a portion of the stator core assembly  108 , including at least a portion of the stator core  114  and/or at least a portion of the stator housing  116 . The rotor core assembly-cooling conduit  128  has a thermally conductive relationship with at least a portion of the rotor core assembly  106 , including the rotor core  110  and/or the rotor shaft  112 . The stator core assembly-cooling conduit  130  has a thermally conductive relationship with at least a portion of the stator core assembly  108 , including the stator core  114  and/or the stator housing  116 . The coolant circulation system  104  may additionally include one or more coolant reservoirs  132 , one or more coolant pumps (e.g., a first coolant pump  134  and/or a second coolant pump  136 ), and one or more heat exchangers  138 . 
     As shown in  FIG. 1A , an air gap control system  100  may include a shared coolant circulation system  104  configured to circulate a shared coolant  126  through a cooling conduit  124  that includes both a rotor core assembly-cooling conduit  128  and a stator core assembly-cooling conduit  130 . The coolant circulation system  104  shown in  FIG. 1  A includes a first coolant pump  134  configured to supply coolant  126  to the rotor core assembly  106  via a rotor core assembly-cooling conduit  128 , and a second coolant pump  136  configured to supply coolant  126  to the stator core assembly  108  via a stator core assembly-cooling conduit  130 . The coolant circulation system  104  shown in  FIG. 1A  additionally includes coolant reservoir  132  and a heat exchanger  138  which are shared between the rotor core assembly-cooling conduit  128  and the stator core assembly-cooling conduit  130 . 
     Alternatively, an air gap control system  100  may include separate coolant circulation systems for the rotor core assembly-cooling conduit  128  and the stator core assembly-cooling conduit  130 . For example, as shown in  FIG. 1B , an air gap control system  100  may include a rotor core coolant circulation system  104 A and a stator core coolant circulation system  104 B. The rotor core coolant circulation system  104 A may include a cooling conduit  124 A which supplies rotor core coolant  126 A circulating through the rotor core assembly  106  to a rotor core heat exchanger  138 A. Rotor core coolant  126 A circulating through the rotor core heat exchanger  138 A flows to a rotor core coolant reservoir  132 A. The first coolant pump  134  supplies rotor core coolant  126 A from the rotor core coolant reservoir  132 A to the rotor core assembly-cooling conduit  128 . Similarly, the stator core coolant circulation system  104 B may include a cooling conduit  124 B which supplies stator core coolant  126 B circulating through the stator core assembly  108  to a stator core heat exchanger  138 B. Stator core coolant  126 B circulating through the stator core heat exchanger  138 B flows to a stator core coolant reservoir  132 B. The second coolant pump  136  supplies stator core coolant  126 B from the rotor core coolant reservoir  132 A to the stator core assembly-cooling conduit  130 . 
     As shown in  FIG. 1A , a coolant circulation system  104  may include separate pumps for the rotor core assembly-cooling conduit  128  and the stator core assembly-cooling conduit  130 . Alternatively, as shown in  FIG. 1C , a coolant circulation system  104  may include a single common coolant pump  135  configured to supply coolant  126  to both the rotor core assembly  106  via the rotor core assembly-cooling conduit  128  and to the stator core assembly  108  via the stator core assembly-cooling conduit  130 . 
     During operation of the electric machine  102 , the one or more pumps (e.g., the first coolant pump  134  and/or the second coolant pump  136  and/or the common coolant pump  135 ) may circulate coolant  126  through the cooling conduit  124 . Heat generated by the electric machine  102  transfers to the coolant  126  flowing through the cooling conduit  124  (e.g., to coolant  126  flowing through the rotor core assembly-cooling conduit  128  and/or to coolant  126  flowing through the stator core assembly-cooling conduit  130 ) at a rate that may depend on the temperature gradient between the coolant  126  and the surface of the cooling conduit  124 . The temperature gradient may depend on, among other things, the temperature of the rotor core assembly  106  and/or of the stator core assembly  108 , the supply temperature of the coolant  126 , and the flow rate of the coolant  126  through the cooling conduit  124 . The coolant  126  exits the rotor core assembly-cooling conduit  128  and/or the stator core assembly-cooling conduit  130  having been heated by the thermally conductive relationship therewith. 
     The laminations and other various components that make up the rotor core assembly  106  and/or the stator core assembly  108  each have a coefficient of thermal expansion (Y), which describes how the size of the component changes with a change in temperature. The coefficient of thermal expansion (Y) may be described with respect to a change in volume, surface area, and/or length with changing temperature. For example, the laminations and other various components that make up a rotor core assembly  106  and/or a stator core assembly  108  each have a coefficient of radial thermal expansion (Y r ), which describes how a radial length changes with a change in temperature. Heat transferring to the coolant  126  from the rotor core assembly  106  and/or from the stator core assembly  108  may affect the temperature of such laminations or other various components. Accordingly, the size of the laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  may depend on the temperature and/or the flow rate of the coolant  126  flowing through the cooling conduit  124  (e.g., the rotor core assembly-cooling conduit  128  and/or the stator core assembly-cooling conduit  130 ), since the temperature and or flow rate affects the heat transfer, temperature, and thermal expansion of such components. 
     Coolant  126  discharging from the rotor core assembly-cooling conduit  128  and/or the stator core assembly-cooling conduit  130  flows through the cooling conduit  124  to one or more heat exchangers  138 , where a heat sink fluid  140  cools the coolant  126 . The one or more heat exchangers  138  may have any desired configuration suitable to transfer heat from the coolant  126  to the heat sink fluid  140 . Suitable heat exchangers include shell and tube, plate and shell, and plate fin configurations, and the like. The heat exchanger  138  may be an external component or integrally formed within at least a portion of the electric machine  102 . The heat sink fluid  140  may be any desired fluid, including a liquid or a gas, or a combination of a liquid and a gas. As shown, coolant  126  passing through the heat exchanger  138  flows to the coolant reservoir  132 , however in some embodiments a coolant reservoir  132  need not be provided. For example, the cooling conduit  124  may itself define a coolant reservoir  132 . The coolant reservoir  132  may be an integral portion of the electric machine  102 , or an external component. 
     Still referring to  FIGS. 1A-1C , a coolant circulation system  104  may additionally or alternatively utilize a stream of cooling air  146  supplied to the electric machine  102 , for example, from an air conduit  148 . The coolant circulation system  104  may include an air conduit  148  in addition, or as an alternative, to the cooling conduit  124  described herein. The air conduit  148  may define a pathway for supplying cooling air  146  to the electric machine  102 , such as to a surface of the rotor core assembly  106  and/or a surface of the stator core assembly  108 . For example, the air conduit  148  may supply a stream of cooling air  146  to the air gap  118 . During operation, a stream of cooling air  146  from the air conduit  148  may flow across the air gap  118 , providing cooling to the rotor core assembly  106  and/or the stator core assembly  108 . The cooling air  146  may flow through the air conduit  148  from an upstream pressure having a higher pressure relative to a downstream pressure. For example, the cooling air  146  may include ambient air introduced into the air conduit  148  through an air scoop (not shown) on a nacelle, cowling, housing or the like surrounding the electric motor. At least a portion of the air conduit  148  may include structures integrally formed within the electric machine  102 . These structures may include one or more of internal or external channels, tubes, pathways, or the like. A controllable damper  150  may be provided to control the flow rate of the cooling air  146  flowing through the air conduit  148  and across the air gap  118 . 
     During operation of the electric machine  102 , the controllable damper  150  may be opened at least a portion so as to circulate cooling air  146  through the air conduit  148 . Heat generated by the electric machine  102  transfers to the cooling air  146  flowing from the air conduit  148  and across the air gap  118  (e.g., from the rotor core  110  to the cooling air  146  flowing across the air gap  118  and/or from the stator core  114  to the cooling air  146  flowing across the air gap  118 ) at a rate that may depend on the temperature gradient between the cooling air  146  and the surface of the rotor core  110  and/or the surface of the stator core  114 . The temperature gradient may depend on, among other things, the temperature of the rotor core assembly  106  and/or of the stator core assembly  108 , the supply temperature of the cooling air  146 , and the flow rate of the cooling air  146  across the air gap  118 . 
     Still referring to  FIGS. 1A-1C , an exemplary air gap control system  100  includes one or more air gap controllers  200 , one or more sensors configured to provide an input  202  to an air gap controller  200 , and one or more controllable devices configured to receive an output  204  that includes a control command from an air gap controller  200 . An exemplary air gap controller  200  may include one or more control models or algorithms that utilize one or more inputs  202  from one or more of the sensors to provide one or more outputs  204  that include a control command to one or more of the controllable devices. A single air gap controller  200  may be provided, or a plurality of air gap controllers  200  may be provided. When multiple air gap controllers  200  are provided, each may have responsibility for a different portion of an electric machine  102  and/or for a different purpose of an air gap control system  100 . 
     An air gap controller  200  may be configured to control the length L of the air gap  118  using a temperature and/or a flow rate of coolant  126  flowing through or around the electric machine  102  (e.g., through the rotor core assembly-cooling conduit  128  and/or the stator core assembly-cooling conduit  130 ). An air gap controller  200  may control the temperature and/or flow rate of the coolant  126 , thereby affecting the amount of thermal expansion (Y) in one or more components of the rotor core assembly  106 , one or more components of the stator core assembly  108 , and/or various other components of the electric machine  102 . Additionally, or in the alternative, an air gap controller  200  may be configured to control the length L of the air gap  118  using a temperature and/or a flow rate of cooling air  146  flowing across the air gap  118 . An air gap controller  200  may control the temperature and/or flow rate of the cooling air  146 , thereby affecting the amount of thermal expansion (Y) in one or more components of the rotor core assembly  106 , one or more components of the stator core assembly  108 , and/or various other components of the electric machine  102 . Further, an air gap controller  200  may be configured, additionally or alternatively, to control the length L of the air gap  118  using a power input and/or rotor shaft speed of the electric machine  102 . An air gap controller  200  may control a power input to the electric machine  102 , thereby affecting the magnetic flux generated by the electric machine  102 , and in turn, the heat transfer, temperature, and thermal expansion of various components of the electric machine  102 . An air gap controller  200  may control a rotor shaft speed of the electric machine  102 , thereby affecting the centrifugal force acting upon the rotor core assembly  106 . 
     Adjustments and/or controls carried out by an air gap controller  200  may change or control the size of the laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . For example, such adjustments or controls may affect the radial length of the laminations or other various components according to their respective coefficient of radial thermal expansion (Y r ). Additionally, or in the alternative, adjustments and/or controls carried out by an air gap controller  200  may control the amount of centrifugal growth and/or flexing of various components of the rotor core assembly  106 . An air gap controller  200  may be configured to change the length L of the air gap  118  to optimize for or accommodate different operating parameters and/or operating conditions. For example, an air gap controller  200  may be configured to control the length L of the air gap  118  to an air gap target value, which air gap target value may vary depending on the particular operating parameters and/or operating conditions. Additionally, or in the alternative, an air gap controller  200  may be configured to prevent or mitigate a change in the length L of the air gap  118 . For example, such adjustments or controls may prevent or mitigate a change in the size of laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  due to changing operating parameters and/or operating conditions that might otherwise affect the amount of thermal expansion, centrifugal growth, and/or flexing of various components of the electric machine  102 . 
     The one or more sensors in an exemplary air gap control system  100  may include any sensor capable of ascertaining information pertaining to the air gap control system  100  (e.g., information pertaining to the electric machine  102  and/or the coolant system  104 ) and capable of providing an input  202  to the air gap controller  200 . For example, as shown in  FIGS. 1A-1C , the one or more sensors may include one or more temperature sensors,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  217 ,  219  one or more power sensors  218 ,  220 , one or more rotor speed sensors  222 , and/or one or more air gap sensors  224 ,  226 . The one or more controllable devices in an exemplary air gap control system  100  may include any device capable of receiving an output  204  that includes a control command from the air gap controller  200 . For example, as shown in  FIG. 1A , the one or more controllable devices may include one or more control valves  228 ,  230 ,  232 , one or more pumps  134 ,  136 , and/or one or more power control units  234 . As shown in  FIG. 1B , the one or more controllable devices may include one or more control valves  228 ,  230 ,  232 A,  232 B, one or more pumps  134 ,  136 , and/or one or more power control units  234 . As shown in  FIG. 1C , the one or more controllable devices may include one or more control valves  229 ,  231 ,  232 , one or more pumps  135 , and/or one or more power control units  234 . 
     In some embodiments, and an air gap controller  200  may determine an air gap length value. The air gap length value may be directly measured, sensed, calculated, ascertained, or otherwise determined. In one embodiment, an air gap controller  200  may determine an air gap length value based at least in part on temperature measurements provided by one or more temperature sensors  206 ,  208 ,  210 ,  212  configured to measure a coolant temperature at one or more locations of the cooling conduit  124  flowing into and out of the rotor core assembly  106  and/or the stator core assembly  108 . Such coolant temperatures may be used to determine the temperature of the rotor core assembly  106  or various components thereof and/or to determine the temperature of the stator core assembly  108  or various components thereof. The length L of the air gap  118  may then be determined based at least in part on these determine temperatures of the rotor core assembly  106  and/or stator core assembly  108  and the respective coefficients of thermal expansion (Y) (e.g., the coefficient of radial thermal expansion (Y r )). 
     As shown in  FIGS. 1A-1C , a first temperature sensor T 1    206  may be configured to ascertain a temperature of coolant  126 ,  126 A flowing into the rotor core assembly  106  and a second temperature sensor T 2    208  may be configured to ascertain a temperature of coolant  126 ,  126 A flowing out of the rotor core assembly  106 . A rate or quantity of heat transfer from the rotor core assembly  106  to the coolant  126 ,  126 A may be ascertained based at least in part on the difference between the temperature ascertained by the first temperature sensor T 1    206  and the temperature ascertained by the second temperature sensor T 2    208 . Additionally, or in the alternative, a temperature of the rotor core assembly  106  (e.g., the rotor core  110  and/or the rotor shaft  112 ) may be determined based at least in part on a temperature of the coolant  126 ,  126 A, a temperature change in the coolant  126 ,  126 A, and/or rate or quantity of heat transfer to the coolant  126 ,  126 A. A third temperature sensor T 3    210  may be configured to ascertain a temperature of coolant  126 ,  126 B flowing into the stator core assembly  108  and a fourth temperature sensor T 4    212  may be configured to ascertain a temperature of coolant  126 ,  126 B flowing out of the stator core assembly  108 . A rate or quantity of heat transfer from the stator core assembly  108  to the coolant  126 ,  126 B may be ascertained based at least in part on the difference between the temperature ascertained by the third temperature sensor T 3   210  and the temperature ascertained by the fourth temperature sensor T 4    212 . Additionally, or in the alternative, a temperature of the stator core assembly  108  (e.g., the stator core  114  and/or the stator housing  116 ) may be determined based at least in part on a temperature of the coolant  126 ,  126 B, a temperature change in the coolant  126 ,  126 B, and/or rate or quantity of heat transfer to the coolant  126 ,  126 B. 
     Additionally, or in the alternative, an air gap controller  200  may determine an air gap length value based at least in part on temperature measurements provide by one or more temperature sensors  214 ,  216  configured to measure a rotor core assembly temperature and/or a stator core assembly temperature. As shown in  FIGS. 1A-1C , a fifth temperature sensor T 5    214  may be configured to ascertain a temperature of the rotor core assembly  106  and/or a sixth temperature sensor T 6    216  may be configured to ascertain a temperature of the stator core assembly  108 . The fifth temperature sensor T 5    214  may ascertain a temperature of the rotor core  110  and/or a temperature of the rotor shaft  112 . In some embodiments, multiple temperature sensors may be provided at various locations about the rotor core  110  and/or the rotor shaft  112 . The sixth temperature sensor T 6    216  may ascertain a temperature of the stator core  114  and/or a temperature of the stator housing  116 . In some embodiments, multiple temperature sensors may be provided at various locations about the stator core  114  and/or the stator housing  116 . Any suitable temperature sensors may be used. A gap length value may be determined based at least in part on the temperature of the rotor core assembly  106  and/or the temperature of the stator core assembly  108 , and the coefficient of thermal expansion (Y) (e.g., the coefficient of radial thermal expansion (Y r )) for the various components thereof. In some embodiments an air gap controller  200  may additionally or alternatively determine an air gap length value based at least in part on rotor speed measurements provided by one or more rotor speed sensors  222 . A rotor core assembly  106  may exhibit centrifugal growth in an amount proportional to the rotational speed of the rotor core assembly  106 . Centrifugal force acting upon the rotor core assembly  106  may be ascertained based at least in part on the speed of the rotor core assembly  106 . The amount of centrifugal growth exhibited by a rotor core assembly  106  may be determined from such centrifugal force, and an air gap length value may be determined from the amount of centrifugal growth. Accordingly, an air gap controller  200  may be configured to ascertain an air gap length value based at least in part on a correlation between rotor speed and centrifugal growth. 
     In some embodiments, an air gap controller  200  may additionally or alternatively determine an air gap length value based at least in part on one or more output power measurements provided by one or more power sensors  218 ,  220 . The temperature and/or centrifugal growth of a rotor core assembly  106  and/or the temperature of a stator core assembly  108  may be correlated to output power of the electric machine  102 . In the case of an electric machine  102  that operates as an electric motor, the air gap control system  100  may include a first output power sensor  218  configured to measure mechanical output power such as a watt meter, a torque sensor, a speed sensor, or the like. In the case of an electric machine  102  that operates as a generator, the air gap control system  100  may include a second output power sensor  220  configured to measure electrical output power. An output power sensor  218 ,  220  may take the form of a watt meter, a torque sensor, a speed sensor, or the like. 
     In some embodiments, an air gap controller  200  may additionally or alternatively determine an air gap length value based at least in part on one or more air gap length measurements provided by one or more air gap length sensors  224 ,  226 . The air gap length sensors may take the form of a capacitive proximity sensor, or any other suitable distance sensor. The air gap length sensors may be non-contact capacitance sensor that generates a signal proportional to an air gap. For example, the air gap length sensor may include a capacitive displacement sensor, such as a capaNCDT  6200  sensor available from Micro-Epsilon of Raleigh, N.C., USA. A plurality of air gap sensors (e.g., a first air gap sensor  224  and a second air gap sensor  226 ) may be positioned at various locations of an electric machine  102  so as to measure an air gap length at such various locations. For example, air gap sensors  224 ,  226  may be positioned at intervals across the longitudinal width and/or radial perimeter of the air gap  118 . An air gap sensor may be positioned on the stator core assembly  108 , as shown with respect to the first air gap sensor  224 , and/or on the rotor core assembly  106 , as shown with respect to the second air gap sensor  226 . 
     As mentioned, the air gap control system  100  may include one or more controllable devices. In some embodiments, as shown in  FIGS. 1A and 1B , an air gap control system  100  may include a rotor core cooling bypass control valve  228  positioned at a rotor core assembly-cooling conduit bypass conduit  142 . The rotor core cooling bypass control valve  228  may be configured to allow a volume of coolant  126 ,  126 A flowing through the cooling conduit  124  to bypass the heat exchanger  138 ,  138 A, thereby changing the temperature of the coolant  126 ,  126 A flowing through the rotor core assembly-cooling conduit  128 . The rotor core cooling bypass control valve  228  may be modulated, opened, or closed in response to a control command from the air gap controller  200 . The rotor core cooling bypass control valve  228  can be opened or modulated in the open-direction to increase the portion of coolant  126 ,  126 A bypassing the heat exchanger  138 , thereby changing (e.g., increasing) the temperature of the coolant  126 ,  126 A flowing through the rotor core assembly-cooling conduit  128 . The rotor core cooling bypass control valve  228  can be closed or modulated in the closed-direction to decrease the portion of coolant  126 ,  126 A bypassing the heat exchanger  138 ,  138 A, thereby changing (e.g., decreasing) the temperature of the coolant  126 ,  128 A flowing through the rotor core assembly-cooling conduit  128 . 
     In another embodiment, as shown in  FIGS. 1A and 1B , an air gap control system  100  may additionally or alternatively include a stator core cooling bypass control valve  230  positioned at a stator core assembly-cooling conduit bypass conduit  144 . The stator core cooling bypass control valve  230  may be configured to allow a volume of coolant  126 ,  126 B flowing through the cooling conduit  124  to bypass the heat exchanger  138 ,  138 B, thereby changing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . The stator core cooling bypass control valve  230  may be modulated, opened, or closed in response to a control command from the air gap controller  200 . The stator core cooling bypass control valve  230  may be opened or modulated in the open-direction to increase the portion of coolant  126 ,  126 B bypassing the heat exchanger  138 ,  138 B, thereby increasing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . The stator core cooling bypass control valve  230  can be closed or modulated in the closed-direction to decrease the portion of coolant  126 ,  126 B bypassing the heat exchanger  138 ,  138 B, thereby decreasing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . 
     In still another embodiment, as shown in  FIG. 1C , an air gap control system  100  may include a rotor core cooling control valve  229  positioned at a rotor core assembly-cooling conduit  128  and/or a stator core cooling control valve  231  positioned at a stator core assembly-cooling conduit  130 . The rotor core cooling control valve  229  may be configured to control the volume of coolant  126  flowing through the rotor core assembly-cooling conduit  128  to the rotor core assembly  106 , thereby changing the temperature of the coolant  126  flowing out of the rotor core assembly  106 . The rotor core cooling control valve  229  may be modulated, opened, or closed in response to a control command from the air gap controller  200 . The stator core cooling control valve  231  may be configured to control the volume of coolant  126  flowing through the stator core assembly-cooling conduit  130  to the stator core assembly  108 , thereby changing the temperature of the coolant  126  flowing out of the stator core assembly  108 . The stator core cooling control valve  231  may be modulated, opened, or closed in response to a control command from the air gap controller  200 . 
     The rotor core cooling control valve  229  can be opened or modulated in the open-direction to increase the flow rate of coolant  126  flowing to the rotor core assembly  106 , thereby changing (e.g., increasing) the amount of heat transfer between the rotor core assembly  106  and the coolant  126  and changing (e.g., decreasing) the temperature of the coolant  126  flowing out of the rotor core assembly  106 . The rotor core cooling control valve  229  can be closed or modulated in the closed-direction to decrease the flow rate of coolant  126  flowing to the rotor core assembly  106 , thereby changing (e.g., decreasing) the amount of heat transfer between the rotor core assembly  106  and the coolant  126  and changing (e.g., increasing) the temperature of the coolant  126  flowing out of the rotor core assembly  106 . Similarly, the stator core cooling control valve  231  can be opened or modulated in the open-direction to increase the flow rate of coolant  126  flowing to the stator core assembly  108 , thereby changing (e.g., increasing) the amount of heat transfer between the stator core assembly  108  and the coolant  126  and changing (e.g., decreasing) the temperature of the coolant  126  flowing out of the stator core assembly  108 . The stator core cooling control valve  231  can be closed or modulated in the closed-direction to decrease the flow rate of coolant  126  flowing to the stator core assembly  108 , thereby changing (e.g., decreasing) the amount of heat transfer between the stator core assembly  108  and the coolant  126  and changing (e.g., increasing) the temperature of the coolant  126  flowing out of the stator core assembly  108 . 
     In another embodiment, as shown in  FIGS. 1A and 1B , an air gap control system  100  may additionally or alternatively include a stator core cooling bypass control valve  230  positioned at a stator core assembly-cooling conduit bypass conduit  144 . The stator core cooling bypass control valve  230  may be configured to allow a volume of coolant  126 ,  126 B flowing through the cooling conduit  124  to bypass the heat exchanger  138 ,  138 B, thereby changing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . The stator core cooling bypass control valve  230  may be modulated, opened, or closed in response to a control command from the air gap controller  200 . The stator core cooling bypass control valve  230  may be opened or modulated in the open-direction to increase the portion of coolant  126 ,  126 B bypassing the heat exchanger  138 ,  138 B, thereby increasing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . The stator core cooling bypass control valve  230  can be closed or modulated in the closed-direction to decrease the portion of coolant  126 ,  126 B bypassing the heat exchanger  138 ,  138 B, thereby decreasing the temperature of the coolant  126 ,  126 B flowing through the stator core assembly-cooling conduit  130 . 
     In yet another embodiment, as shown in  FIGS. 1  A and  1 C, an air gap control system  100  may additionally or alternatively include a third control valve  232  configured to start, stop, increase, or decrease a volume of heat sink fluid  140  flowing through or around the heat exchanger  138  in response to a control command from the air gap controller  200 , thereby changing the amount of heat transfer with the coolant  126  flowing through the heat exchanger  138 , which in turn may change the temperature of the coolant  126  exiting the heat exchanger  138 . The third control valve  232  may be opened or modulated in the open-direction to increase the volume of heat sink fluid  140  flowing through or around the heat exchanger  138 , thereby decreasing the temperature of the coolant  126  exiting the heat exchanger  138 . The third control valve  232  may be closed or modulated in the closed-direction to decrease the volume of heat sink fluid  140  flowing through or around the heat exchanger  138 , thereby increasing the temperature of the coolant  126  exiting the heat exchanger  138 . 
     The temperature of the coolant  126  may be controlled using the rotor core cooling bypass control valve  228 , the stator core cooling bypass control valve  230 , and/or the third control valve  232 . In some embodiments, the temperature of the coolant  126  may be increased or decreased to change the rate or quantity of heat transfer from the rotor core assembly  106  to the coolant  126  and/or from the stator core assembly  108  to the coolant  126 . With a change in the rate or quantity of heat transfer, the size of one or more laminations or other various components of the rotor core assembly  106  or of the stator core assembly  108  may be changed according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . 
     Similarly, as shown in  FIG. 1B , an air gap control system  100  may additionally or alternatively include a third control valve  232 A configured to start, stop, increase, or decrease a first volume of heat sink fluid  140 A flowing through or around the rotor core heat exchanger  13   8 A in response to a control command from the air gap controller  200 , thereby changing the amount of heat transfer with the rotor core coolant  126 A flowing through the rotor core heat exchanger  138 A, which in turn may change the temperature of the rotor core coolant  126 A exiting the rotor core heat exchanger  138 A. The third control valve  232 A may be opened or modulated in the open-direction to increase the first volume of heat sink fluid  140 A flowing through or around the rotor core heat exchanger  138 A, thereby decreasing the temperature of the rotor core coolant  126 A exiting the rotor core heat exchanger  138 A. The third control valve  232 A may be closed or modulated in the closed-direction to decrease the first volume of heat sink fluid  140 A flowing through or around the rotor core heat exchanger  138 A, thereby increasing the temperature of the rotor core coolant  126 A exiting the rotor core heat exchanger  13   8 A. 
     The temperature of the rotor core coolant  126 A may be controlled using the rotor core cooling bypass control valve  228  and/or the third control valve  232 A. In some embodiments, the temperature of rotor core coolant  126 A may be increased or decreased to change the rate or quantity of heat transfer from the rotor core assembly  106  to the rotor core coolant  126 A. With a change in the rate or quantity of heat transfer, the size of one or more laminations or other various components of the rotor core assembly  106  may be changed according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . 
     As additionally shown in  FIG. 1B , an air gap control system  100  may additionally or alternatively include a fourth control valve  232 B configured to start, stop, increase, or decrease a second volume of heat sink fluid  140 B flowing through or around the stator core heat exchanger  138 B in response to a control command from the air gap controller  200 , thereby changing the amount of heat transfer with the stator core coolant  126 B flowing through the stator core heat exchanger  138 B, which in turn may change the temperature of the stator core coolant  126 B exiting the heat exchanger  138 . The fourth control valve  232 B may be opened or modulated in the open-direction to increase the second volume of heat sink fluid  140 B flowing through or around the stator core heat exchanger  138 B, thereby decreasing the temperature of the stator core coolant  126 B exiting the stator core heat exchanger  138 B. The fourth control valve  232 B may be closed or modulated in the closed-direction to decrease the second volume of heat sink fluid  140 B flowing through or around the stator core heat exchanger  138 B, thereby increasing the temperature of the stator core coolant  126 B exiting the stator core heat exchanger  138 B. 
     The temperature of the stator core coolant  126 B may be controlled using the stator core cooling bypass control valve  230  and/or the third control valve  232 B. In some embodiments, the temperature of the stator core coolant  126 B may be increased or decreased to change the rate or quantity of heat transfer from the stator core assembly  108  to the stator core coolant  126 B. With a change in the rate or quantity of heat transfer, the size of one or more laminations or other various components of the stator core assembly  108  may be changed according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . 
     In even further embodiments, as shown in  FIGS. 1A and 1B , an air gap control system  100  may include one or more pumps  134 ,  136  configured to change the flow rate of coolant  126  (or the flow rate of rotor core coolant  126 A and/or stator core coolant  126 B) flowing through the cooling conduit  124  in response to a control command from the air gap controller  200 . As shown in  FIGS. 1A and 1B , a first cooling pump  134  may be configured to supply a flow of coolant  126  (or rotor core coolant  126 A) through the rotor core assembly-cooling conduit  128 , and a second cooling pump  136  may be configured to supply a flow of coolant  126  (or stator core coolant  126 B) through the stator core assembly-cooling conduit  130 . A flow rate of coolant  126  (or rotor core coolant  126 A) flowing through the rotor core assembly-cooling conduit  128  may be modulated, started, or stopped using the first coolant pump  134 . Likewise, a flow rate of coolant  126  (or stator core coolant  126 B) flowing through the stator core assembly-cooling conduit  130  may be modulated, started, or stopped using the second coolant pump  136 . Additionally, or alternatively, a flow rate of coolant  126  (or rotor core coolant  126 A and/or stator core coolant  126 B) flowing through the cooling conduit  124  (e.g., rotor core assembly-cooling conduit  128  and/or the stator core assembly-cooling conduit  130 ) may be modulated, started, or stopped using a control valve  228 ,  230 . The flow rate of coolant  126  (or rotor core coolant  126 A and/or stator core coolant  126 B) may be increased or decreased to change the rate or quantity of heat transfer from the rotor core assembly  106  to the coolant  126  and/or from the stator core assembly  108  to the coolant  126 . With a change in the rate or quantity of heat transfer, the size of one or more laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  may be changed according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . 
     As shown in  FIG. 1C , an air gap control system  100  may include one a common coolant pump  135  configured to change the flow rate of coolant  126  flowing through the cooling conduit  124  in response to a control command from the air gap controller  200 . As shown in  FIG. 1C , a common coolant pump  135  may be configured to supply a flow of coolant  126  through both the rotor core assembly-cooling conduit  128  and the stator core assembly-cooling conduit  130 . The common coolant pump  135  may work in concert with the rotor core cooling control valve  229   229  and the stator core cooling control valve  231 . A flow rate of coolant  126  flowing through the rotor core assembly-cooling conduit  128  and/or through the stator core assembly-cooling conduit  130  may be modulated, started, or stopped using the common coolant pump  135 . Additionally, or alternatively, the proportion of coolant  126  flowing through the rotor core assembly-cooling conduit  128  and the stator core assembly-cooling conduit  130  may be increased or decreased using the rotor core cooling control valve  229  and/or the stator core cooling control valve  231 . The flow rate of coolant  126  may be increased or decreased to change the rate or quantity of heat transfer from the rotor core assembly  106  to the coolant  126  and/or from the stator core assembly  108  to the coolant  126 . With a change in the rate or quantity of heat transfer, the size of one or more laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  may be changed according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . 
     In still further embodiments, as shown in  FIGS. 1A-1C , an air gap control system  100  may include one or more power control units  234  configured to change a power input or a power output of the electric machine  102  in response to a control command from the air gap controller  200 . For example, the power control unit  234  may increase or decrease the speed of the rotor shaft  112  and/or the magnetic flux generated by the electric machine  102 . Additionally, or alternatively, the power control unit  234  may restrict the permissible rate of change (e.g., acceleration) of the rotor shaft  112 . An increase or decrease in the power input or power output of the electric machine  102  may change the rate or quantity of heat generation in the rotor core assembly  106  and/or the stator core assembly. Such a change in the rate or quantity of heat generation may change the size of one or more laminations or other various components of the rotor core assembly  106  and/or of the stator core assembly  108  according to their respective coefficient of thermal expansion (Y), thereby adjusting or controlling the length L of the air gap  118 . Additionally, an increase or decrease in the speed of the rotor shaft  112  may change the amount of centrifugal growth exhibited by a rotor core assembly  106 , thereby adjusting or controlling the length L of the air gap  118 . 
     The air gap control systems  100  shown in  FIGS. 1A-1C  may be configured to adjust or control the length L of the air gap  118  at least in part by allocating cooling capacity between the rotor core assembly  106  and the stator core assembly  108 . The length of the air gap  118  may be widened by decreasing the temperature of the coolant  126  flowing to the rotor core assembly  106  and/or increasing the temperature of the coolant flowing to the stator core assembly  108 . Conversely, the length of the air gap  118  may be narrowed by increasing the temperature of the coolant  126  flowing to the rotor core assembly  106  and/or decreasing the temperature of the coolant flowing to the stator core assembly  108 . When the temperature of the coolant  126  flowing through the rotor core assembly  106  decreases, thermal contraction of the rotor core assembly  106  widens the length L of the air gap  118 . When the temperature of the coolant  126  flowing through the stator core assembly  108  increases, thermal expansion of the stator core assembly  108  also widens the length L of the air gap  118 . Conversely, when the temperature of the coolant  126  flowing through the rotor core assembly  106  increases, thermal expansion of the rotor core assembly  106  narrows the length L of the air gap  118 ; and/or when the temperature of the coolant  126  flowing through the stator core assembly  108  decreases, thermal contraction of the stator core assembly  108  also narrows the length L of the air gap  118 . 
     With the air gap control system  100  shown in  FIGS. 1A and 1B , the length of the air gap  118  may be increased using the rotor core cooling bypass control valve  228  and/or the stator core cooling bypass control valve  230 . The length L of the air gap  118  may be widened by increasing the proportion of coolant  126  that bypasses the heat exchanger  138  and returns to the stator core assembly  108  via the stator core assembly-cooling conduit  130  and/or increasing the proportion of coolant that flows through the heat exchanger  138  before returning to the rotor core assembly  106 . The length L of the air gap  118  may be narrowed by increasing the proportion of coolant  126  that bypasses the heat exchanger  138  and returns to the rotor core assembly  106  via the rotor core assembly-cooling conduit  128  and/or increasing the proportion of coolant that flows through the heat exchanger  138  before returning to the stator core assembly  108 . In some embodiments, an air gap control system  100  may include a rotor core cooling bypass control valve  228  but not a stator core cooling bypass control valve  230 , or a stator core cooling bypass control valve  230  but not a rotor core cooling bypass control valve  228 . 
     With the air gap control system  100  shown in  FIG. 1B , the temperature of the rotor core coolant  126 A and the temperature of the stator core coolant  126 B can be controlled independently from one another. Additionally, in some embodiments the rotor core coolant  126 A and the stator core coolant  126 B may include different types of coolant. 
     With the air gap control system  100  shown in  FIG. 1C , the length of the air gap  118  may be increased using the rotor core cooling control valve  229  and/or the stator core cooling control valve  231 . The length L of the air gap  118  may be widened by increasing the proportion of coolant  126  that flows to the rotor core assembly  106  via the rotor core assembly-cooling conduit  128  relative to the proportion of coolant  126  that flows to the stator core assembly  108  via the stator core assembly-cooling conduit  130 . The length L of the air gap  118  may be narrowed by decreasing the proportion of coolant  126  that flows to the rotor core assembly  106  via the rotor core assembly-cooling conduit  128  relative to the proportion of coolant  126  that flows to the stator core assembly  108  via the stator core assembly-cooling conduit  130 . In some embodiments, an air gap control system  100  may include a rotor core cooling control valve  229  but not a stator core cooling control valve  231 , or a stator core cooling control valve  231 but not a rotor core cooling control valve  229 . 
     In some embodiments, as shown in  FIGS. 1A-1C , an air gap control system  100  may include one or more temperature sensors  217 ,  218  configured measure a cooling air temperature at one or more locations of the air conduit  148 . A seventh temperature sensor T 7    217  may be configured to ascertain a temperature of cooling air  146  flowing through an inlet side of the air conduit  148  and an eighth temperature sensor T 8   219  may be configured to ascertain a temperature of cooling air  146  flowing through an outlet side of the air conduit  148 . A rate or quantity of heat transfer from the rotor core assembly  106  and/or from the stator core assembly  108  to the cooling air  146  may be ascertained based at least in part on the difference between the temperature ascertained by the seventh temperature sensor T 7   217  and the temperature ascertained by the eighth temperature sensor T 8    219 . 
     Such cooling air temperature may be utilized to control the flow rate or the cooling air  146 . An air gap control system  100  may include a controllable damper  150  that may be modulated, opened, or closed in response to a control command from the air gap controller  200 . The controllable damper  150  may be opened or modulated in the open-direction to increase the portion of cooling air  146  flowing across the air gap, thereby changing the rate of heat transfer between the rotor core  110  and the cooling air  146  and/or between the stator core  114  and the cooling air  146 . 
     Now referring to  FIG. 2 , an exemplary air gap controller  200  will be described in further detail. An air gap controller  200  may include one or more computing devices  250 , which may be located on or within an electric machine  102 , adjacent to an electric machine  102 , or at a remote location relative to the electric machine  102 . The one or more computing devices  250  may include one or more processors  252  and one or more memory devices  254 . The one or more processors  252  may include any suitable processing device, such as a microprocessor, microcontroller, integrated circuit, logic device, and/or other suitable processing device. The one or more memory devices  254  may include one or more computer-readable media, including but not limited to non-transitory computer-readable media, RAM, ROM, hard drives, flash drives, and/or other memory devices. 
     The one or more memory device  254  may store information accessible by the one or more processors  252 , including computer-readable instructions  256  that can be executed by the one or more processors  252 . The instructions  256  may be any set of instructions which when executed by the one or more processors  252  cause the one or more processors  252  to perform operations. In some embodiments, the instructions  256  may be configured to cause the one or more processors  252  to perform operations for which the air gap controller  200  and/or the one or more computing devices  250  are configured. Such operations may include controlling a temperature and/or a flow rate of the coolant  126  flowing through the cooling conduit  124 , so as to adjust and/or control the length L of the air gap  118 . Such operations may additionally or alternatively include controlling a flow rate of cooling air  146  flowing through the air conduit  148  and across the air gap  118 , so as to adjust and/or control the length L of the air gap  118 . The length L of the air gap  118  may be adjusted and/or controlled so as to optimize for or accommodate different operating parameters and/or operating conditions, and/or so as to prevent or mitigate a change in the length L of the air gap  118  as operating parameters or operating conditions change. Additionally, such operations may include adjusting air gap target values and/or adjusting air gap control models  274  used for determining air gap length targets. Such operations may be carried out according to control commands provided by an air gap control model  274 . The instructions  256  can be software written in any suitable programming language or can be implemented in hardware. Additionally, and/or alternatively, the instructions  256  can be executed in logically and/or virtually separate threads on processors  252 . 
     The memory devices  254  may store data  258  accessible by the one or more processors  252 . The data  258  can include current or real-time data, past data, or a combination thereof. The data  258  may be stored in a data library  260 . As examples, the data  258  may include data associated with or generated by an air gap control system  100 , including data associated with or generated by an electric machine  102 , and/or a coolant circulation system  104  for the electric machine  102 , including data  258  associated with or generated by an air gap controller  200 , a computing device  250 , and/or an air gap control model  274 . The data  258  may also include other data sets, parameters, outputs, information, etc. shown and/or described herein. The past operating data may include an operating history for various operating intervals such as, for example, Operating Interval A, Operating Interval B, Operating Interval C, and so on up to the Nth Operating Interval. The past operating data associated with each Operating Interval may include past operating parameters, past operating conditions, past mission management system (MMS) data (e.g., flight management system (FMS) data), and/or past Operating Interval profiles indicative of the nature and conditions of the particular Operating Interval. 
     In one embodiment, such as when the air gap control system  100  is implemented in an aircraft, marine vessel, motor vehicle, or the like, the data  258  may include past mission data (e.g., flight missions, marine missions, land missions) for the electric machine  102  or other similar electric machines stored in a data library  260 . The past mission data may include a past mission history for one or more various missions such as, for example, Mission A, Mission B, Mission C, and so on up to the Nth Mission. The past mission data associated with each Mission may include past operating parameters, past operating conditions, past MMS data (e.g., FMS data), and/or past Mission profiles indicative of the nature and conditions of the particular Mission. 
     In another embodiment, the operating intervals may be maintenance or service intervals for the electric machine  102  or various components thereof. The past operating data may contain a past maintenance or service history for various maintenance or service intervals, such as, for example, Service Interval A, Service Interval B, Service Interval C, and so on up to the Nth Service Interval. The maintenance or service intervals may include intervals between any form of maintenance or service typically performed on electric machines. The past service data associated with each Service Interval may include past operating parameters, past operating conditions, past MMS data (e.g., FMS data), and/or past Service Interval profiles indicative of the nature and conditions of the particular Service Interval. 
     The one or more computing devices  250  may also include a communication interface  262 , which may be used for communications with a communications network  264  wired or wireless communication lines  266 . The communication network  264  may include, for example, a local area network (LAN), a wide area network (WAN), SATCOM network, VHF network, a HF network, a Wi-Fi network, a WiMAX network, a gatelink network, and/or any other suitable communications network for transmitting messages to and/or from the air gap controller  200  across the communication lines  266 . The communication interface  262  may allow the computing device  250  to communicate with one or more sensors and with one or more controllable devices of an air gap control system  100 . The communication interface  262  may additionally allow the computing device  250  to communicate with the other components of the electric machine  102  and/or other components of an aircraft, marine vessel, motor vehicle, or facility where the electric machine  102  has been implemented. 
     The communication interface  262  may additionally or alternatively allow the computing device  250  to communicate with one or more external resources, such as a server  268  or a data warehouse  270 . As an example, at least a portion of the data  258  may be stored in the data warehouse  270 , and the server  268  may be configured to transmit data  258  from the data warehouse  270  to the computing device  250 , and/or to receive data  268  from the computing device  250  and to store the received data  268  in the data warehouse  270  for further purposes. In some embodiments, the server  268  and/or the data warehouse  270  may be implemented as part of a mission management system (MMS)  272  such as a flight management system (FMS). The communication interface  262  may include any suitable components for interfacing with one or more network(s), including for example, transmitters, receivers, ports, controllers, antennas, and/or other suitable components. The communication lines  266  of communication network  264  may include a data bus or a combination of wired and/or wireless communication links. 
     Still referring to  FIG. 2 , an exemplary air gap controller  200  includes one or more air gap control models  274  configured to control the length L of the air gap  118  based at least in part on one or more model inputs, including inputs pertaining to operating parameters and/or operating conditions pertaining to the electric machine  102 . An exemplary air gap control model  274  may be configured provide a determined air gap length value and compare the determined air gap length value to an air gap target value. If the determined air gap length value differs from the air gap target value (e.g., by more than a predefined threshold amount), the air gap control model  274  may output a control command and the air gap controller  200  may transmit an output  204  that includes the control command to one or more controllable devices to adjust the length L of the air gap  118  so as to become closer to the target value. The air gap controller  200  may transmit a series of outputs  204  that include one or more control commands to the one or more controllable devices, for example, until the determined length L of the air gap  118  agrees with the air gap target value (e.g., until the determined length L of the air gap  118  differs from the air gap target value by less than a predefined threshold amount). The outputs  204  that include the control commands may be routed from the air gap controller  200  to the one or more controllable devices via the communication interface  262 . Exemplary air gap control models  274  include an Active Air Gap Control (AAGC) model  276  and a High Efficiency Air Gap Control (HEAGC) model  278  as described herein. 
     Now referring to  FIG. 3 , an exemplary air gap control model  274  is shown. By way of example, the exemplary air gap control model  274  shown in  FIG. 3  is an AAGC model  276 . As shown, the AAGC model  276  may include an air gap length model  300  and an air gap target model  302 . The air gap length model  300  receives one or more model inputs  304 . Using the model inputs  304 , the air gap length model  300  determines and outputs a determined air gap length value  306 . The air gap target model  302  receives one or more model inputs  304  and outputs an adjusted air gap target value  328 . In one embodiment, the air gap length model  300  may be based at least in part on a coefficient of thermal expansion for one or more components of a rotor core assembly  106  and/or a stator core assembly  108 . For example, the coefficient of thermal expansion may include a coefficient of radial thermal expansion for laminations of the rotor core  110  and/or for laminations of the stator core  114 . In another embodiment, the air gap length model  300  additionally or alternatively may be based at least in part on an amount of centrifugal force exhibited by a rotor core assembly  106 . 
     The determined air gap length value  306  and/or the adjusted air gap target value  328  may depend at least in part on the way the electric machine  102  is currently being operated and/or how the electric machine  102  has been uniquely operated in the past. The determined air gap length value  306  and/or the adjusted air gap target value  328  may include one or more values, including a current or real-time value, an average, a maximum, a minimum, and/or a range. The determined air gap length value  306  and/or the adjusted air gap target value  328  may additional include one or more statistical parameters, such as a distribution value (e.g., a variance, a standard deviation) and/or a regression coefficient value. 
     The AAGC model  276  performs a compare operation  308 . The compare operation  308  includes one or more operations configured to compare the determined air gap length value  306  to an air gap target value  310 . The compare operation  308  may include a PID controller or any other suitable controller. The compare operation  308  may utilize linear or non-linear control algorithms, and any analytical technique including frequency domain and/or time-domain state space representation techniques. The air gap target value  310  may be derived or determined from one or more model inputs  304  and/or from the air gap target model  302 . Based on the compare operation  308 , the AAGC model  276  provides one or more control commands  312  which may be included in an output  204  from an air gap controller  200 . For example, if the determined air gap length value  306  differs from the air gap target value  310  by a predefined threshold amount, then the AAGC model  276  provides one or more control commands  312  to adjust the length L of the air gap  118  so as to become closer to the air gap target value  310 . The one or more control commands  312  may include a command configured to change one or more operating parameters of the electric machine. For example, such a control command  312  may be configured to adjust a temperature of the coolant  126  and/or a flow rate of the coolant  126  ( 314 ), to adjust a flow rate of the cooling air  146  ( 315 ), and/or to adjust a power input and/or speed of the rotor shaft  112  ( 316 ). Such a control command  312  may be effective to adjust a length L of an air gap  118 . 
     Any number of model inputs  304  may be used by an air gap control model  274 , including current or real-time data  258 , past data  258 , or a combination thereof. The one or more model inputs  304  may include data  258  associated with or generated by an air gap control system  100 , including data  258  associated with or generated by an electric machine  102  and/or a coolant circulation system  104  for the electric machine  102 . Such data associated with an air gap control system  100  may additionally or alternatively including data  258  associated with or generated by an air gap controller  200 , a computing device  250 , and/or an air gap control model  274 . The model inputs  304  may include data  258  associated with, comprising, or generated by an air gap controller  200 , data  258  associated with, comprising, or generated by a computing device  250 , and/or data associated with, comprising, or generated by an air gap control model  274 . As examples, the model inputs  304  may include data  258  associated with or comprising one or more user inputs  318 , data  258  associated with or comprising design specifications  320  for an electric machine 102 , data  258  associated with or comprising operating parameters  322 , data  258  associated with or comprising operating conditions  324 , data  258  associated with or comprising mission management system (MMS) data  326 , and combinations thereof. The model inputs  304  may themselves be regarded as data  258 , which may be stored in the data library  260  and/or the data warehouse  270 , and which may be included in subsequent model inputs  304 . The model inputs  304  may additionally include other data sets, parameters, outputs, information, etc. shown and/or described herein. 
     Exemplary user inputs  318  may include data  258  associated with or comprising one or more set points for the operation of the electric machine  102 , including a power input set point or a power output set point for the electric machine  102 , a temperature set point for the coolant  126 , a flow rate set point for the coolant  126 , a flow rate set point for the cooling air  146 , a temperature set point for the rotor core assembly  106  or various components thereof, and a temperature set point for the stator core assembly  108  or various components thereof. 
     Exemplary design specifications  320  may include data  258  associated with or comprising a nominal air gap length when the electric machine  102  is at idle or at steady state under certain operating conditions  324 , a maximum air gap length, a minimum air gap length, coefficients of thermal expansion (Y) for various components of the electric machine  102 , and/or other design information pertaining to the electric machine  102 . 
     Exemplary operating parameters  322  may include data  258  associated with or comprising parameters associated with the operation of the electric machine  102  and/or the coolant circulation system  104 . Such operating parameters  322  may include sensed, measured, determined, or predicted operating speeds, flow rates, power input levels, power output levels, and the like. For example, exemplary operating parameters  322  may include data from one or more sensors, including one or more temperature sensors,  206 ,  208 ,  210 ,  212 ,  214 ,  216 ,  217 ,  219  one or more power sensors  218 ,  220 , one or more rotor speed sensors  222 , one or more air gap sensors  224 ,  226 . Additionally, or in the alternative, exemplary operating parameters  322  may include operating states or set points associated with one or more controllable devices, including one or more control valves  228 ,  230 ,  232 , one or more controllable dampers  150 , one or more pumps  134 ,  136 , and/or one or more power control units  234 . Any one or more of such operating parameters  322  may be included in a model input  304 . 
     In one embodiment, a model input  304  may include data  258  associated with or comprising one or more coolant temperature inputs, which may be provided by one or more coolant temperature sensors  206 ,  208 ,  210 ,  212 , and/or one or more cooling air temperature sensors  217 ,  219 . The one or more coolant temperature inputs may include a first temperature input from a first temperature sensor  206  configured to ascertain a temperature of coolant  126  flowing into a rotor core assembly  106 , and/or a second temperature input from a second temperature sensor  208  configured to ascertain a temperature of coolant  126  flowing out of the rotor core assembly  106 . Additionally, or in the alternative, the one or more coolant temperature inputs may include a third temperature input from a third temperature sensor  210  configured to ascertain a temperature of coolant  126  flowing into a stator core assembly  108 , and/or a fourth temperature input from a fourth temperature sensor  212  configured to ascertain a temperature of coolant  126  flowing out of the stator core assembly  108 . 
     In another embodiment, a model input  304  may additionally or alternatively include data  258  associated with or comprising one or more rotor core assembly temperature inputs and/or one or more stator core assembly temperature inputs, which may be provided by one or more coolant temperature sensors  214 ,  216 . A rotor core assembly temperature input may include a fifth temperature input from a fifth temperature sensor  214  configured to ascertain a temperature of a rotor core assembly  106 . The temperature of the rotor core assembly  106  may include a temperature of a rotor core  110  and/or a temperature of a rotor shaft  112 . A stator core assembly temperature input may include a sixth temperature input from a sixth temperature sensor  216  configured to ascertain a temperature of a stator core assembly  108 . The temperature of the stator core assembly  108  may include a temperature of a stator core  114  and/or a temperature of a stator housing  116 . 
     In yet another embodiment, a model input  304  may additionally or alternatively include data  258  associated with or comprising one or more cooling air temperature inputs, which may be provided by one or more cooling air temperature sensors  217 ,  219 . The one or more cooling air temperature inputs may include a seventh temperature input from a seventh temperature sensor  217  configured to ascertain a temperature of cooling air  146  flowing through an inlet side of the air conduit  148 . The one or more cooling air temperature inputs may additionally or alternatively include an eighth temperature input from an eighth temperature sensor  219  configured to ascertain a temperature of cooling air  146  flowing through an outlet side of the air conduit  148 . 
     In yet another embodiment, a model input  304  may additionally or alternatively include data  258  associated with or comprising one or more power sensor inputs provided by one or more power sensors  218 ,  220 , and/or one or more rotor speed inputs provided by one or more rotor speed sensors  222 . 
     In yet another embodiment, a model input  304  may additionally or alternatively include data  258  associated with or comprising one or more air gap length inputs provided by one or more air gap sensors  224 ,  226 . 
     Exemplary operating conditions  324  may include data  258  associated with or comprising operating modes, as well as sensed, measured, calculated, or predicted conditions internal or external to the electric machine  102  which may affect the operation thereof. Such internal conditions include temperatures, pressures, heat transfer rates, vibration levels, magnetic flux, etc. Such external conditions include ambient temperature, pressure, humidity, wind speed, etc. 
     Exemplary MMS data  326  may include data  258  associated with or comprising current, planned, or proposed mission plans. For example, when the electric machine  102  is implemented in an aircraft, marine vessel, or motor vehicle, or the like, the MMS data  326  can include data  258  associated with or comprising current or past missions (e.g., flight missions, marine missions, land missions) for the electric machine  102 , including GPS coordinates or waypoints such as the origin and destination and various intermediate GPS coordinates or waypoints, projected path of travel, altitude information, weather information, estimated time in route, as well as various operator inputs. 
     In some embodiments, the air gap target model  302  may include a machine-learned model  330  that allows the AAGC model  276  to control the length L of the air gap  118  without being explicitly programmed to carry out any particular control protocol. The adjusted air gap target value  328  may be applicable for an entire operating period or for a portion of an operating interval, such as all or a portion of a mission, or all or a portion of a service interval. For example, when the electric machine  102  is implemented in an aircraft, the adjusted air gap target value  328  output by the air gap target model  302  (e.g., the machine-learned model  330 ) may apply to particular altitude or range of altitudes of a flight. 
     A machine-learned model  330  may use any suitable machine learning technique, operating regime, or algorithm. A machine-learned model  330  may be configured to use pattern recognition, computational learning, artificial intelligence, or the like to derive algorithms that allow the machine-learned model  330  to determine an air gap target value  310 . A machine-learned model  330  may include an unsupervised or a supervised learning regime, including a semi-supervised learning regime, an active learning regime, a reinforcement learning regime, and/or a representation learning regime. A machine-learned model  330  may utilize neural networks, decision trees, association rules, inductive logic algorithms, cluster analysis algorithms, and the like. In some embodiments, the model inputs  304  may include data  258  associated with or generated by a machine-learned model  330 . 
     By way of example, the machine-learned model  330  shown in  FIG. 3  includes a neural network. However, an exemplary machine-learned model  330  may include any other suitable model, including a linear discriminant analysis model, a partial least squares discriminant analysis model, a support vector machine model, a random tree model, a logistic regression model, a naive Bayes model, a K-nearest neighbor model, a quadratic discriminant analysis model, an anomaly detection model, a boosted and bagged decision tree model, an artificial neural network model, a C4.5 model, a k-means model, and combinations thereof. Even further additional suitable types of machine or statistical learning models are also contemplated. It will also be appreciated that the machine-learned model  330  can use certain mathematical methods alone or in combination with one or more machine or statistical learning models. 
     In some embodiments, the compare operation  308  of an AAGC model  276  may be configured to compare determined air gap length values  306  to an air gap target schedule  310  which may include a series of air gap target values  310  corresponding to different points of an Operating Interval, such as a Mission or a Service Interval. The air gap target schedule may be provided by or derived from one or more model inputs  304 , and/or an air gap target model  302 . For example, the machine-learned model  330  may output an adjusted air gap target schedule  328 . An air gap target schedule  310  or an adjusted air gap target schedule  328  may include air gap target values  310  and/or adjusted air gap target values  328  for all or a portion of an Operating Interval, such as all or a portion of a Mission, or all or a portion of a Service Interval. For example, when the electric machine  102  is implemented in an aircraft, an air gap target schedule or an adjusted air gap target schedule may be provided for all or a portion of a flight mission, including for various flight phases, such as takeoff, climb, cruise, descent, approach and land, missed approach, etc. As another example, an air gap target schedule or an adjusted air gap target schedule may be provided for all or a portion of a Service Interval, including for various periods between services or maintenance, such as a break-in period, a normal operating period, and a wear-out period. 
     In addition to outputting an adjusted air gap target value  328 , in some embodiments an air gap target model  302  (e.g., a machine-learned model  330 ) may output a confidence score  332 , which may provide an indication as to a level of confidence attributable to one or more outputs. The confidence score  332  can be used, for example, to set a margin of error to be used by the air gap target model  302  in determining an adjusted air gap target value  328 . For example, in the event of a low confidence score  332  the air gap target model  302  may account for a more conservative or wide margin for error when determining an adjusted air gap target value  328 , whereas in the event of a high confidence score  332  the air gap target model  302  may allow for a more aggressive or narrow margin for error when determining an adjusted air gap target value  328 . In some embodiments, the model inputs  304  may include data  258  associated with or comprising a confidence score  332 . 
     Now referring to  FIG. 4 , an exemplary correlation  400  between an air gap target value  310  and an operating parameter  322  is shown. An air gap control model  274  may utilize an operating parameter  322  as a model input  304  to determine an air gap target value  310  based at least in part on such a correlation  400 . The correlation  400  may be predetermined, for example, based on user input  318  and/or design specifications  320 . Alternatively, or in addition, a correlation  400  between an operating parameter  322  and an air gap target value  310  may be provided by an air gap target model  302 . For example, such a correlation  400  may be provided as a machine-learned output from a machine-learned model  330 . As shown in  FIG. 4 , the operating parameter  322  shown in the exemplary correlation  400  is a rotor shaft speed. It will be appreciated, however, that an air gap target value  310  may be correlated with numerous other operating parameters  322 , among other model inputs  304 , and the example shown in  FIG. 4  is not intended to be limiting. 
     An air gap control model  274  may utilize a correlation  400  to determine an air gap target value  310 . In some embodiments, the air gap target values  310  provided in the correlation  400  may be selected so that the electric machine  102  can accelerate from any given rotor shaft speed to a maximum continuous speed (e.g., so that a snap acceleration can be performed) without causing the rotor core  110  to contact the stator core  114  and/or without introducing an unacceptable risk that the rotor core  110  might contact the stator core  114 . As shown in  FIG. 4 , when a determined air gap length value  306  differs from an air gap target value  310 , for example, as determined by a compare operation  308  ( FIG. 3 ), the air gap control model  274  may provide one or more control commands  312  ( FIG. 3 ) so as to adjust the length L of the air gap  118  so as to become closer to the air gap target value  310 , as shown by the downwards arrow  402 . The correlation  400  may include an upper threshold  404  and a lower threshold  406 . The compare operation  308  may be configured to output a control command  312  only if the determined air gap length value  306  falls outside of the range defined by the upper threshold  404  and the lower threshold  406 . 
     Referring again to  FIG. 2 , another exemplary air gap control model  274  may include an HEAGC model  278 . An HEAGC model  278  may be configured to restrict the acceleration rate of the electric machine  102  and or to increase the length L of the air gap  118  to reduce the risk of the rotor core  110  contacting the stator core  114  during an unexpected or a planned high acceleration event. The HEAGC model  278  may be utilized generally, or under various specific operating conditions  324  and/or during various specific stages of a mission such as a flight mission. When an electric machine  102  is expected to maintain relatively stable operation, the HEAGC model  278  may be utilized to restrict the acceleration rate of the electric machine  102  so as to reduce the risk of the rotor core  110  contacting the stator core  114  during an unexpected high acceleration event. When a high acceleration event is expected, the HEAGC model  278  may disable the restriction on the acceleration rate of the electric machine  102  and correspondingly adjust the air gap target value  310  so as to increase the length L of the air gap  118 , thereby reducing the risk of the rotor core  110  contacting the stator core  114  during the expected high acceleration event. After the expected high acceleration has passed, the HEAGC model  278  can reduce the length L of the air gap  118 , and/or once again restrict the rate of acceleration. 
     The HEAGC model  278  and its control logic may be used separately or in conjunction with the AAGC model  276 . For example, the AAGC model  276  can operate to adjust the air gap target value  310  and reduce the length L of the air gap  118  so as to become closer to the air gap target value  310 . The HEAGC model  278  can be complimentary to the AAGC model  276 , but the HEAGC model  278  is not required. 
     Now referring to  FIG. 5 , an exemplary training computing device  500  is shown. In some embodiments an air gap target model  302  (e.g., a machine-learned model  330 ) may be trained using a training computing device  500 . The training computing device  500  may be communicatively coupled with the air gap controller  200  via the communications network  264  ( FIG. 2 ). Alternatively, the training computing device  500  may be included as a part of the air gap controller  200 . For example, the training computing device  500  may be part of the computing device  250  included as part of the air gap controller  200 . The training computing device  500  may include one or more processors  502  (e.g., a processor  252 ) and one or more memory devices  504  (e.g., memory device  254 ). The one or more memory devices  504  can store information accessible by the one or more processors  502 , including computer-readable instructions  506  that may be executed by the one or more processors  502 . The memory devices  504  can further store data  508  that may be accessed by the one or more processors  502 . The training computing device  500  can also include a communication interface  510  used to communicate with resources on the communication network  264  ( FIG. 2 ). The hardware, implementation, and functionality of the components of the training computing device  500  may operate, function, and include the same or similar components as those described with respect to the one or more computing devices  250  of the one or more air gap controllers  200 . 
     The training computing device  500  includes a model trainer  512  configured to train one or more air gap control models  274 , including an air gap length model  300  and/or an air gap target model  302  (e.g., a machine-learned model  330 ). The model trainer  512  may use any one or more various training or learning techniques such as backwards propagation of errors, which may include performing truncated backpropagation through time. In some embodiments, supervised training techniques may be used on a set of labeled training data. The model trainer  512  may perform a number of generalization techniques (e.g., weight decays, dropouts, etc.) to improve the generalization capability of the control model  274  being trained. 
     In some embodiments, the model trainer  512  can train one or more air gap control models  274  based on a set of training data  514 . The training data  514  may include past operating data  516 , which may include, for example, previous operating parameters  322 , previous operating conditions  324 , and previous MMS data  326 . In some embodiments, the training data  514  may include at least a portion of the data library  260 . Alternatively, the data library  260  may include the training data  514  or at least a portion thereof. 
     The model trainer  512  may utilize past operating data  516  to train one or more air gap control models  274  how the electric machine  102  has been actually and/or uniquely operated in the past under particular operating conditions  324 , such as during a particular Operating Interval, such as a particular Mission or Service Interval. Additionally, or in the alternative, the model trainer  512  may utilize past operating data  516  to validate or test an air gap control model  274 , including the determined air gap length model  300  and/or the air gap target model  302  (e.g., the machine-learned model  330 ). 
     A specific subset of training data  514 , such as a specific subset of past operating data  516 , may be selected when training an air gap control model  274 . For example, as shown in  FIG. 5  past operating data  516  for one or more particular Missions may be selected from the data library  260  or other memory device. The past operating data  516  may be representative of several occurrences of one or more particular Missions. The past operating data  516  for Mission A may include mission data for Mission A  1 , Mission A  2 , and so on to the Nth occurrence of Mission A. Such past operating data  516  for Mission A may be utilized by the model trainer  512  to train the air gap target model  302  (e.g., the machine-learned model  330 ) how the electric machine  102  has been actually and/or uniquely operated over the course of several occurrences of the particular Mission. Likewise, the model trainer  512  can train the machine-learned model  330  so that the machine-learned model  330  is able to machine-learn how the electric machine  102  has been actually and uniquely operated with respect to Mission B, and so on up to Mission N. 
     While the example of a Mission is provided, it will be appreciated that the model trainer  512  may train an air gap control model  274  with respect to any other Operating Interval (e.g., with respect to a Service Interval). Additionally, it will be appreciated that the model trainer  512  can process or pre-process the past operating data  516 , for example, to disregard outlier data so that it will not be used to train, test, and/or validate the air gap control model  274 . Such outlier data may include outlier occurrences of a particular Mission or other Operating Interval, and/or particular Missions or other Operating Intervals that differ significantly from a group or category of Missions or other Operating Intervals. 
     Now referring to  FIG. 6 , an exemplary method  600  of controlling a length L of an air gap  118  in an electric machine  102  using an air gap controller  200  will be discussed. The exemplary method  600  commences with receiving at an air gap controller, one or more model inputs comprising data associated with an air gap control system  100 , including data associated with an electric machine and/or data associated with a coolant circulation system  104  for the electric machine  602 . The data associated with the coolant circulation system  104  may include data associated with a coolant circulation system  104  that provides cooling utilizing a stream of coolant  126  from a cooling conduit  124  and/or a stream of cooling air from an air conduit. The exemplary method continues with determining with the air gap controller an air gap length value for the electric machine using an air gap length model  604 , and/or determining with the air gap controller an adjusted air gap target value for the electric machine using an air gap target model  606 . The air gap length model and/or the air gap target model may be configured to utilize the one or more model inputs. The air gap length value and/or the adjusted air gap target value may be based at least in part on the one or more model inputs. An exemplary method continues by comparing with the air gap controller, an air gap length value to an air gap target value  608 . The air gap length value may be a pre-existing air gap length value or a determined air gap length value from step  604 . The air gap target value may be a pre-existing air gap target value, or an adjusted air gap target value determined in step  606 . The comparison ascertains whether the air gap length value differs from the air gap target value by a predefined threshold. The exemplary method continues by outputting with the air gap controller, one or more control commands to one or more controllable devices associated with the air gap control system  100 , including one or more controllable devices associated with the electric machine  102  and/or the coolant circulation system  104 , when the air gap length value differs from the air gap target value by a predefined threshold  610 . The one or more control commands are intended to adjust a length L of an air gap  118  between an outer surface of a rotor core  110  and an inner surface of a stator core  114  of an electric machine  102  so as to become closer to the air gap target value. As such, the one or more control commands  312  may be configured to impart a change to one or more operating parameters  322  of the electric machine  102  and/or of the coolant circulation system  104  to adjust the length L of an air gap  118 . 
     This written description uses exemplary embodiments to describe the presently disclosed subject matter, including the best mode, and also to enable any person skilled in the art to practice such subject matter, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the presently disclosed subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.