Patent Publication Number: US-11646623-B2

Title: Fractional slot electric motors with coil elements having rectangular cross-sections

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 17/029,906, entitled: “Fractional Slot Electric Motors with Coil Elements Having Rectangular Cross-Sections”, filed on 23 Sep. 2020, which claims the benefit under 35 USC § 119(e) of U.S. Provisional Patent Application No. 62/904,502, entitled: “Electric Vehicle Motors”, filed on 23 Sep. 2019. Both of these applications are incorporated herein by reference in their entirety for all purposes. 
    
    
     BACKGROUND 
     Electric motors are devices that convert electrical energy into mechanical energy. A typical electric motor operates through the interaction between the motor&#39;s magnetic field and the electric current in the wire winding to generate rotation forces. Electric motors can be powered by direct current (DC) sources (e.g., batteries, rectifiers) or by alternating current (AC) sources (e.g., power grid, inverters, or electrical generators). In general, electric motors may be classified based on power source types, internal construction, application, and motor output. For example, motors may be brushed or brushless. Furthermore, motors may be of various phases, e.g., a single-phase motor, a two-phase motor, or a three-phase motor. 
     A typical electric motor includes a rotor assembly, which rotates within a stator assembly. Both assemblies generate respective magnetic fields that interact with each other causing the rotor assembly to rotate relative to the stator assembly, thereby converting electrical energy into mechanical energy. A stator assembly includes a stator core having multiple slots with coil elements protruding through these slots and wound around the stator core. These elements may be collectively referred to as stator winding. Specifically, each stator slot may house multiple coil elements, arranged in a radial direction and away from the center axis of the stator core. Finally, routing, interconnecting, and assembly of coil elements in fractional slot motors are typically more challenging than in integer slot motors. 
     SUMMARY 
     Described herein are fractional slot electric motors with compact crowns. A motor comprises multiple coil elements protruding through a stator core and forming electrical connections with each other and/or with a lead assembly. The lead assembly comprises phase busbars connected to selected coil elements and comprising terminals for connecting to an external power supply. The lead assembly also comprises neutral busbars, with no external connections and internally connected to other coil elements. Each coil element has a rectangular cross-sectional profile to maximize the slot-fill-ratio of the motor. Each coil element is electrically coupled to two other components. For example, each looped coil element is coupled to two other coil elements at a stator side, opposite the lead assembly. Each extended coil element is coupled to another coil element at that same side and coupled to another coil element or a busbar at the lead assembly side. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The description will be more fully understood with reference to the following figures, which are presented as exemplary examples of the invention and should not be construed as a complete recitation of the scope of the invention, wherein: 
         FIG.  1 A  is a schematic view of conventional coil elements, each having a circular cross-section, protruding through the same stator slot, in accordance with some examples. 
         FIG.  1 B  is a schematic view of four coil elements, each having a rectangular cross-section, protruding through the same stator slot, in accordance with some examples. 
         FIG.  1 C  illustrates cogging torque plots for different designs of electrical motors, including a 48-slot integer slot motor and two different designs of 60-slot fractional slot motors. 
         FIG.  2 A  is a perspective view of a fractional slot electric motor, shown without a rotor, in accordance with some examples. 
         FIG.  2 B  is a top view of a stator core of the fractional slot electric motor in  FIG.  2 A , in accordance with some examples. 
         FIG.  2 C  is an expanded view of a stator slot, illustrating different positions for coil elements in the stator slot, in accordance with some examples. 
         FIG.  2 D  is another perspective view of the fractional slot electric motor in  FIG.  2 A , illustrating electrical connections of coil elements at a first side of the stator core, opposite to the lead assembly, in accordance with some examples. 
         FIG.  2 E  is an expanded view of two coil elements connected at the first side of the stator core, in accordance with some examples. 
         FIG.  2 F  is a cross-sectional profile of a coil element, in accordance with some examples. 
         FIG.  3 A  is a perspective view of two pairs of coil elements, extending from the stator core at the first side, illustrating electrical connections in each pair, in accordance with some examples. 
         FIG.  3 B  is a top view of the two pairs of coil elements in  FIG.  3 A , with other coil elements not shown, in accordance with some examples. 
         FIG.  3 C  is a top view of the two pairs of coil elements in  FIG.  3 A  and  FIG.  3 B , with all other coil elements shown, in accordance with some examples. 
         FIG.  4 A  is a perspective view of a lead assembly for connecting to coil elements at the second side of the stator core and forming external electrical connections to the fractional slot electric motor, in accordance with some examples. 
         FIG.  4 B  is a perspective view of the lead assembly in  FIG.  4 A  without a busbar insulator, in accordance with some examples. 
         FIG.  4 C  is a perspective view of two neutral busbars of the lead assembly in  FIG.  4 A  and  FIG.  4 B , in accordance with some examples. 
         FIG.  4 D  is a perspective view of three stacked phase busbars of the lead assembly in  FIG.  4 A  and  FIG.  4 B , in accordance with some examples. 
         FIG.  4 E ,  FIGS.  4 F, and  4 G  are individual perspective views of each of the three phase busbars in  FIG.  4 D , in accordance with some examples. 
         FIG.  4 H  is a schematic side view of any one of the phase busbars in  FIG.  4 D , illustrating the flexible connection between a primary connection terminal and a hoop, in accordance with some examples. 
         FIG.  5 A  is a perspective view of the fractional slot electric motor without a busbar insulator, illustrating electrical connections between coil elements, phase busbars, and neutral busbars, in accordance with some examples. 
         FIG.  5 B  is an expanded perspective view of a portion of the fractional slot electric motor in  FIG.  5 A , showing electrical connections in one group, in accordance with some examples. 
         FIG.  6 A  is a perspective view of the fractional slot electric motor without a lead assembly, showing the orientation of coil elements at the second side of the stator core, in accordance with some examples. 
         FIG.  6 B ,  FIG.  6 C ,  FIG.  6 D , and  FIG.  6 E  are perspective views of selected individual coil elements in  FIG.  6 B , in accordance with some examples. 
         FIG.  7 A  is a perspective view of the stator core with one of the coil elements inserted into the core, in accordance with some examples. 
         FIG.  7 B  is a top view of a portion of the stator core in  FIG.  7 A , showing the shape of the coil element, in accordance with some examples. 
         FIG.  7 C  is a perspective view of a portion of the stator core, showing the shape of two coil elements adjacent to each other, in accordance with some examples. 
         FIG.  7 D  is a perspective view of the stator core with another one of the coil elements inserted into the core, in accordance with some examples. 
         FIG.  7 E  is a top view of a portion of the stator core in  FIG.  7 D , showing the shape of the coil element, in accordance with some examples. 
         FIG.  7 F  is a perspective view of the stator core with yet another one of the coil elements inserted into the core, in accordance with some examples. 
         FIG.  7 G  is a top view of a portion of the stator core in  FIG.  7 F , showing the shape of the coil element, in accordance with some examples. 
         FIG.  7 H  is a perspective view of the stator core with another one of the coil elements inserted into the core, in accordance with some examples. 
         FIG.  7 I  is a top view of a portion of the stator core in  FIG.  7 H , showing the shape of the coil element, in accordance with some examples. 
         FIG.  8 A  illustrates a stator wiring schematic, in accordance with some examples. 
         FIG.  8 B ,  FIG.  8 C ,  FIG.  8 D ,  FIG.  8 E , and  FIG.  8 F  illustrate different portions of the stator wiring schematic in  FIG.  8 A , in accordance with some examples. 
         FIG.  9    illustrates a stator wiring table, in accordance with some examples. 
         FIG.  10    is a schematic representation of an electric vehicle, comprising a fractional slot motor, in accordance with some examples. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are outlined in order to provide a thorough understanding of the presented concepts. In some examples, the presented concepts are practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail so as not to unnecessarily obscure the described concepts. While some concepts will be described in conjunction with the specific examples, it will be understood that these examples are not intended to be limiting. 
     Introduction 
     Electrical motors are core components of various power systems, such as drive systems of electric vehicles. The space available for electrical motors is typically limited, while the power requirements can be substantial. Overall, high efficiency and small size are important considerations for many applications of electrical motors. One approach for reducing the size and increasing the efficiency of an electric motor is by increasing the volume of coils passing through the stator core of the motor. This volume is typically represented by a slot-fill-ratio (SFR), which is defined as a ratio of the total cross-sectional area of all coils passing through the stator core to the total cross-sectional area of all slots available for these coils. Increasing the SFR value helps to decrease the resistance of the windings, thereby decreasing power loss and increasing efficiency. 
     Convention electrical motors often use wires with a round cross-section to form coil windings. Round wires are easy to route and bend, resulting in their wide adoptions for different types of electrical motors. However, round wires leave unfilled space in stator slots between the wires (e.g., unfilled corners) even when the stator slot is tightly packed with the round wires as, for example, is shown in  FIG.  1 A . Specifically,  FIG.  1 A  is a schematic view of conventional coil elements  141 , each having a circular cross-section, protruding through the same slot  114  of stator core  110 . One can easily see much of the unfilled space in slot  114 . The insulation of coil elements  141  is not shown for clarity. As a result, the SFR value for round wires is typically less than 50%. Rectangular conductors yield much high SFR values (e.g., greater than 50%, greater than 60%, or even greater than 70%) as, for example, is shown in  FIG.  1 B . Specifically,  FIG.  1 B  is a schematic view of coil elements  140 , each having a rectangular cross-section, protruding through the same slot  114  of stator core  110 . While rectangular coil elements provide a much higher SFR, these coil elements are more difficult to route due to their limited bendability, at least in some directions. 
     The routing difficulty often results in larger winding extensions at each side of the stator, increasing the overall size of the motor. For example, a coil may be formed by individual coil elements inserted into stator slots. The coil elements are bent on each side of the stator core to interconnect with each other and/or with busbars. These two portions of the stator winding, one at each side of the stator core, may be referred to as coil crowns. All bends of the coil elements and connections among the coil elements are provided with these coil crowns. The heights of these crowns and, in some examples, the external diameters of these crowns are typically greater for rectangular conductors than for round wires due to the limited bendability of the rectangular conductors. Furthermore, using individual coil elements, as oppose to a continuous wire, requires a large number of connections among the coil elements, adding to the complexity and the side of the crowns. 
     It should be noted that the routing and connections among the coil elements depend on the motor type. The routing and connections are typically much more complicated for fractional slot electric motors than, for example, for integer slot electric motors. However, fractional slot electric motors provide various advantages over integer slot electric motors, such as reduced cogging torque. 
     Cogging torque is a phenomenon in which the magnetic poles of the rotor align with magnetic features on the stator. During operation, the alignment of the magnetic poles and the magnetic features can result in an oscillatory torque, also known as cogging torque. In some cases, the cogging torque is large enough such that it is transmitted through structures supporting the motor and can be felt by end-users (e.g., drivers and passengers of electrical vehicles). Using a fractional slot per pole winding reduces cogging torque as, for example, is schematically shown in  FIG.  1 C . Specifically,  FIG.  1 C  illustrates cogging torque plots for three different electrical motors, including a 48-slot integer slot motor (line  200 ) and two different designs of 60-slot fractional slot motors (line  210  and line  220 ). Specifically, line  220  corresponds to an example of a fractional slot motor described below. The torque values are obtained using a finite element analysis and have been normalized for simplicity. The cogging torque of the 48-slot integer slot motor is reduced by approximately a factor of 2 or more in both 60-slot fractional slot motors. Furthermore, different winding configurations of 60-slot fractional slot motors result in different cogging torque performance. This reduction in cogging torque results in reduced noise, vibration, and harshness (NVH). However, as noted above, the coil routing of fractional slot electric motors are quite complicated. The winding complexity presents various challenges in maintaining compact mechanical packaging of the wire connections, especially with rectangular coil elements. 
     Described herein are fractional slot electric motors, which use coil elements with a rectangular cross-sectional profile. These coil elements include looped coil elements and extended coil elements. Each looped coil element has two extensions protruding from a stator core and extending from the core at the first end of the core. Each looped coil element also has a loop end, interconnecting the two extensions at the second end of the core, opposite the first end. Each extended coil element has one end protruding from the stator core at the first end and another end protruding at the second end, opposite of the first end. The coil elements are interconnected directly with each other at the first end of the stator core. For example, the coil elements form two rows of interconnected pairs. Each pair is formed by one coil element, bent in a clockwise direction, and another coil element bent in a counterclockwise direction. The adjacent pairs are radially offset relative to each other. More specifically, each looped coil element is connected to two other coil elements at the first end. Each extended coil element is connected to one other coil element at the first end. 
     Furthermore, each extended coil element is connected to a lead assembly or another coil element at the second end of the stator core. The lead assembly comprises a combination of three phase busbars and two neutral busbars. The looped coil elements are not connected to any components at the second end. Instead, the end loops of the looped coil elements are positioned between the second end and the lead assembly. This configuration of the coil elements and the lead assembly allows complex connections between coil elements, needed for fractional slot motors while maintaining compact packaging on each side of the stator core. For example, the crown height at the first end may be less than 50 millimeters or even less than 45 millimeters, such as about 40 millimeters. The crown height at the second end, not accounting the lead assembly, may be less than 45 millimeters or even less than 40 millimeters, such as about 36 millimeters. The crown height at the second end, accounting for the lead assembly, may be less than 80 millimeters or even less than 70 millimeters, such as about 61 millimeters. 
     Fractional Slot Electric Motor Examples 
       FIG.  2 A  illustrates a perspective schematic view of fractional slot electric motor  100 , in accordance with some examples. Fractional slot electric motor  100  is shown without a rotor to better illustrate other components of the motor. As shown in  FIG.  2 A , fractional slot electric motor  100  comprises stator core  110 , coil  130 , and lead assembly  150 . Coil  130  is formed by coil elements  140 , each protruding through stator core  110  and insulated from each other with a varnish coating (e.g., polyester varnish, epoxy varnish). The thickness of this insulation may be less than 300 micrometers or, more specifically, less than 250 micrometers, such as about 200 micrometers. Even with the insulating varnish, coil  130  provides some space between coil elements  140 , e.g., for cooling or, more specifically, for circulating a cooling fluid among coil elements  140 . In some examples, the average space between adjacent coil elements is between 0.5 millimeters and 2 millimeters or, more specifically, between 0.75 millimeters and 1.25 millimeters, such as about 1 millimeter. In some examples, stator core  110  is formed from multiple ferromagnetic annular plates arranged as a stack. 
     Coil elements  140  are directly interconnected with each other (e.g., by welding), primarily at first side  111  of stator core  110 . However, a selected few coils are also directly interconnected with each other (e.g., by welding) at second side  112  of stator core  110 . Furthermore, some of coil elements  140  are also connected to lead assembly  150  (e.g., also by welding). 
     In some examples, coil  130  has a 3 phase-2 parallel winding configuration. For the voltage to be balanced across the parallel legs, which are referred to as Parallel 1 and Parallel 2, various examples of coil elements  140  are used to form the two parallel windings with a balanced impedance. In some examples, this balanced impedance is achieved by a specific coil configuration, in which each parallel leg contains coils in both layers of the winding. More specifically, each winding has coils in every slot possible for their respective phase groups as shown and further described below with reference to  FIGS.  8 A- 8 F  and  FIG.  9   . Without this balanced winding, the current distribution between the parallel legs may be uneven and will result in different heating rates for the different parallel windings. The reduction in cogging torque was observed through the electromagnetic finite element analysis presented above with reference to  FIG.  1 C . 
     Referring to  FIG.  2 B , stator core  110  comprises stator slots  114 , extending between first side  111  and second side  112 . In some examples, stator core  110  comprises 60 stator slots. Stator slots  114  are used to protrude coil elements  140  through stator core  110 . In some examples, each stator slot is configured to receive four coil elements  140 . The position of each coil element in the stator slot may be identified with a distance from center axis  113  of stator core  110 . For example,  FIG.  2 B  illustrates four concentric circles having diameters D 1 , D 2 , D 3 , and D 4 , each corresponding to a different coil position in each slot.  FIG.  2 C  is an expanded view of one stator slot  114 , identifying each position. The outermost (from center axis  113 ) position is identified as first position  115   a  and may be referred to as an “A” position. This position corresponds to the largest circle (D 1 ). Second position  115   b , which may be referred to as a “B” position, corresponds to a slightly smaller circle (D 2 ). Third position  115   c , which may be referred to as a “C” position, corresponds to an even smaller circle (D 3 ). Finally, the innermost position or fourth position  115   d , which may also be referred to as a “D” position, corresponds to the smallest circle (D 4 ). As such, each of 240 different positions (60 stator slots×4 positions in each slot) may be identified by the slot number and the specific position in each slot. This identification is used, for example, in  FIG.  9   , described below. Overall, each coil element may protrude through one of 240 positions. Some types of coil elements (e.g., looped coil elements) protrude through two of 240 positions, which is further described below. 
     Referring back to  FIG.  2 A , each of coil elements  140  protrudes through one or two of the stator slots  114  between first side  111  and second side  112  of stator core  110 . In some examples, coil elements  140  are made from copper or a copper alloy. An electrical current passing through these coil elements  140  generates electromagnetic flux, which can be modulated to control the speed of fractional slot electric motor  100 . 
     As noted above, each coil element occupies one or two positions in stator slots  114 . With 240 different positions, the number of coil elements  140  is at least 120. In some examples, this number is 144. In other words, coil  130  is formed by 144 individual coil elements  140 , which are interconnected in accordance with the specific design of fractional slot electric motor  100 , described below with reference to  FIGS.  8 A- 8 F  and  FIG.  9   . 
     Referring to  FIG.  2 D , coil elements  140  are interconnected at first side  111  of stator core  110 . More specifically, each of coil elements  140  is electrically coupled to at least one other of coil elements  140  adjacent to first side  111  of stator core  110  as, e.g., is shown in  FIG.  2 E . Some examples of coil elements  140  (e.g., looped coil elements) are each coupled to two other coil elements, as further described below with reference to  FIG.  6 A - FIG.  6 C . These connections are formed, e.g., by welding or, more specifically, by laser welding. 
     Referring to  FIG.  2 F , each of coil elements  140  has a rectangular cross-sectional profile. The rectangular cross-sectional profile allows increasing the SFR, as was described above with reference to  FIG.  1 A  and  FIG.  1 B . In some examples, each coil element  140  has a thickness (CE T ) of between 3.0 millimeters and 4.0 millimeters, such as about 3.4 millimeters, and a width (CE W ) of between 2.5 millimeters and 3.5 millimeters, such as about 3.0 millimeters. However, other examples are within the scope. 
     During assembly of fractional slot electric motor  100 , coil elements  140  are inserted into stator slots  114  from second side  112  of stator core  110 . At this stage, portions of coil elements  140 , protruding through stator slots  114  and extending from first side  111  of stator core  110  are straight. After that, the connections are formed between these ends of coil elements  140  (e extending from first side  111 ) by bending these ends, which will now be described with reference to  FIG.  3 A ,  FIG.  3 B , and  FIG.  3 C . Specifically,  FIG.  3 A  illustrates four coil elements  140  extending from two stator slots  114  and forming electrical connections at first side  111  of stator core  110 . Other coil elements are not shown in  FIG.  3 A  for clarity. These stator slots  114  are separated by six other stator slots, effectively representing first and eight stator slots  114 . Each of coil elements  140  is bent to form electrical connections with a corresponding coil element. 
       FIG.  3 B  is a top view corresponding to  FIG.  3 A  illustrating the degree of circumferential and radial bends of each coil element.  FIG.  3 B  also illustrates four arcs (D 1 , D 2 , D 3 , and D 4 ) corresponding to different positions in each slot, which are described above with reference to  FIG.  2 B  and  FIG.  2 C . Around the stator circumference, each coil element is bent the distance corresponding to 3.5 slot spaces, with 1 slot space corresponding to an arc length between the centers of two adjacent slots. It should be noted that that the arc length corresponding to D 4  is longer than the arc length corresponding to D 1 . However, the bending direction is different or, more specifically, opposite for the two interconnected coil elements. For example, first coil element  140   a  is bent clockwise, while second coil element  140   b  is bent counterclockwise. First coil element  140   a  and second coil element  140   b  are interconnected above the portion of stator core  110 , located between the third and fourth stator slots. Similarly, third coil element  140   c  is bent clockwise, while fourth coil element  140   d  is bent counterclockwise. Third coil element  140   c  and fourth coil element  140   d  are also interconnected above the same portion of stator core  110 . 
     However, first coil element  140   a  and second coil element  140   b  are also bent radially away from the stator core center axis to provide some space from third coil element  140   c . For example, second coil element  140   b  extends from stator slot  114  at second position  115   b  (“B” position, corresponding to D 2 ). However, the end of second coil element  140   b , which forms an electrical connection with first coil element  140   a , is positioned over the arc, corresponding to the first position (“A” position) and having a diameter of D 1 . Similarly, first coil element  140   a  extends from stator slot  114  at first position  115   a  (“A” position, corresponding to D 1 ). The end of first coil element  140   a , which forms an electrical connection with second coil element  140   b , radially extends outside of the designated positions. On the other hand, third coil element  140   c  extends from stator slot  114  at third position  115   c  (“C” position, corresponding to D 3 ) and forms an electrical connection with fourth coil element  140   d  at that position. Similarly, fourth coil element  140   d  extends from stator slot  114  at fourth position  115   d  (“D” position, corresponding to D 4 ) and forms an electrical connection with third coil element  140   c  at that position. In other words, first coil element  140   a  and second coil element  140   b  are both bent radially away from the stator axis by one position. In comparison, third coil element  140   c  and fourth coil element  140   d  are not bent radially. 
       FIG.  3 C  illustrates all remaining coil elements surrounding first coil element  140   a , second coil element  140   b , third coil element  140   c , and fourth coil element  140   d . It should be noted that the connection and bent features, described above with reference to first coil element  140   a , second coil element  140   b , third coil element  140   c , and fourth coil element  140   d , are repeated 60 times around the circumference of stator core  110 . This configuration consistency allows greatly reducing the crown height, formed by interconnected coil elements  140 , at first side  111  of stator core  110 . Briefly referring to  FIG.  2 D , coil elements  140  extend (L 1 ) by less than 50 millimeters or even less than 45 millimeters at first side  111  of stator core  110 . In some examples, this coil extension/crown height (L 1 ) is between 38 millimeters and 42 millimeters, such as about 40 millimeters. 
     Referring to  FIG.  4 A , lead assembly  150  comprises three phase busbars  152 , each corresponding to a different phase of fractional slot electric motor  100 . Three phase busbars  152  provide external connections to fractional slot electric motor  100  and are also connected to some of coil elements  140 . In some examples, lead assembly  150  comprises two neutral busbars  160 , used for interconnecting other coil elements  140 . Unlike phase busbars  152 , two neutral busbars  160  do not form external connections. 
     Phase busbars  152  and neutral busbars  160  are insulated from each other and also supported with respect to each other by busbar insulator  158 . In some examples, busbar insulator  158  molded over phase busbars  152  and neutral busbars  160 . In other words, phase busbars  152  and neutral busbars  160  are integrated into busbar insulator  158 . However, various connecting terminals extend from busbar insulator  158  to form connections to phase busbars  152  and neutral busbars  160 . 
       FIG.  4 A  also illustrates busbar thermocouple  159 , which is an optional component of lead assembly  150 . When present, busbar thermocouple  159  is connected to one busbar  152 , which extends from busbar insulator  158 , and is configured to measure the temperature of this busbar  152 . This temperature measurement is communicated to a motor controller, e.g., to reduce the current through fractional slot electric motor  100 , when the temperature exceeds a certain threshold. 
       FIG.  4 B  illustrates lead assembly  150  without busbar insulator  158 , to show the arrangement of phase busbars  152  and neutral busbars  160  in lead assembly  150 . Phase busbars  152  form top layers of lead assembly  150 . These layers are further away from stator core  110 . Neutral busbars  160  form the bottom layer, closest to stator core  110 . 
       FIG.  4 C  illustrates neutral busbars  160  without other components of lead assembly  150  shown for clarity. In this example, each neutral busbar  160  comprises three neutral terminals  163  and neutral hoop  165 , interconnecting neutral terminals  163 . Neutral terminals  163  are used to connect three coil different elements  140 , interconnected by neutral busbar  160 . As shown in  FIG.  4 C , neutral terminals  163  are radially offset from neutral hoop  165  to avoid interference with other components of lead assembly  150  and coil elements  140 . Furthermore, referring to  FIG.  4 A , neutral terminals  163  extend through and away from busbar insulator  158 , thereby allowing to form electrical connections to coil elements  140 . Neutral hoop  165  remains enclosed within and insulated by busbar insulator  158 . 
       FIG.  4 D  illustrates phase busbars  152  without other components of lead assembly  150  shown.  FIGS.  4 E- 4 G  provide separate illustrations of each of these three phase busbars  152 . Each busbar  152  comprises two phase terminals  153  to form connections to two coil elements  140 . Two phase terminals  153  of each phase busbar  152  are joined or connected by hoop  155 , which may have a planar semi-circular shape. Furthermore, unlike neutral busbars  160 , each phase busbar  152  comprises external terminal  154  for connecting fractional slot electric motor  100  to an external power source (e.g., an inverter). External terminal  154  is connected to hoop  155  by neck  156 . In some examples, neck  156  provides a flexible connection between external terminal  154  and hoop  155  such that primary connection terminal  154  is able to move at least in the direction perpendicular to the plane of hoop  155  as, for example, is schematically shown in  FIG.  4 H . For example, neck  156  and, in some examples, external terminal  154  is formed from a stack of thin metal strips. This flexibility preserves internal connections (e.g., between phase terminals  153  and coil elements  140 ) and/or external connections (e.g., between external terminals  154  and the external power source). 
     Various connections between lead assembly  150  and coil elements  140  will now be described with reference to  FIG.  5 A  and  FIG.  5 B . Specifically,  FIG.  5 A  illustrates fractional slot electric motor  100  without busbar insulator  158 , showing various connections at second side  112  of stator core  110 . These connections may be conceptually divided into six groups, identified as Group 1-Group 6 in  FIG.  5 A . The same types of connections exist in each group, which are further illustrated in  FIG.  5 B  for Group 3. Specifically,  FIG.  5 B  shows five different connections  501 - 502  between lead assembly  150  and coil elements  140  or just between coil elements  140 . Connection  501  is between phase busbar  152  and coil element  140 , while connection  502  between neutral busbar  160  and another coil element  140 . Each of connection  503 , connection  504 , and connection  505  is between a pair of different coil elements  140 . 
     It should be noted that in this example, only 30 connections are formed by coil elements  140  at second side  112  of stator core  110 . 18 of the 30 connections are among coil elements  140 , 6 connections between coil elements  140  and phase busbars  152  (2 connections to each phase busbar  152 ), and 6 connections between coil elements  140  and neutral busbars  160  (3 connections to each neutral busbar  160 ). These connections are specific types of coil elements  140 , which are referred to as extended coil elements. Overall, 48 extended coil elements are used for these connections: two coil elements for each of 18 element-element connections (or a total of 36 coil elements), 6 coil elements for element-neutral busbar connections, and 6 coil elements for element-phase busbar connections. The remaining coil elements  140  do not form any connections at second side  112  of stator core  110 . Instead, these coil elements  140  protrude from one stator slot  114  and extend toward and into another stator slot  114 . These coil elements  140  are referred to as looped coil elements. In this example, there are 96 looped coil elements. Both types of coil elements will now be explained with reference to  FIG.  6 A - FIG.  6 E . 
       FIG.  6 A  illustrates fractional slot electric motor  100  without lead assembly  150 , showing the arrangement of coil elements  140  at second side  112  of stator core  110 .  FIGS.  6 B- 6 E  illustrates individual coil elements  140 . More specifically,  FIGS.  6 B- 6 D  illustrate three examples of looped coil elements  141 .  FIG.  6 E  illustrates an example of extended coil elements  142 . 
     Referring to  FIG.  6 B- 6 D , each looped coil element  141  comprises two loop extensions  143 , each terminating with loop extension end  145 . Loop extensions  143  are interconnected by end loop  144 . Loop extensions  143  protrude through stator core  110  between first side  111  and second side  112 , through different stator slots  114 . Loop extension ends  145  extend from stator core  110  at first side  111  and are connected to ends of other coil elements  140  as described above with reference to  FIG.  2 D . End loop  144  extends from stator core  110  at second side  112  and between different stator slots  114 . As noted above, looped coil element  141  is not connected to any other elements at second side  112 . 
     Referring to  FIG.  6 E , each extended coil element  142  comprises one extension  146 , terminating with first extension end  147  and second extension end  148 . Extension  146  protrudes through stator core  110 . First extension ends  147  extends from stator core  110  at first side  111  and is connected to first extension end  147  of another extended coil element  142  or loop extension  143  of looped coil element  141 . Second extension end  148  extends from stator core  110  at second side  112  and is connected second extension end  148  of another extended coil elements  142 , connected to busbar  152 , or connected to neutral busbar  160 . 
       FIG.  7 A  is a schematic illustration of first-type coil element  171 , which is an example of looped coil elements  141 .  FIG.  7 B  is a top view corresponding to  FIG.  7 A . Specifically,  FIG.  7 B  illustrates first-type coil element  171  protruding into first stator slot  114   a  (at the “D” position corresponding to D 4 ) and second stator slot  114   b  (also at the “D” position corresponding to D 4 ). Second stator slot  114   b  is separated by five other slots from first stator slot  114   a . Furthermore, first-type coil element  171  extends over “D” positions, thereby having no substantial radial offset, at least at second side  112  of stator core  110 . Finally, first-type coil element  171  extends circumferentially to end point  149 , which is past second stator slot  114   b , and then forms a return loop to second stator slot  114   b . This feature is used to avoid interference from other coil elements, e.g., extending from other slots between first stator slot  114   a  and second stator slot  114   b . First-type coil element  171  may be referred to as a “slot D—large span” coil element. In some examples, coil elements  140  comprise six different instances of first-type coil element  171 . 
       FIG.  7 C  is a schematic illustration of first-type coil element  171  together with second-type coil element  172 . Second-type coil element  172  is an example of extended coil elements  142 . Second-type coil element  172  extends from third stator slot  114   c , adjacent to first stator slot  114   a , and follows the shape of first-type coil element  171 , before extending away from second side  112  of stator core  110 . This extension is used to form an electrical connection to second-type coil element  172 . 
       FIG.  7 D  is a schematic illustration of third-type coil element  173 , which is another example of looped coil elements  141 .  FIG.  7 E  is a top view corresponding to  FIG.  7 D . Specifically,  FIG.  7 E  illustrates third-type coil element  173  protruding into first stator slot  114   a  (at the “D” position corresponding to D 4 ) and second stator slot  114   b  (also at the “C” position corresponding to D 3 ). It should be noted that first stator slot  114   a  and second stator slot  114   b  in  FIG.  7 E  may be different from those identified in other figures (e.g.,  FIG.  7 B  and  FIG.  7 C ) and are used to describe a specific coil element example. In  FIG.  7 E , second stator slot  114   b  is separated by seven other slots from first stator slot  114   a . Furthermore, third-type coil element  173  extends radially across all coil positions (from the “D” position to the “A” position) before returning back to the “C” position. This feature is used to avoid interference from other coil elements. Third-type coil element  173  may be referred to as a “slot C-D—crown span 8 slots” coil element. In some examples, coil elements  140  comprise 30 different instances of third-type coil element  173 . 
       FIG.  7 F  is a schematic illustration of fourth-type coil element  174 , which is another example of looped coil elements  141 .  FIG.  7 G  is a top view corresponding to  FIG.  7 F . Specifically,  FIG.  7 G  illustrates fourth-type coil element  174  protruding into first stator slot  114   a  (at the “B” position corresponding to D 2 ) and second stator slot  114   b  (also at the “C” position corresponding to D 3 ). As before, first stator slot  114   a  and second stator slot  114   b  in  FIG.  7 F  may be different from those identified in other figures. In  FIG.  7 G , second stator slot  114   b  is separated by six other slots from first stator slot  114   a . Furthermore, fourth-type coil element  174  extends radially across all coil positions (from the “B” position past the “A” position) before returning back to the “C” position. This feature is used to avoid interference from other coil elements. Fourth-type coil element  174  may be referred to as a “slot B-C—crown span 7 slots” coil element. In some examples, coil elements  140  comprise 30 different instances of fourth-type coil element  174 . 
       FIG.  7 H  is a schematic illustration of fifth-type coil element  175 , which is an example of extended coil elements  142 .  FIG.  7 I  is a top view corresponding to  FIG.  7 H . Specifically,  FIG.  7 I  illustrates fifth-type coil element  175 , comprising extension  146 , which protrudes into first stator slot  114   a  (at the “A” position corresponding to D 1 ). First end  147  of fifth-type coil element  175  extends from stator core  110  for connection another coil element or lead assembly  150 . More specifically, first end  147  extends over second stator slot  114   b , separated by three other slots from first stator slot  114   a . Furthermore, first end  147  extends over the “C” position in second stator slot  114   b . In some examples, first end  147  extends over the “D” position in second stator slot  114   b . This feature is used to avoid interference from other coil elements. Fifth-type coil element  175  may be referred to as an “I pin slot D-A” an “I pin slot D-A.” In some examples, coil elements  140  comprise 18 different instances of fifth-type coil element  174 . 
     Overall, in some examples, fractional slot electric motor  100  comprises 144 separate coil elements  140 . These coil elements  140  may be represented by 7 different types or configurations, five of which are described above with reference to  FIG.  7 A- 7 I . In some examples, coil elements  140  comprise 96 looped coil elements  141  and 48 extended coil elements  142 . 
       FIG.  8 A  illustrates a stator wiring schematic, in accordance with some examples.  FIG.  8 B ,  FIG.  8 C ,  FIG.  8 D , FIG. BE, and  FIG.  8 F  illustrate different portions of the stator wiring schematic in  FIG.  8 A , in accordance with some examples. The schematic in figure  FIG.  8 A  shows each of the 4 layers of the winding. The connections between coils can be seen as well as the connections to the busbars and the neutral bus bar. 
       FIG.  9    illustrates a stator wiring table, in accordance with some examples. More specifically,  FIG.  9    illustrates the contents of each stator slot, indicating the phase of each coil, which parallel circuit each coil belongs to, and the order of the connection. 
     Electrical Vehicle Examples 
     One application of fractional slot electric motor  100  is electrical vehicles or, more specifically, hybrid electric vehicles, plug-in hybrid electric vehicles, and all-electric vehicles. For example,  FIG.  10    is a schematic illustration of electric vehicle  1000 , which comprises battery pack  1010 , inverter  1020 , and fractional slot electric motor  100 . Other components of electric vehicle  1000  are not shown for simplicity. Battery pack  1010  is configured to receive energy received from inverter  1020  (e.g., from an external source, such as a charger, or regenerative braking of electric vehicle  1000 ) and store this electrical energy for future use. Furthermore, battery pack  1010  is configured to release the stored electrical energy to inverter  1020 , e.g., to drive fractional slot electric motor  100  and to operate other systems of electric vehicle  1000  (e.g., heating-ventilation, lighting, and the like). 
     Further Examples 
     Further, the description includes examples according to the following clauses: 
     Clause 1. A fractional slot electric motor comprising:
         a stator core, comprising a first side and a second side and stator slots, extending between the first side and the second side;   a coil, formed by coil elements, wherein:   each of the coil elements through one or two of the stator slots between the first side and the second side of the stator core,   each of the coil elements is electrically coupled to at least one other of the coil elements at the first side of the stator core, and   each of the coil elements has a rectangular cross-section; and   a lead assembly, comprising three phase busbars, wherein:   each of the phase busbars is electrically coupled to at least one of the coil elements at the second side of the stator core, and   each of the phase busbars comprises an external terminal ( 154 ) for connecting the fractional slot electric motor to an external power supply.       

     Clause 2. The fractional slot electric motor of clause 1, wherein the lead assembly further comprises two neutral busbars, each electrically coupled at least one the coil elements at the second side of the stator core. 
     Clause 3. The fractional slot electric motor of clause 2, wherein the neutral busbars and the phase busbars are stacked within the lead assembly and are electrically isolated from each other. 
     Clause 4. The fractional slot electric motor of clause 3, wherein the lead assembly comprises a busbar insulator, molded over each of the neutral busbars and the phase busbars and mechanically supporting the neutral busbars and the phase busbars with respect to each other. 
     Clause 5. The fractional slot electric motor of clause 4, wherein each of the phase busbars comprises one or more phase terminals, protruding from the busbar insulator and electrically coupled to one or more of the coil elements. 
     Clause 6. The fractional slot electric motor of clause 4, wherein each of the neutral busbars comprises one or more neutral terminals, protruding from the busbar insulator and electrically coupled to one or more of the coil elements. 
     Clause 7. The fractional slot electric motor of clause 4, wherein the lead assembly further comprises a busbar thermocouple, supported on the busbar insulator and thermally coupled to one of the phase busbars, partially protruding through the busbar insulator. 
     Clause 8. The fractional slot electric motor of any one of clauses 1-7, wherein each of the three phase busbars further comprises a hoop such that the external terminal is flexibly connected to the hoop. 
     Clause 9. The fractional slot electric motor of clause 8, wherein:
         each of the three phase busbars further comprises a neck, flexibly connected to the hoop to the external terminal, and   the neck is formed by a plurality of metal strips.       

     Clause 10. The fractional slot electric motor of clause 8, wherein the three phase busbars are stacked in the lead assembly. 
     Clause 11. The fractional slot electric motor of any one of clauses 1-10, wherein the coil elements comprise looped coil elements and extended coil elements such that the looped coil elements are only connected at the first side of the stator core while the extended coil elements are connected at both the first side and the second side of the stator core. 
     Clause 12. The fractional slot electric motor of clause 11, wherein:
         each of the looped coil elements comprises an end loop and two loop extensions, interconnected by the end loop and each terminating with loop extension end,   the loop extension end of each of the two loop extensions extends from the stator core at the first side and connected to one of the coil elements, and   the end loop extends from the stator core at the second side between two different ones of the stator slots.       

     Clause 13. The fractional slot electric motor of clause 12, wherein the end loop of each of the looped coil elements is positioned between the second side of the stator core and the lead assembly. 
     Clause 14. The fractional slot electric motor of clause 12, wherein the two loop extensions of one of the of the looped coil elements extend through the two different ones of the stator slots at same positions. 
     Clause 15. The fractional slot electric motor of clause 12, wherein the two loop extensions of one of the of the looped coil elements extend through the two different ones of the stator slots at different positions. 
     Clause 16. The fractional slot electric motor of clause 11, wherein:
         each of the extended coil elements comprises a first end, an extension, and a second end,   the extension protrudes through the stator core between the first side and the second side and interconnects the first end and the second end,   the first end extends from the stator core at the first side and is connected to one of the coil elements, and   the second end extends from the stator core at the second side and is connected to one of the coil elements or the lead assembly.       

     Clause 17. The fractional slot electric motor of clause 16, wherein the second end of each of the extended coil elements is at least partially protrudes past the lead assembly. 
     Clause 18. The fractional slot electric motor of clause 16, wherein the second end is radially offset relative to the extension. 
     Clause 19. The fractional slot electric motor of any one clauses 1-18, wherein the coil elements are arranged into in a three-phase, two-parallel configuration. 
     Clause 20. The fractional slot electric motor of any one of clause 1-19, wherein the rectangular cross-sectional of each of the coil elements has a thickness of between 3.0 millimeters and 4.0 millimeters and a width of between 2.5 millimeters and 3.5 millimeters. 
     CONCLUSION 
     Although the foregoing concepts have been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatus. Accordingly, the present examples are to be considered as illustrative and not restrictive.