Abstract:
An apparatus and corresponding method for maintaining an air gap between a stator and rotor in an electro-mechanical energy converter is provided. The apparatus includes a structural sleeve and a plurality of stator sections attached to an inner surface of the structural sleeve. A hub is enclosed by the structural sleeve and is concentric with the structural sleeve. A plurality of rotor sections is flexibly coupled to the hub and is enclosed by the structural sleeve. A rail system is positioned within the structural sleeve and is concentric with the structural sleeve. The rail system guides the rotor sections in a substantially circular path and defines an air gap between the plurality of stator sections and plurality of rotor sections.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present invention relates generally to electro-mechanical energy converters. 
     2. Discussion of Background Information 
     One type of electro-mechanical energy converter, called an “electric motor,” converts electric energy into mechanical work. Another type of electro-mechanical energy converter, called an “electric generator,” converts mechanical work into electric energy. Both types of electro-mechanical energy converters come in a range of sizes and are often interchangeable in operation, which is to say that a motor can act as a generator and vice versa when the process is reversed. In all cases, mechanical work is required to drive an electric generator that can come from a variety of sources, amongst which is the work provided by ocean waves. 
     Motors and generators typically operate at high speed (1000 to 4000 rpm) and low torque because this combination reduces the overall cost to manufacture for a given power level. The relatively slow speed and large forces from ocean waves result in challenging requirements for electro-mechanical energy conversion. Direct mechanical coupling of these low speed (less than 5 rpm revolution per minute) and high torque (millions of Newton-meter) mechanical forces and converting it to electrical energy can be efficiently and cost effectively achieved with a large-diameter direct-drive generator. This direct coupling requires that certain electromagnetic and mechanical design challenges be addressed. 
     A common industry practice to address low-speed and high torque motor/generator requirements is to increase diameter. An increase in diameter improves both efficiency and reduces the unit material cost for the same torque delivered by the motor/generator. Torque is improved by a large diameter machine due to the increased machine radius and a longer lever arm acting on the same electromagnetic force. 
     In a conventional design, a generator/motor consists of two primary components: a fixed element, called a “stator,” against which a rotational element, called a “rotor” electromagnetically reacts. The stator and rotor are separated by a small radial clearance (air gap) that provides mechanical clearance between the moving parts. Through numerous machine design types, understood by those skilled in the art, magnetic flux is directed through the air gap between stator and rotor and through one or more sets of metallic coils. The relative rotation between stator and rotor causes a time rate of change of the magnetic flux through the metallic coils and generates voltage directly proportional to that rate of change. The time rate of change of magnetic flux can be increased either by faster rotation at the air gap and/or by higher flux density. For a given rotational speed, the velocity is proportional to the radius, which means that the larger the diameter of the generator/motor, the faster the relative motion between the rotor and stator at the air gap. It can be shown that when all other machine parameters are assumed constant a higher speed translates into higher flux velocity and improved generator efficiency. 
     As the diameter of an electro-mechanical energy converter increases, the ability to manufacture these parts precisely (i.e., with smaller or “tighter” tolerances) and therefore the ability to maintain a small air gap becomes increasingly difficult and more expensive. Tolerances of approximately 5 to 10 millimeters (mm) have been achieved with existing large diameter direct-drive generators/motors. Large air gaps, such as 5 to 10 mm, decrease the efficiency (and/or increase the cost) of a motor/generator. 
     A need therefore exists for an increased motor/generator diameter and a need for a design that allows this large diameter with a reduced air gap (e.g., 0.5 mm to 3 mm) between stator and rotor in an electro-mechanical energy converter. 
     SUMMARY 
     Example embodiments of the present invention provide a mechanical assembly for maintaining an air gap between a stator and rotor in an electro-mechanical energy converter. In one embodiment, the mechanical assembly includes a structural sleeve serving as a frame that concentrically arranges other components of the assembly and enclosing these components. A plurality of stator sections is attached to an inner surface of the structural sleeve. A plurality of rotor sections is flexibly coupled to a hub. The hub transfers torque to or from the rotor sections in an electro-mechanical energy conversion process. A rail system, which may comprise two axially separated rails, is positioned within the structural sleeve. The rail system guides the rotor sections in a substantially circular path. The rail system also defines an air gap between the plurality of stator sections and plurality of rotor sections. 
     In another aspect, the present invention provides a method for maintaining an air gap between a stator and rotor in an electro-mechanical energy converter. In one embodiment, the method includes providing a structural sleeve and attaching a plurality of stator sections to an inner surface of the structural sleeve. The method includes providing a hub that is enclosed by the structural sleeve and is concentric with the structural sleeve. The method includes flexibly coupling a plurality of rotor sections to the hub. The plurality of rotor sections being flexibly coupled is enclosed by the structural sleeve. The method includes positioning a rail system within the structural sleeve. The rail system being so positioned guides the rotor sections in a substantially circular path and to define an air gap between the plurality of stator sections and plurality of rotor sections. The rail system is concentric with the structural sleeve. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       One will better understand these and other features, aspects, and advantages of the present invention following a review of the description, appended claims, and accompanying drawings in which: 
         FIGS. 1A-1C  are diagrams of an example electro-mechanical energy converter environment. 
         FIG. 2  is a diagram of an example electro-mechanical energy converter environment. 
         FIGS. 3A and 3B  are cross sectional views of an electro-mechanical energy converter according to one embodiment of the present invention. 
         FIG. 4  is a cross sectional view of one end of an electro-mechanical energy converter according to one embodiment of the present invention. 
         FIGS. 5A-5C  are diagrams of a rail system and car design according to one embodiment of the present invention. 
         FIGS. 6A and 6B  are diagrams of a flexible coupling according to one embodiment of the present invention. 
         FIGS. 7A-7C  are diagrams of a rotor section flexibly coupled to a hub according to example embodiments of the present invention. 
         FIGS. 8A and 8B  are diagrams a drive hub according to the example embodiments of the present invention. 
         FIGS. 9A-9C  are diagrams of an example permanent magnetic generator according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS. 1A-C  show an example electro-mechanical energy converter environment  100  including a wave energy converter (WEC)  105  and waves  110 . The WEC  105  includes an electro-mechanical energy converter  115 , as shown in relation to a nacelle  120 , rotatable forward float  125  and rotatable aft float  130 . The electro-mechanical energy converter  115  is disposed concentrically with the longitudinal central axis and at an end of the nacelle  120 . In a wave energy extraction process, waves  110  interact with the floats  125 ,  130 , which in turn rotate a drive hub  135  in a reciprocating manner with the rise and fall of the waves  110 . The drive hub  135  rotates a rotor (not shown) of the electro-mechanical energy converter  115  to generate electricity. 
       FIG. 2  shows example electro-mechanical energy converter environment  200  including a tunnel boring machine  205  and rock face  210 . The tunnel boring machine  205  includes an electro-mechanical energy converter  215 , cutter head  220 , and driveshaft  225  connecting the electro-mechanical energy converter  215  and cutter head  220 . In a tunnel boring process, the electro-mechanical energy converter  215  uses electricity to turn a rotor (not shown) of the electro-mechanical energy converter  215 . The rotor turns the driveshaft  165  which in turn rotates the cutter head  220 . The rotating cutter head  220  engages the rock face  210  breaking off chunks of rock, which are then conveyed away leaving a hole to form a tunnel. In contrast to the WEC  105  discussed above, the rotation of the cutter head  220  is not reciprocating but rather is run in a continuous manner in both forward and reverse directions. 
     The electro-mechanical energy converter  115  of the WEC  105  (which is a type of electric generator) and the electro-mechanical energy converter  215  of the tunnel boring machine  205  (which is a type of electric motor) share the characteristics of being large in size (e.g., 5 to 10 meters (m) in diameter or greater), high torque (e.g., several million Newton-meters), and slow speed (e.g., 1 revolution per minute). These characteristics present challenges to the mechanical design of such electro-mechanical energy converters. 
     As depicted in the cross sectional side view of  FIG. 3A  and the cross section end view of  FIG. 3B , one embodiment of an electro-mechanical energy converter  300  comprises a rotor  305 , a stator  310 , a drive hub  315  for driving the rotor  305 , a rail system  335  on which the rotor  305  travels, and an outer structural sleeve  330  that encloses the foregoing components, as well as other components of the electro-mechanical energy converter  300 . 
     In one example shown in  FIG. 4 , the stator  310  is attached to the outer structural sleeve  330 , which serves as the assembly frame of the electro-mechanical energy converter  300 . The stator  310  is comprised of between 40 and 80 stator sections  311  that make up a full 360 degree circumference of the electro-mechanical energy converter  300  or portion thereof. In a convenient embodiment, each of the stator sections  311  has a section length of approximately 300 mm and an axial length between 1 to 2 m. 
     In one embodiment depicted in  FIGS. 5A-5C , the electro-mechanical energy converter  300  comprises a “rail and car” system design rather than a conventional fixed rotor design. In a conventional electro-mechanical energy converter, the rotor is a fixed and precisely-machined rotary cylinder, and the rotor must spin within the stator, which is a precisely machined cylindrical bore. As the diameter of an electro-mechanical energy converter increases, tolerances must be increased to prevent the rotor (cylinder) from impacting the stator (bore). These increases in tolerance translate into a less efficient (and/or more expensive) design. The rail and car system of the electro-mechanical energy converter  300  enables a substantial reduction in the clearance between the rotor  305  and stator  310 , which leads to major advantages discussed below with regard to functionality. 
     In an example embodiment, the electro-mechanical energy converter  300  comprises a pair of rails  335 A,  335 B (as labeled in  FIG. 5A  and collectively referenced as  335 ) at the extreme axial ends of the stator  310 . The rails  335  are positioned between the rotor  305  and stator  310 . In a convenient embodiment shown in  FIG. 4 , each rail  335  is attached to either end of the structural sleeve  330  by supports  337 . In another embodiment, the rail  335  is attached to the stator  310 . In other embodiments, the electro-mechanical energy converter  300  incorporates a single rail (monorail system) or a plurality of rails (multi-rail system). In some of the foregoing rail systems, the rails  335  are made up of several sections, which may be arranged with or without gaps between the sections. For example, there may be gaps between the sections of rail to accommodate for expansion and contraction of individual sections. The sectional design of the rails  335  also facilitates their manufacture (e.g., machined to a tighter tolerance), assembly, and installation. 
     In a convenient embodiment, each rail  335  is a track that approximates a circle or portion thereof (i.e., an arc) but need not be perfect. Such an imperfect circle or portion thereof is expected with a large diameter electro-mechanical energy converter because of difficulty in achieving small machine tolerances, exposure to massive loads, thermal expansion and contraction of all components. 
     In the rail and car design described herein, the structural sleeve  330 , stator  310 , and/or rail  335  are designed such that machined tolerances between the face of the stator  310  and the running surface of the rail  335  are achieved through the application of localized machine tolerances in which the stator  310 , rail  335 , and/or rotor  305  interfaces occur. 
     As depicted in  FIGS. 5A and 5B , the rotor  305  in the rail and car design is not a rigid cylinder but is instead comprised of a large number of smaller rotor sections  307 . The rotor sections  307  are arranged end to end to form a substantially complete circle. In one example embodiment, the rotor  305  is comprised of between 40 and 80 rotor sections  307  that complete a full 360 degree circumference of the electro-mechanical energy converter  300 . 
     The rotor sections  307  follow the rail  335 . Because the rail  335  may not be a perfect circle, as described above, the path followed by the rotor sections  307  may not be a perfect circle as well. This design accommodates any non-circular characteristic that is inherent to a large diameter electro-mechanical energy converter or any non-circular characteristic that is experienced by a large diameter electro-mechanical energy converter during operation, such as load, and thermal expansion and contraction of components. 
     In some applications, such as a wave energy converter  105  of  FIGS. 1A-1C , the rotation of the electro-mechanical energy converter  115  is limited to less than 360 degrees. For example, the forward float  125  would rise and reach a maximum position of 45 degrees clockwise (CW) from horizontal and then fall counterclockwise (CCW) to a position that is 45 degrees below horizontal. This cyclic CW and CCW operation continually repeats to produce electricity. Thus, the full range of motion of the forward float  125  and drive hub  135  would be limited to 90 degrees, which also limits the range of the electro-mechanical energy converter  115  motion to 90 degrees in this example application. This range of motion can be limited by either mechanical end stops on the wave energy converter  105  or by electronic controls of the electro-mechanical energy converter  115 . 
     In applications where a limited range of rotation exists, the electro-mechanical energy converter  300  may be designed without a complete 360 degrees of components. This reduced size might be implemented to reduce cost or to accommodate other mechanical clearance requirements. In such a design, the rail  335  of  FIG. 5A , for example, is designed to be 350 degrees in length with enough stator sections  311  to achieve 350 degrees of stator  310  length while designing the number of rotor sections  307  to achieve a rotor  310  length of 260 degrees. This configuration would allow the rotor  305  a CW to CCW range of motion limited to 90 degrees between extreme ends of the rail  335  or stator  310 . The circular length (circumference), or arc length, of the electro-mechanical energy converter  300  could range between 10 degrees and 360 degrees depending on the application. 
     In a convenient embodiment shown in  FIG. 5B , each of the rotor sections  307  is supported by four wheels  340 . As shown, two wheels  340  ride on one rail  335 A and the other two wheels  340  ride on the other rail  335 B. This rotor section and wheel configuration can be thought of as a car that rides on two tracks (for example, a train or rollercoaster). A rotor axle  345  supports each rotor wheel  340  and is held in place by an axle bearing  347  (shown in  FIG. 5C ). The axle bearing  347  holds a tight radial tolerance but allows for axial play in the system; that is to say, the rotor axle  345  is allowed to slide in the axial direction to allow for axial variance in the construction of the electro-mechanical energy converter  300 . 
     In another embodiment, the rail  335  may provide a sliding surface (bearing surface) and a low friction guide is attached to the rotor  305  to control tolerance between stator  310  and rotor  305 . In another embodiment, the rail  335  may be machined with a bearing race and a roller bearing assembly is attached to the rotor sections  307  to control tolerance between stator  310  and rotor  305 . 
     In the configuration shown in  FIG. 5B , the space (air gap) between the rotor  305  and stator  310  is controlled by the mechanical tolerances of the rail  335  and the rotor-wheel  340 . Because the mechanical size of each of the rotor sections  307  is small (on the order of 0.5 m), controlling the clearances and tolerances between the stator and rotor is easier to achieve. A tight tolerance between the rotor car wheels  340  and face of each of the rotor sections  307  is in a range of 0.25 mm (i.e., 0.010″), which results in a substantially reduced air gap  312  of approximately 1 mm for the electro-mechanical energy converter  300  having a diameter of approximately 10 m. 
     A reduced air gap  312  allows for reduced air gap reluctance and increased magnetic circuit permeability, which in turn reduces the amount of magnetic material (permanent or electromagnetic) for a given electro-mechanical energy converter rating. For example, in the linear range of magnetic circuit design, a 1 mm air gap will require 10 times less magnetic material than a machine that has a 10 mm air gap. A reduction in air gap reduces overall magnetic circuit reactance, which improves the power factor of the electro-mechanical energy converter  300  and stabilizes the operational performance of a variable frequency machine, such as the WEC  105   FIG. 1A  and tunnel boring machine  205  of  FIG. 2 . 
     The tolerances of the rotor wheels  340  and/or rails  335  can be easily machined using smaller cost-effective computer numerically controlled (CNC) machine tools. One result of the foregoing rail and car design is the allowance for a small air gap between stator  310  and rotor  305  even when size of the electro-mechanical energy converter becomes very large (e.g., greater than 5 to 10 m in diameter). As discussed below in more detail, this reduction in air gap consequentially leads to a cost reduction. The reduction in air gap  312  is further improved by the tight tolerance between the face of the stator and contact surface of the rail. In one embodiment, the rail  335  is attached to the stator  310  and the tolerance between the two elements is 0.25 mm (0.010 inch). This technique of controlling air gap tolerance can apply to all sizes and types of electro-mechanical energy converters, for example, converters less than 1 m in diameter. 
     Examples of possible electro-mechanical energy converters (i.e., electric motors and generators) include both alternating current (AC) and direct current (DC) electro-mechanical energy converters. Within the general classification of AC and DC electro-mechanical energy converters, a multitude of electromagnetic designs exist, all of which may benefit from the examples described herein, and includes designs, such as but not limited to brushed DC, brushless DC, shunt wound, separately excited, series wound, compound wound, single phase, three phase, poly phase, synchronous, asynchronous, axial flux, radial flux, transverse flux, permanent magnet, shaded pole, reluctance, switched reluctance, coreless, ironless, squirrel cage, induction, doubly fed induction, singly fed electric, doubly fed electric, etc. 
     In a convenient embodiment shown in  FIGS. 6A and 6B , each of the rotor section  307   s  is flexibly coupled end-to-end by a flexible coupling  369  between each pair of the rotor sections  307 . The flexible coupling  369  comprises a rotor pivot joint  370  and rotor pivot pin  375 , as best depicted in  FIG. 6B . In other embodiments, the flexible coupling  369  may take on a ball-socket or hinge-type configuration as required by design. The flexible couplings allow for articulation between adjacent rotor sections  307  and allow each of the rotor sections  307  to precisely ride the rail  335 . This configuration also allows for the steel contact surfaces between rotor sections to be touching allowing for high magnetic permeability between each section (a necessity for proper operation of some types of electro-mechanical energy converters). In this embodiment, the flexible coupling  369  is designed to efficiently couple magnetic flux from one rotor section  307  to the next rotor section  307 . 
     In another embodiment, there is a physical gap between each of the rotor sections  307 . The physical gap allows for clearance between adjacent rotor sections  307  as they ride the rail  335 . In this embodiment, each of the rotor sections  307  is independently attached to the drive hub  315 . 
       FIGS. 7A-C  show the rotor sections  307  flexibly coupled to the drive hub  315 . The drive hub  315  transfers torque to or from the rotor sections  307 . The figures also show the rotor sections  307  are held in an outward radial direction by a radial force (Fr) such that each rotor section  307  is forced tightly against the rail  335 . By holding each rotor section  307  (and in some embodiments, the rotor wheels  340 ) of  FIG. 5B  tight against the rail  335 , a small air gap  312  between the rotor section  307  and stator section  311  is maintained. The following are examples of configurations in which the rotor sections  307  are flexibly coupled to the drive hub  315  and examples of configurations for producing the radial force (Fr). 
     In the example configuration depicted in  FIG. 7A , each rotor section  307  is held tightly against the rail  335  by a rotor holding spring  380 . The rotor holding spring  380  pushes against the drive hub  315  to push the rotor section  307  against the rail  335 . The springs  380  are located between drive dogs  308  that are fixed to the rotor section  307 . The drive dogs  308  transfer toque to or from the rotor sections  307  and the drive hub  315 . In a convenient embodiment, the rotor holding spring  380  is folded in an accordion-like configuration and slid between the rotor section  307  and drive hub  315 . The spring  380  may be made of fiber reinforced plastic (FRP) or other metallic or composite material. 
     In another example configuration depicted in  FIGS. 7B and 7C , a drive arm  390  and arm spring  395  are used to both force the rotor sections  307  (and in some embodiments, the rotor wheels  340 ) tightly against the rail  335  and to transfer torque to or from the rotor sections  307  and the drive hub  315 . 
     In yet another example configuration in which the stator and/or rotor are magnetic is a permanent magnet or electromagnet, the magnetic attraction between the stator and rotor provides the radial force to hold each rotor section  307  (and in some embodiments, the rotor wheels  340 ) tight against the rail  335 . This configuration may be used in conjunction with any one of the other embodiments shown in  FIG. 7A  (rotor holding spring and drive dog) and  FIGS. 7B and 7C  (drive arm and arm spring). In such a combination, each component need only provide some a portion of the radial force, which may lead to an optimization of cost and materials. 
     In a convenient embodiment, at least one of the components of the foregoing configurations shown in  FIGS. 7A-7C  is of a sectional design allowing removal of an individual rotor section  307 . Removal of rotor section  307  may be accomplished by using one or more actuators or forcing cylinders  395  shown in  FIG. 7B  to controllably remove the rotor section  307  from the rotor  305 . Similarly, once the rotor sections  307  are removed, the stator section  311  can be removed using the same apparatus shown in  FIG. 7B . 
     In addition to a reduction in air gap  312  leading to overall reduction in cost, the electro-mechanical energy converter  300  is modular for accommodating variations arising from use of the electro-mechanical energy converter  300 , such as load, and thermal expansion and contraction, as well as for enabling repairs. For example, as described above, the rotor  305  is designed in a plurality of rotor sections  307  to allow for dimensional variance due to loading and thermal expansion/contraction. The sectional design of the rotor  305  also allows for mechanical tolerance variation, assembly, disassembly, maintenance, and repair. In a convenient embodiment, the stator  310  also is designed in sections  311  to allow for dimensional variance due to loading and thermal expansion/contraction. The sectional design of the stator  310  also allows for mechanical tolerance variation, assembly, disassembly, maintenance, and repair. Other components of the electro-mechanical energy converter may also be designed in sections. 
     The design of the electro-mechanical energy converter  300  therefore enables repair of discrete rotor sections  307  and/or stator sections  311  without requiring removal of the entire electro-mechanical energy converter  300  from a machine. This is of particular utility for the WEC  105  of  FIG. 1A  and tunnel boring machine  205  of  FIG. 2  for which removal and replacement of their respective electro-mechanical energy converter,  115  and  215 , is a costly and time consuming procedure. 
     Returning now to the structural elements of the electro-mechanical energy converter  300 , as discussed above with regard to  FIGS. 3A and 3B , the electro-mechanical energy converter  300  includes the structural sleeve  330  that encapsulates the components of the electro-mechanical energy converter  300 , such as the rotor  305  and stator  310 , and provides a frame for these elements. The structural sleeve  330  may be inserted into and/or bonded to a machine, such as WEC  105  of  FIG. 1A  and tunnel boring machine  205  of  FIG. 2 . 
     The choice of material for the structural sleeve  330 , and other components for that matter, depends largely on the operating environment of the machine of which the electro-mechanical energy converter  300  is a part. For example, given that the WEC  10  operates in a high salinity environment (viz., the ocean), fiber reinforced plastic (FRP), also known as fiberglass, is a suitable material for manufacturing the structural sleeve  330 . Of course, other materials such as aluminum, steel, other metal alloys, and composites are possible. 
     Some of the components of the electro-mechanical energy converter  300  may be designed to withstand high torque. For example, for added rigidity, the drive hub  315  may incorporate one or more stiffeners  385  (as shown in  FIG. 8A ) and/or a “stepped” profile  386  (as shown in  FIG. 8B ). In another example, the drive hub  315  includes a central hub, which has a smaller diameter than the drive hub  315 , and spokes radiating outward from the central hub and ending at a wall of the drive hub  315 . In this “hub and spoke” configuration, there is no material between the spokes. As such, the rotational mass the hub and spoke configuration may be less than the configurations of  FIGS. 8A and 8B , which ultimately leads to lower cost and more efficient operation of the electro-mechanical energy converter  300 . 
     It should be readily apparent that the design and its features described above may be applied to any one of a variety of electro-mechanical energy converters. For example,  FIGS. 9A-9C  show the foregoing design applied to a permanent magnetic generator  900 . In one embodiment, the permanent magnetic generator  900  includes a plurality of rotor sections  907  (one of which is shown) and a plurality of stator sections  911  (one of which is shown). The plurality of rotor sections  907  and plurality of stator sections  911  complete a full 360 degree circumference of the permanent magnetic generator  900 . 
     The permanent magnetic generator  900  further includes a pair of rails  935 , and each rail  935  is attached to either end of a generator  900 . Each rail  935  is a track that approximates a circle, but need not be a perfect circle. In one embodiment, each of the rotor sections  907  is supported by four wheels  940 , two wheels ride on one of the rails  935  and the other two wheels  940  ride on the other rail  935 . 
     As shown in  FIG. 9B , each of the stator sections  911  includes a stator back iron that couples magnetic flux from one stator pole to the next to provide a low reluctance flux path. A stator coil  950  converts a changing magnetic field (such as that caused by the rotor sections  907  turning magnets, as described below) into an induced electromotive force and current. A stator coil slot (not shown) located in the stator back iron holds the stator coil  950  in place. A bus bar provides for series or parallel electrical connection between the stator coil phases and output terminals of the generator  900 . The output terminals in turn may be connected with a battery(s) to store the generated electricity and/or a transmission line(s) to carry the generated electricity to another location. The generator  900  may also include a cooling jacket  955  attached to the stator as part of a cooling heat exchange system. The cooling jacket  955  may be either air or air cooled. 
     As shown in  FIG. 9C , each of the rotor sections  907  includes a rotor back iron  909  and rotor magnets  965  attached to the rotor back iron  909 . The rotor back iron  909  couples magnetic flux from one rotor magnet  965  to the next to provide a low reluctance flux path. The rotor magnet  965  may be surface or embedded magnets attached to the rotor back iron  909 . The rotor magnet  965  may be made of several different materials, such as the neodymium-iron-boron, Alnico, samarium-cobalt, iron-ferrite. Because the foregoing design provides a small air gap  912 , lower cost magnets such as iron-ferrite and Alnico may be used in manufacturing the permanent magnetic generator  900 . 
     In an electro-mechanical energy converting process, as the rotor sections  907  travel around the rails  935 , a time varying magnetic field with respect to the stator coil  950  is created. The stator coils  950  convert the changing magnetic field into electricity. 
     In keeping with the modular design enabling mechanical tolerance variation, assembly, disassembly, maintenance, and repair, as described above, in a convenient embodiment, the stator coils  950  and bus bar connections  960  are removable to allow repair or troubleshooting of discrete stator sections  911 . Additionally, the stator coils  950  and bus bar  960  are located outside of the rails  935  to allow for accessibility for assembly, disassembly, maintenance, and repair. 
     The stator and rotor sections of the design (e.g., the rotor sections  307  and stator sections  311  of  FIGS. 5A and 5B ) are adapted to accommodate a particular type and/or electromagnetic design of an electro-mechanical energy converter. For example, in some cases each of the stator sections is magnetically coupled to another stator section and each of the rotor sections is magnetically coupled to another rotor section. In other cases, each of the stator sections is magnetically coupled to another stator section while each of the rotor sections is not magnetically coupled to another rotor section. In yet other cases, each of the stator sections is not magnetically coupled to another stator section while each of the rotor sections is magnetically coupled to another rotor section. In still yet others cases, each of the stator sections is not magnetically coupled to another stator section and each of the rotor sections is not magnetically coupled to another rotor section. 
     In addition to the wave energy converter  105  of  FIG. 1A  and the tunnel boring machine  205  of  FIG. 2 , there are other industry applications for low speed and high torque, such as a tram bull wheel drive, Ferris wheel, low speed wind energy conversion, large turrets/tables used for machining and equipment handling, and very large turrets (VLT) used for the offshore oil and gas industry. 
     As discussed above, increasing the diameter of electro-mechanical energy converter increases both the air gap magnetic flux speed and for a fixed amount of electromagnetic material, an increased diameter also increases the drive shaft torque. Increasing flux speed by increasing the diameter produces the same effect as increasing rotational speed and has a proportional effect on cost reduction. Conventional industrial solutions for large-diameter electro-mechanical energy converters typically require a large air gap (5 mm to 10 mm) which increases the volume requirements and cost of electromagnetic materials. These large air gaps are required to allow for mechanical clearance between stator and rotor that are paired in a large diameter electro-mechanical energy converter. Conventional electro-mechanical energy converters are approximately 6 m in diameter and become increasingly expensive to manufacture as the size and diameter increases. 
     The diameter of electro-mechanical energy converter  300 , according to the examples described herein, can be increased to 10 m or more, which exceeds conventional technologies. 
     A large diameter increases torque and allows for reduced rotor  305  and stator  310  materials for the same torque rating. This is because the Machine torque (T)=Magnetic shear force (F)×machine radius (r). For the equation T=F×r and an increased radius (r), the same torque (T) can be achieved by using less magnetic force (F) and consequently less magnetic materials. 
     Increasing the machine diameter will increase the linear speed of the rotor magnetic flux and allow for an increased EMF, which can be used to reduce magnetic materials and machine costs. The equation Voltage(V)=N×A(dB/dt) [1] describes the voltage produced as a function of the number of stator coil turns (N), the area of magnetic circuit flux (A), and the time rate of change of magnetic flux density (dB/dt). The term dB/dt is directly proportional (∝) to linear magnet speed at the surface of the outer rotor diameter; dB/dt∝ωr(P/2π) [2] [2]; where ω is the radial machine speed, r is the machine radius, and P is the number of magnetic poles in the machine. According to [2], for a given machine speed (ω), an increase in machine diameter or radius (r) will increase the rate of change of flux density (dB/dt) and as shown in [1] a reduction in the required magnet surface area (A) and/or a reduction in the number of stator coil turns (N). A reduction in N or A will translate into a reduction in machine cost by making the machine shorter in the axial direction or by reducing the number of stator coil turns. 
     It is noted that the foregoing examples have been provided merely for the purpose of explanation and are in no way to be construed as limiting the present invention. While the present invention has been described with reference to an exemplary embodiment, it is understood that the words, which have been used herein, are words of description and illustration, rather than words of limitation. Changes may be made, within the purview of the appended claims, as presently stated and as amended, without departing from the scope and spirit of the present invention in its aspects. Although the present invention has been described herein with reference to particular means, materials and embodiments, the present invention is not intended to be limited to the particulars disclosed herein; rather, the present invention extends to all functionally equivalent structures, methods and uses, such as are within the scope of the appended claims.