Abstract:
A system for transferring torque between a pair of independently, concurrently rotating shafts of a turbofan engine includes a magnetic gearbox. The magnetic gearbox has a first ring structure, a second ring structure and an intermediate ring structure. Each ring structure has an annular aperture therethrough and a plurality of permanent magnets embedded therein. The intermediate ring structure is disposed between the first and the second ring structures. Each ring structure is coaxially concentric with, and independently rotatable with respect to the remaining ring structures. The first and second ring structures are each coupled to separate ones of the rotating engine shafts, and the intermediate ring is operable to transfer torque between the pair of shafts. Preferably, the intermediate ring structure is coupled to a rotating machine. The rotating machine has a controller, and is operable for adjusting a ratio of torque transferred between the pair of shafts.

Description:
FIELD OF THE INVENTION 
   The present invention is directed to a method and apparatus for coupling of rotating machines, and more specifically to coupling of high pressure (HP) and low pressure (LP) turbine shafts of a turbofan machine 
   BACKGROUND OF THE INVENTION 
   A gas turbine engine generally includes one or more compressors followed in turn by a combustor and high and low pressure turbines. These engine components are arranged in serial flow communication and disposed about a longitudinal axis centerline of the engine within an annular outer casing. The compressors are driven by the respective turbines and compressor air during operation. The compressor air is mixed with fuel and ignited in the combustor for generating hot combustion gases. The combustion gases flow through the high and low pressure turbines, which extract the energy generated by the hot combustion gases for driving the compressors, and for producing auxiliary output power. 
   The engine power is transferred either as shaft power or thrust for powering an aircraft in flight. For example, in other rotatable loads, such as a fan rotor in a by-pass turbofan engine, or propellers in a gas turbine propeller engine, power is extracted from the high and low pressure turbines for driving the respective fan rotor and the propellers. 
   It is well understood that individual components of turbofan engines, in operation, require different power parameters. For example, the fan rotational speed is limited to a degree by the tip velocity and, since the fan diameter is very large, rotational speed must be very low. The core compressor, on the other hand, because of its much smaller tip diameter, can be driven at a higher rotational speed. Therefore, separate high and low turbines with independent power transmitting devices are necessary for the fan and core compressor in aircraft gas turbine engines. Furthermore since a turbine is most efficient at higher rotational speeds, the lower speed turbine driving the fan requires additional stages to extract the necessary power. 
   Many new aircraft systems are designed to accommodate electrical loads that are greater than those on current aircraft systems. The electrical system specifications of commercial airliner designs currently being developed may demand up to twice the electrical power of current commercial airliners. This increased electrical power demand must be derived from mechanical power extracted from the engines that power the aircraft. When operating an aircraft engine at relatively low power levels, e.g., while idly descending from altitude, extracting this additional electrical power from the engine mechanical power may reduce the ability to operate the engine properly. 
   Traditionally, electrical power is extracted from the high-pressure (HP) engine spool in a gas turbine engine. The relatively high operating speed of the HP engine spool makes it an ideal source of mechanical power to drive the electrical generators connected to the engine. However, it is desirable to draw power from additional sources within the engine, rather than rely solely on the HP engine spool to drive the electrical generators. The LP engine spool provides an alternate source of power transfer, however, the relatively lower speed of the LP engine spool typically requires the use of a gearbox, as slow-speed electrical generators are often larger than similarly rated electrical generators operating at higher speeds. 
   However, extracting this additional mechanical power from an engine when it is operating at relatively low power levels (e.g., at or near idle descending from altitude, low power for taxi, etc.) may lead to reduced engine operability. Traditionally, this power is extracted from the high-pressure (HP) engine spool. Its relatively high operating speed makes it an ideal source for mechanical power to drive electrical generators that are attached to the engine. However, it is desirable at times to increase the amount of power that is available on this spool, by transferring torque and power to it via some other means. 
   Another source of power within the engine is the low-pressure (LP) spool, which typically operates at speeds much slower than the HP spool, and over a relatively wider speed range. Tapping this low-speed mechanical power source without transformation result in impractically large generators. 
   Many solutions to this transformation have been proposed, including various types of conventional transmissions, mechanical gearing, and electromechanical configurations. One such solution is a turbine engine that utilizes a third, intermediate-pressure (IP) spool to drive a generator independently. However, this third spool is also required at times to couple to the HP spool. The means used to couple the IP and HP spools are mechanical clutch or viscous-type coupling mechanisms. 
   U.S. Pat. No. 6,895,741, issued May 24, 2005, and entitled “Differential Geared Turbine Engine with Torque Modulation Capacity”, discloses a mechanically geared engine having three shafts. The fan, compressor, and turbine shafts are mechanically coupled by applying additional epicyclic gear arrangements. Electromagnetic machines can be controlled for selectively modulating the torque versus speed characteristic of the compressor and the fan, and for modulating the rotational speed relationship between the turbine, compressor and the fan. The machines can be used as electric starters. Either or both of the compressor rotor shaft and the fan rotor shaft can be rotated by machines which receive electrical power and operate as motors to electrically start the engine. However, this does not provide a solution to coupling of concurrently rotating HP and LP turboshafts without using a mechanical gearbox. 
   Therefore, there is a need for a system of controllably transferring power between independently rotating machines operating at different speeds, without a mechanical gearbox, and for transferring torque between independently rotating machines using magnetic coupling techniques. 
   SUMMARY OF THE INVENTION 
   A new method is described for variably transferring mechanical torque from one rotating machine to another, relying solely upon magnetic effects in a planetary magnetic gearbox, to couple one machine to another. A second means is used to variably control the torque transfer. The present invention is applicable to turbofan engines to couple rotating shafts at differing speeds within the turbofan engine for controllably transferring power. By applying the relatively high- and low-speed engine shafts to an epicyclic magnetic gearbox, an effective gear ratio can be modulated by variably operating on a third input of the epicyclic gearbox. At ranges of normal operating speeds, the torque modulation provides controllable power transfer between shafts. The present invention can be particularly useful for extracting greater amounts of mechanical power from an engine, or in enhancing dynamic engine performance. Optionally, additional gearing may be employed to achieve a desired range of operability. 
   In one aspect, the present invention is directed to a system for transferring torque between a pair of independently, concurrently rotating shafts of an engine. The system includes a magnetic gearbox. The magnetic gearbox has a first ring structure, a second ring structure and an intermediate ring structure. The first, second and intermediate ring structures each include an annular aperture therethrough and have a plurality of magnetic pole pieces embedded therein. The intermediate ring structure is disposed between the first ring structure and the second ring structure. The first, second and intermediate ring structures are coaxially disposed, are concentric with, and are independently rotatable, with respect to the remaining ring structures. The first and second ring structures are coupled to separate ones of the rotating engine shafts. The intermediate ring structure cooperates with the first ring structure and second ring structure and determines the level of torque transferred torque between the pair of shafts. 
   The intermediate ring structure may be coupled to a rotating machine. The rotating machine has a controller and is operable for adjusting a ratio of angular speed and hence the ratio of power transferred between the pair of shafts. In one embodiment, the rotating machine is a motor/generator configured to receive power from and to supply power to the intermediate ring structure in response to a signal generated by the controller, such that the power is distributed variably between the pair of rotating shafts. 
   In another aspect, the present invention is directed to a gas turbine engine. The gas turbine engine includes a low pressure turbine spool and a high pressure turbine spool, at least one compression stage, a combustion chamber, an exhaust system and a rotary fan blade arrangement. The low pressure turbine spool and the high pressure turbine spool are magnetically coupled through a magnetic gearbox. The magnetic gearbox has a first ring structure, a second ring structure and an intermediate ring structure. The first, second and intermediate ring structures each include an annular aperture therethrough and have a plurality of magnetic pole pieces embedded therein. The intermediate ring structure is disposed between the first ring structure and the second ring structure. The first, second and intermediate ring structures are coaxially disposed, are concentric with, and are independently rotatable, with respect to the remaining ring structures. The first and second ring structures are coupled to separate ones of the rotating engine shafts. The intermediate ring is operable to transfer torque between the high pressure turbine spool and the low pressure turbine spool. The intermediate ring structure may be coupled to a rotating machine. The rotating machine has a controller and is operable for adjusting a ratio of power transferred between the high pressure turbine spool and the low pressure turbine spool. The rotating machine may be a motor/generator configured to receive power from and to supply power to the intermediate ring structure in response to a signal generated by the controller, such that the torque is distributed variably between the pair of rotating shafts. 
   The present invention is also directed to a method of transferring torque between first and second independently rotating shafts of a gas turbine engine. The method includes the steps of providing an adjustable magnetic gearbox having a first ring structure, a second ring structure and an intermediate ring structure, each of the first, second and intermediate ring structure having an annular aperture therethrough and a plurality of magnetic pole pieces embedded therein, the intermediate ring structure disposed between the first ring structure and the second ring structure, each of the first, second and intermediate ring structure being coaxially disposed, concentric with, and independently rotatable with respect to the remaining ring structures; coupling the first shaft of the magnetic gearbox to the first ring structure; coupling the second shaft of the magnetic gearbox to the second ring structure; coupling a rotating machine to the intermediate ring structure, concurrently rotating at least two of the first and second shafts and the rotating machine; and controlling the ratio of power transferred between the first and second ring structures by operating the rotating machine to supply or extract power from the intermediate ring structure. 
   Many other power coupling techniques deal with arrangements of multiple motor-generators that use generating to convert mechanical power to electrical power, which electrical power is then applied to a motor that converts the electrical power back into mechanical power on another shaft. Such systems rely heavily on electrical power wiring, power connectors, and auxiliary control systems to accomplish the power conversion, with increased cost and unreliability. 
   An advantage of the present invention is that no mechanical linkage or contact is required between the engine spools, reducing vibration transfer. 
   Another advantage of the present invention is that it may be internal or external to the engine. 
   Yet another advantage of the present invention is that it may be applied on other types of mechanical device that require variable torque transmission (e.g., hybrid automotive transmissions, etc.) 
   Still another advantage of the present invention is that it permits constant torque variable power coupling of one rotating shaft to another without any mechanical contact. The present invention further permits torque or power coupling without an intermediate conversion to electrical power. 
   A further advantage of the present invention is that it provides a method for controllably transferring mechanical power between spools of any multi-spool turbine engine which is tolerant to faults caused otherwise when mechanical gearing is used. Since there is no mechanical contact between input and output, there is less chance of catastrophic failure or jamming. 
   The present invention provides a system for extracting greater amounts of mechanical power from turbine engines in a manner that minimizes impact on engine capability, and can potentially enhance engine operability by variably selecting the source of the power extraction 
   Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic illustration of a generic turbofan engine. 
       FIG. 2  is a schematic illustration of a magnetic epicyclic gear arrangement. 
       FIG. 3  is a schematic illustration of a mechanical epicyclic planetary gearbox. 
       FIG. 4  is a nomograph illustrating the vector mathematical relationship for the epicyclic gearbox of  FIG. 3 . 
       FIG. 5  is a schematic illustration of a turbofan engine  10  having a variable magnetic gearbox. 
       FIG. 6  is an exploded view showing the three-ring arrangement of the variable magnetic gearbox. 
       FIG. 7  is a schematic illustration of a preferred embodiment showing a turbofan engine having a variable magnetic gearbox with a variable torque controller. 
       FIG. 8  is a schematic diagram of the variable torque controller shown in  FIG. 7 . 
       FIG. 9 , shows a simplified arrangement wherein the magnetic gearbox is coupled to the HP spool and the LP spool in a fixed ratio arrangement. 
   

   Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. 
   DETAILED DESCRIPTION OF THE INVENTION 
   Illustrated in  FIG. 1  is an exemplary generic turbofan engine  10  having a fan  35 , booster  11 , high-pressure compressor  20 , a combustor  22 , a high-pressure turbine  23  and a how-pressure turbine  27 , all arranged in a serial, axial flow relationship. The fan  35 , booster  11 , and how-pressure turbine are serially connected by the how-pressure spool  29 . The high-pressure compressor  20 , combustor  22  and high-pressure turbine are serially connected by the high-pressure spool  21 . 
   A combustor  22  in the core engine mixes pressurized air from the high-pressure compressor  20  with fuel and ignites the resulting fuel and air mixture to produce combustion gases. Some work is extracted from these gases by the high-pressure turbine blades (not shown), which drive the high-pressure compressor  20 . The combustion gases are discharged from the core engine into a power turbine or low-pressure turbine (not shown) having a row of low-pressure turbine blades. 
   Referring next to  FIG. 2 , a magnetic epicyclic gear arrangement or gearbox  110  includes an inner magnet ring  112 , a middle or intermediate ring  114  and an outer magnet ring  116 . Each ring (inner  112 , middle  114  and outer  116 ) is constructed of a predetermined number of magnetic pole pieces  112   a ,  114   a  and  116   a  embedded at intervals along the ring structures  112 ,  114  and  116 . The pole pieces  112   a  and  116   a  are composed of permanent magnets, and pole pieces  114   a  are composed of magnetically permeable material, the pole pieces  114   a  separated by magnetically non-permeable sectors  114 b. The inner and outer ring structures  112 ,  116  are composed of magnetically permeable material. The magnetic gear box  110  has a fixed torque ratio defined by the number of poles in each of the rings  112 ,  114  and  116 . The magnetic gear box  110  couples the HP and LP spools  21 ,  29  of a turbofan engine  10 . Note that the fixed torque ratio applies but only in the situation where the rotational velocities of the rings  112 ,  114  and  116 , satisfy Equation  1 , which is set forth below. Since the forces in the engine are such that the speeds of the rotors are independent of one another, the ideal torque split is realized only when the intermediate ring is free to rotate with low enough load to preclude slip. The fixed torque ratio is an ideal property that describes where the magnetic gear box  110  tends to operate. Placing a load on the magnetic gear box  110  causes the magnetic gear box  110  to deviate from the ideal relationship, but the resultant forces for small speed variations [from Equation 1 below] are in a direction to restore the ideal torque ratio and bring the speeds into agreement [with Equation 1]. 
   The magnetic gear box  110  provides a fully passive magnetic equivalent of a mechanical epicyclic planetary gearbox  120 , shown in  FIG. 3 . A high degree of magnetic coupling is achieved in the magnetic gear arrangement  110 , which enables torque densities comparable to the mechanical epicyclic planetary gearbox  120 . The magnetic gear box  110 , however, operates without mechanical contact between the rings  112 ,  114  and  116 , thus reducing mechanical vibration. The pullout torque allows the magnetic gear box  110  to slip, which permits a factor of safety compared with mechanical gear arrangements that jam and break when subjected to excessive torques. The magnetic gear box  110  presents additional advantage over a mechanical epicyclic gearbox since the gear ratio of the magnetic gearbox can be less than 1 (i.e., i 0 &lt;1.0, whereas the gear ratio of a mechanical epicyclic gearbox (see, e.g., Equation 2 below) is limited to i 0 &gt;1.0 (i 0 =1.0 for a differential gearbox). 
   Referring again to  FIG. 3 , the magnetic gear box  110 , as described above, has an inner ring  112 , an intermediate ring  114  and an outer ring  116 , which are analogous with the three main components of the mechanical epicyclic gearbox  120 , i.e., the innermost “sun” gear  122 , the middle “planet” carriers  124 , and the outermost ring gear  126  correspond to the inner ring  112 , an intermediate ring  114  and an outer ring  116 , respectively. Each of the gears  122 ,  124  and  126  has teeth indicated as  122   a ,  124   a  and  126   a , respectively. A planet carrier  121  is used to maintain uniform positions of the middle “planet” carriers  124  around the “sun” gear  122 . 
   Referring to  FIG. 4 , the angular velocity of the sun gear  122  (designated by ω s ), the planetary gear  124  (designated by ω p ) and the ring gear  126  (designated by ω r ) are illustrated by the nomograph shown in  FIG. 4 . Each gear angular velocity ω s , ω p  and ω r  is represented by a vertical axis  132 ,  134  and  136 , respectively. The angular velocities ω s , ω p  and ω r  are determined by equation 1 as follows:
 
ω c (1 −i   0 )=ω s −( i   0 )(ω r )  Equation 1
 
   where the ratio of the planetary gear set is i 0 ,
 
 i   0   =−z   r   /z   s   Equation 2
 
   z r =number of teeth of the ring gear and 
   z s =number of teeth of the sun gear. 
   The ordinate  140  of the carrier velocity ω c  is disposed along the horizontal axis  138  between the ordinates  142 ,  144  of the sun gear velocity ω s , and the ring gear velocity ω r . The carrier ordinate  140  divides the distance between the two outer ordinates  142 ,  144 , in the ratio of 1 to −i 0 , as indicated by arrows  146 ,  148  beneath the horizontal axis  138 . The intersection points  152 ,  154  and  156  of line  158  with vertical axes  132 ,  134  and  136 , respectively, define values of the angular velocities ω s , ω c  and ω r . By varying the velocity of the epicyclic gearing the sloping line  158  is rotated. The torques T s , T c  and T r  act on shafts of the sun gear  122 , the carriers  124  and the ring gear  126  according to equation 3:
 
 T   s ω s   +T   c ω c   +T   r ω r =0  Equation 3
 
   Referring next to  FIG. 5 , there is a schematically illustrated turbofan engine  10  having a variable magnetic gearbox  110  disposed between the HP spool  21  and the LP spool  29 . The outer ring  116  of the magnetic gear box  110  is coupled to the HP spool  21 , the inner ring  112  is coupled to the LP spool  29 , and the third ring  114  is coupled to a motor/generator (M/G)  160 . The engine  10  also includes a conventional gearbox  162  coupled with HP spool  21  for driving starter/generators  164 ,  166 , which provide the primary source of electrical power for the aircraft systems. It should be understood that the present invention could be applied to engines having more than two spools, in order to connect a higher speed and a lower speed spool of the engine. The intermediate ring  114  is coupled to the M/G  160 . The intermediate ring  114  may be split into two effective rings  114   b  and  114   c , which may be controllably adjusted to advance or retard the phasing of the magnetic fields, for cancellation so that the intermediate ring  114  is allowed to free-wheel such that no coupling is provided between the engine spools  21 ,  29  by the magnetic gear box  110 , or for adjusting the degree or percentage of coupling between the inner ring  112  and the outer ring  116 . This may be accomplished by mounting two individual rings  114   b ,  114   c , on a common, keyed shaft, and providing a control means (not shown) for phase adjustment of the respective magnetic fields. An alternate method is to provide a single, integrated ring  114  having interleaved poles  114   b ,  114   c  with an external phase control (not shown) for the respective magnetic fields. The ability to control the magnetic coupling between the shafts connected to the magnetic gear box  110 , particularly by allowing the intermediate ring to free-wheel such that no coupling is provided between the shafts  21 ,  29  by the magnetic gear box  110 , provides a safety feature when it is necessary to separate the two shafts, such as the HP spool  21  and the LP spool  29 . It should be noted that the arrangement of the HP spool  21 , LP spool  29  and the M/G may be selectively modified as discussed in further detail below, such that the inner ring  112 , intermediate ring  114  and outer ring  116  are coupled with any of the HP spool  21 , LP spool  29  and the M/G, to achieve alternate torque transfer characteristics. 
   Referring to  FIG. 6 , an exploded view shows the three-ring arrangement of the magnetic gear box  110 . Inner ring  112  contains the LP spool magnets  112   a . In this embodiment the poles  114   b ,  114   c  of the intermediate ring  114  are interleaved, as indicated by the coaxial striations  114   b ,  144   c . The intermediate ring  114  is coupled to the M/G  160  and is used to control the relative distribution of torque between the HP spool  21  and the LP spool  29 . The outer magnet ring  116  is coupled to the HP spool  21 , and the inner magnet ring  112  is coupled to the LP spool  29 . Each of the rings  112 ,  114  and  116  is coaxial and concentric, and is independently rotatable with respect to the others. 
   Referring next to  FIGS. 7 and 8 , the preferred embodiment of the invention is shown. The magnetic gear box  110  is configure so that the HP spool  21 , which normally rotates at a higher speed than the LP spool  29 , is coupled to the inner ring magnets  112  for driving the inner ring magnets  112  at a higher speed, and the LP spool  29  is coupled to the outer magnet ring  116  operating at lower speed than the HP spool  21 . This relationship follows the natural gearing ratio of the magnetic gear box  10 , for a fixed set of planetary teeth  122 a, for example. However, the HP &amp; LP spools  21 ,  29  may operate independently of one another, and at varying speeds. Under such circumstances a fixed-ratio coupling is not acceptable, so the intermediate magnet ring  114  is arranged to rotate independently as well. The intermediate magnet ring  114  will rotate at the angular velocity defined by Equations 1 and 2. Line  158  (See  FIG. 4 ) could pass through zero speed at some particular ratio of HP to LP speed. By applying a torque to a movable intermediate ring  114 , torque is transferred to the inner ring  112  and outer ring  116  according to the magnetic gear box equations 1 &amp; 2. The applied torque on the movable intermediate ring  114  can be positive or negative to transfer power in the desired direction. This torque is applied to the spinning intermediate ring  114 , and therefore power will flow in either direction at this location. The power source and the load for this transferred power must be supplied variably. For example, a motor-generator  160  could be used to supply or to expend power in the intermediate ring  114 , as shown in  FIG. 5 . However, this electrical power must be obtained from the existing electrical power system. In doing so, circulating power can result, in both mechanical and electrical form. Increases in power system weight and size may result if not carefully taken into account in the design. To minimize undesirable circulating power, a variable torque coupler  200  varies the output torque applied to the intermediate ring  114  or other input to the epicyclic magnetic gear box  110 . 
   As shown in  FIG. 8  the variable torque coupler  200  has rotating permanent magnets  202  for inputs, a rotating squirrel-cage-type induction rotor  204  drives an output shaft  206 , and a sliding magnetic shield control (not shown). The rotor  204  slides on a keyed output shaft  206  to selectively control the output torque of the variable torque coupler  200 . By sliding the variable torque coupler axially with respect to the engine centerline, as indicated by arrow  208 , the induced current and torque on the rotor is adjusted. The output of this coupler  200  is applied through shaft  206  to the intermediate ring  114  input of the magnetic gear box  110 , which thus controls the flow of power therethrough. 
   In addition to the variable torque coupler  200 , several other configurations may be employed, including continuously- and infinitely-variable mechanical transmissions, hydraulic transmissions, motor-generator combinations, and novel electromechanical arrangements. 
   The present invention discloses an epicyclic magnetic gear box  110  in combination with a continuously variable method for controlling the flow of power therethrough. The application of this continuous variability may optionally be applied throughout the operating ranges of the HP and LP spools  21 ,  29 , so that, for example, in some operating schemes the intermediate ring  114  may be fully free to rotate without any applied torque. Conversely, in other operating schemes, the intermediate ring  114  may be constrained to rotate in one direction, providing the torque necessary to maintain the speed ratio of the HP spool  21  to the LP spool  29  fixed. The fixed gear ratio of a magnetic gear box is made variable. The magnetic gear box  110  with a variable gear ratio provides the ability to control the transfer of power from a first mechanical rotating shaft to a second mechanical rotating shaft. Optionally, additional gearing stages may be provided to allow optimization of engine spool operating speeds. 
   Table 1 below sets forth the possible permutations in which the LP spool  29 , HP spool  21  and M/G set  160  may be configured for torque-coupling between them. 
   
     
       
             
             
             
             
           
         
             
               TABLE 1 
             
             
                 
             
             
                 
               Inner (Sun) 
               Intermediate 
                 
             
             
                 
               Ring Coupled 
               (Planetary) Ring 
               Outer Ring 
             
             
               Case 
               to: 
               Coupled to: 
               Coupled to: 
             
             
                 
             
           
           
             
               1 
               LP 
               M/G 
               HP 
             
             
               2 
               HP 
               M/G 
               LP 
             
             
               3 
               LP 
               HP 
               M/G 
             
             
               4 
               HP 
               LP 
               M/G 
             
             
               5 
               M/G 
               HP 
               LP 
             
             
               6 
               M/G 
               LP 
               HP 
             
             
                 
             
           
        
       
     
   
   In another aspect, the magnetic gear box  110  may provide a fixed ratio of torque transfer between the HP spool  21  and the LP spool  29  of the turbofan engine, wherein the third inputs such as the M/G  160  or the variable torque coupler  200  are not connected to the intermediate magnetic ring  114 . E.g., in  FIG. 9 , there is a simplified arrangement wherein the magnetic gear box  110  is coupled to the HP spool  21  and the LP spool  29 . This arrangement extracts power from the HP spool  21  through the standard mechanical gearbox arrangement  162 , wherein the gearbox  162  is coupled to one or more starters  168  or starter/generators  164 . As power extraction by the starter-generator  164  increases, the LP spool  29  can provide the additional power via magnetic gearbox  110  such that the amount of power provided by the HP spool  21  is approximately constant, allowing the speed of the HP spool  21  to remain constant.” This has at least three beneficial effects, namely, (1) low acceleration time for increasing emergency thrust, (2) a high stability margin of the HP spool, and (3) reduced idle thrust due to decreasing LP spool speed. The torque coupling magnetic gear box  110  can be used internally to enhance performance and operability in other ways, e.g., transferring torque to optimize the power split between the HP turbine  23  and the LP turbine  27 , or to utilize LP spool  21  as a power source/sink to aid in acceleration or deceleration of the core engine  18 . Note that this configuration would not be used as a thrust control device for steady-state airspeed stability, as moving power to the fan while under fan speed control would rebalance the cycle. This could change the fuel flow, but it would not create more thrust. 
   Various turbofan arrangements may employ the magnetic gearbox  110 , other than those examples shown in the figures. As an example, a three-spool turbofan (e.g., HP, LP and auxiliary) may be configured with the magnetic gearbox  110 , such that any two spools of the three spools can be connected. Alternately, in a turbofan having three spools, two magnetic gearboxes  110  may be configured to connect any spool to the remaining two spools. In yet another arrangement, a three spool turbofan may be configured such that one magnetic gearbox  110  can be connected to all three spools, with one ring  112 ,  114  and  116 , connected to each spool. The input power to the magnetic gearbox  110  may be sourced from the third spool of a turbofan. Or in a two spool turbofan, a third, dependent spool that is formed by use of a gearbox may be employed to separate parts of the HP spool, such as with a geared fan or a geared booster. 
   The invention is may also be applied to rotating machines other than turbofan engines. For example, an automotive-style differential drive system may employ a magnetic gearbox  110  wherein the left axle is connected to one ring structure, the right axle is connected to the other of ring structures, and the intermediate ring is fixed [i.e., i 0 =1]. In straight-line driving with good traction, both rings rotate at the same speed in opposite directions. On curves, the ring speeds deviate slightly, one faster and one slower, and small restoring torques will tend to slow the fast axle and accelerate the slow axle. If one wheel breaks away, there will be some slip speed at which substantial torques act to transfer power from the slipping wheel to the wheel with traction generated to return the wheels to the same rotational speed. 
   In another application of the invention, a multi-generator power system may be driven from one prime mover, wherein there is one master generator and the rest of the generators are slave generators. The coupling from the prime mover to the master generator is rigid and it is connected to a first ring of multiple magnetic gearboxes  110 . The coupling or couplings between the prime mover and the slave generators will be torsionally flexible with limited range of tangential displacement from nominal. The slave generators are connected to one or more secondary rings of a similar number of magnetic gearboxes  110 . The intermediate rings  114  are controlled to retard or advance the slave generators within said limited range of tangential displacement such that all slave generators are operating at identical phase angles to the master generator. This arrangement eliminates the problems of gearbox misalignment and wear, rotor and stator manufacturing tolerances, etc., which can cause considerable losses in the power system, and distributes the electrical load more evenly across the generators. 
   This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to make and use the invention. The patentable scope of the invention 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 have structural elements with insubstantial differences from the literal languages of the claims.