Patent Publication Number: US-10763736-B2

Title: Long distance power transmission with magnetic gearing

Description:
TECHNICAL FIELD 
     The present disclosure relates to long distance power transmission with magnetic gearing. More particularly, the present disclosure relates to systems using long distance, low frequency power transmission, such as subsea systems that make use of magnetic gearing with rotating machinery. 
     BACKGROUND 
     When operating a single electric motor in a remote location, such as a subsea pump or compressor, the electrical power can be transmitted from an adjustable speed drive located topside (e.g. above the sea surface) to the remote location. When the distance between the topside adjustable speed drive and the remote location is long, high voltages and low frequencies can be used to reduce electrical power losses. However, in some cases the load (e.g. pump or compressor) relies on a relatively high rotational speed in order to be effective. In some cases, the rotational speed required by the pump or compressor is higher than the synchronous speed at which the electrical motor can provide when driven by the low frequency transmitted power. Thus in many cases there can be a trade off between: (1) tie-back distance over which the power is transmitted; (2) power and voltage losses due to the tie-back distance; and (3) desired load shaft speeds for the remote machinery. 
     One solution is to use a mechanical gear system at the remote location to increase the rotational speed from the electric motor driven by the low frequency transmitted power to a level that can be effectively used by the remote equipment. However, the power losses due to the mechanical gear system may be too great. This is especially true where the remote location is subsea, since the rotating equipment often needs to be liquid filled. In such cases, the liquid-filled mechanical gear systems may have unacceptable viscous losses. An alternative to a mechanical gear system is an in-series configuration of motor-generator-motor. For example, a two-pole motor can be directly mechanically coupled to an eight-pole generator. The output frequency from the generator would be four times greater than the supply frequency. By connecting a normal two pole pump motor to the generator output, the required pump speed can be achieved. This type of solution does, however, introduce two rotating electrical machines subsea, in addition to the ordinary pump motor. The resulting overall system is therefore much larger in size. For further details see, e.g. Intl. Patent Publ. No. WO 2013/039404 A1. 
     SUMMARY 
     This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining or limiting the scope of the claimed subject matter as set forth in the claims. 
     According to some embodiments, a system for powering a subsea rotating machine is described. The system includes elements at both a surface facility and a subsea location. At the surface facility an electrical power supply is configured to supply multi-phase high-voltage low-frequency alternating-current electrical power. A power transmission cable runs between the power supply and the subsea location. At the subsea location, a subsea motor is configured to convert the low-frequency electrical power into torque on a motor output element thereby causing the motor output element to rotate at a drive rotational frequency rotation. A subsea magnetic gear module is also at the location and includes: a first rotating element mechanically coupled to the motor output element; and a second rotating element mechanically coupled to an output shaft. The first and second rotating elements are magnetically coupled to each other such that torque from the motor output element rotating at the drive rotational frequency is converted to torque on the output shaft rotating at a working rotational frequency. An effective gear ratio of the subsea magnetic gear module can be defined as the ratio of the working rotational frequency to the drive rotational frequency, which according to some embodiments, is greater than one. Also at the subsea location is a subsea rotating machine that is mechanically coupled to the output shaft and configured to use the torque on the output shaft at the working frequency to operate the rotating machine. 
     According to some embodiments, the subsea magnetic gear module is liquid filled and includes a plurality of permanent magnets on the first, second and/or other rotating elements. According to some embodiments, the subsea magnetic gear module is configured such that its effective gear ratio is 2:1, 3:1, 4:1, 6:1 or higher, or other intermediate values. The gear motor cable can be at least 20 kilometers in length between the power supply and the subsea location. According to some embodiments, this distance can be more than 20, 50, 100 or 200 kilometers. According to some embodiments, the low-frequency alternating-current electrical power is at most 30 hertz. In some examples it is 20 Hz or 16⅔ Hz. 
     The electrical power supply can include a step-up transformer and the subsea location can include a step down transformer configured to reduce voltage of the low-frequency electrical power for use by the subsea motor. Examples of the rotating machine type include: pump, compressor and separator. According to some embodiments, the subsea rotating machine is configured to process a hydrocarbon bearing fluid produced from a subterranean reservoir. 
     According to some embodiments, the surface power supply is configured to adjust the voltage and frequency of the electrical power, and the subsea magnetic gear module is configured such that a ratio of the working frequency to the drive frequency is fixed. In such cases, the subsea magnetic gear module can include an outer stationary housing having a plurality of alternating polarity permanent magnet pieces mounted thereon, the first rotating element can include a plurality of magnetic and non-magnetic material pieces alternatingly mounted therein, and the second rotating element can include a plurality of alternating polarity permanent magnet pieces mounted thereon. 
     According to some embodiments, the surface power supply is configured to supply the electrical power at a fixed low-frequency and the subsea magnetic gear module is configured such that the effective gear ratio is adjustable. In such cases, the subsea motor can be configured such that the drive frequency is fixed, and a variable speed drive located at the subsea location is configured to supply variable frequency electrical power to a plurality of stator windings in the subsea magnetic gear module to create rotating magnetic fields. The rotating magnetic fields can be used to rotate a variable electric motor element at a variable motor frequency. The subsea magnetic gear module may be configured to combine (e.g. sum) the rotation of the variable electric motor element with the rotation of the motor output element to rotate the output shaft at the working rotational frequency. In some cases, the sum is multiplied by an inherent, built-in, effective magnetic gear ratio which can be 2:1, 3:1, 4:1, 5:1, 6:1 or more or some other intermediate value. According to some embodiments, the rotating magnetic fields can at times rotate the variable electric motor element in a direction opposite to the rotation of the first rotating element and the motor output element. When the variable electric motor element and the first rotating element rotate in opposite directions, energy can be generated by the stator windings and then used to partially power the subsea motor. According to some embodiments, the subsea magnetic gear module is liquid filled and includes the variable electric motor element, and at high values of working frequency the variable electric motor element and the first and second rotating elements all rotate in the same direction such that viscous losses are reduced when compared to cases where some of the elements contra-rotate. According to some embodiments, the stator windings create rotating magnetic fields that directly interact with magnetic material in the first rotating element thereby rotating the output shaft at the working rotational frequency. 
     According to some embodiments, a method of powering a subsea rotating machine is described. The method includes supplying low-frequency high-voltage electrical power from a surface facility through a power transmission cable to a subsea location. At the subsea location: a subsea motor uses the low frequency power to rotate a motor output element at a drive frequency; the drive frequency is stepped-up to a higher working frequency on an output shaft using a subsea magnetic gear module that includes a plurality of rotating elements and a plurality of permanent magnet pieces; and the rotating output shaft is used to operate the rotating machine at the higher working frequency. 
     According to some embodiments, a system for transmitting low frequency electrical power over long distances is described. The system includes: a long distance power transmission cable at least 20 kilometers in length; and an electrical transducer electrically connected to one end of the power transmission cable with a rotating element configured to rotate synchronously with the frequency of power transmitted over the transmission cable. A magnetic gear module is mechanically coupled to the electrical transducer and includes: a first rotating element mechanically coupled to the rotating element of the transducer; and a second rotating element mechanically coupled to a shaft, the first and second rotating elements being magnetically coupled to each other such that a first rotational speed and a second rotational speed of the rotating element of the transducer are related to each other. According to some embodiments, the shaft can be mechanically coupled to a rotating machine that can be of a type selected from the following: wind turbine, water turbine, pump, compressor and separator. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject disclosure is further described in the following detailed description, and the accompanying drawings and schematics of non-limiting embodiments of the subject disclosure. The features depicted in the figures are not necessarily shown to scale. Certain features of the embodiments may be shown exaggerated in scale or in somewhat schematic form, and some details of elements may not be shown in the interest of clarity and conciseness. 
         FIG. 1  is a diagram illustrating a subsea environment in which magnetic gearing can be used in connection with long distance power transmission, according to some embodiments; 
         FIG. 2  is a schematic diagram of a single remote pump in subsea location being driven by a surface-located adjustable frequency drive (AFD), according to some embodiments; 
         FIG. 3  is a perspective view of a permanent-magnet liquid-filled step-up gear, according to some embodiments; 
         FIG. 4  is a cross-section of a permanent-magnet liquid-filled step-up gear, according to some embodiments; 
         FIG. 5  is a cross section along the A-A′ of the permanent-magnet liquid-filled step-up gear shown in  FIG. 4 ; 
         FIG. 6  is a schematic diagram of multiple remote pumps in subsea location being driven by a surface-located fixed frequency drive and subsea variable magnetic gearing, according to some embodiments; 
         FIG. 7  is a perspective view of a liquid-filled variable magnetic gear, according to some embodiments; 
         FIG. 8  is a cross-section of a liquid-filled variable magnetic gear, according to some embodiments; 
         FIG. 9  is a cross section along the B-B′ of the liquid-filled variable magnetic gear shown in  FIG. 8 ; 
         FIG. 10  is a diagram illustrating various states during operation of a magnetic variable ratio gear, according to some embodiments; 
         FIG. 11  is a graph showing several plots showing aspects of various stages of operation of a magnetic variable ratio gear, according to some embodiments; 
         FIG. 12  is a diagram illustrating magnetic gearing being used to enable efficient power transmission from windmills and seawater turbines, according to some embodiments; 
         FIG. 13  is a perspective view of an alternative liquid filled variable magnetic gear, according to some embodiments; 
         FIG. 14  is a cross section of an alternative liquid-filled variable magnetic gear, according to some embodiments, and 
         FIG. 15  is a cross section along C-C′ of the alternative liquid-filled variable magnetic gear shown in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific embodiments of the present disclosure will be described below. The particulars shown herein are by way of example, and for purposes of illustrative discussion of the embodiments of the subject disclosure only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the subject disclosure. In this regard, no attempt is made to show structural details of the subject disclosure in more detail than is necessary for the fundamental understanding of the subject disclosure, the description taken with the drawings making apparent to those skilled in the art how the several forms of the subject disclosure may be embodied in practice. Additionally, in an effort to provide a concise description of these exemplary embodiments, all features of an actual implementation may not be described in the specification. It should be appreciated that in the development of any such actual implementation, as in any engineering or design project, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which may vary from one implementation to another. Moreover, it should be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of design, fabrication, and manufacture for those of ordinary skill having the benefit of this disclosure. 
     When introducing elements of various embodiments of the present invention, the articles “a,” “an,” “the,” and “said” are intended to mean that there are one or more of the elements. The terms “comprising,” “including,” and “having” are used in an open-ended fashion, and thus should be interpreted to mean “including, but not limited to.” Also, any use of any form of the terms “connect,” “engage,” “couple,” “attach,” or any other term describing an interaction between elements is intended to mean either an indirect or a direct interaction between the elements described. In addition, as used herein, the terms “axial” and “axially” generally mean along or parallel to a central axis (e.g., central axis of a body or a port), while the terms “radial” and “radially” generally mean perpendicular to the central axis. For instance, an axial distance refers to a distance measured along or parallel to the central axis, and a radial distance means a distance measured perpendicular to the central axis. The use of “top,” “bottom,” “above,” “below,” and variations of these terms is made for convenience, but does not require any particular orientation of the components. 
     Certain terms are used throughout the description and claims to refer to particular features or components. As one skilled in the art will appreciate, different persons may refer to the same feature or component by different names. This document does not intend to distinguish between components or features that differ in name, but not function. 
     According to some embodiments, techniques are described for power transmission from an adjustable speed drive located topside to a single motor in a remote location, typically a subsea pump or compressor. The techniques combine power transmission at high voltage and low frequency to a pump that makes use of a rotational speed that is higher than the synchronous speed at which a two pole induction or permanent magnet (PM) motor can provide. A magnetic step-up gear is used to multiply the load shaft speed by a given ratio. The “gear” ratio of the magnetic gear system can be selected to suit the application. 
     According to some embodiments, a system is described that uses magnetic gearing principles in a novel way to overcome challenges and limitations in conventional subsea power transmission systems. Some challenges in conventional solutions are related to power and voltage loss due to increasing tie-back distance. Further, the relatively high frequency used to achieve the desired load shaft speed limits the step-out distance due to added reactive voltage drop and resistive losses caused by the skin effect. The receiving-end voltage regulation becomes poor due to Ferranti effect that limits the ability to control the receiving-end voltage with varying load. 
     According to some embodiments, by transmitting the power at low frequency and increasing the load shaft speed locally using a magnetic step-up gear, the step-out distance can be increased significantly, while reducing the negative effects described above. The system according to some embodiments has the following features: (1) reducing the power transmission frequency limits the effects of added AC reactance and cable resistance due to the skin effect; (2) the low transmission frequency limits the consequence from Ferranti effect on the load related voltage regulation; (3) long step-out distance become practical due to low transmission frequency and high transmission voltage; (4) step-up and step-down transformers are used to provide high voltage transmission; and (5) using a subsea step-up magnetic gear system, the output shaft speed can be optimized to fit the load speed requirements. 
     According to some embodiments, the magnetic gear system has smooth surfaces that limit viscous losses and avoid undesirable displacement or pumping effects associated with conventional liquid-submerged mechanical gear systems. 
     In applications where multiple receiving-end loads are in a remote location, and it is desirable to operate each load with its own individual and adjustable speed, conventional topside systems and umbilicals are relatively complicated and may be impractical. According to some other embodiments, techniques are described for transmitting power from a topside-located fixed-frequency source to one or more pump or compressor loads in a remote location, such as a subsea pumping station. High voltage and low fixed frequency electrical power is transmitted to one or more pumps that rely on rotational speeds that are higher than the synchronous speed at which a two pole induction or PM motor can achieve at the low transmission frequency. In order to achieve adjustable speed, according to some embodiments, the magnetic gear system includes a speed control motor (SCM). The main drive motor operates at fixed, relatively low rotational speed. The SCM, which might be integrated with the magnetic gear, operates at variable speed by means of a local adjustable speed drive (ASD). A PM step-up gear with an adjustable gear “ratio” might be used to multiply the load shaft speed by a variable ratio. The effective gear ratio can be selected to suit the application. 
     According to some embodiments, systems are described that include the following features: (1) a single power supply cable feeding several pumps in one remote location; (2) electrical power is supplied at fixed low frequency from topside to remote (e.g. subsea) location; (3) long step-out distances are possible due to low fixed frequency and high transmission voltage; (4) the majority of the load power is supplied by an external induction, reluctance or PM motor operating at low fixed speed; (5) a minority of the load power is supplied by an integrated relatively small variable speed torque motor; and (6) a variable speed output shaft provides the sum of the power from the main fixed speed motor and the variable speed torque motor to the load. 
       FIG. 1  is a diagram illustrating a subsea environment in which magnetic gearing can be used in connection with long distance power transmission, according to some embodiments. On sea floor  100  a subsea station  120  is shown which is downstream of several wellheads being used in this example to produce hydrocarbon-bearing fluid from a subterranean rock formation. Station  120  includes a subsea pumping module  140 , which is powered by an electric motor such as an induction motor or permanent magnet motor. The station  120  is connected to one or more umbilical cables, such as umbilical  132 . The umbilicals in this case are being run from a surface platform  112  through seawater  102 , along sea floor  100  and to station  120 . In other cases, the umbilicals may be run from some other surface facility such as a floating production, storage and offloading unit (FPSO), or a shore-based facility. The distance between platform  112  and station  120  is referred to as the “step out” distance. In this case the step out distance might be relatively large, for example greater than 30 kilometers. In some cases the step out distance might be greater than 50 km, and in some cases the step out distance might be 200 km or more. The umbilical  132  can also be used to supply barrier and other fluids, and control and data lines for use with the subsea equipment in station  120 . Although a pumping module  140  is shown in  FIG. 1 , according to some embodiments the module  140  can be configured for other subsea fluid processing functions, such as a subsea compressor module and/or a subsea separator module. In embodiments described herein, it is understood that references to subsea pumps and pumping modules can alternatively refer to subsea compressors and compressor modules. Furthermore, references herein to subsea pumps and subsea compressors should be understood to refer equally to subsea pumps and compressors for single phase liquids, single phase gases, or multiphase fluids. According to some embodiments, the subsea magnetic gear system described herein is used in connection with an electrical submersible pump (ESP)  150  which can either be located downhole, as shown wellbore  154  in  FIG. 1  or in a subsea location such as on the sea floor in a christmas tree at a wellhead  152  or other equipment. Thus in embodiments described herein, it is understood that references to subsea pump and pumping modules can alternatively refer to ESPs whether deployed downhole or in a subsea location. 
     According to some embodiments, the further pumping modules  142  and  144  are included in station  120  and might also be driven by electrical power from platform  112  via umbilical  132 . The pumping modules  142  and  144  may be used, for example, to pump fluids from other wells such as well  164  via wellhead  162 . In cases where it is desirable to run the pumping modules  140 ,  142  and  144  at different speeds, they can be driven by separate electric motors within station  120 . 
     Referring to embodiments where there is a single remote pumping module  140  in subsea station  120 , an adjustable frequency drive (AFD) is located on platform  112 . The AFD transmits power through umbilical  132  at various frequencies to an electric motor that directly drives the subsea pump. Challenges arise as the step-out distance between the platform  112  and station  120  increases. Many pump and compressor applications require relatively high speed for optimum operation and efficiency. With ordinary motors, being either synchronous or asynchronous, the motor speed is directly related to the supply frequency. 
     A typical conventional drive system for subsea pumps and compressors uses a two-pole induction motor operating at nominal speeds between about 3000 rpm and 6000 rpm. This speed range corresponds to a transmission frequency of about 50 Hz to 100 Hz, depending on the power rating and particular application. The step-out distance from the surface AFD to the subsea pump motor might be below about 15 km-20 km. Most conventional subsea pumps and compressors are direct driven, i.e. the motor and pump shafts have the same rotational speed. Since the motor speed is closely linked to the supply frequency, higher speed means higher transmission frequencies. 
       FIG. 2  is a schematic diagram of a single remote pump in subsea location being driven by a surface-located adjustable frequency drive (AFD), according to some embodiments. AFD  210  is located on surface platform  112 . The AFD  210  is connected to high voltage main grid  200  via circuit breaker  202 . In one example, main grid  200  is at 50 Hz and AFD  210  is configured to supply variable power from 0 to 16⅔ Hz at 6.6 kV. In cases where the main grid is 60 Hz the AFD can be configured to supply power from 0 to 20 Hz, for example. The power from AFD  210  is routed through a step-up transformer  212  that is configured to increase the voltage from 6.6 kV to 33 kV. Conductors  220  run through umbilical  132  and, according to some embodiments, is 30 to over 200 kilometers in length. In the subsea station  120 , the power from conductors  220  is routed through step-down transformer  242  that reduces the voltage from 33 kV to 6.6 kV. The power is then used to drive electric motor  250 . The 0 to 16⅔ Hz power translates to 0-1000 rpm in the electric motor  250 . Motor  250  is connected via shaft  252 , which may include a coupling, to PM step-up gear  254 . The PM gear  254  in this example has a fixed 6:1 step up ratio such that shaft  256 , which also may include a coupling, is driven at 0-6000 rpm. Shaft  256  is used to directly drive pump  258 . 
     For long distances between the AFD  210  and the pump module  140 , significant challenges may arise from voltage and power loss in the transmission lines. A large part of the transmission losses are related to the power frequency. The inductive voltage drop is proportional to the supply frequency and the resistive loss is closely related to the skin-effect cause by the frequency induced current displacement in the conductors. If the transmission frequency is kept very low and the transmission voltage high, the step-out distance can be increased several times with limited voltage and power loss. 
     With reduced transmission frequency over conductors  220 , the rotational speed of motor  250  will be reduced proportionally. A higher output speed from a motor supplied from a power source with low frequency can be obtained using a mechanical step-up gear between the low-speed motor and the high-speed pump shaft. However, a subsea mechanical gear has a limited efficiency due to viscous losses when operated completely submerged in a barrier fluid. Further, mechanical gears are also prone to wear and tear over time, and cannot be expected to have a lifetime of 25 years of continuous operation without service. 
     According to some embodiments, a planetary or epicyclical step-up gear system is used instead of PM step-up gear  254 . For further details on subsea epicyclical gearing, refer to U.S. patent application Ser. No. 14/715,514, which is incorporated herein by reference. 
     According to some embodiments, power is transmitted from the remotely located AFD  210  over a long distance through conductors  220  to the subsea station  120  at a low frequency, while still running the pump at desirable speed in the range of about 3000 to 6000 rpm without the use of a traditional mechanical gear. According to some embodiments, the system combines the use of a low speed motor with a long tie-back cable and adjustable low frequency power supply. The adjustable frequency will normally be provided by a static frequency converter located topside such as AFD  210  in  FIG. 2 . The low speed motor  250 , which can be an induction motor, reluctance motor or permanent magnet type, will typically have two poles and will be combined with PM step-up gear  254 . 
     In the example shown in  FIG. 2 , motor  250  has one pole-pair and rotates at 1000 rpm when the supply frequency is 16⅔ Hz. Magnetic step-up gear  254  might have a ratio of 1:6, which drives pump  258  at a full speed of about 6000 rpm. With this example system, the speed of motor shaft  252  is adjusted by controlling the frequency and voltage at the sending end of the transmission conductors  220 . This embodiment provides the ability to reduce losses related to step-out length and transmission frequency in order to achieve a longer step-out distance compared to a system operating at a frequency directly related to the synchronous speed of the motor/pump rotor. 
       FIG. 3  is a perspective view of a permanent-magnet liquid-filled step-up gear, according to some embodiments. PM gear  254  has an outer housing that includes outer shell  310 , upper end piece  312  and lower end piece  314 . The gear includes two shafts, namely input shaft  252  and output shaft  256 . In the case of  FIG. 3 , the PM gear is configured as a step-up gear, such that rotating the input shaft  252  one full revolution about axis  300  results in more than one full rotation of output shaft  256  about axis  300 . According to some embodiments, the gear ratio, the ratio of input shaft rotations (or rpm) to output shaft rotations (or rpm), is 1:6. Note that other gear ratios can be configured as will be discussed in greater detail, infra. Note also that the gear  254  can also be configured as a step-down gear, such that one revolution of the input shaft results in less than one revolution of the output shaft. 
       FIG. 4  is a cross-section of a permanent-magnet liquid-filled step-up gear, according to some embodiments. The outer shell  310 , which remains stationary, includes a plurality of magnetic north and magnetic south permanent magnets  410  arranged along the inner surface of shell  310 . Magnetic south piece  412  and magnetic north piece  414  are labeled in  FIG. 4 . The combination of outer shell  310  and permanent magnets  410  make up the magnetic gear stator  400 . In the example shown, there are 16 permanent magnet pieces alternatingly arranged on stator  400 . Within stator  400  is a pole piece rotor  430  that is made up of alternating sections of magnetic material and non-magnetic material. Labeled in  FIG. 4  are non-magnetic material  432  and magnetic material  434 . According to some embodiments, the magnetic material may be magnetic sheet steel and the non-magnetic material may be polymer. In the example shown, there are 20 alternating pieces of magnetic and non-magnetic material in pole piece rotor  430  (i.e. 10 pieces of each material). Pole piece rotor  430  is fixedly mounted to input shaft  254  (shown in  FIG. 3 ) which rotates pole piece rotor  430  as shown by arrow  436 . Between stator  400  and pole piece rotor  430  is narrow space  420  that filled with barrier fluid. Within pole piece rotor  430  is magnetic gear rotor  450 . The outer portion of rotor  450  includes a plurality of alternative magnetic north and magnetic south pieces as shown, while the inner portion of magnetic gear rotor  450  is the output shaft  256 . In the example shown, magnetic gear rotor  450  includes four permanent magnet pieces. Through the interaction of the magnetic fields and rotation of pole piece rotor  430 , the magnetic gear rotor  450  is rotated as shown by arrow  456 . Between pole piece rotor  430  and magnetic gear rotor  450  is a narrow space  440  that is filled with barrier fluid. Note that since the PM gear  254  is part of a subsea rotating machine, the housing is completely filled with a barrier fluid and the pressure is compensated relative to the surrounding ambient and/or process pressure. Note that a conventional mechanical gear would suffer from significant viscous losses, mainly caused by viscous shear loss and fluid displacement between the teeth at the high speeds required. By the using permanent magnets (PM), the rotating members  430  and  450  of PM step-up gear  254  can be made with smooth surfaces that minimize the viscous shear and eliminate displacement losses. In particular, the inner surface of stator  400 , the inner and outer surfaces of pole piece rotor  430  and the outer surface of magnetic gear rotor  450  are all very smooth so as to minimize such losses. 
       FIG. 5  is a cross section along the A-A′ of the permanent-magnet liquid-filled step-up gear shown in  FIG. 4 . In  FIG. 5  it can be seen that input shaft  252  is fixedly attached to pole piece rotor  430  using a flanged portion. Also visible are two permanent magnet pieces  550  that are fixedly attached to output shaft  256  which make up a portion of magnetic gear rotor  450  (visible in  FIG. 4 ). The axial and radial alignment of the various rotating members might be maintained using a number of bearings. In the case of input shaft  252  and pole piece rotor  430 , bearings  572  and  574  are labeled and in the case of output shaft  256  and magnetic gear rotor  450  bearings  558  and  560  are labeled. Note that other bearings are shown but not labeled and other bearings are not shown for purposes of clarity. Furthermore, the internal volumes of PM gear  254  might be filled with barrier fluid although it is not shown in  FIG. 5  for purposes of clarity. 
     With the development of rare earth magnets, which might be used in PM gear  254 , significant torque can be transferred over a relatively large gap without mechanical contact. By replacing parts of the gap with magnetically soft pole pieces, with a pole number different from the external stator PM pole number and a PM rotor with a lower pole number than the stator pole number, a “gear ratio” between the two can be achieved. By selection of the individual pole numbers, the pole piece rotor  430  and the internal magnetic gear rotor  450  can be made to rotate in the same direction. Rotation of the different rotating members  430  and  450  in the same direction enables keeping the relative movement between the different rotating members low, which limits viscous losses. 
     In the case of PM gear  254 , the multi-pole PM stator  400  might be kept stationary while the passive pole-piece rotor  430  is coupled to an external low speed motor  250  (shown in  FIG. 2 ) via a rotating input shaft  252 . The inner high speed magnetic gear rotor  450  drives output shaft  256  which is coupled to the driven pump or compressor  258  (also shown in  FIG. 2 ). 
     According to some embodiments, the gear ratio of the magnetic gear is given by the following equation:
 
 n   shaft   =n   PPC   *N   stator   /N   Rotor  
 
Where: n shaft  is speed of output shaft (e.g. shaft  256 ); n PPC  is speed of pole piece rotor (e.g. shaft  252 ); N stator  is the number of PM poles in stator (e.g. stator  400 ); and N Rotor  is number of PM poles in magnetic gear rotor (e.g. rotor  450 ). The direction of the output shaft will be the same direction as the pole piece carrier shaft if the number of pole pieces in the pole piece rotor (e.g. rotor  430 ) are the sum of N stator  and N Rotor  or in opposite direction if the number of pole pieces is the difference between N stator  and N Rotor .
 
     The magnetic gearing by means of intermediate passive poles are referred to as second, third and fourth order magnetic gears depending on the magnetic and drive shaft configuration complexity. 
     According to some embodiments, the PM gear  254  is configured as a fixed “gear” ratio in the range 1:1.6 to 1:6. For example, if the external motor  250  is operated at 50 Hz and 1:1.6 ratio will produce an inner rotor output speed of about 4800 rpm and operated at 16⅔ Hz and 1:6 ratio will produce an inner rotor output speed of about 6000 rpm. According to some embodiments, other gear ratios and motor speeds can be provided. 
     According to some embodiments, a variable ratio PM gear can be used instead of, or in addition to a fixed PM gear. When there is a combination of long step-out distance and multiple motors/pumps in a single location that are each driven at different speeds, the traditional one-to-one power transmission configuration becomes both expensive and unpractical. In addition, long step-out distances give rise to significant challenges from voltage and power loss in the transmission cable. 
       FIG. 6  is a schematic diagram of multiple remote pumps in subsea location being driven by a surface-located fixed frequency drive and subsea variable magnetic gearing, according to some embodiments. A single 3-phase high voltage (HV) cable feeder  620  might be used between a surface platform  112  and remote located subsea station  120 . Using such an arrangement with a single 3-phase HV feeder in umbilical  132  to drive provides a cost effective power transmission system for multiple remote pumps. A large part of the transmission losses are related to transmission voltage and power frequency. The inductive voltage drop is proportional to the supply frequency and the resistive loss is closely related to the skin-effect caused by the frequency induced current displacement in the conductors. Also, the Ferranti effect is frequency dependent and will give rise to negative voltage regulation when a combination of long cable, high voltage and high frequency is applied. 
     When the transmission frequency is kept low and constant, without going to DC transmission, the step-out distance can be increased several times with minimum voltage and power loss, while the Ferranti effect is limited. A driver for using low frequency and not DC is the ability to use step-up and step-down transformers. This means that conventional power electronics components can be used to convert the low frequency HV transmitted power to low frequency, medium-voltage (MV) power suitable for driving a variable speed motor in the subsea location. 
     A fixed-frequency drive (FFD)  610  is located on surface platform  112 . The FFD  610  is connected to high voltage main grid  200  via circuit breaker  202 . In one example, main grid  200  is at 50 Hz and FFD  610  is configured to supply fixed frequency power at 16⅔ Hz at 11 kV. The power from AFD  610  is routed through a step-up transformer  612  that is configured to step up the voltage from 11 kV to 66 kV. Conductors  620  run through umbilical  132  and, according to some embodiments, are about 30 to over 200 kilometers in length. 
     As part of the subsea station  120 , a single step-down power transformer  642  might be used to reduce the transmission voltage to a suitable MV distribution level, e.g. 11 kV. The MV distribution side might include a switch gear unit  640  that has several feeders. Each pump, compressor, or other load might require two feeders. The two feeders provide the functionality for starting, operating and fault clearing for each of the pumps (or other loads). In the example shown in  FIG. 6 , only the two feeders for “Pump 2” are shown for clarity although the other pumps may have similar or identical components. One feeder is connected to subsea VSD  660 , while the other feeder is connected to fixed speed drive motor  650 . Subsea VSD  660  is used to generate a variable frequency that will determine the ultimate drive speed for pump  658 . Within magnetic variable-ratio (MVR) gear  654 , fixed speed drive motor  650  is driven by the fixed frequency of the feeder from switch gear  640 . According to some embodiments, the fixed frequency may be 16⅔ Hz. The motor converts the fixed frequency power to a fixed speed rotation in input shaft  652  to MVR gear  654 . A second variable speed motor  670 , driven by VSD  660 , is included within MVR gear  654 . As will be described in further detail, infra, the MVR gear  654  combines the fixed drive speed from fixed drive motor  650  with variable speed from variable motor  670  to rotate output shaft  656  at a variable speed from 0-6000 rpm in this example. 
       FIG. 7  is a perspective view of a liquid-filled variable magnetic gear, according to some embodiments. MVR gear  654  has an outer housing that includes outer shell  710 , upper end piece  712  and lower end piece  714 . MVR gear  654  includes two shafts that rotate about central axis  700 : fixed-speed input shaft  652 ; and variable-speed output shaft  656 . MVR gear  654  uses a magnetic gear principle that is similar to the principle of PM gear  254  shown and described with respect to  FIGS. 2-5 , supra. MVR gear  654  uses rare earth magnets to transfer significant torque over a relatively large gap without mechanical contact. In the gap, magnetically soft pole pieces might be used and the number of poles in the various components can be selected to achieve the desired “gear ratio.” 
       FIG. 8  is a cross-section of a liquid-filled variable magnetic gear, according to some embodiments. This inner structure is similar or identical to PM gear  254  shown and described with respect to  FIGS. 2-5 , supra. The output shaft  656  forms part of variable magnetic gear rotor  850 . The outer portion of rotor  850  includes a plurality of alternative magnetic north pieces  852  and  854 , and magnetic south pieces as shown, while the inner portion of magnetic gear rotor  850  is the output shaft  656 . Pole piece rotor  830  surrounds magnetic gear rotor  850  and might be made up of alternating sections of magnetic material and non-magnetic material. Labeled in  FIG. 8  are non-magnetic material pieces  832  and  834  and magnetic material piece  836 . According to some embodiments, the magnetic material might be steel or another soft magnetic material and the non-magnetic material might be brass or another non-magnetic material. In the example shown there are 20 alternating pieces of magnetic and non-magnetic material in pole piece rotor  830  (i.e. 10 pieces of each material). Pole piece rotor  830  is fixedly mounted to fixed-speed input shaft  652  (shown in  FIG. 7 ) that rotates pole piece rotor  830  as shown by the arrow. Between magnetic gear rotor  850  and pole piece rotor  830  is narrow space  840  that might be filled with barrier fluid. Outside of pole piece rotor  830  is speed control rotor  800 . The inner surface of speed control rotor  800  includes 16 permanent magnet pieces with alternating polarity including magnetic north pieces  812  and  814  and magnetic south piece  816 . The outer surface of speed control rotor  800  includes 6 alternating polarity pieces, including magnetic south piece  802  and magnetic north piece  804 . Between speed control rotor  800  and pole piece rotor  830  is a narrow space  820  that might be filled with barrier fluid. Within outer shell  710  and outside of speed control rotor  800  is speed control stator  870  that includes a plurality of stator windings  882 . Between speed control stator  870  and speed control rotor  800  is a narrow space  860  that is filled with barrier fluid. 
       FIG. 9  is a cross section along the B-B′ of the liquid-filled variable magnetic gear shown in  FIG. 8 . In  FIG. 9  it can be seen that input shaft  652  is fixedly attached to pole piece rotor  830  using a flanged portion. The axial and radial alignments of the various rotating members are maintained using a number of bearings as shown. Note that other bearings may be included but are not shown for purposes of clarity. Furthermore, the internal volumes of MVR gear  654  are filled with barrier fluid although it is not shown in  FIG. 9  for purposes of clarity. 
     By selection of the individual pole numbers of the various components of MVR gear  654 , the outer speed control rotor  800 , the pole piece rotor  830  and the inner PM rotor  850  can be made to rotate in the same direction when operating at high rotational speeds. Rotation in the same direction reduces the relative speed between the different rotating members, which in turn limits viscous losses. 
     One example to conceptualize the design of MVR gear  654  is that a multi-pole PM “stator” might be configured as a rotating member, the speed control rotor  800 . Attached to the external face of speed control rotor  800  is a second set of permanent magnets. These PMs, which are placed at a relatively large diameter, are in fact the active parts of the rotor of the variable speed motor  670 . The speed control stator  870 , with a 3-phase winding, is placed outside the speed control rotor  800 , and also surrounds all the other rotating machine elements. 
     Magnetic gears based on intermediate passive pole pairs can be configured to rotate in the same or opposite direction relative to one another. For pressure compensation and other reasons, in subsea applications it might be desirable to fill the motor and magnetic gear assemblies with liquid (e.g. barrier fluid). The liquid might cause viscous loss when trapped between two surfaces with relative motion. 
     In the case of MVR gear  654  embodiments shown in  FIGS. 6-9 , the passive pole-piece rotor  830  might be coupled to external constant (low) speed motor  650  via rotating shaft  652  (both shown in  FIG. 6 ). The inner high speed PM rotor  850  includes output shaft  656  that is coupled to the driven load. The direction of rotation of the output shaft  656  will be the same direction as the pole piece rotor  830  and input shaft  652 , if the number of pole pieces are the sum of N stator  and N Rotor . The directions will be opposite if the number of pole pieces is the difference between N stator  and N Rotor . 
     Note that fixed speed drive motor  650  can be a conventional motor of induction, PM or other type, such as a reluctance motor. According to various embodiments, the external drive motor  650  can either be integrated with the MVR gear  654  or it can be a separate conventional motor that is mechanically coupled to the MVR gear  654 . In the embodiments shown in  FIGS. 6-9 , the MVR gear  654  has an outer stator, the speed control stator (SCS)  870  with a number of poles that matches the synchronous speed of the external motor at a convenient frequency. The stator  870  might be equipped with a 3-phase winding to produce a controllable rotating magnetic field. The stator  870  and windings might be designed to produce nominal flux at a given frequency, which coincides with the rated speed of the external fixed speed motor  650 . The SCS  870  is powered from a local VSD  660 . According to some embodiments, the VSD  660  is configured for operation in at least two quadrants in order to run in both rotational directions and to both source and sink energy to and from the SCS  870 . In this way, the VSD  660  acts as a gear-ratio controlling device, and it also contributes to the total power conversion of the unit. 
     The SCS  870  interacts with a speed control PM rotor (SCR)  800 , which might be free to rotate with no mechanical shaft to tap off or feed in mechanical energy. Concentrically inside the SCR  800  is a passive pole rotor (PPR)  830 , with a pole number that gives a “gear” ratio to produce the required output speed of the inner high-speed output rotor (HSR)  850 . As can be seen in embodiments of  FIG. 9 , the PPR  830  might be coupled to and rotate with the main motor drive shaft  652 . This design will have a “gear” ratio in the range from 1:2 to 1:6. The three rotating members  800 ,  830  and  850  in the in MVR gear  654  act similarly to an epicyclical gear with a given ratio, but without any mechanical contact. 
     By means of the MVR gear  654 , an external two-pole motor  650 , operated at 16⅔ Hz (with a fixed speed of about 1000 rpm) can produce a variable output speed from about 0 to 6000 rpm. The external motor  650  can therefore be driving at fixed speed from a fixed frequency power grid. By reducing the transmission frequency to e.g. 16⅔ Hz (50/3) the losses related to the power transmission can be reduced. Additionally, a subsea grid might be used to supply several subsea consumers can be established with a common fixed low frequency transmission cable. 
     As mentioned, the main fixed speed motor  650  can be of PM, induction or other type, such as a reluctance motor. In order to eliminate a direct on-line start of the main motor  650 , the MVR gear  654  in combination with the SCS  870  can be used to rotate the SCR  800  in a reverse direction during the initial spin up of the fixed speed motor  650 . This allows for synchronous speed matching when initially bringing the pump on line. This example technique is schematically illustrated in the state diagram of  FIG. 10  and the plots of  FIG. 11 . 
       FIG. 10  is a diagram illustrating various states during operation of a magnetic variable ratio gear, according to some embodiments.  FIG. 11  is a graph showing several plots illustrating aspects of various stages of operation of a magnetic variable ratio gear, according to some embodiments.  FIG. 10  shows six states of the MVR gear. The relative rotational directions and rotational speeds of the three rotating members SCR  800 , PPR  830  and HSR  850  are shown for each state with solid arrows. The six states  1010 ,  1012 ,  1014 ,  1016 ,  1018  and  1020  shown in  FIG. 10  correspond to various output drive shaft speeds that are shown in parenthetical reference numbers along the horizontal axis of  FIG. 11 . In  FIG. 11 , curves  1114 ,  1116  and  1118  plot the power applied (or generated) by the SCS  870 , PPR  830  and HSR  850 , respectively. Curves  1110  and  1112  plot the rotational speed of PPR  830  and SCR  800  respectively. 
     The following description refers to both  FIGS. 10 and 11 . State  1010  shows the MVR gear in an “all stopped” state where SCR  800 , PPR  830  and HSR  850  are all stationary. The fixed speed drive motor  650  (shown in  FIG. 6 ) which drives PPR  830 , and SCS  870  which drives SCR  800  are both unpowered. In  FIG. 11  state  1010  is shown at drive shaft speed=0. There is no power applied (or generated) by SCS  870 , PPR  830  and HSR  850  as shown in curves  1114 ,  1116  and  1118 , respectively. Additionally, the rotational speeds of SCR  800  and PPR  830  are both zero as can be seen in curves  1110  and  1112 , respectively. As part of the start-up procedure, the SCR  800  is initially rotated in a direction that is opposite to the drive motor  650  and PPR  830 . If the speeds of both SCR  800  and PPR  830  are increased synchronously from about 0 to 1000 rpm but in opposite directions, the output shaft and HSR  850  will remain stationary. According to some embodiments, an anti-rotation device (not shown) may be included in the HSR shaft to prevent reverse rotation during acceleration of PPR  830  and the external motor  650 . This state  1012  is shown in  FIG. 10 . In  FIG. 11  this is shown by the curves  1110  indicating PPR  830  spins to +1000 rpm and curve  1112  indicating SCR  800  spins to −1000 rpm, while the output shaft HSR  850  remains stationary. Note that from this point forwards the PPR  830  is driven at +1000 rpm, as indicated by curve  1110 , when the drive motor  650  is operating at a fixed speed by a fixed frequency power feed. In order to start rotating HSR  850  and therefore the pump, the reverse spin of SCR  800  is gradually decreased. State  1014  illustrates the situation where the reverse spin of SCR  800  has been reduced to −750 rpm. This causes the shaft HSR to rotate at +1000 rpm. Note that the power curve  1114  for SCS  870  dips into negative values in region  1120  of  FIG. 11 . This indicates that the SCS  870  is actually generating power that according to some embodiments is fed back into the drive motor  650  via the subsea switch gear  640  (both shown in  FIG. 6 ). In this example, for HSR  850  shaft speeds of less than 4000 rpm, the SCS  870  generates power. State  1016  illustrates the situation where the reverse spin of SCR  800  is reduced to −500 rpm, which causes HSR  820  to rotated at +2000 rpm. State  1018  illustrates the point at which SCR  800  is stationary and is therefore neither generating or using power. The power of both PPR  830  (i.e. the drive motor  650 ) and the output shaft HSR  850  are equal at about 1800 kW since no power is being contributed or used by SCS  870 . At this point, the HSR shaft  850  is driven at +4000 rpm. In order to drive HSR  850  at speeds greater than 4000 rpm, VSD  660  (shown in  FIG. 6 ) is used to rotate SCR  800  in the same direction as PPR  830  and HSR  850  (i.e. all clockwise in the example of  FIG. 10 ). In state  1020  SCR  800  spins at +500 rpm, causing the shaft HSR  850  to spin at +6000 rpm. Note that in the region  1122  of  FIG. 11 , where the HSR is driven at its highest speeds (4000 to 6000 rpm), all of the rotating members  800 ,  830  and  850  rotate in the same direction which is beneficial in reducing viscous losses. 
     As mentioned, when operating in region  1122 , MVR motor (i.e. SCS  870  and SCR  800 ) will contribute additional power to the main motor (motor  650  driving PPR  830 ) to drive HSR  850  and pump  658  via output shaft  656  (shown in  FIG. 6 ). At speeds below the rated 4000 rpm, (in region  1120  of  FIG. 11 ) the SCR  800  and SCS  870  will in effect feed energy back to the grid while rotating in a reverse direction. This energy can simply be recycled back to drive the main motor  650  and only the marginal losses in the components will be lost. This feature can contribute to increase the overall efficiency of the system. 
     Thus, according to some embodiments, the majority of the power to the high speed load (pump  658  in  FIG. 6 ) is provided through an external high efficiency motor  650 , running directly off the fixed low frequency MV power supply at constant low speed (e.g. 16⅔ Hz). The rotational torque from the external low speed, high torque motor  650  is transferred via a shaft to the Pole Piece Rotor (PPR)  830 . Starting of the external low speed motor will be performed by means of the MVR gear  654  and the subsea VSD  660 . By rotating the SCR  800  in negative direction while the pump shaft  656  is at rest, the main motor  850  will accelerate. Once the main motor  650  has been accelerated to synchronous speed (i.e. state  1012  in  FIGS. 10 and 11 ) it is tied to the MV distribution bus through the dedicated circuit breaker (CB). 
     Once started, the PPR  830  will rotate at constant or near constant speed. The outer rotor, the Speed Control Rotor (SCR)  800 , is energized by the three-phase stator  870  and an external inverter to rotate both ways, or even be at rest, at full torque. For example, by arranging the SCR  800  with  6  exterior poles, the external inverter can operate within an operating frequency range (0 Hz-50 Hz), and the magnetic stator backing can be kept thin to optimize the overall diameter of the machine. The higher frequency compared to the transmission frequency is beneficial and will have no negative effects due to the short distance between the inverter and the motor terminals. The number of poles in the SCR  800  and the inverter frequency range can be selected to suit the application. Direction and speed of rotation of the SCR  800  dictate the effective gear ratio and output speed of the HS shaft of HSR  850 . The HSR  850  will rotate with a speed given by the following equation:
 
 N   HS =( N   PPR   +N   SCR )* R  
 
Where: N HS =Speed of HS output shaft (HSR  850 ); N PPR =Speed of Pole Piece Rotor (PPR  830 ); N SCR =Speed of Speed Control Rotor (SCR  800 ); and R=“Gear” Ratio between PPR and HSR. The “Gear” Ratio is given by:
 
     
       
         
           
             
               
                   
               
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     The inherent gear direction of rotation is given by the number of pole pieces in the PPR  830 . By setting the number of pole pieces equal to the sum of the number of pole pairs  812  (eight in this case) in the SCR and the number of pole pairs  852  on the HSR (two in this case) the HSR will rotate in the same direction as the PPR. By setting the number of pole pieces equal to the difference between the number of pole pairs  812  in the SCR and the number of pole pairs  852  on the HSR, the inner ring will rotate in the opposite direction of the PPR 
     By changing the direction of rotation of the SCR  800 , the HSR  850  can run slower or faster than the PPR  830  multiplied by R (the gear ratio). Hence, the variable high speed pump shaft power might be provided by the sum of the fixed low speed motor power and the gear control motor power via the variable ratio magnetic gear. In  FIG. 11  this embodiment is indicated by curve  1118  being the sum of curves  1116  and  1114 . The main low speed motor might provide more than ⅔ of the power and the gear control motor might provide less than ⅓ of the total power supplied to the pump shaft at maximum speed. However, the power split between the two motors may be selected to suit the application. The power from both motors is summed, and the speed is multiplied in the MVR gear to provide the desired shaft output power and speed to the load shaft. 
     Thus, the techniques described enable the transmission of power from a fixed low frequency supply placed topside to the subsea installation over a single 3 phase cable, while still running one or more pump(s) at desirable high, variable and individual speed in the range of about 3000 to 6000 rpm. 
     Although some of the embodiments described supra have related to using magnetic gearing to implement long distance power transmission to subsea stations such as subsea pumping modules, the techniques are not limited to such applications. For example, according to some embodiments, magnetic gearing might be used to implement long distance power transmission to and/or from other rotating machinery. The techniques described herein can be applied to applications where long distance transmission benefits from transmission frequencies that are lower than the rotating machinery can use or supply. Additionally, the techniques described herein can be beneficial in applications where it is desirable to have a liquid-filled transmission since viscous losses can be reduced over mechanical gear systems. 
       FIG. 12  is a diagram illustrating magnetic gearing being used to enable efficient power transmission from windmills and seawater turbines, according to some embodiments. Shown are a plurality of wind turbines  1210  deployed in a location that is remote from a station  1200  that may form part of an electric power transmission and/or distribution grid. According to some embodiments, each of the wind turbines  1210  have a magnetic gear such as PM gear  254  or MVR gear  654  which converts rotational speeds (i.e. either step-up or step-down) between rotating shafts of, for example, the turbine and the generator. By selecting appropriate pole numbers and intermediate passive pole flux paths as described herein, the magnetic gearing can be configured to suit the particular application. Through the use of the magnetic gearing, the power transmission through cables  1212  can be made at a suitable frequency to reduce losses. Similarly,  FIG. 12  shows a plurality of seawater turbines  1220  that are being used, for example, for generating power from tidal currents. According to some embodiments, each of the water turbines  1220  have a magnetic gear such as PM gear  254  or MVR gear  654  which converts rotational speeds (i.e. either step-up or step-down) between rotating shafts of the turbine and the generator, for example. Through the use of the magnetic gearing, the power transmission through cables  1222  can be made at a suitable frequency to reduce losses. Additionally, in the case where the magnetic gearing is implemented in a subsea location, the gearing modules can be liquid filled with lower viscous losses when compared to mechanical gearing through the techniques described supra such as by using smooth surfaces and rotating components in the same direction. 
       FIG. 13  is a perspective view of another example of liquid filled variable magnetic gear, according to some embodiments. MVR  1354  has an out shell  1310 . Visible are fixed-speed input shaft  652  and variable speed output shaft  656  which correspond to the input and output shafts shown in  FIG. 6 .  FIG. 14  is a cross section of another example of liquid-filled variable magnetic gear, according to some embodiments. It can be seen that MVR  1354  uses a simpler structure than MVR  654  shown in  FIGS. 8 and 9 . In particular the rotating SCR  800  shown in  FIGS. 8 and 9  is replaced by a direct Magnetic Speed Controller (MSC)  1400 . MSC  1400  is similar to the Speed Control Stator  870  (shown in  FIGS. 8 and 9 ) except that MSC  1400  is configured to provide a number of electromagnetic stator poles using a plurality of stator windings  1482 . The number of electromagnetic stator poles in MSC  1400  might be equal to the number of PM poles on the inner diameter of the mechanical SCR  800  shown in  FIG. 8 . The flux provided by the electromagnetic stator poles will interfere with the passive pole pieces on pole piece rotor  1430  which is analogous to the pole piece rotor  830  shown in  FIG. 8 . The interaction between the electromagnetic stator poles and passive pole pieces on rotor  1430  provides a given “gear” ratio of MVR  1354 . A difference between the rotating SCR MVR  654  and MVR  1354 , apart from the mechanical simplification, is the frequency of the variable electric power used. Since MSC  1400  provides a number of electromagnetic stator poles that are used to achieve the gear ratio directly, the number of electromagnetic poles tends to be higher. In order to provide the same rotating flux vector as in the PM counterpart of MVR  654 , the stator frequency of MSC  1400  might be higher. In the given example, the rated stator frequency of MSC  1400  may be increased from 50 Hz to 133 Hz to achieve the same rotational speed capabilities. 
       FIG. 15  is a cross section along C-C′ of the example of liquid-filled variable magnetic gear shown in  FIG. 14 . In  FIG. 14  it can be seen that input shaft  652  is fixedly attached to pole piece rotor  1430  using a flanged portion. The axial and radial alignments of the various rotating members might be maintained using a number of bearings as shown. Note that other bearings may be included but are not shown for purposes of clarity. Furthermore, the internal volumes of MVR gear  1354  might be filled with barrier fluid although it is not shown in  FIG. 15  for purposes of clarity. 
     According to some embodiments, the operation of MVR  1354  is similar to that of MVR  654  in several respects. In particular,  FIGS. 10 and 11 , along with the descriptions, supra, apply equally to MVR  1354  by substituting analogous structures (e.g. substituting MSC  1400  for SCR  800 ). 
     While the subject disclosure is described through the above embodiments, it will be understood by those of ordinary skill in the art that modification to and variation of the illustrated embodiments may be made without departing from the inventive concepts herein disclosed. Moreover, while some embodiments are described in connection with various illustrative structures, one skilled in the art will recognize that the system may be embodied using a variety of specific structures.