Dual-rotor permanent magnet generator

Abstract

A dual-rotor permanent magnet generator is provided. The dual-rotor permanent magnet generator includes: a stator having a plurality of stator slots; an inner rotor having a plurality of rotor poles; an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and a winding set installed on the stator through the plurality of stator slots such that the winding set is shared (i.e., simultaneously / concurrently shared) by a magnetic-geared machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage. There is also provided a power generation system including the dual-rotor permanent magnet generator, a method of operating the power generation system, and a method of manufacturing a dual-rotor permanent magnet generator.

Description

DUAL-ROTOR PERMANENT MAGNET GENERATORCROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority of Singapore Patent Application No. 10202400443R filed on 19 February 2024, the content of which being hereby incorporated by reference in its entirety for all purposes.TECHNICAL FIELD

[0002] The present invention generally relates to a dual-rotor permanent magnet (PM) generator and a power generation system including the dual-rotor permanent magnet generator.BACKGROUND

[0003] Power generation systems are crucial in various sectors, including transportation (e.g., automobiles, ships, airplanes) and renewable energy (e.g., wind turbines), where they generate electric power for various technical applications, such as charging batteries, powering on-board devices, and energizing electrical infrastructures. A key requirement in these power generation systems is the variable- speed constant-amplitude (VSCA) voltage operation, which seeks to maintain a constant (or near-constant) voltage across different speeds (e.g., outer rotor speeds under varying operational conditions). This is vital as significant voltage fluctuations can damage electrical loads and power converters, and diminish power supply efficiency.

[0004] Traditionally, asynchronous or induction generators have been employed to achieve the VSCA voltage operation. However, these generators often suffer from low torque / power density, low efficiency, and complex control mechanisms. In contrast, permanent-magnet (PM) synchronous generators are known for their high torque / power density and efficiency, along with simpler control. Nevertheless, PM synchronous generators typically require full-scale controllable converters for the VSCA voltage operation, leading to increased costs and efficiency losses due to converter operations.

[0005] An attractive solution has emerged with dual-rotor PM generators, which can achieve the VSCA voltage operation using an uncontrollable diode rectifier. This is achieved by adjusting, for example, the position of the inner rotor relative to the speed of the outer rotor under varying operational conditions. However, conventional designs of these dual-rotor PM generators require two distinct sets of windings with different pole -pairs for two machine parts, namely, the magnetic-geared machine (MGM) part and the vernier permanent magnet (VPM)machine part. This conventional design suffers from various technical problems, including complex winding design, increased manufacturing cost, complicated control, reduced reliability, and more importantly, unsatisfactory electromagnetic performance due to the inefficient use of slot conductor area and PM fields in the machine. For illustration purpose, FIG. 1 depicts a schematic drawing of a topology of such conventional dual-rotor PM generators.

[0006] A need therefore exists to provide a dual-rotor permanent magnet (PM) generator that seeks to overcome, or at least ameliorate, one or more deficiencies in conventional dualrotor PM generator, and more particularly, that is able to achieve a VSCA voltage operation in an efficient and effective manner, such as efficient / simpler winding design, good / improved electromagnetic performance, and reduced manufacturing cost. It is against this background that the present invention has been developed.SUMMARY

[0007] According to a first aspect of the present invention, there is provided a dual-rotor permanent magnet generator comprising: a stator having a plurality of stator slots; an inner rotor having a plurality of rotor poles; an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and a winding set installed on the stator through the plurality of stator slots such that the winding set is shared by a magnetic-geared machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage.

[0008] According to a second aspect of the present invention, there is provided a power generation system comprising: the dual-rotor permanent magnet generator according to the above-mentioned first aspect of the present invention; and a speed-dependent device connected to one of the inner and outer rotors for controlling it.

[0009] According to a third aspect of the present invention, there is provided a method of operating the power generation system according to the above-mentioned second aspect of the present invention, the method comprising: controlling, using the speed-dependent device of the power generation system, an angular position of said one of the inner and outer rotors based ona speed of the other one of the inner and outer rotors to perform a variable-speed constant amplitude (VSCA) voltage operation in relation to the induced voltage.

[0010] According to a fourth aspect of the present invention, there is provided a method of manufacturing a dual-rotor permanent magnet generator comprising: providing a stator having a plurality of stator slots; providing an inner rotor having a plurality of rotor poles; providing an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and installing a winding set on the stator through the plurality of stator slots such that the winding set is shared by a magnetic-geared machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Embodiments of the present invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which:FIG. 1 depicts a schematic drawing of a topology of a conventional dual-rotor permanent-magnet (PM) generator with two distinct sets of windings;FIG. 2 depicts a schematic drawing of a topology of a dual-rotor PM generator, according to various embodiments of the present invention;FIG. 3 depicts a schematic drawing of a power generation system, including the dualrotor PM generator, according to various embodiments of the present invention;FIG. 4A depicts a method of operating the power generation system, according to various embodiments of the present invention;FIG. 4B depicts a schematic block diagram of a control system for operating the power generation system, according to various embodiments of the present invention;FIG. 5 depicts a method of manufacturing the dual-rotor PM generator, according to various embodiments of the present invention;FIG. 6 depicts a schematic drawing illustrating a configuration of a power generation system with VSCA voltage operation in various industrial applications, showing the case of the conventional dual-rotor PM generator and the case of the present dual-rotor PM generator according to various example embodiments of the present invention;FIG. 7 A depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for stand-alone wind power generation utilizing partial-scale converters for a DFIG (doubly fed induction generator);FIG. 7B depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for stand-alone wind power generation utilizing a full-scale converter for an IG / SG (induction generator or synchronous generator);FIG. 7C depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for stand-alone wind power generation utilizing a diode rectifier for a DRPMG (dual-rotor PM generator);FIGs. 8A and 8B show schematic drawings illustrating the winding and rotor dcsigns / configurations of the basclinc / convcntional DRPMG and the present DRPMG according to various example embodiments of the present invention;FIG. 9 depicts a schematic drawing illustrating a configuration of a stand-alone wind power generation system with VSCA voltage operation, showing the case of the baseline / conventional DRPMG and the case of the present DRPMG according to various example embodiments of the present invention;FIG. 10 depicts a vector diagram of induced voltages at different speeds of the outer rotor in FIG. 9 for the VSCA voltage operation;FIG. 1 1 shows a table (Table I) including example key design parameters of the three generators investigated, namely, the PMSG (permanent-magnet synchronous generator), the baseline DRPMG and the present DRPMG according to various example embodiments of the present invention;FIG. 12 shows the no-load field distributions including the flux density and flux line of the above-mentioned three generators at the no-load condition;FIGs. 13A and 13B show the corresponding flux densities in the outer airgap, including the waveform and harmonic spectrum;FIGs. 14A and 14B show the no-load induced voltages of the above-mentioned three generators at a rated outer rotor speed of 200 r / min under the no-load condition, including the waveform and harmonic spectrum;FIGs. 15A and 15B show plots of no-load induced voltages of the present DRPMG at varying inner rotor positions and outer rotor speeds for amplitude of induced voltages versus inner rotor position at a rated outer rotor speed of 200 r / min (FIG. 15 A) and waveform of induced voltage at varying outer rotor speeds (FIG. 15B);FIG. 15C show plots of the measured dynamic VSCA operation of the present DRPMG 200 at load condition, for the outer rotor speed, inner rotor position, AC induced voltage at phase A, rectified DC voltage and DC output current;FIG. 16 shows the torque waveforms of the above-mentioned three generators at the rated current density of 5 A / mm2;FIG. 17 shows a Table (Table II) presenting various other performances of the above- mentioned three generators, such as power, loss, efficiency, power factor, PM utilization, and so on;FIGs. 18A to 18E show images of the prototype of the present DRPMG, including its stator, outer rotor, inner rotor, PMs and coils, as well as the test rig for VSCA operation;FIGs. 19A and 19B compare the measured no-load induced voltages at a rated speed of 200 r / min with the FEA-predicted induced voltages;FIGs. 20A and 20B depict plots showing the outer rotor torque for the torque versus current angle at 20 A (FIG. 20A) and the torque versus current amplitude at zero current angle (FIG. 20B); andFIGs. 21 A and 2 IB show the measured and simulated (FEA-predicted) voltage envelopes of no-load AC induced voltages at a rated outer rotor speed of 200 r / min.DETAILED DESCRIPTION

[0012] Various embodiments of the present invention provide a dual-rotor permanent magnet (PM) generator and a power generation system including the dual -rotor PM generator.

[0013] As discussed in the background, a key requirement in various power generation systems is the variable-speed constant-amplitude (VSCA) voltage operation, which seeks to maintain a near-constant voltage across different speeds (e.g., outer rotor speeds under varying operational conditions). In this regard, an attractive solution has emerged with dual-rotor PM generators, which can achieve the VSCA voltage operation using an uncontrollable diode rectifier. However, conventional designs of these dual-rotor PM generators require two distinct sets of windings with different pole-pairs for two machine parts, namely, the magnetic -geared machine (MGM) part and the vernier permanent magnet (VPM) machine part. This conventional design suffers from various technical problems, including complex winding design, increased manufacturing cost, complicated control, reduced reliability, and more importantly, unsatisfactory electromagnetic performance due to the inefficient use of stator slot conductor area and PM fields in the machine. A schematic drawing of a topology of suchconventional dual-rotor PM generators is shown in FIG. 1. In this regard, various embodiments of the present invention provide a dual-rotor PM generator that seeks to overcome, or at least ameliorate, one or more deficiencies in conventional dual-rotor permanent magnet generator, and more particularly, that is able to achieve the VSCA voltage operation in an efficient and effective manner, such as efficient / simpler winding design, good / improved electromagnetic performance, and reduced manufacturing cost.

[0014] FIG. 2 depicts a schematic drawing of a topology of a dual-rotor PM generator 200, according to various embodiments of the present invention. The dual-rotor PM generator 200 comprises: a stator 210 having a plurality of stator slots 214; an inner rotor 220 having a plurality of rotor poles 224; an outer rotor 230 comprising a plurality of permanent magnets 234, the outer rotor 230 being arranged between the stator 210 and the inner rotor 220; and a winding set 240 installed on the stator 210 through the plurality of stator slots 210 such that the winding set 240 is shared by a magnetic -geared machine (MGM) and a vernier permanentmagnet machine (VPM) of the dual-rotor permanent magnet generator 200 for producing an induced voltage.

[0015] The dual-rotor PM generator 200 is advantageously able to achieve the VSCA voltage operation in an efficient and effective manner. In particular, the winding set 240 (i.e., a single winding set or a single set of unified winding) is installed on the stator 210 through the plurality of stator slots 214 such that the winding set 240 is shared simultaneously / concurrently by the MGM and the VPM of the dual-rotor PM generator 200 for producing an induced voltage during operation. Accordingly, the dual-rotor PM generator 200 is advantageously configured to achieve the VSCA voltage operation with only a single winding set in the stator 210. In this regard, the winding set 240 simultaneously functions as both MGM winding and VPM winding, and thus, enables the MGM and the VPM to share (or effectively utilize) the entire stator slot conductor area, which significantly improves the utilization of the PM magnetic field and boosts power output from the dual-rotor PM generator 200. In contrast, as described in the background and illustrated in FIG. 1, conventional dual-rotor PM generators require two distinct sets of windings with different pole-pairs for the MGM and the VPM. As a result and as can be seen from FIG. 1, the stator slot conductor area has to be divided into two sections for the MGM and VPM windings respectively, along with additional insulation (not shown) being required between the MGM and VPM windings. Such conventional dual winding sets in the stator not only complicates the winding design but also results in suboptimal utilization of the stator slot conductor area as the respective winding set is restricted to only a portion / section of the totalavailable stator slot conductor area. Accordingly, the dual-rotor PM generator 200 is advantageously able to achieve the VSCA voltage operation in an efficient and effective manner, including efficient / simpler winding design, good / improved electromagnetic performance, and reduced manufacturing cost. These advantages or technical effects, and / or other advantages or technical effects, will become more apparent to a person skilled in the art as the dual-rotor PM generator 200, as well as the corresponding power generation system, is described in more detail according to various embodiments and example embodiments of the present invention.

[0016] In various embodiments, the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. In this regard, various embodiments advantageously found that the conventional dual winding sets can be unified into a single winding set 240 by matching the number of stator slots 214 to the number of inner rotor poles 224. In particular, various embodiments found that by matching the number of stator slots 214 to the number of inner rotor poles 224, the pole-pair numbers of the MGM and VPM windings are equal and thus, the conventional MGM and VPM winding sets can advantageously be unified into a single winding set 240 in the dual-rotor PM generator 200 according to various embodiments of the present invention. Accordingly, as shown in FIG. 2, in the dual-rotor PM generator 200, the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. In contrast, as shown in FIG. 1 , in the conventional dualrotor PM generator, the number of stator slots at the stator and the number of rotor poles at the inner rotor are different. Therefore, in the conventional dual-rotor PM generator, the pole-pair numbers of the MGM and VPM windings are different and thus, the MGM and VPM winding sets cannot be unified into a single winding set in the conventional dual-rotor PM generator. In other words, based on conventional teaching / understanding, the MGM and VPM winding sets cannot be unified into a single winding set in the conventional dual-rotor PM generator as it would render the conventional dual-rotor PM generator inoperable or result in the conventional dual -rotor PM generator having unsatisfactory performances. Furthermore, it will be understood by a person skilled in the art that in the conventional dual-rotor PM generator, the two sets of windings have different pole-pairs numbers, resulting in two entirely different winding distributions for each distinct winding set in the stator slots within the generator. Therefore, the conventional dual-rotor PM generator has and require two distinct sets of windings with different pole-pairs for the MGM and the VPM, and which do not constitute a single winding set (not a single set of unified winding). In contrast, the dual-rotor PM generator200 according to various embodiments of the present invention has a winding set 240 (a single set of unified winding) installed on the stator 210 that is shared simultaneously / concurrently by the MGM and the VPM for producing an induced voltage. Therefore, in the unified winding set 240 of the dual-rotor PM generator 200, for example, only one consistent winding distribution in the stator slots 214 is required to achieve the pole-pair number that corresponds to the difference between the number of stator slots 214 and the PM pole-pairs of the outer rotor 230.

[0017] It will be appreciated by a person skilled in the art that the topology of the dual-rotor PM generator 200 shown in FIG. 2 is an example illustration and the number of stator slots 214 at the stator 210, the number of rotor poles 224 at the inner rotor 220 and the number of permanent magnets 234 embedded in the outer rotor 230 are not limited to that illustrated in FIG. 2. For example, various numbers of stator slots 214 at the stator 210 and various numbers of rotor poles 224 at the inner rotor 220 may be provided or configured as long as the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. For example, the number of stator slots 214 and the number of rotor poles 224 may both be any value that is a multiple of the phase number (e.g., 12, 15 or 18 for a 3-phase machine) as long as they are equal.

[0018] In various embodiments, the stator 210, the outer rotor 230, the inner rotor 220 and the winding set 240 together constitute the MGM. In various embodiments, the stator 210, the outer rotor 230 and the winding set 240 together constitute the VPM. In the MGM, the PM field from the outer rotor 230 is modulated by the rotor poles 224 at the inner rotor 220, generating magnetic harmonics absorbed by the winding set 240 to induce voltage. Similarly, in the VPM, the PM field from the outer rotor 230 is modulated by the stator 210, creating harmonics absorbed by the same winding set 240 to induce voltage.

[0019] In various embodiments, as shown in FIG. 2, the stator 210, the inner rotor 220 and the outer rotor 230 are arranged concentrically about a common axis. In this regard, in various embodiments, the plurality of stator slots 214 is circumferentially spaced equally (or spaced regularly) around the stator 210, the plurality of rotor poles 224 is circumferentially spaced equally around the inner rotor 220, and the plurality of permanent magnets 234 is circumferentially spaced equally around the outer rotor 230.

[0020] In various embodiments, the winding set 240 is connected to an AC -DC rectifier (e.g., a diode rectifier) for rectifying the induced voltage of the winding set 240 produced during operation for charging an energy storage.

[0021] In various embodiments, one of the inner and outer rotors 220, 230 is connected to and controlled by a speed-dependent device (e.g., a servo motor), and the other one of the inner and outer rotors 220, 230 is coupled to and controlled by a rotatable device configured to be rotatable by an external kinetic energy source (e.g., wind or hydro power). In various embodiments, to achieve higher power output, the inner rotor 220 is connected to and controlled by the speed-dependent device and the outer rotor 230 is coupled to and controlled by the rotatable device. For example, the outer rotor 230 may be mechanically connected to a shaft, which is then coupled to another shaft installed in the rotatable device using a shaft coupling. This arrangement facilitates efficient transmission of rotational energy to the outer rotor 230.

[0022] FIG. 3 depicts a schematic drawing of a power generation system 300 according to various embodiments of the present invention. The power generation system 300 comprises: the dual-rotor PM generator 200 as described herein according to various embodiments of the present invention; and a speed-dependent device 320 (e.g., a servo motor) connected to one of the inner and outer rotors 220, 230 (e.g., the inner rotor 220) for controlling it.

[0023] In various embodiments, as described hereinbefore, the other one of the inner and outer rotors 220, 230 (e.g. , the outer rotor 230) is coupled to and controlled by a rotatable device 330 configured to be rotatable by an external kinetic energy source (e.g., wind or hydro power). For example, the power generation system 300 may be wind power generation system or a hydro power generation system (e.g., for off-grid application).

[0024] In various embodiments, the power generation system 300 further comprises a control system 420 (to be described later below) communicatively coupled to the speeddependent device 320 for operating the power generation system 300. In particular', the control system 420 is configured to control the speed-dependent device 320 to control an angular position of one (the above-mentioned one) of the inner and outer rotors 220, 230 (e.g., the inner rotor 220) based on a speed of the other one of the inner and outer rotors 220, 230 (e.g., the outer rotor 230) to perform a VSCA voltage operation in relation to the induced voltage (i.e., total or net induced voltage). As described herein, the VSCA voltage operation seeks to maintain a constant (or near-constant) voltage across different speeds (e.g., different outer rotor speeds under varying operational conditions (e.g., due to speed variation of wind turbine blades at different wind velocities and pressures)).

[0025] In various embodiments, the power generation system 300 further comprises: an AC-DC rectifier connected to the winding set 240; and an energy storage connected to the AC- DC rectifier. In this regard, the AC-DC rectifier is configured to rectify the induced voltage ofthe winding set 240 produced during operation for charging the energy storage. In various embodiments, the AC-DC rectifier is a diode rectifier.

[0026] FIG. 4A depicts a method 400 of operating the power generation system 300 according to various embodiments of the present invention. The method 400 comprises controlling (at 406), using the speed-dependent device 320 of the power generation system 300, an angular position of one (the above-mentioned one) of the inner and outer rotors 220, 230 (e.g., the inner rotor 220) based on a speed of the other one of the inner and outer rotors 220, 230 (e.g., the outer rotor 230) to perform a VSCA voltage operation in relation to the induced voltage. According to various embodiments, the amplitude of the total or net induced voltage in the winding set 240 is determined by the vector sum of the induced voltages from the MGM and VPM. In this regard, for example in the case of the spccd-dcpcndcnt device 320 being connected to the inner rotor 220, by adjusting the angular position of the inner rotor 220, the vector angle between these two induced voltages can be controlled, thus enabling precise regulation of the total induced voltage to achieve the VSCA operation. According to various embodiments, the speed of the outer rotor 230 does not need to be directly measured, as its effect is reflected in the rectified output voltage from the AC / DC rectifier. In this regard, for example in the case of the outer rotor 230 being coupled to and controlled by the rotatable device, when the speed of the outer rotor 230 changes in response to variations in the rotatable device, the rectified output voltage is dynamically monitored using a voltage sensor. For example, the angular position of the inner rotor 220 may be detected by a position sensor (e.g., encoder or resolver), which provides feedback to a control system for operating the power generation system 300 (e.g., a standard PI (proportional and integral) controller may be embedded in the control system). The control system may take as input the difference between the measured rectified output voltage (e.g., which is based on the speed of the outer rotor 230) and the desired constant output voltage. Based on this voltage difference and the feedback on the inner rotor position, the control system may generate and send a control signal to the speeddependent device 320 (e.g., a servo motor) to control the speed-dependent device 320 to adjust the angular position of the inner rotor 220. Therefore, such an example closed-loop control system is able achieve consistent VSCA voltage operation, despite variations in the outer rotor speed.

[0027] FIG. 4B depicts a schematic block diagram of a control system 420 for operating the power generation system 300 according to various embodiments of the present invention, corresponding to the above-mentioned method 400 of operating the power generation system300 as described above with reference to FIG. 4A according to various embodiments of the present invention. The control system 420 comprises: at least one memory 422; and at least one processor 424 communicatively coupled to the at least one memory 422 and configured to perform the method 400 of operating the power generation system 300 as described above according to various embodiments of the present invention. Accordingly, the at least one processor 424 is configured to control, using the speed-dependent device 320 of the power generation system 300 (by generating and sending a control signal thereto), an angular position of one (the above-mentioned one) of the inner and outer rotors 220, 230 based on a speed of the other one of the inner and outer rotors 220, 230 to perform a VSCA voltage operation in relation to the induced voltage.

[0028] It will be appreciated by a person skilled in the art that the at least one processor 424 may be configured to perform various functions or operations through set(s) of instructions (e.g., software modules) executable by the at least one processor 424 to perform various functions or operations. Accordingly, as shown in FIG. 2, the control system 420 may comprise: an angular position controlling module (or an angular position controlling circuit) 426 configured to control, using the speed-dependent device 320 of the power generation system 300, an angular position of one (the above-mentioned one) of the inner and outer rotors 220, 230 based on a speed of the other one of the inner and outer rotors 220, 230 to perform a VSCA voltage operation in relation to the induced voltage.

[0029] In various embodiments, the control system 420 for operating the power generation system 300 corresponds to the method 400 of operating the power generation system 300 as described hereinbefore with reference to FIG. 4 A, therefore, various operations, functions or steps configured to be performed by the least one processor 424 may correspond to various operations, functions or steps of the method 400 described hereinbefore according to various embodiments, and thus need not be repeated with respect to the control system 420 for clarity and conciseness. In other words, various embodiments described herein in context of methods (e.g., the method 400 of operating the power generation system 300) are analogously valid for the corresponding systems or devices (e.g., the control system 420 for operating the power generation system 300) and vice versa. For example, in various embodiments, the at least one memory 422 may have stored therein the angular position controlling module 426, which corresponds to the corresponding operation, function or step of the method 400 of operating the power generation system 300 as described hereinbefore according to various embodiments,which are executable by the at least one processor 204 to perform the corresponding operation, function or step as described herein. roo3o] A computing system, a controller, a microcontroller or any other system providing a processing capability may be provided according to various embodiments in the present invention. Such a system may be taken to include one or more processors and one or more computer- readable storage mediums. For example, the control system 420 described hereinbefore may include at least one processor (or controller) 424 and at least one computer- readable storage medium (or memory) 422 which are for example used in various processing carried out therein as described herein. A memory or computer-readable storage medium used in various embodiments may be a volatile memory, for example a DRAM (Dynamic Random Access Memory) or a non-volatile memory, for example a PROM (Programmable Read Only Memory), an EPROM (Erasable PROM), EEPROM (Electrically Erasable PROM), or a flash memory, e.g., a floating gate memory, a charge trapping memory, an MRAM (Magnetoresistive Random Access Memory) or a PCRAM (Phase Change Random Access Memory).

[0031] In various embodiments, a “circuit” may be understood as any kind of a logic implementing entity, which may be special purpose circuitry or a processor executing software stored in a memory, firmware, or any combination thereof. Thus, in an embodiment, a “circuit” may be a hard-wired logic circuit or a programmable logic circuit such as a programmable processor, e.g., a microprocessor (e.g., a Complex Instruction Set Computer (CISC) processor or a Reduced Instruction Set Computer (RISC) processor). A “circuit” may also be a processor executing software, e.g., any kind of computer program, e.g., a computer program using a virtual machine code, e.g., lava. Any other kind of implementation of various functions or operations may also be understood as a “circuit” in accordance with various other embodiments. Similarly, a “module” may be a portion of a system according to various embodiments in the present invention and may encompass a “circuit” as above, or may be understood to be any kind of a logic-implementing entity therefrom.

[0032] Some portions of the present disclosure are explicitly or implicitly presented in terms of algorithms and functional or symbolic representations of operations on data within a computer memory. These algorithmic descriptions and functional or symbolic representations are the means used by those skilled in the data processing arts to convey most effectively the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities, such as electrical, magnetic oroptical signals capable of being stored, transferred, combined, compared, and otherwise manipulated.

[0033] The present specification also discloses a system (e.g., which may also be embodied as one or more devices or apparatuses), such as the control system 420, for performing various operations, functions or steps of various methods described herein. Such a system may be specially constructed for the required purposes or may comprise a general purpose computer system selectively activated or reconfigured by a computer program stored in the computer system. In general, various algorithms that may be presented herein are not limited to being implemented or executed by any particular computer system. Alternatively, the construction of more specialized computer system to perform various operations, functions or steps of various methods described herein may be provided as desired or as appropriate without going beyond the scope of the present invention.

[0034] In addition, the present specification also at least implicitly discloses computer program(s) or software / functional module(s), in that it would be apparent to a person skilled in the art that various operations, functions or steps of various methods described herein may be put into effect by computer code. The computer program(s) is not intended to be limited to any particular programming language and implementation thereof, and it will be appreciated by a person skilled in the art that a variety of programming languages and coding thereof may be used to implement the computer program(s). Moreover, the computer program(s) is not intended to be limited to any particular control flow as there are a variety of programming languages which can use different control flows. It will be appreciated by a person skilled in the ait that a computer program may be stored on any computer-readable storage medium (non- transitory computer-readable storage medium), such as but not limited to, a magnetic disk, an optical disk or a memory chip. For example, a computer program stored on a computer-readable storage medium may be loaded and executed on a computer system to implement various operations, functions or steps of various methods described herein according to various embodiments of the present invention.

[0035] Accordingly, in various embodiments, there is provided a computer program product, embodied in one or more computer-readable storage mediums (non-transitory computer-readable storage medium), comprising instructions (e.g., the angular position controlling module 426) executable by one or more computer processors to perform the method 400 of operating the power generation system 300 as described hereinbefore with reference to FIG. 4 A according to various embodiments of the present invention. Accordingly, variouscomputer programs or software modules described herein may be stored in a computer program product receivable by a system therein, such as the control system 420 as shown in FIG. 4B, for execution by at least one processor 204 of the control system 200 to perform various operations, functions or steps of various methods described herein according to various embodiments of the present invention.

[0036] It will be appreciated by a person skilled in the art that various modules described herein (e.g., the angular position controlling module 426) may be software module(s) realized by computer program(s) or set(s) of instructions executable by a computer processor to perform various functions or operations. Various modules described herein (e.g., the angular position controlling module 426) may also be implemented as hardware module(s) being functional hardware unit(s) designed to perform various functions or operations. More particularly, in the hardware sense, a module is a functional hardware unit designed for use with other components or modules. For example, a module may be implemented using discrete electronic components, or it can form a portion of an entire electronic circuit such as an Application Specific Integrated Circuit (ASIC). Numerous other possibilities exist. It will also be appreciated by a person skilled in the art that a combination of hardware and software modules may be implemented. Furthermore, various operations, functions or steps of various methods described herein may be performed in parallel rather than sequentially as desired or as appropriate (e.g., as long as it does not render the method(s) inoperable or unsatisfactory for its intended purpose).

[0037] FIG. 5 depicts a method 500 of manufacturing the dual-rotor PM generator 200 according to various embodiments of the present invention. The method 500 comprises: providing (at 502) a stator 210 having a plurality of stator slots 214; providing (at 504) an inner rotor 220 having a plurality of rotor poles 224; providing (at 506) an outer rotor 230 comprising a plurality of permanent magnets 234, the outer rotor 230 being arranged between the stator 210 and the inner rotor 220; and installing (at 508) a winding set 240 on the stator 210 through the plurality of stator slots 214 such that the winding set 240 is shared by the MGM and the VPM of the dual-rotor PM generator 200 for producing an induced voltage. In various embodiments, the single winding set 240 is configured with a pole -pair number equal to the difference between the number of stator slots 214 in the stator 210 and the number of PM polepairs in the outer rotor 230. In various embodiments, design parameters for the stator 210, the inner rotor 220, the outer rotor 230 and the winding set 240 may be optimized (e.g., using a genetic algorithm) as appropriate. As an illustrative example and without limitation, variousexample design parameters for the dual-rotor PM generator 200 will be described later below with reference to Table 1 shown in FIG. 11 according to various example embodiments of the present invention.

[0038] In various embodiments, the method 500 is for manufacturing the dual-rotor PM generator 200 as described herein with reference to FIG. 2 according to various embodiments of the present invention, therefore, various steps or operations of the method 500 may correspond to forming, providing or configuring various components, parts or portions of the dual-rotor PM generator 200 as described herein according to various embodiments, and thus such corresponding steps or operations need not be described or repeated with respect to the method 500 for clarity or conciseness. In other words, various embodiments described herein in the context of the dual-rotor PM generator 200 arc analogously valid for the method 500 (e.g., for manufacturing the dual-rotor PM generator 200 having various components, parts, portions and / or configurations as described hereinbefore according to various embodiments), and vice versa. It will also be appreciated by a person skilled in the art that the method 500 for manufacturing the dual-rotor PM generator 200 is not limited to any particular order of operations / steps. For example, as described hereinbefore, the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. Therefore, in the method 500, the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. For example, as described hereinbefore, the stator 210, the inner rotor 220 and the outer rotor 230 are arranged concentrically about a common axis. Therefore, in the method 500, the inner rotor 220 and the outer rotor 230 are arranged concentrically about a common axis.

[0039] It will be appreciated by a person skilled in the art that the terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and / or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0040] Any reference to an element or a feature herein using a designation such as “first”, “second” and so forth does not limit the quantity or order of such elements or features, unless stated or the context requires otherwise. For example, such designations may be used herein asa convenient way of distinguishing between two or more elements or instances of an element. Thus, a reference to first and second elements does not necessarily mean that only two elements can be employed, or that the first element must precede the second element, unless stated or the context requires otherwise. In addition, a phrase referring to “at least one of’ a list of items refers to any single item therein or any combination of two or more items therein.

[0041] In order that the present invention may be readily understood and put into practical effect, various example embodiments of the present invention will be described hereinafter by way of examples only and not limitations. It will be appreciated by a person skilled in the ail that the present invention may, however, be embodied in various different forms or configurations and should not be construed as limited to the example embodiments set forth hereinafter. Rather, these example embodiments arc provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art.

[0042] In particular, for better understanding of the present invention and without limitation or loss of generality, various example embodiments of the present invention will now be described with respect to a power generation system whereby the inner rotor is connected to and controlled by the speed -dependent device (based on a control signal from the control system) and the outer rotor is coupled to and controlled by the rotatable device for clarity and conciseness. It will be understood by a person skilled in the art that the power generation system is not limited to such a specific configuration and as described hereinbefore, one of the inner and outer rotors may be connected to and controlled by the speed-dependent device and the other one of the inner and outer rotors may be coupled to and controlled by the rotatable device configured to be rotatable by an external kinetic energy source.

[0043] Various example embodiments provide a unified winding design in a dual-rotor PM generator for a power generation system. In particular, various example embodiments introduce a single set of unified winding that can function as both MGM and VPM windings at the same time. As described hereinbefore, FIGs. 1 and 2 depict schematic drawings of topologies of a conventional dual-rotor PM generator and a dual-rotor PM generator 200 according to various embodiments of the present invention (which may be herein referred to as the present dual-rotor PM generator), respectively. As can be seen, in the conventional dual-rotor PM generator, the conductor area in the stator slot is divided into two sections for the MGM and VPM windings, respectively. In stark contrast, in the present dual-rotor PM generator 200, only a single winding set 240 is installed in the stator 210, which is shared by the MGM and the VPM simultaneouslyfor producing an induced voltage (total or net induced voltage) during operation. The dual-rotor PM generator 200 comprises: a stator 210 having a plurality of stator slots 214; an inner rotor 220 having a plurality of rotor poles 224; an outer rotor 230 comprising a plurality of permanent magnets 234, the outer rotor 230 being arranged between the stator 210 and the inner rotor 220; and a winding set 240 installed on the stator 210 through the plurality of stator slots 210 such that the winding set 240 is shared (simultaneously / concurrently shared) by the MGM and the VPM of the dual-rotor PM generator 200 for producing an induced voltage. Therefore, the single winding set 240 simultaneously functions as both MGM winding and VPM winding, which enables the MGM and the VPM to share (or effectively utilize) the whole stator slot conductor area, thereby yielding higher torque / power production from the present dual-rotor PM generator 200. Furthermore, such a unified winding design (single winding set 240) significantly simplifies the winding design, and hence significantly reduces manufacturing cost and control complexity.

[0044] FIG. 6 depicts a schematic drawing illustrating a configuration of a power generation system with VSCA voltage operation in various industrial applications, showing the case of the conventional dual-rotor PM generator and the case of the present dual-rotor PM generator 200 according to various example embodiments for illustration purpose. In particular, according to various example embodiments, there is provided a power generation system comprising the dual-rotor PM generator 200 and a speed-dependent device 320 (e.g., a servo motor) connected to the inner rotor 220 for controlling the inner rotor 220. Various example embodiments found that the unified winding design in the present dual-rotor PM generator 200 significantly improves generator performance, including induced voltage, torque / power density, efficiency, and PM utilization, compared to the conventional dual winding sets design. Thus, the present dual-rotor PM generator 200, and more particularly, the unified winding design, has great potential for widespread application in various power generation systems requiring the VSCA voltage operation, such as electric vehicles, electric aircraft, electric ships, and wind turbines.

[0045] Accordingly, various example embodiments introduce a dual-rotor PM generator 200 that is equipped with a single set of unified winding 240 in the stator 210, a winding design not found in existing technologies for dual-rotor PM generators. This developed single set of unified winding 240 is capable of producing induced voltage and power from both the MGM part and the VPM part inside the dual-rotor PM generator 200. As a result, the single set of unified winding 240 advantageously simplifies winding design, reduces control complexity,and moreover, enhances utilization of stator slot conductor area and PM fields significantly within the dual-rotor PM generator 200. According to various example embodiments, by controlling the inner rotor position (i.e., the angular position of the inner rotor 220, which may be defined by the mechanical angle between the centreline of a rotor pole in the inner rotor 220 and the centreline of a slot in the stator 210), the contribution by the MGM part to the voltage and power can be flexibly regulated in the developed unified winding 240, ensuring a high- performance and cost-effective VSCA operation with uncontrollable diode rectifier 610. According to various example embodiments, the amplitude of the total or net induced voltage in the winding set 240 is determined by the vector sum of the induced voltages from the MGM and VPM. In this regard, by adjusting the angular position of the inner rotor 220, the vector angle between these two induced voltages can be adjustcd / controllcd to achieve constant voltage (or near constant) at varying outer rotor speeds, thus enabling precise regulation of the total induced voltage to achieve the VSCA operation.

[0046] Accordingly, the developed unified winding design (the single unified set of windings 240) according to various example embodiments advantageously controls and produces the voltages and powers from the two machine parts (namely, the MGM and the VPM) inside the dual-rotor PM generator 200, yielding marked improvements in electromagnetic performance over conventional dual-rotor PM generators. Therefore, the single winding set 240 is capable of controlling power contributions from both rotors (inner and outer rotors 220, 230) using a single set of windings 240. Moreover, the developed unified winding design advantageously enables the dual-rotor PM generator 200 to provide a high-performance and low-cost VSCA operation using the uncontrollable diode rectifier 610. In particular, the developed unified winding design enables low-cost VSCA operation with a single unified set of windings 240 in the dual-rotor PM generators by controlling the power contribution from one machine part (namely, the MGM), without resorting to two sets of windings as required in conventional dual-rotor PM generators. In particular, conventional dual-rotor PM generators require two sets of windings for managing power from both rotors (inner and outer rotors) and the two machine parts (namely, the MGM and the VPM). It will be understood by a person skilled in the ail that in the conventional dual-rotor PM generator, the two sets of windings have different pole-pairs numbers, resulting in two entirely different winding distributions for each distinct winding set in the stator slots within the generator. Therefore, the conventional dualrotor PM generator has and require two distinct sets of windings with different pole-pairs for the MGM and the VPM, and which do not constitute a single winding set (not a single set ofunified winding). In contrast, the dual-rotor PM generator 200 has a winding set 240 (a single set of unified winding) installed on the stator 210 that is shared simultaneously / concurrently by the MGM and the VPM for producing an induced voltage. Therefore, in the unified winding set 240 of the dual-rotor PM generator 200, for example, only one consistent winding distribution in the stator slots 214 is required to achieve the pole-pair number that corresponds to the difference between the number of stator slots 214 and the PM pole-pairs of the outer rotor 230. This advancement associated with the developed unified winding design substantially reduces winding design complexity and manufacturing costs, simplifies control mechanisms, and enhances stator slot conductor utilization, leading to increased power density, efficiency and material utilization.

[0047] For better understanding and as an illustrative example, a stand-alone wind power generation system, including the dual-rotor PM generator 200, will now be described according to various example embodiments of the present invention. It will be appreciated by a person skilled in the art that the dual-rotor PM generator 200 is not limited to being implemented in a wind power generation system and as described hereinbefore, the dual-rotor PM generator 200 may be implemented in any power generation system as desired or as appropriate, especially when the VSCA operation is required.

[0048] For example, stand-alone wind power generation systems are essential for supplying electricity to remote areas, such as rural communities and offshore islands. In these wind power generation systems, it is crucial to maintain VSCA voltage over a wide range of wind speeds. In this regard, various example embodiments provide a dual-rotor PM generator (DRPMG) 200 featuring a single winding set 240 in the stator 210, specifically designed to achieve VSCA voltage which may be utilized in stand-alone wind power applications. A comprehensive finite- element analysis (FEA) is conducted to compare the present DRPMG 200 according to various example embodiments of the present invention (e.g., illustrated in FIG. 2) with the conventional DRPMG that incorporates dual winding sets (e.g., illustrated in FIG. 1 , which may also be referred to as the baseline design or the baseline DRPMG). The operation principle of the present DRPMG 200 will also be described in detail according to various example embodiments of the present invention, and analytical models for both the single and dual winding set configurations are provided to highlight the performance benefits of the single winding design according to various example embodiments of the present invention. The results demonstrate that the present DRPMG 200 outperforms both the regular PM synchronous generator (PMSG) and the baseline DRPMG in terms of induced voltage, torque / power density, efficiency, andPM material utilization. Furthermore, a prototype of the present DRPMG 200 is constructed and tested, validating the analysis with experimental data.

[0049] With ever increasing concern about the depletion and pollution of conventional centralized fossil-based energies, such as coal, oil, and natural gas, much effort has been devoted to the development of distributed renewable energies, such as wind power, solar photovoltaic, hydropower, and bioenergy. Due to the decentralized nature, stand-alone power generation systems can effectively harness these distributed renewable energies for electricity supply, especially in remote areas such as rural communities and offshore islands where establishing a power grid is unfeasible or costly but a local power supply is desired. Among the stand-alone power generations, wind power generation is one of the most favorable choices owing to its easy accessibility, huge untapped capacity of wind power, and the flexibility of equipment location for power generation.

[0050] In stand-alone wind power generation systems, batteries typically serve as the energy storage bank and necessitate an almost or near constant voltage for efficient and safe charging. Therefore, considering the speed variation of the wind turbine blades at different wind velocities and pressures, it is crucial for such wind power generation systems to provide an almost or near constant voltage, i.e., the VSCA voltage operation.

[0051] Recent decades have witnessed significant advancements in stand-alone wind power generation systems, with the emergence of various wind power generators, which convert the captured kinetic energy into electric power. Depending on the type of generators and the scale of the power converter, the traditional systems can be typically divided into a system with partial- scale power converters based on the doubly fed induction generator (DFIG) as shown in FIG. 7A, and a system with a full-scale power converter based on induction generator (IG) or synchronous generator (SG), as shown in FIG. 7B. In particular, FIG. 7A depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for standalone wind power generation utilizing partial-scale converters for a DFIG, and FIG. 7B depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for stand-alone wind power generation utilizing a full-scale converter for a 1G / SG. The former system enjoys the merits of the small capacity of power converters, and hence, reduced cost and power loss in power electronics. However, the brushes and slip rings in the DFIG and the gearbox in the system lead to regular maintenance, low reliability, and low efficiency of the system. On the other hand, in the latter system, the squirrel-cage IG (SCIG), wound-field SG (WFSG), and permanent-magnet SG (PMSG) are commonly applied.Although SCIG and WFSG have the advantage of structure simplicity, they exhibit inherently low torque / power density and low efficiency as well as the need for the gearbox to match the wind turbine speed. The above issues can be resolved in the multi-pole PMSG, but the full- scale power converter is unavoidable to provide the VSCA voltage, and hence, significantly raises the cost and power loss of the whole system.

[0052] More recently, DRPMGs are developed with VSCA voltage operation for standalone wind power generation systems (e.g., as described in Wang, et al., “A Novel Dual-Rotor Bidirectional flux-modulation PM generator for stand-alone DC power supply”, IEEE Transactions on Industrial Electronics, vol. 66, no. 1, pp. 818-828, Jan 2019 (hereinafter referred to as the Wang reference) and in Chen, et al., “Design and Analysis of a Variable- Speed Constant- Amplitude Wind Generator for Stand-alone DC Power Applications”, IEEE Transactions on Industrial Electronics, vol. 70, no. 8, pp. 7731-7742, Aug 2023 (hereinafter referred to as the Chen reference)), and a schematic drawing of such a wind power generation system is illustrated in FIG. 7C. In particular, FIG. 7C depicts a schematic drawing showing a conventional system configuration with VSCA voltage operation for stand-alone wind power generation utilizing a diode rectifier for a DRPMG. In such a system, one rotor is directly coupled to the wind turbine blades without gearboxes. Moreover, the other rotor is connected to a low-cost speed dependent device (SPD), such as a cam-spring governor or a small highspeed servo motor. This angular position of this rotor can be regulated in such a way that the induced voltage of the windings can be kept constant (or near constant), i.e., the VSCA voltage operation. Then, a low-cost uncontrolled diode rectifier can be simply adopted to rectify the voltage of winding for battery charging. However, various example embodiments note that in conventional DRPMGs (e.g., as described in the above-mentioned Wang and Chen references), dual winding sets with different pole-pair numbers are required and then connected in series in the stator. It will be understood by a person skilled in the art that the pole-pair number of a winding set refers to half the total number of magnetic poles in the rotating fundamental magnetic field generated by the currents flowing through the winding set. In this regard, the requirement for dual winding sets complicates the winding design and results in suboptimal utilization of the stator slot conductor area, as each winding set is restricted to only a portion / section of the total available space.

[0053] Various example embodiments introduce a new DRPMG 200 which may be implemented in stand-alone wind power generation systems. Unlike conventional designs, the present DRPMG 200 achieves VSCA voltage operation with only a single winding set (a singleset of unified winding) in the stator 210. This design significantly improves the utilization of the permanent magnet (PM) magnetic field in the airgap, resulting in higher induced voltage, torque / power density, efficiency, and enhanced PM material utilization compared to its counterparts. The operation principle and the analytical models of the DRPMG with single and dual winding sets will be described below. A comprehensive finite-element analysis (FEA) comparison between a regular PMSG, the present single winding DRPMG 200, and the baseline dual-winding DRPMG will also be described later below. Thereafter, for illustration purpose, the prototyping and experimental testing of the present DRPMG 200 will be described to validate the analysis.MACHINE TOPOLOGY AND OPERATION PRINCIPLEMachine Topologies

[0054] As described hereinbefore, FIGs. 1 and 2 illustrate the topologies of the baseline DRPMG and the present DRPMG 200 which may be utilized for stand-alone wind power generation. Both designs feature a stator, an outer rotor with spoke-array permanent magnets (PMs) and steel segments, and an inner rotor with salient poles. In an example baseline DRPMG, the baseline DRPMG is a variant of the design with split teeth described in the above- mentioned Chen reference, utilizing regular open stator slots and a slot-pole combination of 24 stator slots and 20 inner rotor poles. This configuration is known to enhance machine performance metrics, including torque / power density, power factor, and PM utilization. However, the baseline DRPMG requires dual winding sets in the stator, namely, one for the MGM and the other for the VPM (e.g., see the above-mentioned Chen reference, as well as Cao et al., “Decoupling Analysis of Brushless Dual-Mcchanical-Port Dual-Elcctrical-Port Machines”, IEEE Transactions on Industrial Electronics, vol. 71, no. 5, pp. 4361-4374, May 2024 (hereinafter referred to as the first Cao reference) and Cao et al., “Analysis of Contra- Rotating Brushless Integrated Flux-Modulation Machine with Open-Slot Structure for Wind Power Generation” in IEEE Transactions on Industrial Electronics, vol. 70, no. 12, pp. 11934- 11947, Dec 2023 (hereinafter referred to as the second Cao reference). Specifically, the MGM part comprises the stator, the MGM winding, the outer rotor, and the inner rotor, while the VPM part comprises the stator, the VPM winding, and the outer rotor. Consequently, the stator slot conductor area must be divided between the MGM and VPM windings, with additional insulation needed between them, leading to suboptimal utilization of the slot conductor area.

[0055] In contrast, various example embodiments demonstrate that these conventional dual winding sets can be unified into a single winding set by matching the number of stator slots to the number of inner rotor poles. This unification allows the MGM and VPM machines parts to share the entire stator slot conductor area, significantly improving the utilization of the PM magnetic field and boosting power output from the generator. This is the first time a DRPMG with a single winding set has been developed for VSCA operation, which for example may be implemented in stand-alone wind power generation systems. The principles behind the performance improvements of the present unified winding design will be described in further detail later below.Evolutionary Process from Dual to Single Winding Set

[0056] For better understanding only and without limitation (e.g., without being bound by theory), the evolutionary process from the baseline DRPMG with dual winding sets to the present DRPMG 200 with a single winding set 240 will now be described. According to fieldmodulation theory, in the MGM part of both generators (i.e., the baseline and the present DRPMGs), the pole-pair number of MGM winding / \IGM, the pole-pair number of PMs in the outer rotor Por, and the number of salient poles in the inner rotor A',, are governed by:MlGM—lM>r—Mr I(Equation 1)

[0057] For the VPM part, the pole -pair number of the VPM winding PVPM> the pole-pair number of PMs in the outer rotor Por, and the number of stator slots Qsshould satisfy:P VPM — I Mr—<?s I(Equation 2)

[0058] In various example embodiments, for example, the present DRPMG 200 may share the same pole-pair number of PMs (Por= 20) and the same number of stator slots (Qs= 24) as that of the baseline DRPMG. However, they differ in the number of salient poles in the inner rotor lVjr, for example, the baseline DRPMG has 18 inner rotor poles , while the present DRPMG 200 may have 24 inner rotor poles. To differentiate between the two designs, the notation (x) is used, where x=l refers to the baseline DRPMG and x=2 refers to the present DRPMG 200. Therefore, A / ]r( 1 ) — 18 T5QSand Mr(2) — 24 = Qsmeaning that the number of salient poles of the inner rotor equals the number of stator slots in the present DRPMG 200, while they differ in the baseline design.

[0059] Accordingly, the pole-pair numbers of MGM and VPM windings for both generators can be derived. In the baseline DRPMG, they are different, e.g., PMGM(1)=2 (i.e., the pole-pair number of MGM winding in the baseline DRPMG is 2) and / Jvi>\i( l )=4 (i.e., the polepair number of VPM winding in the baseline DRPMG is 4), whereas they are always equal in the present DRPMG 200, e.g.„ PMGM(2)=PVPM(2)=4. This difference arises from the disparity of the relationship between the numbers of inner rotor poles and stator slots in both generators, as indicated by Equations (1) and (2) above. FIGs. 8A and 8B show schematic drawings illustrating the winding and rotor designs / configurations of the baseline DRPMG and the present DRPMG 200. In particular, in the present DRPMG 200, the winding set 240 is installed on the stator 210 through the plurality of stator slots 210 such that the winding set 240 is shared by the MGM and the VPM of the present DRPMG 200 for producing an induced voltage. Furthermore, in the present DRPMG 200, the number of stator slots 214 at the stator 210 and the number of rotor poles 224 at the inner rotor 220 are equal. In this regard, various example embodiments advantageously found that the conventional dual winding sets can be unified into a single winding set 240 by matching the number of stator slots 214 to the number of inner rotor poles 224. In particular, various embodiments found that by matching the number of stator slots 214 to the number of inner rotor poles 224, the pole-pair numbers of the MGM and VPM windings are equal and thus, the conventional MGM and VPM winding sets can advantageously be unified into a single winding set 240 in the dual-rotor PM generator 200 according to various embodiments of the present invention. In contrast, as shown in FIG. 8A, in the conventional DRPMG, the number of stator slots at the stator and the number of rotor poles at the inner rotor are different. Therefore, in the conventional DRPMG, the pole-pah numbers of the MGM and VPM windings arc different and thus, the MGM and VPM winding sets cannot be unified into a single winding set in the conventional DRPMG.

[0060] In particular, the frequency of the induced voltage for MGM winding / MGMcanbe expressed as:(Equation 3) where HMGM is the equivalent mechanical angular speed of induced voltage for MGM winding; norand nirare the mechanical angular speeds of outer and inner rotors, respectively. Meanwhile, the frequency of the induced voltage for VPM winding / VPMcan he expressed as:(Equation 4)where nVPM>sthe equivalent mechanical angular speed of induced voltage for VPM winding.

[0061] According to Equations (3) and (4) above, when the inner rotor is at a standstill, i.e., Hjr= 0, the frequencies / MGMan<-t / VPM both equal Polnol / 60 in the baseline and present DRPMGs. Therefore, the MGM and VPM windings with different pole-pair numbers can be connected in series in the baseline DRPMG, while the present DRPMG 200 can unify the MGM and VPM windings with equal pole-pair numbers into only a single winding set.System Configuration

[0062] FIG. 9 depicts a schematic drawing illustrating a configuration of a stand-alone wind power generation system with VSCA voltage operation, showing the case of the baseline / conventional DRPMG 200 and the case of the present DRPMG 200 according to various example embodiments for illustration purpose. As can be seen, the wind turbine 930 is directly coupled to the outer rotor and the inner rotor is controlled by a SPD 320 such as a small servo motor as illustrated. In the baseline DRPMG, the dual winding sets for MGM and VPM machine parts arc connected in scries. The AC voltage of the connected winding is rectified by a three-phase uncontrollable diode rectifier into DC voltage, which is used to charge the energy storage bank, such as batteries. In contrast, the present DRPMG 200 adopts a single winding set 240 for both the MGM and VPM machine parts, which may be directly connected to the diode rectifier 610. Due to the variable speed of the wind turbine blades 930, caused by fluctuations in wind velocity and pressure, stand-alone wind power generation systems require an almost or near constant output voltage (i.e., VSCA voltage operation) for efficient and safe charging. The VSCA voltage operation of the wind power generation system shown in FIG. 9 will now be described below.VSCA Voltage Operation

[0063] To illustrate the VSCA voltage operation of the baseline and present DRPMGs, the induced voltages of windings are calculated assuming that: 1) the inner rotor held by the servo motor is at a standstill, i.e., n jr= 0; and 2) only the fundamental component is considered while neglecting the high-order ones.

[0064] In the MGM and VPM parts, the salient poles of the inner rotor and the stator teeth act as the flux modulators to modulate the PM field generated by the PMs in the outer rotor, respectively. However, these flux modulators are located on opposite sides of the outer rotor.Thus, the fundamental component of magnetomotive force (MMF) of the PM field FPM(0, t) for MGM and VPM parts has a phase shift of it, which may be expressed as:(Equation 5) where 6 is the mechanical angular position in the outer airgap, t is the time, FPM1is the amplitude of MMF, moris the mechanical angular speed of the outer rotor and equals 27rFornQr / 60, 0orOis the initial mechanical angular position of the outer rotor, and k is 0 for VPM pail and 1 for MGM pail.

[0065] In the MGM part, the sahent poles of the inner rotor work as the flux modulators to modulate the PM field, and hence, the permeance of the MGM pait AMGM(0) may be written as:(Equation 6) where AMGM0and AMGM1are the amplitudes of the DC constant and the fundamental components of, respectively, 0 is the mechanical angular speed of theinner rotor and equals zero due to is the initial mechanical angular position of theinner rotor. Then, the dominant magnetic flux density of the MGM part in the outer airgapcanbe expressed as:■(Equation 7) where BPorand F|p01.+ / vir| arc the amplitudes of flux density due to the modulation of AMGM0and AMGMI in MGM pail, respectively.

[0066] Due to the orthogonality property of trigonometric functions, the induced voltage of the MGM winding eMGM(t) according to the winding function theory may be calculated as Equation (8), where Rosis the outer airgap radius, Lsis the stack length of machine, NMGM(F) is the winding function of MGM winding expressed as Equation (9), VPh.MGM is the number of turns per phase in scries for MGM winding, , kp0Iand k| por+wir| arc the winding factors of corresponding harmonic orders in MGM winding, ZMGM is the machine period of MGM winding and equals to GCDfQs, PMGM), GCD is the greatest common divisor, PMGMO is the initialmechanical angular position of VMGM(9). AS can be seen in Equation (8), the contribution by the harmonicsis amplified by the corresponding factors which arethe so-called gear ratios of harmonics. d ( I(Equation 9)

[0067] Considering that AM GMO is much smaller than AMGM1in the MGM part (sec the above-mentioned Wang and Chen references), and thatismuch smaller thanare close and relatively large to produce high induced voltage and torque / power density (e.g., see the above-mentioned first Cao reference), the first and third terms in Equation (8) are negligible in MGM part. Hence, the induced voltage of the MGM winding CMGM (O may be simplified as:(Equation 10) where EMGM1is the amplitude of eMGM(t). Then, eMGM(t) may be rewritten in vector form(Equation 11)

[0068] In the VPM part, the stator teeth work as the flux modulators that modulate the PM field, and hence, the permeance of the VPM part AVPM(Θ) may be written as:(Equation 12)where AVPMo and AVPM1are the amplitudes of the DC constant component and the fundamental component of AVPM(6), respectively. Then, the dominant magnetic flux density of the VPM part in the outer airgap SVPM(0, t) may be expressed as:(Equation 13) where arc the amplitudes of flux density due to the modulation of AVPMo andAVPMI in the VPM part, respectively.

[0069] Due to the orthogonality property of trigonometric functions, the induced voltage of the VPM winding according to the winding function theory may be calculated asEquation (14), where) is the winding function of VPM winding as Equation (15), is the number of turns per phase in series for VPM winding, kare the winding factors for corresponding harmonic orders in VPM winding, tVPMis the machine period of VPM winding and equals GCD (Qs. PVPM). AS can be seen in Equation (14), the contribution by the harmonicsis amplified by their gear ratios of(Equation 14)(Equation 15)

[0070] Similarly, the induced voltage of the VPM winding eVPM(t) may be simplified as:(Equation 16)where EVPM1islhe amplitude of eVPM(t) . Then, eVPM(t) may be rewritten in vector form as:(Equation 17)

[0071] Consequently, the resultant induced voltage etotal (0 °f the generators is the sum ofeMGM(t)and 6VPM (0, i-c->(Equation 18)

[0072] Then, etotai(t) may be rewritten in vector form etota] as:(Equation 19)

[0073] The electrical angular difference may be expressed as:(Equation 20)

[0074] As can be seen in Equation (20), the electrical angular difference 3 is only dependent on the initial mechanical angular position of the inner rotor 0irOadjusted by the speeddependent device (e.g., servo motor). Thus, the amplitude of the resultant induced voltage Etota[ ranges from Eare obtained with 3 equal to odd multiples of n and 0, respectively.

[0075] Furthermore, when the outer rotor speed morvaries, Etota| can be maintained constant by adjusting the electrical angular differenceThis can be further illustrated by the vector diagram of induced voltages at different speeds of the outer rotor in FIG. 10 for the VSCA voltage operation, where conditions with different speeds arc denoted with and without the apostrophe. As can be seen, the speed increase of the outer rotor from morto a>0'Yresults in higher amplitudes of induced voltages of MGM and VPM windings. However, by adjusting the angle of two vectors the resultantinduced voltagesotaiat different speeds fall on the voltage circle with a constantradius, i.c., the amplitude of resultant induced voltage maintains constant at various speeds of outer rotor. This is the so-called VSCA voltage operation for the stand-alone wind power generation system.Improvements

[0076] As the winding factors of MGM and VPM windings are both designed with high values and close to 1 in both the baseline and present DRPMGs, and hence, they are simply considered equal, i.c., ^|por— jvir| ~ k\por-Qs\=^w- Then, according to Equations (18) and (20), when iVjr0irOequals odd multiples of ?r, the resultant induced voltage £total reaches the maximum value of fmax(ij for the baseline and present DRPMGs as:(Equation 21)

[0077] Supposing the stator slot conductor area is evenly divided into MGM and VPM windings in the baseline DRPMG, they have an equal number of turns per phase in series, which is half of their sum Nph / Sum, i.e., However, in thepresent DRPMG 200, the MGM and VPM windings share the whole stator slot conductor area in the form of the single set of unified winding 240, and hence, lVph MGM(2) = Vph VPM (2) = ^ph,sum- In addition, due to, the gear ratio for theharmonicis larger than the gear ratio l forthe harmonic issmaller thanin Equation (21) since a higher gear ratio typically exhibits flux leakages and leads to a lower flux density.

[0078] Then, the difference between the maximal induced voltages of the present and baseline DRPMGscan he calculated and have the following relationship as:(Equation 22)

[0079] The first term of Equation (22) is larger than 0 when. The second term of Equation (22) is always positive because the flux density of the unmodulated harmonic or in the VPM part is larger than that of the high-order modulated harmonicwhich also has a gear ratio of- Besides, the second term ofEquation (22) is actually the component contributed by the VPM part since it equals the second term of Equation (21) for the baseline DRPMG. Therefore, is significantly larger thanAs an illustrative example and without limitation, for the baseline and present DRPMGs described in this paper,rand are 24, 20 and 18,respectively, which satisfyAs a result, the present DRPMG 200 can exhibit a significant boost in induced voltage and torque / power density compared to the baseline DRPMG.PERFORMANCE COMPARISON

[0080] To assess the performance of the present DRPMG 200 with a single winding set 240, a comparative study is carried out among the regular surface-mounted PMSG, the baseline DRPMG with dual winding sets and the present DRPMG 200 with a single winding set 240 for the stand-alone wind power generation.

[0081] For a fair comparison, all the generators have the same number of stator slots, polepair number of PMs, machine diameter, stack length, slot filling factor, current density, and materials, as summarized in Table I shown in FIG. 11. In particular, FIG. 11 shows a table (Table I) including example key design parameters of the above-mentioned three generators investigated (namely, the PMSG, the baseline DRPMG and the present DRPMG 200). Then, the geometry parameters are globally optimized using the non-dominated sorting genetic algorithm (NSGA-IT) coupled with finite element analysis (FEA). The optimization aims to maximize the output power Poutat the rated outer rotor speed of 200 r / min and minimize the PMs consumption VPM under the constraints of torque ripple T,7?<10%. The key design parameters of the optimal designs are summarized in Table I in FIG. 11.

[0082] It should be noted that, while various criteria, such as fixing PM volume or copper loss, can be employed to optimize and compare different machine structures, the use of specific fixed values for these criteria is not always appropriate or applicable across varying machine designs. In the comparative study, a criterion is adopted based on fixed current density without constraining the PM volume, and global optimization is utilized to automatically identify the optimal electric and magnetic loadings for each design. Finally, the performance of each design can be further evaluated in terms of the output performance per PM volume and copper loss.No-load Field Distribution

[0083] The no-load field distributions including the flux density and flux line of the above- mentioned three generators at the no-load condition are shown in FIG. 12. The corresponding flux densities in the outer airgap (no-load airgap flux densities), including the waveform and harmonic spectrum, are presented in FIGs. 13A and 13B. In the regular PMSG, the working harmonic is the 20th harmonic as the pole-pair number of PMs POr is 20. By contrast, the baseline and present DRPMGs integrate MGM and VPM parts that utilize multiple working harmonics to produce higher induced voltage and power. In the baseline and present DRPMGs, the working harmonics of the VPM part arc mainly 20th (Pol=20), 4th, and 44th harmonics due to the modulation of stator teeth on the PM field. However, theydiffer in the working harmonics of the MGM part due to different numbers of salient poles of the inner rotor Vjr. Specifically, the working harmonic is 2ndharmonic120 — 181) for the baseline counterpart but 4th-order harmonic (for the present DRPMG 200.

[0084] Although the baseline DRPMG produces a slightly higher amplitude of 20th harmonic (mainly due to its higher PM volume) and an additional 2nd harmonic, the present DRPMG 200 exhibits a significantly higher amplitude of 4th harmonic produced by both MGM and VPM parts. Moreover, in the present DRPMG 200, the MGM and VPM windings share the whole stator slot conductor area, whereas in the baseline DRPMG, each winding only uses a portion / section of the stator slot conductor area. This design improvement markedly amplifies the contributions by these modulated harmonics in MGM and VPM parts of the DRPMG. Hence, it is shown that a significantly higher induced voltage and torque / power density is produced in the present DRPMG 200.No-Load Induced Voltage

[0085] FIGs. 14A and 14B show the no-load induced voltages of the above-mentioned three generators at the rated outer rotor speed of 200 r / min under the no-load condition, including the waveform and harmonic spectrum. As can be seen, the present DRPMG 200 produces the highest fundamental component of the induced voltage £i among all the generators, i.e., 83.3% and 82.0% higher than the regular PMSG and baseline DRPMG, respectively. In the baseline DRPMG, the amplitude of the 20th harmonic is reduced, and the stator slot conductor area is split into two parts for the MGM and VPM windings, leading to a less noticeable increase in induced voltage. In contrast, the notable rise of the present DRPMG 200 in induced voltage isdue to its single winding design, which significantly enhances the utilization of the stator slot conductor area and the PM field.

[0086] To further consider the effect of the total number of turns per phase and PMvolume VpM, the fundamental amplitude of the induced voltage is divided by -As can be seen, the present DRPMG 200 demonstrates the highest utilization of conductor and PMs to produce induced voltage. Specifically, the present DRPMG 200 exhibits 14.4% higher and 59.4% higher than the regular PMSG and the baseline DRPMG,respectively. This further corroborates the design improvement by the present winding design as analyzed in the sub-section “Improvements" described above.

[0087] FIGs. 15A and 15B show the induced voltage of the present DRPMG 200 at varying inner rotor positions and outer rotor speeds. In particular, FIGs. 15A and 15B show plots of no- load induced voltages of the present DRPMG 200 at varying inner rotor positions and outer rotor speeds for amplitude of induced voltages versus inner rotor position at a rated outer rotor speed of 200 r / min (FIG. 15 A) and waveform of induced voltage at varying outer rotor speeds (FIG. 15B). As can be seen in FIG. 15A, at a rated outer rotor speed of 200 r / min, the amplitude of induced voltage varies with the position of the inner rotor, and the maximal and minimal values are obtained withrespectively, which agrees well with the previous analysis associated with Equations (19) and (20) described hereinbefore. Therefore, as shown in FIG. 15B, by adjusting the position of the inner rotor from / Vjr0jrO= 0 to (Vjr^iro=it, the induced voltage can be maintained constant (or near constant) even as the speed of the outer rotor varies, reaching up to 2.7 times the rated speed. Thus, the present DRPMG 200 is able to provide the VSCA voltage operation using the uncontrolled diode rectifier 610 at low cost and high reliability.

[0088] FIG. 15C show plots of the measured dynamic VSCA operation of the present DRPMG 200 at load condition, for the outer rotor speed, inner rotor position, AC induced voltage at phase A, rectified DC voltage and DC output current.Torque Performance

[0089] FIG. 16 shows the torque waveforms of the above-mentioned generators at the rated current density of 5 A / mm2. It can be observed that the present DRPMG 200 performs the best in the production of output torque Tor. To be specific, the average outer rotor torque of the present DRPMG 200 is 131.6 Nm, which is about 82% higher than the average torques of regular PMSG and the baseline DRPMG. Considering the copper loss and PM volume, thepresent DRPMG 200 can offer about 17% and 117% more torque per copper loss and PM volume than the regular PMSG and the baseline counterpart, respectively.Other Performances

[0090] Various other performances of the above-mentioned generators, such as power, loss, efficiency, power factor, PM utilization, etc., are compared and summarized in Table II shown in FIG. 17. As can be seen, the present DRPMG 200 exhibits an over 80% increase in power compared to the regular' PMSG and baseline DRPMG due to its significantly higher induced voltages (sec FIGs. 14A and 14B). Regarding loss and efficiency, the total losses generated by the baseline and present DRPMGs are in a similar level and higher than those of the regular PMSG, mainly due to higher copper loss and iron loss. However, the present generator 200 exhibits a 3.3% and 6.7% higher efficiency than the regular PMSG and the baseline counterpart, respectively, thanks to its significantly higher output power. Moreover, the present DRPMG 200 can provide the VSCA operation using a low-cost uncontrolled diode rectifier instead of the high-cost full-scale controllable converter in the regular PMSG.EXPERIMENTAL VALIDATION

[0091] To validate the prior analysis and proposed single winding design in DRPMGs, a prototype of the present DRPMG 200 is constructed and tested. As an illustrative example, the design parameters of the prototype are detailed in Table I shown in FIG. 11. FIGs. 18A to 18E show images of the prototype, including its stator, outer rotor, inner rotor, PMs and coils, as well as the test rig for VSCA operation. The dSPACE system (corresponding to the control system 420 as described hereinbefore with reference to FIG. 4B according to various embodiments of the present invention) shown in FIG. 18D controls the inverters to drive servo motor I, which connects to the outer rotor of the prototype via a torque sensor, rotating at various speeds to mimic the variations in wind power input. Additionally, the dSPACE system also manages servo motor II, which is connected to the inner rotor of the prototype, to facilitate precise dynamic adjustments to the inner rotor's position based on the real-time speeds of the outer rotor. In this regard, when the speed of the outer rotor 230 changes in response to variations in the rotatable device 930, the position of the inner rotor 220 may be detected using a position sensor (e.g., encoder or resolver). This position sensor provides feedback to the dSPACE, which may deploy a standard PI (proportional and integral) controller. The PI controller may take as input the difference between the desired constant output voltage and themeasured output voltage from the AC / DC rectifier as detected by a voltage sensor. Using this voltage difference and the feedback on the inner rotor position, the PI controller in dSPACE may generate and send a control signal to the speed-dependent device 320 (e.g., a servo motor) to adjust the angular position of the inner rotor 220 accordingly. Therefore, such an example closed-loop control system dynamically regulates the position of the inner rotor 220, ensuring that the VSCA operation is maintained consistently across varying outer rotor speeds. Subsequently, the induced three-phase AC voltages are converted into DC voltage using an uncontrollable diode rectifier. The DC output is then linked to an 80Q ceramic highpower resistor, which operates in parallel with a 33000pF electrolytic capacitor, serving as an energy storage bank.

[0092] In the no-load induced voltages test, servo motor I drives the outer rotor of the prototype at various speeds and the inner rotor fixed at 180 electric degrees. During this test, the rectifier, capacitor and resistance in FIG. 18E are disconnected from the test setup. FIGs. 19A and 19B compare the measured no-load induced voltages at a rated speed of 200 r / min with the FEA-predicted ones, showing that they are in good agreement with a difference of 1.5% in the fundamental amplitude.

[0093] The on-load average torque of the outer rotor is measured and compared with the values predicted by FEA. In this test, the prototype's armature windings are powered by an inverter, controlled by the dSPACE system, with the inner rotor locked at 180 electrical degrees. In this setup, servo motor I shown in FIG. 18E is replaced with a magnetic power brake, serving as the load. FIGs. 20A and 20B illustrate the measured and simulated torque across various current angles and amplitudes. In particular, FIGs. 20A and 20B depict plots showing the outer rotor torque for the torque versus current angle at 20 A (FIG. 20A) and the torque versus current amplitude at zero current angle (FIG. 20B). To ensure safe operation within the limits of the torque sensor (HBM-T21WN, nominal torque: 100 Nm, accuracy: 0.2 Nm), the current is tested up to 20 A, with the measured output torque reaching about 104.5 Nm. As can be seen, a good agreement can be found between the measured and simulated average output torque.

[0094] To further experimentally validate the VSCA voltage operation, a comparison is made between the measured and simulated (FEA-predicted) voltage envelopes of no-load AC induced voltages at a rated outer rotor speed of 200 r / min and the result is shown in FIGs. 21 A and 2 IB. In this test, the outer rotor, driven by servo motor I, maintains a constant speed of 200 r / min, while the inner rotor, driven by servo motor II, rotates at a low speed of 1 r / min. The results show that the range of the measured and simulated induced voltages spans from about30 V to 78 V, despite a slight discrepancy mainly due to manufacturing imperfections and controlled speed variations of the two rotors during the experiments. Additionally, dynamic VSCA operation under load conditions is assessed, with the outer rotor's speed varying from 150 r / min to 200 r / min and then returning to 200 r / min. Notably, the inner rotor's position is regulated correspondingly to maintain nearly constant output DC voltage, despite the variations in the outer rotor's speed. This effectively demonstrates the VSCA voltage operation of the present generator 200 in practical test.

[0095] Accordingly, a DRPMG 200 with a single winding set 200 is developed according to various example embodiments of the present invention and compared with the baseline DRPMG with dual winding sets for the VSCA operation in stand-alone wind power generation systems. By adjusting the inner rotor position, the induced AC voltages can be simply rectified using a low-cost uncontrollable diode rectifier, while maintaining constant voltage output at various wind speeds, which is an important feature for stand-alone applications in remote areas.

[0096] The analysis described hereinbefore demonstrates that the single winding set design according to various example embodiments can be effectively implemented when the number of stator slots Q5matches the number of poles on the inner rotor N\r. Furthermore, the analysis and case study demonstrate significant performance improvements with this single winding set design, especially for DRPMGs where the pole-pair number of the outer rotor Porsatisfies the slot-pole relationship Por> (Qs+ 2Njr) / 3.

[0097] Compared to the regular PMSG, the present DRPMG 200 offers about 80% increase in torque / power density, 3.3% higher efficiency and 17% better PM utilization. Compared to the baseline DRPMG with dual winding sets, it offers about 80% higher torque / power density, 6.7% improvement in efficiency, and a remarkable 117% boost in PM utilization. These significant performance enhancements arc attributed to the higher utilization of the stator slot conductor area and PM field by the design of the DRPMG 200 with a unified single winding set. Accordingly, various example embodiments provide an improved DRPMG design which for example may be implemented in stand-alone wind power generation and provides useful / practical design guidance for DRPMGs with a single winding set. As described hereinbefore, a prototype of the present DRPMG 200 is built and experimentally tested to validate the analysis.

[0098] While embodiments of the invention have been particularly shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the scope ofthe invention as defined by the appended claims. The scope of the invention is thus indicated by the appended claims and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced.

Claims

CLAIMS1. A dual-rotor permanent magnet generator comprising: a stator having a plurality of stator slots; an inner rotor having a plurality of rotor poles; an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and a winding set installed on the stator through the plurality of stator slots such that the winding set is shared by a magnctic-gcarcd machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage.

2. The dual-rotor permanent magnet according to claim 1, wherein the number of stator slots at the stator and the number of rotor poles at the inner rotor are equal.

3. The dual -rotor permanent magnet generator according to claim 1 or 2, wherein the stator, the outer rotor, the inner rotor and the winding set together constitute the magnetic-geared machine, and the stator, the outer rotor and the winding set together constitute the vernier permanentmagnet machine.

4. The dual-rotor permanent magnet generator according to any one of claims 1 to 3, wherein the stator, the inner rotor and the outer rotor are arranged concentrically about a common axis.

5. The dual-rotor permanent magnet according to any one of claims 1 to 4, the winding set is connected to an AC-DC rectifier for rectifying the induced voltage of the winding set produced during operation for charging an energy storage.

6. The dual-rotor permanent magnet according to any one of claims 1 to 5, wherein one of the inner and outer rotors is connected to and controlled by a speed-dependent device, and the other one of tire inner and outer rotors is coupled to and controlled by a rotatable device configured to be rotatable by an external kinetic energy source.

7. A power generation system comprising: a dual-rotor permanent magnet generator comprising: a stator having a plurality of stator slots; an inner rotor having a plurality of rotor poles; an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and a winding set installed on the stator through the plurality of stator slots such that the winding set is shared by a magnetic-geared machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage; a speed-dependent device connected to one of the inner and outer rotors for controlling it.

8. The power generation system according to claim 7, wherein the number of stator slots at the stator and the number of rotor poles at the inner rotor are equal.

9. The power generation system according to claim 7 or 8, wherein the stator, the outer rotor, the inner rotor and the winding set together constitute the magnetic-geared machine, and the stator, the outer rotor and the winding set together constitute the vernier permanentmagnet machine.

10. The power generation system according to any one of claims 7 to 9, wherein the stator, the inner rotor and the outer rotor arc arranged concentrically about a common axis.

11. The power generation system according to any one of claims 7 to 10, wherein the other one of the inner and outer rotors is coupled to and controlled by a rotatable device configured to be rotatable by an external kinetic energy source.

12. The power generation system according to any one of claims 7 to 11, further comprising a control system communicatively coupled to the speed-dependent device for operating the power generation system, wherein the control system is configured to control the speeddependent device to control an angular position of said one of the inner and outer rotors basedon a speed of the other one of the inner and outer rotors to perforin a variable -speed constant amplitude (VSCA) voltage operation in relation to the induced voltage.

13. The power generation system according to any one of claims 7 to 12, further comprising: an AC-DC rectifier connected to the winding set; and an energy storage connected to the AC-DC rectifier, wherein the AC-DC rectifier is configured to rectify the induced voltage of the winding set produced during operation for charging the energy storage.

14. The power generation system according to any one of claims 7 to 13, wherein the AC- DC rectifier is a diode rectifier.

15. A method of operating the power generation system according to any one of claims 7 to 14, the method comprising: controlling, using the speed-dependent device of the power generation system, an angular position of said one of the inner and outer rotors based on a speed of the other one of the inner and outer rotors to perform a variable-speed constant amplitude (VSCA) voltage operation in relation to the induced voltage.

16. A method of manufacturing a dual-rotor permanent magnet generator comprising: providing a stator having a plurality of stator slots; providing an inner rotor having a plurality of rotor poles; providing an outer rotor comprising a plurality of permanent magnets, the outer rotor being arranged between the stator and the inner rotor; and installing a winding set on the stator through the plurality of stator slots such that the winding set is shared by a magnetic-geared machine and a vernier permanent-magnet machine of the dual-rotor permanent magnet generator for producing an induced voltage17. The method according to claim 16, wherein the number of stator slots at the stator and the number of rotor poles at the inner rotor are equal.

18. The method according to claim 16 or 17, whereinthe stator, the outer rotor, the inner rotor and the winding set together constitute the magnetic-geared machine, and the stator, the outer rotor and the winding set together constitute the vernier permanentmagnet machine.

19. The method according to any one of claims 16 to 18, wherein the stator, the inner rotor and the outer rotor are arranged concentrically about a common axis.

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