Air-gap Winding Research Articles

Conventional and TFPM Linear Generators for Direct-Drive Wave Energy Conversion

The archimedes wave swing (AWS) is a system that converts ocean wave energy into electric energy. The goal of the research described in this paper is to identify the most suitable generator type for this application. Of the conventional generator types, the three-phase permanent-magnet synchronous generator with iron in both stator and translator is most suitable, because it is cheaper and more efficient than the induction generator, the switched reluctance generator, and the permanent-magnet (PM) generator with an air-gap winding. The paper also proposes a new transverse-flux PM (TFPM) generator topology that could be suitable for this application. This new double-sided moving-iron TFPM generator has flux concentrators, magnets, and conductors on the stator, while the translator only consists of iron.

Extension of winding function theory for nonuniform air gap in electric machinery

This paper extends the winding function theory for nonuniform air gap in rotating electric machinery. It shows that the winding function differs from that used in the symmetrical case, although several papers employ the uniform air-gap winding function to study electric motor performance under fault conditions. The extended theory will be particularly helpful in the study of squirrel-cage induction motors with a nonuniform air gap such as that caused by eccentricity of the rotor and stator.

380 kW synchronous machine with HTS rotor windings––development at Siemens and first test results

Applying HTS conductors in the rotor of synchronous machines allows the design of future motors or generators that are lighter, more compact and feature an improved coefficient of performance. To address these goals a project collaboration was installed within Siemens, including Automation & Drives, Large Drives as a leading supplier of electrical machines, Corporate Technology as a competence center for superconducting technology, and other partners. The main task of the project was to demonstrate the feasibility of basic concepts. The rotor was built from racetrack coils of Bi-2223 HTS tape conductor, these were assembled on a core and fixed by a bandage of glass-fibre composite. Rotor coil cooling is performed by thermal conduction, one end of the motor shaft is hollow to give access for the cooling system. Two cooling systems were designed and operated successfully: firstly an open circuit using cold gaseous helium from a storage vessel, but also a closed circuit system based on a cryogenerator. To take advantage of the increased rotor induction levels the stator winding was designed as an air gap winding. This was manufactured and fitted in a standard motor housing. After assembling of the whole system in a test facility with a DC machine load experiments have been started to prove the validity of our design, including operation with both cooling systems and driving the stator from the grid as well as by a power inverter.

Research and development of slow response type 70 MW class superconducting generator with high-current density superconductors

Some 70 MW class superconducting generators have been developed in Japan to verify the basic technologies for a 200 MW class superconducting pilot generator. The manufacture of a 70 MW rotor with a slow response excitation began from 1992, and factory tests were conducted in 1996. Succeeding the factory tests, the rotor was combined to the stator with air gap windings at the verification test facility in Osaka power station, and verification tests were conducted in 1998. The advantages and reliability of superconducting generators had been clarified by these tests. In this paper, verification test results including stability of superconducting field windings and helium cooling characteristics are described. The main results are summarized as follows: (1) the advantages of the superconducting generators, such as increasing generation efficiency and improving power system stability were verified, and the operational reliability was confirmed by the long-term operation, (2) the superconducting stability against constant current and rapid current variations corresponding to the load conditions of the 200 MW class slow response pilot generator were confirmed to be satisfactory and (3) helium cooling characteristics such as cooling down and heat loss were confirmed to be also satisfactory.

Development of superconducting generator having highly stabilized superconducting field winding––70 MW class superconducting generator

The development of a practical superconducting generator has been carried out in Japan since 1988. The authors have developed a rotor with a highly stabilized superconducting field winding and a stator with an air gap winding composed of double transposed copper coils. A series of tests was completed at the end of 1997. The output of 78.8 MW was recorded, which was the highest value obtained to date worldwide. The development of the generator and the test results are described.

高安定低速応超電導発電機の開発 ７０ＭＷ級超電導発電機

Since 1988, the development of a practical superconducting generator has been carried out in Japan. The authors have developed a rotor with a highly stabilized superconducting field winding and a stator with an air gap winding composed of double transposed copper coils. A series of tests were completed at the end of 1997. The output of 78.8MW was recorded, which was the highest value obtained worldwide. The development of the generator and the test results are described.

A 30 kVA superconducting generator development and basic tests

A 30 kVA superconducting generator has been designed, developed and successfully tested by the Korea Electrotechnology Research Institute (KERI). The design is based on a 2-dimensional electromagnetic field analysis of cylindrical coordinates. The field winding of rotor has been wound with superconducting wire of NbTi alloy and tested after assembly with other rotor components. The stator has air-gap type armature windings, which produce higher terminal voltage, and a more sinusoidal voltage waveform than conventional iron cored machines. Steady-state open and short-circuit tests and lamp load tests have also been conducted. The results of these tests are given in this paper and compared with design quantities. Moreover, the cooling scheme and characteristics of the test system are included.

Conceptual design of a high temperature superconducting generator

This paper describes the conceptual design of a high temperature superconducting (HTS) generator for mobile radar (MR) applications. The power system for MR consists of a diesel engine, a generator and power conditioning electronics. Significant improvements in power density, performance and system efficiency result if the generator and the power electronics are cryogenically cooled. The generator supplies power to the radar and the auxiliaries of the system. The generator output required is less than 50 volt AC (RMS phase to neutral) to maximize the efficiency of the cryogenically cooled power electronics. The auxiliaries of the system consume power at 120 V AC, 3-phase, at 60 Hz. The generator is designed to meet both requirements. The diesel engine provides the prime power to the generator at 1800 RPM. The generator is a four-pole two stator winding machine supplying power at 50 V AC, 60 Hz, 12-phase to the power electronics, and 120 V AC, 60 Hz, 3-phase to the auxiliaries of the system. The field winding of the generator consists of HTS coils; the stator has two ambient temperature air gap windings. The cooling of the field winding is provided by heat exchange with helium gas cooled by a Gifford McMahon cryocooler.

70 MW class superconducting generator test

Since 1989 development of a practical superconducting generator has been carried out in Japan. The authors have developed a rotor having a superconducting field winding and a stator having an air gap winding composed of a double transposed copper coil. A series of tests were completed at the end of 1997. The output of 78.7 MW was recorded which is the highest value yet obtained world wide. Test results of the shop test in 1996 and the site test in 1997 and analysis are described.

Armature reaction field and winding inductances of slotless permanent-magnet brushless machines

The paper describes an improved two-dimensional (2-D) analytical model for predicting the winding inductances and the armature reaction field of slotless permanent magnet brushless machines. Results from the model are compared to those deduced from finite element analyses, existing one dimensional (1-D) and 2-D analytical models and measurements. It is shown that the radial thickness of the airgap winding can have a significant effect on both the winding inductances and the armature reaction field.

Development of a 70 MW class superconducting generator

A national project to develop a superconducting generator was begun in 1988 in Japan under the New Sunshine Project of AIST, MITI. This generator has merits of higher efficiency and compactness for transmission lines. A 70 MW class superconducting generator is the goal of the project. The authors have been developing a rotor having a superconducting field winding and a stator having an air-gap armature winding. The field winding was designed to be cryostable so as to recover from a partial transition to normal conduction. The field winding was tested in a nonrotating cryostat, and excitation performance and stability were confirmed to be enough for the 70 MW class superconducting generator. The rotor and the stator have been completed and a shop test is being made.

Development and performance testing of a 200 kVA damperless superconducting generator

A 200 kVA, 3000 RPM superconducting generator has been developed and tested. The rotor has been wound with superconducting wire of Nb-Ti alloy. A closed-circuit liquid helium system has been designed and installed for cooling the superconducting windings. The stator carries the air-gap type armature windings and a laminated-iron flux-shield. A new concept in the design of superconducting generators with high short-circuit ratio (more than 5) has been introduced. This eliminates the requirement of an electromagnetic damper and quick response excitation system. The generator has been comprehensively tested in the superconducting state. Open-circuit and sustained short-circuit tests, three-phase sudden short-circuit tests, synchronization with the grid and parallel operation with power systems have been conducted. The synchronous machine was operated up to its rated kVA in the four quadrants-as a generator and as a condenser with leading and lagging power factors. A few special tests on superconducting generators, which were not reported earlier, such as direct-online starting of a 20 hp squirrel-cage induction motor and negative phase sequence tests have also been performed successfully. Test results and conclusions are given.

Design of an in-wheel motor for a solar-powered electric vehicle

The design of an in-wheel electric motor for the solar-powered vehicle ‘Aurora’, entered in the 1996, 3010 km Darwin – Adelaide World Solar Challenge solar car race is described. Compared to other entrants in the race, the brushless DC motor is more efficient (97.5% compared to 92–95%) and lighter (8.3 kg compared to 12–16 kg) than all other direct-drive motors, and more efficient than all motor/gear combinations. This is achieved by the use of high flux-density rare-earth magnets, and computer aided optimisation of an axial-flux configuration consisting of a Halbach magnet array and an ironless air-gap winding.

Ten MVA air-gap armature winding: thermal, structural and dielectric results

This paper describes the armature winding built as part of MIT's 10 MVA superconducting generator project. The winding has a helical winding form, is an air-gap winding having no iron teeth in the active region, employs a limited voltage gradient insulation scheme and is cooled by an insulating fluid. Structurally, the winding employs bonded joints between the conductors and structural tubes, which are of filament-wound fiber-reinforced plastic. Preliminary tests indicated that thermal performance of this winding was satisfactory. On the other hand, the winding suffered a catastrophic insulation failure after only a short time at high voltage. The resulting fault gave the structural system of the armature winding (and the rest of the machine) a good test. In this paper the authors review the major aspects of the design of the winding, describe the thermal tests and then tell of the insulation failure. They can, at best, speculate about the cause of that failure, but they can, on the basis of this experience, make recommendations for further development of machines of this type.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Small axial flux motor with permanent magnet excitation and etched airgap winding

Disc-type motors with sintered Nd-Fe-B magnets benefit from the high remanence and near-ideal loop shape of the hard magnetic material. Consequent advantages include a high torque for a given motor size and improved efficiency. Special airgap windings manufactured as multilayer etched windings contribute to flat package motors and to low torque harmonics. Different back iron designs were examined. For the operation of the brushless DC motor, instead of the classical Hall sensors, small magnetoresistive sensors can be used effectively for rotor position sensing.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Structure and Characteristics of a High Speed Axial Gap Motor with Two Rotor Discs.

Super high speed electrical machines for electrical vehicles, compressors and co-generation systems have been studied. The speed range of the machines is 10, 000_??_30, 000 rpm. This paper describes a basic structure, analysis and characteristics of a newly developed super high speed machine, which rotates at 20, 000 rpm. The new machine is an axial-field type with a high energy permanent magnet (Nd-Fe-B). The stator has mold coils of air gap winding which can be divided in half. The rotor has two discs sandwiched by the mold coils. Many columnar magnets are embeded in the duralumin discs (non magnetic material) of the rotor. As the rotor is yokeless, the magnetic circuit of field is composed of back yokes of the stator and the permanent magnets of the rotor. This construction allows us to make the considerably light rotor which can withstand large centrifugal force at high speed. Then a little amature reaction, little cogging torque and small noise are realized. The analysis and test results show that the machine has the good stability at high speed and the capacity of about 7kW at 20, 000 rpm is achieved.

A low-loss permanent-magnet brushless DC motor utilizing tape wound amorphous iron

An axial field permanent-magnet brushless DC motor that utilizes tape wound amorphous iron and an air gap winding is proposed. Simplified waveforms and performance equations for this type of machine are presented. The machine equations and waveforms are verified with a proof-of-concept machine. No-load iron losses are compared with manufacturer's data, and full-load iron losses are also presented. Output torque for rectangular and trapezoidal current waveforms are measured and compared. Machine designs from a computer program that demonstrate the possible benefits of this type of machine could be considerably more efficient than induction machines, and, additionally. cost competitive when a variable speed drive is already a system requirement.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Novel disc type motors

The use of high energy permanent magnets can lead to significantly improved efficiency and performance, especially for subfractional horsepower motors. Obviously motors with air gap windings equipped with Nd-Fe-B magnets benefit from the high remanence of this new material. Integrated designs using Nd-Fe-B magnets are cost effective. The trend towards high flux density drives will continue.

Performance evaluations of 50 MVA airgap winding stator for superconducting generator

Structural features of a trial-manufactured air-gap winding stator for a 50 MVA superconducting generator and results of various characteristic tests are described. The stator has an improved winding section, strengthened against electromagnetic force and smaller due to use of a four-layer water-cooled helical air-gap armature winding. The axial flux through the stator core, assumed to be generated by the circumferential current of the helical winding, is too low to pose any practical problems, as shown by a three-dimensional magnetic-field analysis taking into account the magnetic anisotropy due to the laminated iron core. The air-gap armature winding electromagnetic force in the radial and axial directions (which does not contribute to the generator output) increases when the running power factor decreases. The helical winding is shown to be applicable to the armature winding of large superconducting generators.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Performance Evaluations of 50 MVA Airgap Winding Stator for Superconducting Generator (Summary)

Performance Evaluations of 50 MVA Airgap Winding Stator for Superconducting Generator (Summary)