Ferromagnetic Stator Research Articles

An Approach to Designing the Stator Windings of High-Capacity Wind Generators

The capacity of a single offshore windfarm turbine has by now reached 10 MW, and its further growth is possible by making a shift for using superconducting synchronous generators. The article considers the factors that determine the choice of classical distributed and concentrated stator windings in designing high capacity superconducting wind generators. A 10 MW, 3300 V, 10 rpm superconducting wind generator with an excitation winding made of high temperature superconducting material Bi-2223 with ferromagnetic stator and rotor is studied. The stator volume and superconducting wire length were adopted as design selection criteria. It is shown that in the range of pole pair numbers 2p = 32-40, the use of a distributed two-layer winding has limitations in regard of the generator operation stability (the short-circuit current is less than the nominal value), and the use of concentrated windings has limitations due to a significant value of the differential leakage component. It has been found that for a distributed winding in the specified range of pole pair numbers, almost equivalent versions can be obtained with the stator volume equal to 78 m3 and superconductor wire length equal to 16 km. For a concentrated winding, it is preferable to choose a single-layer winding with the embodiment conditions Z = 12 + 6k, 2p = Z – 2, k = 0, l, 2,...; however, the obtained indicators are inferior to those in the case of using a classical distributed two-layer winding.

A Fully Coupled Numerical Method for Coated Conductor HTS Coils in HTS Generators

Superconducting rotational generators are a promising application of the high temperature superconductor (HTS). Typically, superconducting rotating generators use HTS coated conductors to form the DC coils in the rotor as they experience lower AC loss compared to the stator windings. However, in the actual operation, the AC loss produced by the harmonic magnetic field is inevitable. Other transient electromagnetic disturbances in the operating generator environment can also cause the AC loss. This paper introduces a fully coupled numerical method for coated conductor HTS coils in operating generators based on the T-A formulation, which is verified by almost the same simulation results with the A-H formulation model. Using this method, the simulation results show that the ferromagnetic stator teeth will increase the AC loss caused by the harmonic magnetic field and the load change, while the shielding layer can decrease the AC loss in a certain amount.

Сверхпроводниковые ветрогенераторы мощностью 10 МВт и более

The overall dimensions and mass of wind power units with capacities larger than 10 MW can be improved and their cost can be decreased by developing and constructing superconducting synchronous generators. The article analyzes foreign conceptual designs of superconducting synchronous generators based on different principles: with the use of high- and low-temperature superconductivity, fully superconducting or only with a superconducting excitation system, and with the use of different materials (MgB2, Bi2223, YBCO). A high cost of superconducting materials is the main factor impeding commercial application of superconducting generators. In view of the state of the art in the technology for manufacturing superconductors and their cost, a conclusion is drawn, according to which a synchronous gearless superconducting wind generator with a capacity of 10 MW with the field winding made of a high-temperature superconducting material (MgB2, Bi-2223 or YBCO) with the «ferromagnetic stator — ferromagnetic rotor» topology, with the stator diameter equal to 7—9 m, and with the number of poles equal to 32—40 has prospects for its practical use in the nearest future.

Core Loss of a Bulk HTS Synchronous Machine at 2 and 3 T Rotor Magnetisation

The large magnetic fields that bulk high-temperature superconductors (HTS) can trap, enable us to develop high specific power electric machines. However, very few bulk HTS machine prototypes have been developed worldwide. To explore the potential of bulk HTS machines, we developed a 30-kW low speed radial-flux synchronous machine comprised of a conventional stator with ferromagnetic teeth. The rotor has four-field poles. Each pole is a rectangular array of QMG melt-growth GdBa2Cu3O7-δ bulk superconductors. The poles are cooled by a 150 W capacity neon thermosyphon, which maintains an operating temperature of around 30 K. The drawback of the ferromagnetic teeth, however, is their saturation and core losses under high and variable magnetic flux. Apart from high flux above 1 T, the trapped field profile of bulk HTS poles exhibits a nonsinusoidal shape, the harmonic components of which increase core losses. Such core losses are normally divided into hysteresis losses and eddy current losses, both of which depend on the magnitude and frequency of the trapped field. Hysteresis loss is associated with domain walls movement, while eddy currents are circulating currents induced in the core. In this paper, core losses in a ferromagnetic stator of the 30-kW machine are determined while the poles are magnetized to 2 and 3 T. The results show that core losses increase with increasing magnetic flux and rotor speed. Core losses due to rotor pole fields at the operating speed of 190 r/min were 1.2% and 1.9% of the machine rated output under 2 and 3 T magnetization, respectively. Finally, the harmonic content of the machine output voltage was obtained.

Design and Development of a Self-Calibration- Based Inductive Absolute Angular Position Sensor

This paper presents an inductive-based sensor for absolute angular position measurement. Its structure is basically similar to a resolver, but it has two groups of pickup coils that use the same magnetic field to produce two different signals for absolute measurement and online self-calibration. The proposed sensor mainly consists of a ferromagnetic stator, two ferromagnetic rotors, and four groups of coils. Two groups of coils in the stator working as excitation coils are supplied with quadrature alternating current (AC) signals to generate quadrature magnetic fields both in temporal and spatial domains. The other two groups of coils in the rotors working as pickup coils receive magnetic fields to produce two sine signals whose phases are proportional to the displacement of the rotors. However, the phases of the sine signals have different periods of variation in the full measurement range. The sensor uses this different phase variation to determine the absolute angular position and perform the online self-calibration. A sensor model has been built and simulated first to verify the feasibility of the sensor. Then, a sensor prototype was developed based on the sensor model. The experimental results of the sensor prototype show that the sensor has big but regular measurement error using any group of pickup coils before conducting calibration, and finally, the measurement error reduced up to 85% by use of the online self-calibration method presented in this paper.

Compensation of Hysteresis in Hybrid Reluctance Actuator for High-Precision Motion

In hybrid reluctance actuators for high precision motion control, the magnetic hysteresis in the ferromagnetic stator and mover usually affects the precision. Therefore, this paper proposes magnetic flux estimation and control to compensate for the hysteresis. To measure the magnetic flux at various frequencies, the output signal of a search coil is combined with that of the current monitor of the employed current amplifier in a magnetic flux estimator. Finally, a magnetic flux controller is designed and implemented. For evaluation of the hysteresis compensation, the performance of the hybrid reluctance actuator is compared with both flux and current control. Experimental results show that flux control successfully compensates for some of the nonlinearities of the hybrid reluctance actuator, as the hysteresis is reduced by up to a factor 8.2 and the phase lag of the mover position is reduced by up to 6 deg in comparison with the case of current control.

An Inductive Linear Displacement Sensor Based on Planar Coils

This paper presents a non-contact inductive linear displacement sensor which is based on planar coils realized by the printed circuit board (PCB) technique. The sensor mainly consists of a ferromagnetic stator, a ferromag-netic mover, several excitation coil-PCBs, and a pickup coil-PCB. Excitation coil-PCBs are embedded in the mover and generate quadrature magnetic fields both in temporal- and spatial-domains. The stator and mover confine the magnetic fields to propagate between them. Pickup coil-PCB is located between the stator and mover to receive the magnetic fields and outputs a signal whose phase is proportional to the displacement of the mover. Simulations of a 3-D sensor model and final experiment of a prototype are introduced and both verify the feasibility of this sensor. In order to overcome inexact quadrature in spatial domain of magnetic fields caused by asymmetric mover structure, a simple but effective compensation method realized in temporal domain is employed by the prototype. Experimental results show that the prototype has original measurement error of less than 50 $\mu \text{m}$ in the range of 0 –208 mm, and moreover, the error is still of great regularity so that the sensor may have better performance if there is digital calibration for measurement data or improvement in sensor structure.

The Programmable Permanent Magnet Actuator: A Paradigm Shift in Efficiency for Low-Speed Torque-Holding Robotic Applications

In this letter, we propose a new type of electromagnetic actuator called the “programmable permanent magnet” (PPM) actuator. The PPM actuator is designed to tackle low-speed high-torque applications where classic electromagnetic actuators are highly inefficient. The PPM actuator is based on the application of pulsed current to magnetize a hard ferromagnetic stator that is made of individual SmCo (custom grade) magnets placed in a Halbach array configuration. The motor creates a springlike torque function that generates up to 0.2 N·m. Unlike electromagnetic actuators, by design, the PPM actuator does not draw current while exerting torque. Instead, the motor's power consumption depends on its rotational speed or torque variations. The PPM actuator is, by consequence, most efficient, whereas classical electromagnetic motors are most inefficient, and is a new alternative to actuators for robotic applications, which tend to operate in low-speed, high-torque conditions. The prototyped PPM actuator measures 47 mm in diameter, with a height of 35 mm and weight of 200 g, and has been successfully integrated in a commercially available 2-finger gripper from Robotiq, Inc.

An Inductive Angular Displacement Sensor Based on Planar Coil and Contrate Rotor

This paper introduces a noncontact, low-cost, and reliable inductive sensor for angular displacement measurement. It is suitable to work in harsh industrial environments. The sensor has simple structure consisting of three key parts: a ferromagnetic stator, which is a pure plane; a ferromagnetic rotor, which has face slots; and four layers of planar copper coils, including primary and secondary coils. Primary coils are supplied with two orthogonal 4000-Hz ac, and secondary coils output a signal whose phase is proportional to angular displacement. Primary coils are designed with sinusoidal shape, so that magnetic field between the stator and rotor has approximately sinusoidal distribution, and finally linearity between the phase variation of output signal and angular displacement is well helped by this design. The structure and working principles of the sensor are explained in detail. Moreover, a sensor model was simulated to verify the feasibility of the sensor working principles and a sensor prototype was designed for actual experiment. At last actual experiment results are given and analyzed, showing that the sensor prototype has achieved accuracy better than ±12 arcsec in the range of 0°-360°, and this kind of sensor may have a better performance by improving the layout of primary coils and front-end signal-process circuit.

Calculation of the Dynamic Characteristics of a Magnetic Gear Based on Reluctance Network Analysis

This paper presents a method for calculating the characteristics of a magnetic gear on the basis of reluctance network analysis (RNA). The magnetic gear has inner and outer permanent magnet rotors, and has ferromagnetic stator pole-pieces arranged between the two rotors. The inner and outer rotors rotate in opposite directions with a certain gear ratio determined by the number of pole-pairs of the rotors. This magnetic gear has a higher performance than others because all the rotor magnets always contribute to the transmission of torque. The steady-state characteristics of the magnetic gear are calculated by RNA, and are then compared to the results obtained by the finite element method (FEM). Finally, the dynamic behavior of the magnetic gear is simulated by RNA.

Evaluation of the normal force of a planar actuator

This paper describes a method to compute the normal force between the mover and stator of an electromagnetic planar actuator. The method is based on the Maxwell stress tensor and depends on the resulting magnetic field. The actuator has a mover with two permanent magnets and a ferromagnetic stator covered by two orthogonal armature coils. The mover relies on a suspension system with linear bearings that enable bi-directional motion. When one permanent magnet is located over an excited coil phase, a magnetic propulsion force will act on the mover and that pushes the latter accordingly. A normal magnetic force is an intrinsic effect and determines the mechanical load the suspension system sustains. It represents an attraction force between the permanent magnets and the armature core that may affect the mechanical structure of the actuator and its movement.