Hassan H. Eldeeb

A gravity energy storage system (GESS) typically adopts the synchronous generator-motor (SGM) that is directly connected to the grid, in order to provide inertia and reactive power support to power systems with high penetration of renewable energy. However, due to the lack of line-starting capability, it is a critical challenge for a GESS to meet the frequent start-stop requirements in practice. To solve this problem, an SGM starting method based on mass block (MB) dragging is proposed in this study, for a chain-rail-based slope GESS (SGESS) as example. Through the analysis of the loss components of GESS, an optimal mass determination method for the starting MB is derived. The MB-dragging-based starting of 1-kW and 500-kW chain-rail-based SGESSs are then simulated and analyzed, and the dynamic grid-connection characteristics of the SGM started by different starting MBs are compared. Finally, the MB-dragging-based starting of the 1-kW chain-rail-based SGESS prototype is tested for validation, and an experiment bench of SGM-grid connection is established to emulate the starting and grid-connection processes and evaluate their impacts on the grid. The experiment results demonstrate that the proposed method suppresses the inrush current successfully and enables smooth starting of SGM.

Nanocrystalline alloy is considered as the potential alternation to ferrite due to its higher saturation flux density and better ductility in application of loosely coupled transformers (LCT) in wireless power transfer systems. However, to reduce the eddy current loss, it generally needs crushing and compressing processes, which may lead to obvious nonlinear characteristics and affect both equivalent conductivity and permeability. In this study, the influence of the nonlinear characteristic on the maximum output power of LCT without saturation is first revealed, at the same time, the influence of the lamination coefficient of nanocrystalline core and its equivalent conductivity on the efficiency of LCT are also analyzed by finite element analysis. Then, a determination method of critical conductivity for LCT with the nanocrystalline core is also proposed. On this basis, a selection method of nanocrystalline material for LCT in the wireless power transfer system is also proposed considering nonlinear characteristics and laminated structure. Experimental validation was performed on 3-kW and 7.7-kW prototypes, and the results show that the nanocrystalline core with higher permeability approaches saturation more quickly as the primary current increases, and the critical conductivity determination method is also verified. In addition, with identical initial permeability, the higher efficiency of LCT can be reached by reducing the lamination coefficient.

For an inductive power transfer (IPT) system, ferromagnetic materials such as ferrite and nanocrystalline are generally adopted to enhance the coupling and misalignment tolerance between the primary and secondary coils. However, it may decrease the transmission efficiency of IPT system due to the different magnetization and loss characteristics of ferromagnetic materials. Therefore, it is necessary to investigate material performance accurately under the real scenario of high frequency supply. In this study, a mathematical model of the IPT system is firstly established and a new test method combining 3-D finite element analysis (FEA) for magnetization and loss characteristics of ferromagnetic materials in IPT system is proposed and the loss variations of different ferromagnetic materials are also revealed. With the measured magnetization and loss data, misalignment tolerance of magnetic coupler with different ferromagnetic materials is investigated by 3-D FEA. It is found that, when the magnetic field conforms to the paramagnetism of the magnetic material, misalignment tolerance can be improved effectively and the higher efficiency of magnetic coupler can be reached under misalignment conditions. Experimental validation is performed on a 1.2-kW coupler prototype under 200-mm misalignment between primary and secondary coils. With the ferrite and nanocrystalline cores of horizontal and vertical laminations, efficiency of magnetic coupler decreases by 0.457%, 1.392% and 1.352%, respectively, and misalignment tolerance of coupler can be improved effectively.

Over-temperature of the rotor may cause a series of insulation deterioration of electrical machines. In severe cases, it can lead to insulation failure and even faults, such as broken rotor bars and end ring fracture. The temperature online monitoring can effectively avoid the aforementioned insulation failure; however, the conventional temperature measurement by embedded sensors is difficult to utilize due to the power and data transmission of sensors at the rotor side. This paper proposes a contactless strategy to supply power and send rotor temperature data between the stator and rotor sides of electrical machines. With the proposed method, contactless power transfer is realized by means of magnetically-coupled resonance, while the data signal is transferred by capacitive radio frequency (RF) technology. An experimental prototype was developed and installed on a 22-kW induction motor. The results show that the embedded sensors are contactless powered effectively. At the same time clear temperature data signal can be received on the stator side without filtering, and rotor temperature can be monitored in real-time accurately.

In an inductive power transfer (IPT) system, the magnetic core is usually used to improve the coefficient between the primary and secondary coils. However, considering the iron losses on magnetic materials, the addition of magnetic core may affect the system efficiency and, at the same time, increase the weight of the IPT system significantly. Therefore, in this study, the aggregate influence of thickness, permeability, and iron losses of magnetic core on IPT system performance is comprehensively investigated. First, the equivalent circuit model of IPT system is established, and the formulas for different types of losses are analyzed. Then, the three-dimensional models of the IPT system with magnetic core in different shapes are established to simulate the aggregate influence of core thickness, permeability, and iron losses. The simulation results reveal that reducing the core thickness requires a larger saturation flux density of magnetic materials. In addition, only if the material losses below a certain value and an appropriate core shape are adopted, the addition of magnetic material can improve the system efficiency. Finally, experimental validation on a 3-kW IPT system is performed under the conditions of no magnetic core, using ferrite core in radial pattern and hollow square shapes, and using nanocrystalline core with the thickness of 22 μ m and 16 μ m in radial pattern and hollow square shapes, respectively. The results show that the high efficiency and lower overall weight of the IPT system can be achieved when nanocrystalline core with suitable permeability and shape is used.

For an inductive power transfer (IPT) system used in electric vehicle (EV) charging, the receiving coil is usually equipped with a shielding structure to reduce iron loss and electromagnetic interference in the chassis. However, the addition of a shielding structure may significantly reduce self-inductance and mutual inductance, and the eddy current on the shielding pad can also produce power loss. To maximize the efficiency of the IPT system used for EV charging and sufficient shielding, this study first analyzed the equivalent circuit model of the IPT system. A 3-D model of an IPT system, with a steel plate and aluminum shielding, is established and investigated in the finite-element analysis (FEA) domain, in which the shielding structure is modeled with some parametric sizes to simplify the modification of the parameters on a shielding structure. The simulation result shows how the ohmic loss on coils and the iron loss on steel plate changes after adding the shielding structure. Afterward, a practical method for optimizing aluminum shielding structure's parameters is proposed to maximize the efficiency of the IPT system, taking iron losses in a steel part and aluminum shielding into account. Furthermore, considering that the optimization procedure becomes complex since the different resonant capacitances are required due to the variable inductances, a new efficiency calculation method by FEA is presented to simplify the optimization procedure. Experimental validation is finally performed on a 3-kW wireless charging prototype; thus, the effectiveness of the proposed method is verified.

Due to the complex harmonic field effects and frequent starting, broken rotor bars (BRB) caused by hotspots at the rotor slot openings are the most common fault in high-voltage and large-power induction motors (IMs) applied to the auxiliary systems of the power plant. This article proposes an improved rotor bar shape of large-power and high-voltage IMs to eliminate the hotspot and avoid the BRB. With the presented rotor bar shape, the skin effect caused by the high-frequency harmonic fields can be weakened effectively. The optimal design of the presented rotor slot shape, the width and height of the slot opening, as well as the air-gap length are also performed by finite-element analysis to achieve the best operating performance of IMs. A 6-kV 1600-kW IM was remanufactured according to the optimal design and applied to the circulator pump in a thermal power plant. The test results showed that the stator winding temperature of the IM was reduced from 80.7 to 62.1 °C; meanwhile, the surface temperature of the motor cooler was also reduced obviously.

With the emerging power electronic technologies, numerous electrical devices use high-frequency and nonsinusoidal supplies. Such sources generate complex harmonic fields that overburden the accurate prediction of core losses using the conventional finite element method (FEM). Thereby, to predict core losses under those high-frequency and nonsinusoidal supplies accurately, this study first built a test system for core losses measurement of ferromagnetic materials (e.g., soft ferrite, laminated steels, and nanocrystalline alloy) fed by the square and pulsewidth modulated (PWM) supplies. Then, the loss prediction problems of the classical core loss model under high-frequency and nonsinusoidal supplies are revealed. Afterward, an improved FEM-based core loss model is proposed, which includes a piecewise variable parameters based hysteresis loss model, an eddy current loss model considering different cross sections of magnetic circuit and supply waveforms, and improved excess loss coefficients. Experimental validations on different materials, the nanocrystalline alloy in an 11-kVA intermediate frequency transformer, the ultrathin silicon sheets in a 10-kW high-speed permanent magnet motor, and the nanocrystalline alloy in a 3-kW wireless power transfer system, are conducted systematically. The effectiveness and generality of the improved core loss model were verified.

This study investigates the influence of the stator’s turn-to-turn failures (TTFs) on the electromagnetic (EM) fields, such as air gap flux density, flux density in the stator, and rotor iron core inside of direct self-control (DSC) driven induction machines (IMs). The purpose of the investigation is to capture the fault signatures in the air gap EM flux for detecting the stator’s fault at its embryonic stage.

This article proposes a rotor temperature estimation method for in-service induction machine (IM) based on parameter identification, which combines the advantage of recursive least squares (RLS) and model reference adaptive system (MRAS). The RLS with forgetting factor is firstly adopted to identify the parameters of motor inductances. Then, the online identification of rotor resistance can be realized via the MRAS based on the instantaneous reactive power (IRP) and the proportional integral (PI) regulation adaptive law designed by the Popov hyper-stable theory. Thereby, the rotor temperature is computed by the resistance-temperature relationship of metals. To achieve a fast convergence of the parameter identification, rotor slot harmonics are extracted from the stator current and used to determine the rotor speed in real time. Furthermore, to obtain a more accurate initial value of the stator and rotor leakage inductances and resistances, the first 5-15 cycles of the IM starting process are used to mimic the locked-rotor test condition. A merit of the proposed method is that it requires significantly less time to estimate the rotor resistance than the traditional methods. With a novel test bench, experimental validation is performed on a 22-kW IM. In the test, the rotor temperature is measured in three different ways: 1) wireless sensors inserted in the rotating rotor core and rotor end rings, 2) infrared sensor for rotor end ring temperature, and 3) PT100 installed in stator end winding. With this test bench, the real rotor temperature was revealed, and the effectiveness of the presented method is also verified.