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Refereed/Peer-reviewed This article proposes a two-stage optimization algorithm to effectively determine the system configuration at one stage, as well as the capacity of components at the other stage in the middle of the former. This algorithm fits in best with the hybrid systems with more possible types of components. The studied system, in this article, includes diesel generators, wind turbines, photovoltaic arrays, and tidal generators as the power generation components, as well as battery banks and flywheels as the energy storage components. It also includes fuel cells and electrolyzers that either work as batteries or generate electricity in the presence of biomass. When the number of components (decision variables) increases, it becomes difficult to find an optimal solution by the conventional methods. Therefore, in this study, a two-stage multiobjective optimization algorithm is applied to design a cost effective and environmental friendly ac minigrid hybrid power system. In each iteration of the proposed algorithm, first, renewable energy sources and energy storage components are selected to form a hybrid power system along with the diesel generator. Then, the capacities of the components are optimized based on the two objective functions, including levelized cost of electricity and emissions. The optimization model uses real annual data in hourly time intervals for the load, solar insolation, ambient temperature, and wind speed. It is found that the proposed algorithm decreases the susceptibility of the solutions to multiple runs compared with the conventional algorithms and achieves a better optimized power system design.

Refereed/Peer-reviewed This paper proposes two structures of dual-stator permanent-magnet vernier machines (VMs) for high-torque low-speed applications. The proposed structures consist of dual-sided rotor which is sandwiched by inner and outer stators. These topologies include 22 and 46 consequent-pole magnets in the rotor and 24 and 48 stator slots for Design A and Design B, respectively. Design A is an improved structure of dual stator VM and Design B is a novel topology of a dual stator VM that can deliver considerably higher back-EMF and torque than Design A. The only drawback in Design B is lower power factor. The characteristics and performance parameters of the proposed VMs are evaluated using FEA. Sensitivity analysis is used to figure out the optimum dimensions of the magnets. In fact, two combinations of slot/pole number of the vernier machines are investigated as Design A and Design B. The capabilities of the proposed vernier machines are presented in a fair comparison with other VMs namely as consequent pole dual rotor VM (CPDRVM), consequent pole dual stator VM (CPDSVM) and dual stator spoke array VM (DSSAVM). In this regard, the key machine features considered for comparison are back-EMF, torque, torque per magnet volume (TPMV), power factor, loss, and efficiency profiles. The proposed structure (Design B) generates higher torque and TPMV compared to the other studied machines while Design A can achieve a higher power factor.

Efficiency maps (EffMs) illustrate the maximum efficiencies of electric machines over a range of torque-speed operating points. This paper reviews the available techniques for efficiency map calculation and gives a general understanding of EffM modelling and interpretation for traction motors. It shows the accuracy versus calculation effort trade-off for different methods. The process of efficiency map calculation for sample induction motor and permanent magnet synchronous machines is explained. Calculation of EffMs using commercial software is also covered. Finally, possible future research opportunities in this field are suggested. Refereed/Peer-reviewed

Refereed/Peer-reviewed In this article, the analysis of induction machines using five separated subdomains including the rotor slot, rotor slot opening, airgap, stator slot opening, and stator slot regions is considered. The Maxwell equations of each region are solved considering the boundary conditions. Uniform distributions of the current density in the rectangular slots of both the rotor and stator are considered to find the energy of the winding and to calculate the machine inductances using the energy equation. The calculated flux density of each subdomain is used to find the flux distribution. The calculated magnetic flux intensities are required to estimate the saturation level of the motor cores and correct the saturation-related values such as the leakage and magnetizing inductances. The machine core losses are calculated based on the Steinmetz equation at the no-load condition. The calculated inductances and resistances are used in the equivalent circuit of the squirrel-cage induction machine (SCIM) for the performance prediction. The results demonstrate the reliability of the saturation model in the prediction of the saturation influence in a wide range of magnetization levels. The accuracy of the proposed model in the prediction of the performance of the SCIM is validated using two-dimensional (2-D) and 3-D finite-element analysis results. A sensitivity analysis with 75 different geometries was conducted to show the capability of the model for the performance prediction of a wide range of induction machines.

The bond-graph (BG) enables to model multi-physics systems in the energy domains. This paper shows that BG models are an alternative, more intuitive and simpler to interpret, graphical tool as compared to existing dynamic models for performance prediction of the line-start permanent-magnet synchronous motors (LSPMSMs). It includes electrical, magnetic, and mechanical energy domains where the stator windings, rotor bars, and permanent magnets are the main components of the model. The dynamical equations of the proposed BG are extracted and solved using the MATLAB ordinary differential equation solver for a sample LSPMSM. The dynamics of the studied LSPMSM is investigated under two operating conditions (1) a healthy motor and (2) a motor with broken rotor bars. The simulation results are compared with the finite-element analysis (FEA) as the most accurate method to simulate the electric motor behaviour. It is shown that the proposed BG model accurately predicts the performance parameters of the motor such as current, torque, and speed under various operating conditions. In addition to the modelling validation, the transient behavior of the LSPMSM are discussed to describe the possible reasons for its behaviour during transient and steady-state operating conditions.

Refereed/Peer-reviewed This paper proposes a quasi-resonant step-up DC-DC converter to provide the DC-link voltage for piezoelectric transmitters (PZETs). The resonance not only provides a soft-switching condition for the converter switches, but also helps to decrease the converter element sizes for marine applications. Operation modes of the proposed converter are discussed. The current and voltage of the converter components are derived analytically, and hence the converter gain is obtained. The performance of the proposed high-step-up, high-power density converter is examined through a comprehensive simulation study. The simulation results demonstrate the soft-switching operation and short transient time required for the converter to reach the reference output voltage. The converter output voltage remains unchanged when an inverter with a passive filter is placed at its output while supplying the PZET. The proposed DC-DC converter is prototyped to validate the converter gain and soft-switching operation experimentally. The converter gain and soft-switching results in simulation are well matched with those of the experimental tests. The converter efficiency in three different switching frequencies is obtained experimentally. The power density of the proposed topology is determined via the designing of a printed circuit board. The experimental results demonstrate the appropriate gain and efficiency of the converter in the tested power range.

Refereed/Peer-reviewed This article presents the optimization of axial-flux induction motors (AFIMs) for the electric vehicles application by solving the Maxwell's equations inside five subdomains. It allows a detailed modelling and accurate steady-state performance prediction of the AFIMs in a short time (under 1 sec). The analytical model is verified against 2D finite-element analysis. Different design options of the AFIM are considered and then optimized based on the proposed analytical model with the aim of minimizing the motor's volume and satisfy a set of predefined performance parameters for operation in a nominal point and also over a driving cycle. For operation in a nominal point, three power levels and for the driving cycle based operation, three driving cycles are considered. In each operation state, maximum torque per ampere (MTPA) constraint is enforced by changing the number of winding turns per slots. Alternatively, similar optimizations are performed without MTPA constraint by considering the number of winding turns as a variable. It is observed that motor design based on the driving cycle benefits from a lower volume while it is feasible for all the operating conditions. Enforcing the MTPA in design leads to a wider torque-speed operating range and simpler control in the expense of additional volume.

This article investigates the accuracy of the estimation of efficiency maps for permanent magnet (PM) machines using the stator resistance, d - and q -axis flux-linkages versus the corresponding axis current and the iron loss versus the speed characteristic. The ultimate goal is to apply this approach to the experimental measurements, but this article performs initial investigation using only the finite-element (FE) data. Detailed FE data for 50-kW surface PM (SPM) and interior PM (IPM) machines are used to determine the “actual” or exact efficiency map and, hence, the accuracy of using approximations. This article examines the effect on the torque–speed capability curve when ignoring cross-saturation effects. It also examines the modeling of the variation of iron losses as a function of load in the constant torque and power regions. A novel approach based on scaling the no-load (NL) losses as a function of load is proposed and shown to give promising results. FE results from two other machines are also provided, which show good correspondence. Refereed/Peer-reviewed

Refereed/Peer-reviewed Decaying DC (DDC) offset current mitigation is a vital challenge in phasor current estimation since it causes malfunctioning/maloperation of measurements and protection systems. Due to the inductive nature of electric power systems, the current during fault inception cannot change immediately and it contains a transient oscillation. The oscillatory component acts similar to an exponential DC signal and its characteristics depend on the X/R ratio of the system, fault location, and fault impedance. DDC attenuates accurate phasor estimation, which is pivotal in protection systems. Therefore, the DDC must be eliminated from the fault current (FC) signal. This paper presents an overview of DDC mitigation methods by considering different groups—before the discrete Fourier transform (pre-DFT), after the discrete Fourier transform (post-DFT), the least square-based (LS-based), and other methods. Through a comprehensive review of the existing schemes, the effects of noise, harmonics, multiple DDCs (MDDCs), and off-nominal frequency (ONF) on the accuracy of DDC estimation, were recognized. A detailed discussion (along with some simulation results) are presented to address the main advantages/disadvantages of the past studies. Finally, this paper presents a few suggestions for future researchers, for researchers to investigate more implementable solutions in this field.

This paper presents an improved analytical model for axial-flux induction motors (AFIMs) based on solving the magnetic vector potential equations. The advantage of the proposed approach is its fast simulation time (fractions of a second against minutes/hours in FEA) while maintaining over 95 % accuracy compared to FEA that makes it suitable for an optimization purpose. This method applies a uniform distribution of the current density in the rectangular shaped rotor/stator slots. The equivalent circuit parameters of the studied AFIMs are extracted using the magnetic energy analysis. The proposed model allows the saturation effect in calculation of the motor performance parameters (power factor and efficiency). The accuracy of analytical model is validated using FEA results. A comprehensive design optimization is presented to show its capability for an optimization purpose. The objective functions in this study are maximum efficiency, maximum power factor, maximum efficiency times power factor, minimum mass, minimum price, maximum efficiency over slip, and maximum power factor over slip. The proposed analytical model can be easily used for AFIMs with different number of poles and phases. Refereed/Peer-reviewed

In some applications such as electric vehicles, electric motors should operate in a wide torque and speed ranges. An efficiency map is the contour plot of the maximum efficiency of an electric machine in torque-speed plane. It is used to provide an overview on the performance of an electric machine when operates in different operating points. The electric machine losses in different torque and speed operating points play a prominent role in the efficiency of the machines. In this paper, an overview about the change of various loss components in torque-speed envelope of the electric machines is rendered to show the role and significance of each loss component in a wide range of torque and speeds. The research gaps and future research subjects based on the conducted review are reported. The role and possibility of the utilization of the computational intelligence-based modeling of the losses in improvement of the loss estimation is discussed. Refereed/Peer-reviewed

This paper examines the performance of a three-phase interior permanent-magnet generator used in a portable ac generator when directly driving isolated single-phase (1-ph) ac loads. The performance of the generator was compared under three alternative connections: 1) 1-ph star; 2) 1-ph delta; and 3) a proposed "open delta" connection. The voltage regulation, total harmonic distortion, and efficiency as three main performance criteria of the portable generators were examined using analytical calculation, finite element simulation, and experimental measurements. Refereed/Peer-reviewed

This paper presents research work on a 12-slot ten-pole tapered axial-flux permanent-magnet machine utilizing amorphous magnetic material in the stator core. Novel loss separation techniques are described, including mechanical loss and locked-rotor tests. Mechanical loss estimation is based on a combination of experimental tests and 3-D finite-element model analysis using an uncut stator. The locked-rotor test is introduced to separate the stator and rotor losses by eliminating the uncertainty associated with mechanical loss. High rotor yoke losses were identified in the baseline design. The rotor design was modified and a significant improvement in efficiency was demonstrated. Refereed/Peer-reviewed

Amorphous magnetic materials (AMMs) have been used in low-frequency power transformer applications for more than two decades but not commercially utilized in high-frequency rotating electrical machines despite their low iron losses, which is an order of magnitude lower than conventional silicon iron. This was primarily due to the lack of a suitable handling method and an economical cutting technique. This paper reports a novel brushless synchronous permanent magnet rotating electrical machine that combines the benefits of the previous machine topologies while offering higher torque (due to the larger air-gap surface area compared with the flat axial-field machine) and ease of manufacturing from AMM material, which is suitable for a range of high-efficiency, high-power density, and high-speed machine applications. This paper provides the details and the performance characteristics of this novel tapered-field AMM machine as a generator at high speeds using a bonded rotor magnet topology. Refereed/Peer-reviewed

This paper compares the performance of three population-based algorithms including particle swarm optimization (PSO), evolutionary programming (EP), and genetic algorithm (GA) to solve the multi-objective optimal power flow (OPF) problem. The unattractive characteristics of the cost-based OPF including loss, voltage profile, and emission justifies the necessity of multi-objective OPF study. This study presents the programming results of the nine essential single-objective and multi-objective functions of OPF problem. The considered objective functions include cost, active power loss, voltage stability index, and emission. The multi-objective optimizations include cost and active power loss, cost and voltage stability index, active power loss and voltage stability index, cost and emission, and finally cost, active power loss, and voltage stability index. To solve the multi-objective OPF problem, Pareto optimal method is used to form the Pareto optimal set. A fuzzy decision-based mechanism is applied to select the best comprised solution. In this work, to decrease the running time of load flow calculation, a new approach including combined Newton-Raphson and Fast-Decouple is conducted. The proposed methods are tested on IEEE 30-bus test system and the best method for each objective is determined based on the total cost and the convergence values of the considered objectives. The programming results indicate that based on the inter-related nature of the objective functions, a control system cannot be recommended based on individual optimizations and the secondary criteria should also be considered. Refereed/Peer-reviewed

This paper presents the design process, detailed analysis, and prototyping of a novel-structured line-start solid-rotor-based axial-flux permanent-magnet (AFPM) motor capable of autostarting with solid-rotor rings. The preliminary design is a slotless double-sided AFPM motor with four poles for high torque density and stable operation. Two concentric unilevel-spaced raised rings are added to the inner and outer radii of the rotor discs for smooth line-start of the motor. The design allows the motor to operate at both starting and synchronous speeds. The basic equations for the solid rings of the rotor of the proposed AFPM motor are discussed. Nonsymmetry of the designed motor led to its 3-D time-stepping finite-element analysis (FEA) via Vector Field Opera 14.0, which evaluates the design parameters and predicts the transient performance. To verify the design, a prototype 1-hp four-pole three-phase line-start AFPM synchronous motor is built and is used to test the performance in real time. There is a good agreement between experimental and FEA-based computed results. It is found that the prototype motor maintains high starting torque and good synchronization. Refereed/Peer-reviewed

This paper presents a state-of-the-art review of axial-flux permanent-magnet (AFPM) machines in the aspects of construction, features, electromagnetic and thermal modeling, simulation, analysis, design, materials, and manufacturing. Some key references on the above-mentioned aspects pertaining to the machine are discussed briefly. Particular emphasis is given on the design and performance analysis of AFPM machines. A comparison among different permanent magnet machines is also provided. Thus, this paper makes a bridge between the currently used permanent magnet machines in industry and the recent developments of AFPM machines. Refereed/Peer-reviewed

This study presents the design and performance analysis of a prototype axial-flux permanent-magnet (AFPM) synchronous machine. First, the design of AFPM machine is optimised by genetic algorithm based sizing equation and finite element analysis. The design objectives of this machine are maximum power density, minimum total harmonic distortion (THD) of the sinusoidal back-electromotive force (back-EMF) waveform and low cogging torque. Based on the optimised design of the machine a prototype 1 kW, three-phase, 50 Hz, four-pole AFPM synchronous machine is built. Then, the performance of the prototype machine is tested to see the cogging torque, torque-speed characteristic, efficiency and the THD of the induced voltage. It is found that the prototype machine validates the design in terms of high-power density, lowest possible THD of the back-EMF, low cogging torque while maintaining high efficiency. Refereed/Peer-reviewed

This study presents the design, analysis and prototyping of a low-cost monopole machine capable of battery charging. The designed monopole machine encompasses a rotor disc including eight permanent magnets with the same magnetising direction and a stator consists of a magnetic core and three windings: power coil winding, trigger coil winding and recovery coil winding. This machine includes an electronic circuit as a trigger and a magnetic circuit that converts electrical energy to mechanical energy and vice versa. The machine captures energy in the form of back-electromagnetic force generated by a collapsing field in the stator and rotor to avoid wasting energy and stores it in a battery. To simulate the machine, electromagnetic and electrical analyses were conducted simultaneously to predict the design parameters and machine performance: three-dimensional finite element analysis via Vector Field Opera, and MATLAB/Simulink. To validate the simulation results accuracy, the machine was fabricated and tested. Experimental data were compared with simulation results, and showed good agreement with that of simulation. Refereed/Peer-reviewed

This paper presents the design and analysis of an inside-out axial-flux permanent-magnet (AFPM) synchronous machine optimized by genetic algorithm (GA) based sizing equation, finite element analysis (FEA) and finite volume analysis (FVA). The preliminary design is a 2-pole-pair slotted TORUS AFPM machine. The designed motor comprises sinusoidal back-EMF waveforms, maximum power density and the best heat removal. The GA is used to optimize the dimensions of the machine in order to achieve the highest power density. Electromagnetic field analysis of the candidate machines from GA with various dimensions is then put through FEA in order to obtain various motor characteristics. Based on the results from GA and FEA, new candidates are introduced and then put through FVA for thermal behavior evaluation of the designed motors. Techniques like modifying the winding configuration and skewing the permanent magnets are also investigated to attain the most sinusoidal back-EMF waveform and reduced cogging torque. The performance of the designed 1 kW, 3-phase, 50 Hz, 4-pole AFPM synchronous machine is tested in simulation using FEA software. It is found that the simulation results fully agree with the designed technical specifications. It is also found from FVA results that the motor temperature reaches at highest temperature to 87 degrees C at the rated speed and full load under steady state condition. Refereed/Peer-reviewed

For portable ac electric generator sets, it is desired to maintain a constant output voltage under load. With wound-field synchronous generators, this is done using field current (excitation) control. This type of control is not possible with permanent magnet (PM) machines; however, theoretical analysis is used to show that acceptable output voltage regulation can be obtained by two alternative design approaches for interior PM machines. The design of an example 4-pole 16-kW machine is described, and experimental results are provided, demonstrating its sinusoidal output waveform, high voltage regulation, and high efficiency. Refereed/Peer-reviewed