Brushless doubly-fed induction machines (DFIMs) have great potential as variable-speed generators in wind turbines. Undesired space-harmonics exist because the special rotor needs to couple to two stator windings with different pole-pair numbers and different frequencies. These undesired space-harmonics could lead to noise, vibrations and low power quality. Applying skewed slots can overcome the above drawbacks. Previously, a two-dimensional (2D) multi-slice finite element (FE) method was applied to study the effects of skew which was time consuming. In the stage of initial design, it is not efficient to use such a model to predict how much the average torque and the torque ripple would be reduced by skewing slots. This study makes use of normal 2D FE results and applies skew factors in post-processing to investigate the influence of rotor skew. It also proves that to apply skew factors appropriately, not only the space-harmonic order needs to be considered, but also the time-harmonic order. The proposed method can give an approximate prediction of skew effects with limited computing time. The results also indicate that skewing the rotor slot over one stator slot pitch could be a good choice to minimise the torque ripple in a brushless DFIM with a nested-loop rotor.

An analytical model is proposed for the prediction of the no-load air gap magnetic flux density and the armature reaction in the slotless surface-inset PM machines. For this purpose, the exact 2D solution of the Poisson equation is derived. In the modelling process, the rotor salient poles are taken away and some surface magnetisation currents are considered at the borders of the removed salient poles. The contribution of this work is finding the value of the surface magnetisation currents such that the rotor saliency is accurately considered. The field solution in the provided surface-mounted PM machine is simply obtained by the separation of variables method. The machine back-EMF and its inductances is obtained by the predicted flux density due to the PMs and the stator currents, respectively. In addition, using the resultant air gap flux density in the Maxwell stress tensor, the developed electromagnetic torque is computed. Finally, the ability of the proposed model is evaluated by finite element analysis as well as experimental tests.

The frequency domain dielectric response method has been one of the most effective approaches for condition assessment of an electrical insulation system. The low-frequency part is more effective for diagnosing the moisture content and aging status of oil-paper insulation. To facilitate the study of the low-frequency dielectric spectrum of oil-paper insulation, a model applicable to the low-frequency dielectric response of insulation oil is proposed in this study, which is based on the ionic motion of insulation oil. The creation of charge carriers under field test conditions mainly derives from the dissociation of ionic pairs. During testing, ionic motion of insulation oil is jointly affected by thermal diffusion and the electric field; moreover, dissociation and recombination of negative and positive ions also occur. Therefore, the above factors were all considered in the modelling process. The applicability of the model is verified against measured results. For further interpreting the proposed model, the characteristics of the ionic concentration distribution, the relation between ionic distribution and dielectric properties, and the application of the proposed model are analysed.

The vibration caused by torque ripple may degrade the performance of drivetrain, thus the reduction of torque ripple are necessary for hybrid electrical vehicle/electrical vehicle application, while an analytical description of torque will be the basis. In this study, two-dimensional Fourier series supplemented by polynomial fitting is introduced to reconstruct the numerical solution of magnetic coenergy (MCE) from finite element analysis. An analytical torque model, which can describe torque ripple precisely at all working points, is derived from the reconstructed MCE. On the basis of the torque model, the expressions of harmonic currents used to reduce torque ripple are obtained. The proposed expression also satisfies the principle of maximum torque per ampere. Then, a feedforward torque controller based on the expression of harmonic currents is introduced. Meanwhile, a feedback torque controller based on a torque observer is developed as a comparison. The results of simulation and experiments show that both two controllers can reduce the torque ripple in steady state, but the feedforward torque controller has obviously better effects when motor speed increases. Furthermore, the feedforward torque controller is also beneficial to decline torque ripple during transient process of torque response.

This study presents analytical formulation used for design of helical coil for induction heating of solid and hollow cylindrical non-magnetic workpiece (graphite crucible). This formulation is applied for estimation of the active power in graphite crucibles and reactive power drawn by the induction coil in presence of different wall thickness (30, 20, 10 and 5 mm) of graphite crucibles at different frequency (1–50 kHz). Magnetic field produced by induction coil is estimated by using empirical factors suggested by kNagaoka et al. Induction heating processes, i.e. electromagnetism and heat transfer physics are mathematically modelled by Maxwell and Fourier equations, respectively. Finite-element method is used to solve the electromagnetic field in frequency domain and heat transfer in transient domain. Numerical analysis is carried out by using two-dimensional axisymmetric geometry. Analytical results and numerical results are compared and are found to be in good agreement with each other. Experimentally obtained coil voltage and graphite crucible temperature were compared with numerical results and the authors found that they also match very well confirming the validity of their assumptions in analytical/numerical approach.

In this study, a novel idea to drive SRM from single-phase AC supply is presented. Each stator winding is connected in series to thyristor to form one branch. All branches are connected in parallel to an AC supply. Thereby, a magnetic flux is generated between each pair of stator pole portions when a current is supplied to stator coil and repeated many cycles as long as the rate of change of inductance of stator phase is positive. The number of cycles of phase current depends on the ratio between rotor speeds to the supply angular frequency in radian per second. A magnetic attractive force occurs between rotor and stator pole portions as they approach one another, which produces motoring torque controlled by controlling switching delay angle of the switch. The advantage of this drive is its lower cost because the usage of simple shaft encoder and one thyristor per phase and its associated firing circuit, while the disadvantage of this drive is the more torque ripple and the deterioration of torque at high speed, and like most machine drives the supply current has high total harmonic distortion. A simulation model is presented and verified by experimental rig for two-phase SRM.

This study proposes a two-term piecewise variable parameter model for precise prediction of iron losses in induction motors. In this model, two additional flux density terms reflect the non-linear magnetisation and the harmonic fields in the motors, and the main loss parameters are piecewisely variable with the magnitude and frequency of flux density, so that higher accuracy of iron loss prediction can be achieved for a large range of flux density frequencies and magnitudes. Meanwhile, the fine analysis of the fundamental and harmonic iron losses can also be carried out with the proposed model, and the influence of the harmonic fields on the hysteresis and eddy current losses can be revealed. Experimental validations are carried out on the laminated steels, DR510 and DW470, as well as, the Y132S-4, 5.5 kW and YX3-250M-4, 55 kW induction motors whose cores are made of the above steels. The experiments on the motors are performed with the voltage supply of different magnitudes, and both the proposed model and Bertotti model are used to predict the iron losses. It is shown that the iron losses predicted by the proposed model agree well with the test results over a wide range of supply voltages. This study contributes the technical support for the development of new premium efficiency motors.

Introducing high-temperature superconductors (HTS) and their outstanding properties stimulates scientists and engineers in various fields to modernise the products with this new technology. Electric power industry also was affected by this finding in different branches including bearings. The idea of HTS bearings was rapidly developed due to its two main features, i.e. self-stability and contactless operation which leads to save remarkable amount of energy. However, a successful implementation of HTS bearing undoubtedly requires an accurate modelling in the beginning. The contribution of this study is an analytical method for simulating the electromagnetic behaviour of a radial HTS magnetic bearing. The governing non-linear system of partial differential equations in the HTS bulks deduced from the H-formulation along with the E–J power law and is efficiently solved using variational iteration method (VIM). The validity of VIM is ascertained by comparing the results with numerical two-dimensional axisymmetric finite-element method simulations.

An optimal design for the interior permanent magnet (IPM) motor is proposed to reduce vibration and increase average torque, based on simultaneous consideration of the rotor d-axis notch shape, the stator chamfer angle, and the magnetisation direction. The notch shape of the rotor and the chamfer of the stator tooth shape were first reviewed in order to reduce the vibration of the IPM motor; however, the optimal shapes for reduced vibration also reduced the average torque. Then, a design optimisation for average torque improvement was performed considering the magnetisation direction and the vibration reduction based on the modelled stator and rotor shapes. Finally, the validity and superiority of the optimised design were confirmed by manufacturing and testing a prototype motor.

Forced convection of insulation oil is the main heat transfer mode in large power and ultra-high voltage (UHV) transformers. The electrification risk caused by friction between mineral oil and pressboard has been suspected to be responsible for several failures. The electrification measurement was carried out in a rotation system. The influences of the oil temperature, rotation speed, time, and applied electric field of fresh and dry transformer oil have been investigated by a measurement system, respectively. A comparison between these results has been made, and the related theory can be used to explain the physical and chemical processes which take place on the interface between the oil and pressboard. The results from the rotation system indicate that interface charge density increases with the increase of the pressboard thickness, rotation time and speed, but decreases with the increase of the temperature. The electrification phenomenon can lead to reduction of the oil/pressboard breakdown strength. It is concluded that some effective measures should be taken to restrain the possible harm caused by the streaming electrification in UHV converter transformer.

The secondary is an important design factor of single-sided linear induction motors (SLIMs), as well as has influence on the cost and performance of SLIMs. In this study, the cap-lamination secondary is designed and relations of the secondary parameter and performance of SLIMs are calculated. First, the aluminium (Al) cap and lamination back iron of the cap-lamination secondary are designed and discussed. Then, improvements in performance caused by two potentially significant effects of the cap-lamination secondary, i.e. the thick cap edge and lamination, are identified. Second, a flat-solid secondary is applied and two SLIMs, which are the same primary and two different secondaries, are calculated by three-dimensional finite-element method. Eddy current densities in the Al and back iron plate, which are two parts of the secondary, are analysed. Owing to different transverse edge effects, variations of the air-gap flux density are calculated with different secondaries. Finally, the numerical results of the thrust, power factor, and efficiency are experimentally validated by measurements on test rig of SLIMs. It shows that the SLIM with cap-lamination secondary, which has higher performance, is affected greatly by the conductor structure of the secondary and the lamination of the iron back plate.

This study presents a novel phase current reconstruction strategy for switched reluctance machines (SRMs) using two cross-winding current sensors. The phase currents are reconstructed by solving the linear equations associated with two adjacent phase currents in the different turn-on regions. The effect of current sensor offset and power transistor fault on the proposed reconstruction method is analysed. On the basis of the current difference at the rising edge of each drive signal, an offset sensor identification method is presented and two online compensation schemes are adopted. For power transistor short-circuit fault, the logic-judgment-based and freewheeling-time-based diagnostic methods are investigated and a virtual current sensor is introduced to ensure the effectiveness of the reconstruction process. The proposed phase current reconstruction strategy is free from power transistor open-circuit fault. In addition, the current reconstruction method is easily extended to SRMs with higher number of phases without additional current sensors. Simulations and experiments validate the effectiveness and flexibility of the proposed reconstruction strategy.

This study investigates the impact of electrical parameter variation of a high-frequency transformer model on its sweep frequency response analysis (SFRA) signature to help in classification and interpretation. The simulations have been done using MATLAB and compared with the reference data. The results of SFRA measurements are repeatable up to and beyond 1MHz. The proposed diagnostic methodology using the Cross-Correlation Coefficient Factor (CCF) is used to identify the transformer faults. CCF used to measure the degree of relationship between two variables that establish a relation between the predicted and actual data set. The results of this proposed methodology using the CCF compared with existing Chinese Standard factor (CSF) indicate that, the proposed method is valid to identify the transformer faults. Characteristics of the proposed scheme are fully analyzed by extensive MATLAB simulation studies that clearly reveal that this method can accurately identify the transformer faults compared with CSF. And also does not affected by different fault conditions such as transformer normal condition, Turn to Turn Fault for both HV, LV sides, Axial Fault and/or Radial Faults on both sides, Short Circuit Fault between H.V and L.V Sides, Short Circuit to Ground Fault for both HV, LV sides.

Transient currents (TCs) including transformer inrush, motor starting and fault currents cause mechanical, thermal and electrical stresses to the transformer windings and the power utility equipments. Of these, the inrush current suppression is crucial due to high second harmonic level, causing inadvertent operation of the protective relays. In this study, a TC limiter (TCL) comprising of two diodes and two reactors in each phase is proposed. The number of diodes in the proposed TCL is the half of those employed in the single-phase diode-bridge type inrush current limiter (ICL); thus, the proposed TCL topology offers much more reliability and lower cost in addition to lower current ripple. Comparing with the three-phase diode-bridge type ICL, the proposed topology is independent of the transformer primary winding connection type. The analysis and design procedure is provided and the performance is verified through simulation and an experimental scaled down prototype for transformer energising, motor starting and fault event.

This study presents an analytical model for a permanent magnet synchronous machine (PMSM) with phase-to-phase fault (PPF). On the basis of the derived model, the expressions of zero sequence voltage components are obtained and analysed. The analytical model is applied to study the PMSM behaviours in the MATLAB/Simulink environment. The experiment platform is established to validate the derived model. The experimental results are close to the simulation results, indicating the effectiveness of the analytical model. Hence, the obtained model maybe well used for condition monitoring and PPF diagnosis of the PMSM.

The mutual inductance and magnetic force between coaxial Bitter coils with rectangular cross section were recently calculated by some authors using semi-analytical expressions based on two integrations (Ren et al.) and by using the Bessel function approach (Conway). In this study, these important electrical quantities are calculated using the analytical and semi-analytical expressions based on elliptical integrals of the first and second kinds, Heuman's Lambda function, and two simple integrals with their kernel functions continuous on the whole integration interval. Simple Gaussian numerical integration is used. This method can be applied to calculate the mutual inductance and magnetic force between two thin Bitter disk coils (pancakes) as well as between two Bitter coils, where one has a rectangular cross section and the other is a thin disk (pancake). The calculations of the mutual inductances and magnetic forces are validated by comparison with values already published in the literature. The presented approach has advantages of high accuracy and low computational time.

In this study, an easily implementable and effective method in loss-minimisation control of interior permanent-magnet synchronous motors (IPMSMs) is proposed. Complication of loss-minimisation condition and its implementation are main problems in model-based control methods of IPMSMs. To resolve these problems, an approximate-model is used for simplification of loss-minimisation condition and a proportional–integral-controller is utilised to directly solve the loss-minimisation condition. Also, saturation and iron loss resistance variation due to frequency variation are considered in the proposed method. Furthermore, results show the high precision of the proposed method in comparison with accurate model. Steady-state performance of the proposed method is compared with look-up table-based loss-minimisation and maximum torque per ampere (MTPA) control methods. Also, experimental results verify performance, good dynamic response and effectiveness in loss reduction of the proposed method under different conditions in comparison with look-up table-based loss-minimisation and MTPA control methods.

The study presents a significant development of an advanced control strategy for stand-alone three-phase induction generators. The system under study consists of a squirrel-cage induction generator driven mechanically by an internal combustion engine and electrically by a fault-tolerant converter coupled with a LC filter. As a novelty, in this study three low-cost electrical contactors are introduced in order to re-arrange the converter topology after a fault occurrence and a proper re-starting procedure is developed, allowing the converter to operate with only two legs. The control architecture is mainly described with respect to this last operating condition, evidencing its intrinsic robustness. The study focuses on the composite control of the converter and the prime mover in order to improve the dynamic assessment of the whole system. The converter control is designed around the sliding mode technique, due to its attractive performance, especially as far as parameter uncertainties are concerned. In order to validate the theoretical framework, this study presents the first ever made experimental tests of this system with reference to a generation unit of about 10 kVA. The results confirm the high capability of the proposed system in regulating voltage and frequency with very good dynamic features.

For permanent magnet synchronous motors with parallel branches, circulating currents are induced by a partial demagnetisation fault. The induced currents increase the losses of the motor. Therefore, detecting such a fault becomes possible by analysing the motor losses. This study analyses the effects of the partial demagnetisation fault and defines an index, which is the ratio of increased losses and total losses when the motor is under no-load conditions, to monitor the condition of the rotor magnets. The method is easily implemented and sensitive to the fault. The experimental results validate the effectiveness of the proposed method.

A fast method to calculate the iron loss of switched reluctance motor (SRM) is proposed. The key of the method is that the flux density waveforms in different parts of the motor can be obtained quickly. First, the static flux density data varying with phase current and rotor position in various parts of SRM are obtained by finite element method (FEM). Then, the static flux density data are stored in 2D look-up tables in Matlab/Simulink to get the flux density waveforms by look-up table method, and the obtained flux density waveforms are verified by FEM results. Since the flux waveforms are non-sinusoidal and non-linear, an accurate and fast model for the iron loss prediction is presented, and different modified factors are applied to calculate the hysteresis loss according to the classification of flux density waveforms. Based on that, the calculation model is established in Matlab/Simulink, and iron losses of a three-phase 6/4 structure SRM under different turn-on and turn-off angles and load conditions are calculated and analysed. Finally, measured results are presented to verify the precision of the calculation model.