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The Roebel bar conventional design of large AC machines uses the classical strand-slot inductance model (CSSIM). Suitable alternatives are missing as the CSSIM is favored for its inherent simplicity based on the ideally permeable iron core hypothesis. However, saturated armature slots can lead to high variations of the slot inductance, where the CSSIM cannot represent this precisely. An accurate prediction of the strand inductances is crucial when optimizing the transpositions of large Roebel bars to be competitive on efficiency and low measurement tolerances. This fact is crucial in under-roebeling, having less than a 360-degree transposition over the active part. In the end, the goal is to compensate the winding overhang parasitic field with the slot-parasitic field. This paper proposes a differential strand-slot inductance model (DSSIM) based on the concept of differential inductance (DI). It is compatible with a circuital lumped-element model (LEM) that considers the strand topology, geometrical dimensions, saturation level, and small-scale effects. Numerical simulations showcase the performance improvement of the DSSIM against known models. Finally, a 20-strand prototype of a slot model with actual Robel bar strands corresponding to a simplified bar cross-section in a large AC machine’s slot demonstrates the presented DSSIM’s precision. [ABSTRACT FROM AUTHOR]

A variable reluctance electric machine with a two-degree of freedom rotor motion characterized by rotation around the center of the rotor and orbiting around the center of the stator is studied. One motivation for adopting such rotor motion would be to allow torque generation through both the radial and tangential components of the electromagnetic forces created in the airgap, as opposed to common rotary machines in which torque is a byproduct of only the tangential component of the airgap electromagnetic forces. Furthermore, the rotor motion provides a built-in gearing action similar to that of a cycloidal drive, which allows natural torque multiplication and the tight integration of the electromagnetic and mechanical systems. The two-degree of freedom rotor motion also results in a counter-intuitive inertia characteristic of this machine concept which is elaborated in this paper for the first time. Due to the reluctance nature of the torque, and because of the distinctive rotor motion profile which resembles the motion of a cycloid, the studied machine is called a cycloidal reluctance (CR) electric machine (CRM). In this paper, the principle of operation of the CRM is explained, and its performance is compared to a counterpart switched reluctance machine (SRM). A prototype is built, and the theoretical torque calculations are validated by experimental measurements. [ABSTRACT FROM AUTHOR]

This article provides an applicable method to predict the distribution of the electric potential inside the winding of an electrical machine with a reasonable set of parameters. It extends the understanding of the winding impedance in terms of the inter-winding behavior and gives us the opportunity to properly design the insulation system during the development phase of an electrical machine. Predictions are backed up by measurements and an in-depth look at the measurement setup. The results proof nonuniform potential distribution and show that the potential difference between individual turns exceeds the amplitude of the phase voltage. Our findings also establish an interrelation between the winding impedance and the potential oscillations inside the winding. The article provides an introduction to the fundamental process of voltage stress on electrical machine windings and discusses drivers for upcoming challenges in the field of winding insulation and partial discharges. [ABSTRACT FROM AUTHOR]

The end-winding region is usually overlooked in the roebel design of large AC machines. However, saturated stator slots cause overhang parts to impact the circulating currents significantly. In fact, a precise knowledge of the winding overhang strand inductances is crucial when optimising the transpositions of large roebel bars, especially in the case of under-roebeling (having less than 360-transposition in the active part) where the goal is to compensate the winding overhang parasitic field with the slot-parasitic field. In this article, a rectangular inductance calculation model (R-ICM) is proposed. It results in a circuital lumped-element model (LEM) that takes the strand dimensions, the bar bending, as well as small-scale effects into account. Moreover, the work describes how to model finite current-carrying rectangular segments (straight and arced) from first principles. Finally, the methodology was demonstrated for two prototype specimens with different bending-shapes corresponding to the fundamental elements on a stator bar in the winding overhang of large electrical machines. [ABSTRACT FROM AUTHOR]

This article proposes an analytical model of modular spoke-type permanent magnet (MSTPM) machines based on air-gap field modulation (AFM) theory. Firstly, a fundamental AFM model of open-circuit MSTPM machines is introduced. The open-circuit air-gap field of MSTPM machines is determined by three fundamental elements including the primitive magnetizing magnetomotive force (MMF) produced by permanent magnet (PM), and two modulators which consist of stator and rotor permeance. The analytical MMF excited by PM (PM-MMF) can be calculated by using magnetic circuit method, while the stator and rotor permeance models are developed based on relative permeance (RP) method. Thereafter, a general model is proposed to calculate the open-circuit back electromotive force (EMF) of MSTPM machines. Further, the winding inductance model is established on the basis of equivalent magnetic circuit method and RP model. Finally, the machine performance is predicted by the analytical model, and verified by both finite element analysis (FEA) and experimental results. [ABSTRACT FROM AUTHOR]

The stator winding acts as a carrier for the electrical load, and its loss accounts for about 50% of the total loss in AC machines. So how to design the stator winding for reducing winding loss is extremely crucial. Some traditional transposition methods are adopted in the winding design for large AC machines. Although they have a certain effect on reducing winding loss, there is still room for the improvement in the winding design and manufacturing technology. In this paper, a new global transposition method of stator winding is proposed. Then a megawatt driving motor is taken as the research object, and the eddy current losses, circulating current losses and total losses of winding strands are calculated by the three dimensional (3-D) finite element method (FEM) which has been validated by the experiment. In addition, by comparison with the two original transposition methods, it is proved that the proposed global transposition method is more effective in further reducing the winding loss, and the winding loss distribution is more uniform. This global transposition method proposed in this paper provides a reference for the optimal deign of the stator winding in AC machines. [ABSTRACT FROM AUTHOR]

During the design process of the asymmetric-paths winding, the optimization between the asymmetric degree and the tidiness of end connections is still a difficulty for the existing methods. In this paper, the connection indices are proposed for the connection evaluation. In addition, a novel directed graph approach is provided for the connection optimization. It turns the connection design into the route optimization of the directed graph, including the modeling of the winding directed graph, the searching rules for a parallel path, and the matching rules between parallel paths. The winding design of a hydrogenerator is presented for the validation. The results show that the optimized scheme presents a tidy end connection, shorter circumferential busses, and a good performance in the electromagnetic field. This approach provides a new idea for the modeling of ac windings and the optimization design of the asymmetric-paths winding. [ABSTRACT FROM AUTHOR]

This paper focuses on the comparative analysis of modular spoke-type permanent magnet machines with two magnetization modes, which are referred to as M-I and M-II types. The analytical models of the proposed machines are built based on the simple magneto-motive-force-permeance method. With the help of finite element analysis and the analytical models, magnetic fields in machines with different magnetization modes are compared. Then, taking as a base an existing commercial in-wheel machine used in an electric motorcycle, two proposed machines with different magnetization modes are designed as in-wheel traction machines and compared with respect to electromagnetic torque, flux-weakening performance, over-load capability, etc. The machines are prototyped and experimentally tested to verify the prediction that the M-II machines exhibit a higher torque output whereas the M-I machines have a wider speed range. [ABSTRACT FROM AUTHOR]

The computational cost for the simulation of detailed models of machine-rectifier systems is expensive because of repetitive diodes switching. Average-value models (AVMs) of machine-rectifier systems have been developed that can alleviate the computational burden by neglecting the details of the switching of each individual diode while retaining the average characteristics. This paper proposes an alternative formulation of numerical AVMs of machine-rectifier systems, which makes direct use of the natural dynamic impedance of the rectifier without introducing low-frequency approximations or algebraic loops. By using this formulation, direct interface of the AVM is achieved with inductive circuitry on both the ac and dc sides allowing traditional voltage-in, current-out formulations of the circuitry on these sides to be used with the proposed formulation directly. This numerical AVM formulation is validated against an experimentally validated detailed model and compared with previous AVM formulations. It is demonstrated that the proposed AVM formulation accurately predicts the system's low-frequency behavior during both steady and transient states, including the cases where previous AVM formulations cannot predict accurate results. Both run times and numbers of time steps needed by the proposed AVM formulation are comparable to those of existing AVM formulations and significantly decreased compared with the detailed model. [ABSTRACT FROM AUTHOR]

This paper investigates different operating points of a ducted rotor brushless doubly fed reluctance machine (BDFRM) in subsynchronous, synchronous, and supersynchronous speed regions. The BDFRM has a ducted rotor and its stator houses two three-phase distributed windings. An example design of the machine is proposed for investigation. A control approach to maximize the torque capability of the given design is introduced. The static structural analysis result of the machine is also presented, which led to the final manufacturing of the machine laminations. Machine assembly and drive implementation steps are discussed. The built machine is experimentally tested in different speed regions. [ABSTRACT FROM AUTHOR]

Numerous models and formulations have been used to study synchronous machines in different applications. Herein, a unified derivation of the various model formulations, which support direct interface to external circuitry in a variety of scenarios, is presented. A synchronous machine model with magnetizing path saturation including cross saturation and an arbitrary rotor network representation is considered. This model has been extensively experimentally validated and includes most existing machine models as special cases. Derivations of the standard voltage-in, current-out formulation as well as formulations in which the stator and/or the field windings are represented in a voltage-behind-reactance form are presented in a unified manner, including the derivation of a field-only voltage-behind-reactance formulation. The formulations are compared in a variety of simulation scenarios to show the relative advantages in terms of time steps, run time, and accuracy. It has been demonstrated that selection of the formulation with the most suitable interface for the simulation scenario has better accuracy, fewer time steps, and less run time. [ABSTRACT FROM PUBLISHER]

In this paper, novel design of a flux-switching permanent magnet machine (FSPMM) with a yokeless stator and two rotors has been proposed. The proposed machine has been found to perform on par with a 6-slot/13-pole C-core conventional FSPMM in terms of torque output. Additionally, the proposed machine has no significant unbalanced magnetic forces, which is a major challenge in conventional FSPMM with odd number of rotor poles. Step-by-step parametric optimization has been performed using finite-element models of the machine in order to achieve the highest torque with the given magnetic and electrical constraints. [ABSTRACT FROM PUBLISHER]

Representation of synchronous machines using constant-parameter voltage-behind-reactance (VBR) formulations improves accuracy and numerical efficiency of power systems transient simulation programs. This paper extends the VBR representation to the rotor circuit and presents two new formulations that achieve direct constant-parameter interfacing of the rotor and stator terminals with arbitrary external networks. In the first model, the entire machine is represented by constant RL branches that have algebraic coupling among the circuit variables. In the second model, all damper windings are implemented in state-space form to increase the numerical efficiency, while the stator and field windings are provided as constant-parameter circuits. The proposed models are validated against the commonly used and some state-of-the-art alternative models using a single machine with a simplified ac excitation system. Computer studies demonstrate the improved accuracy and efficiency of the proposed models when external rotor circuitry is considered. [ABSTRACT FROM AUTHOR]

Interfacing of ac electrical machine models in power system transient simulation programs is receiving increasing attention in the literature. Models based on the voltage-behind-reactance (VBR) formulation have been recently proposed to achieve a direct interface with external power networks. However, the rotor-position-dependent interfacing inductances due to dynamic saliency in synchronous machine models increase the computational cost of the overall system solution and limit the application of most VBR formulations. This paper presents new methods for elimination of dynamic saliency using continuous- and discrete-time approximation techniques to achieve explicit formulations. The proposed models have simple interfacing circuit consisting of decoupled constant-parameter RL branches. The new models are implemented in MATLAB/Simulink and the PLECS toolbox, and are shown to offer simple and easy-to-use interface, high accuracy, and numerical efficiency as compared to the existing models. The proposed models can find wide application in common state-variable-based transient simulation programs. [ABSTRACT FROM PUBLISHER]

Inclusion of magnetic saturation in low-order qd models of induction and synchronous machines improves the accuracy of system studies. Over the years, numerous explicit and implicit methods of representing magnetic saturation have been proposed in the literature, some of which find their use in commonly used transient simulation packages. This paper presents a straightforward and improved flux correction method of modeling saturation explicitly in classical qd state variable ac machine models. The approach requires limited additional information and avoids the need for a dynamic inductance, which is present in many other approaches. The method is first developed for induction machines and then extended to salient-pole synchronous machines as a more general case. Computer studies demonstrate the accuracy of the proposed method for predicting the dynamic and steady-state cross-saturation phenomena, as well as its computational advantages. [ABSTRACT FROM PUBLISHER]

Reluctance machines (RM) have desirable features including low rotor and no-load rotational losses. In this paper, a new type of RM is introduced, namely the asymmetrical RM. This machine has higher torque density than a conventional RM and is useful in situations where low rotor and no-load rotational losses are desirable. A design model for the new type of machine is presented and validated using finite-element analysis. The machine is compared to other machine types by comparing Pareto-optimal fronts of loss versus electromagnetic mass for a given torque. [ABSTRACT FROM AUTHOR]

An electric machine equipped with n stator phases is characterized by a set of n \times n phase inductances as concerns the stator section. In the case of nonuniform air gap, these inductances vary as the rotor moves. For their numerical determination as functions of the rotor position an accurate computation algorithm is proposed in this paper based on winding function theory. For permeance function identification, a numerically efficient method is employed based on the magnetostatic finite-element analysis of only three appropriately simplified machine models. In the presence of a field circuit, computation of mutual inductances among it and stator phases is also covered with the same approach. An extension of the method to permanent-magnet machines is also presented. Results are assessed on a six-phase synchronous generator prototype equipped with either a salient-pole wound-field rotor or with an interior permanent-magnet rotor. [ABSTRACT FROM AUTHOR]

Protective relay settings for wind turbine generators often depend on short-circuit calculations performed using sequence network circuits in rms-based protection software. Traditional assumptions made in deriving the sequence network representation of induction machines result in some error in short-circuit calculations, which could potentially cause incorrect relay settings. A more accurate sequence network model of induction machines is derived in this paper to aid in short-circuit calculations in induction-generator-based wind plants. A rigorous mathematical approach to defining the stator short-circuit current equations under general faults is described and validated with transient simulations. The sequence network model of the induction machine is derived from the closed-form mathematical solutions. An example short-circuit calculation is performed on a simple network in this paper, and found to have a marked improvement in accuracy from traditional sequence network calculation methods for unbalanced faults. Resonant effects of power factor correction capacitors are also described. [ABSTRACT FROM AUTHOR]

Transient simulation programs, either nodal analysis-based electromagnetic transient program (EMTP-like) or state-variable-based, are used very extensively for modeling and simulation of various power and energy systems with electrical machines. It has been shown in the literature that the method of interfacing machine models with the external electrical network plays an important role in numerical accuracy and computational performance of the overall simulation. This paper considers the state-variable-based simulation packages, and provides a constant-parameter decoupled RL-branch equivalent circuit for interfacing the ac induction and synchronous machine models with the external electrical network. The proposed interfacing circuit is based on the voltage-behind-reactance formulation which has been shown to have advantageous properties. For the synchronous machines, this paper describes both implicit and explicit (approximate) interfacing methods. The presented case studies demonstrate the advantages of using the proposed interfacing method over the traditional qd-models that are conventionally used in many simulation packages. [ABSTRACT FROM AUTHOR]

This paper presents a high-gain observer of the rotor position for the control of surface permanent magnet synchronous machines under loaded conditions, e.g., saturation. Determining the rotor flux position of permanent magnet AC machines under no load is simpler, since its flux and rotor position coincide. However under load and in the presence of saturation in particular, the flux is comprised of two components: one due to the rotor magnets and the other due to the stator current. This causes the magnetic axis to shift from the rotor position, which in turn causes an error in a control algorithm that uses conventional rotor position estimation. This paper characterizes this error through a nonlinear model of the machine and finite element analysis. Design and analysis of a general position observer algorithm for PMAC machines accounting for saturation is presented, along with its implementation for a control methods using back EMF measurement and high-frequency Injection. Experimental results validating the nonlinear model of the machine, existence of the saturation-induced error, and performance of the proposed observer are also given. [ABSTRACT FROM PUBLISHER]

In the field of wind energy, electrical generator solutions have converged on a small number of technologies for specific technical and economic reasons. This paper investigates whether a similar rationale exists within the field of oscillating water column, wave energy converters. The suitability or otherwise of the various generator options in the offshore marine environment is examined in detail. Each generator configuration is then modeled and the annual energy output for a typical wave climate assessed with consideration being given to the impact of speed control on the energy output. Finally, some conclusions are drawn regarding the relative suitability of the different generator options. [ABSTRACT FROM AUTHOR]

The objective of this paper is to analyze, test, and compare machines and drives in oscillating applications. In particular, this paper is focused on low-power wave energy generator applications, such as autonomous weather and monitoring buoys with power requirements in the 100 W and less range. Due to the oscillating motion of waves, the ocean environment can require bidirectional and variable speed operation of the generator. In this research, the efficiency of a set of small brushed dc, induction, brushless dc, and synchronous reluctance drives and machines were compared in constant and oscillating operation. The presented results show that drives and machines used in low-power oscillating applications (e.g., ocean wave energy) should not expect a significant derating with respect to their nameplate rating. In addition, it is shown that the frequency of oscillation (e.g., ocean wave frequency) has little impact on efficiency. [ABSTRACT FROM AUTHOR]

In this paper, a new approach for the calculation of high-frequency losses in induction motors is presented. The input to the motors is assumed to be supplied from a sinusoidal voltage source. The method is based on the two-dimensional (2-D) field solutions of the magnetic circuit, obtained by using a nonlinear ‘harmonic’ solution. Hence, the solution time is very short. From the ‘harmonic’ solution, the airgap field distribution as well as the fundamental frequency eddy current losses are determined. The high-frequency loss calculation is based on the assumption of a path for eddy currents within a lamination. A constant k is introduced that defines the width of the current flow path. The empirically found κ value is verified by a theoretical calculation. The new method is applied to the calculation of losses of two smooth rotor induction motors. The prediction accuracy is found to be very good. The method is also applied to two open rotor slot motors to observe the change in the prediction accuracy. It is found that due to the small slot openings accurate predictions are still possible. The approach presented in this paper requires little time for loss calculation, and is very suitable for minimizing losses at the design stage. [ABSTRACT FROM AUTHOR]

A comparison between two inter-laminar stator core insulation failure detection techniques for generators based on low flux core excitation is presented in this paper. The two techniques under consideration are: 1) the iron core probe based method developed recently and 2) the existing air core probe based method. A qualitative comparison of the two techniques is presented along with an experimental comparison on a 120-MW generator stator core. The test results are compared in terms of fault detection sensitivity, signal to noise ratio, and ease of interpretation, which are the main requirements for stator core inspection. In addition to the comparison, the performance of the iron core probe technique for machines with short wedge depression depth is presented along with the recent improvements in the algorithm. It is shown that the main requirements for stator core inspection are significantly enhanced with the new iron core probe-based core fault detector. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study of the detection of mechanical faults in an induction motor. As is reasonably well known, by means of analysis of combinations of permeance and magneto-motive force (MMF) harmonics, it is possible to predict the frequency of air gap flux density harmonics which occur as a result of certain irregularities in an induction motor. In turn, analysis of flux density harmonics allows the prediction of induced voltages and currents in the stator windings. Reviewing this theory, equations which may aid in the identification of mechanical faults are presented. These equations include both those which indicate eccentric conditions and those which have been suggested to help identify bearing faults. The development of test facility to create eccentricity faults and bearing fault conditions is described. This test facility allows rapid access to the motor bearings, allowing an investigation into the ability to detect faulted hearing conditions using stator current monitoring. Experimental test results are presented, indicating that it may be possible to detect bearing degradation using relatively simple and inexpensive equipment. [ABSTRACT FROM AUTHOR]

A novel online technique for monitoring the insulation condition of ac machine stator windings is proposed in this paper. The concept is to measure the differential leakage currents of each phase winding from the terminal box in a noninvasive manner to assess the insulation condition during motor operation. The conventional differential CTs used for phase fault protection can be replaced with high performance current sensors to measure the leakage current with higher accuracy. Indicators for insulation condition such as the capacitance and dissipation factor are calculated based on the measurements to provide a low cost solution for online insulation condition assessment A simplified online insulation system model is derived for analysis and interpretation of the measured data. Experimental results on a 15-hp induction motor under simulated insulation degradation conditions show that the proposed technique is a very sensitive method capable of detecting incipient signs of insulation degradation. [ABSTRACT FROM AUTHOR]

In this paper, the distribution pattern of harmonic waves in ac electric motors is investigated. On the basis of this, a novel exact and universal approach for calculating differential leakage related to the harmonic waves was developed. This approach applies to any ac electric motors, regardless of winding type, single-phase or three-phase, regular to nonregular, integral-slot or fractional-slot, symmetrical or asymmetrical. Since this novel approach substitutes the approximation method adopted by conventional approaches with an exact one, it distinguishes itself by offering precise results,general applicability, and fast calculation. [ABSTRACT FROM AUTHOR]

A new state-space constant-parameter voltage-behind-reactance synchronous machine model has recently been proposed. Several efficient explicit formulations were derived using continuous- and discrete-time approximations. This letter complements the previous publication and presents a simple and flexible procedure for selecting the parameters of the continuous-time filters. A case study is included to demonstrate the proposed procedure. [ABSTRACT FROM PUBLISHER]

A new scheme for reducing the fluctuations in power to a grid-connected wave energy plant is proposed. It achieves this by physically decoupling the turbine from the induction generator and by using a secondary rotating scheme. Experimental results from a wave energy plant are reported, which led to the solution. [ABSTRACT FROM AUTHOR]