Waqas M. Arshad

Analytical design and analysis procedure for a transverse flux machine

A transverse flux machine (TFM) offers a higher specific torque and power density than a conventional radial flux machine, though at a cost of a poorer power factor. This paper provides an analytical approach to dimension an application-specific TFM and analyse its performance for first order calculations. Important features of TFMs are also outlined to facilitate an understanding of its operation and design. The design is explained with reference to an arbitrarily chosen application; a TFM wind generator. Only a surface mounted, single sided, outer rotor TFM generator design is treated. The method can however be equally applied to other types of TFMs, with proper adjustments of the analytical models.

Use of transverse-flux machines in a free-piston generator

This paper investigates the use of transverse-flux machines (TFMs) in a free-piston generator, an integration of a combustion engine and a linear electrical machine. For hybrid vehicles (the intended application), this integrated device has benefits in efficiency, emissions, reliability, etc. Conventional TFMs are dimensioned and their shortcomings illustrated. Several surface-mounted TFM designs are suggested. These designs, initially believed to be promising, are later discovered to be poor during the three-dimensional finite-element method (3-D FEM) analysis. These designs suffered from a large axial pole-to-pole armature-flux leakage in the stator. This problem is solved in the proposed buried-magnets TFM variants, for which patents have been filed as well. A 5-kW proof-of-concept prototype of one such design is dimensioned that allowed the near fulfilment of 3-D FEM verifiable system demands. One phase of this dimensioned three-phase prototype is built. Measurements on this prototype showed that the use of short pole lengths in the machine had led to manufacturing defects. Nevertheless, the electromagnetic viability of the design could still be approximately verified.

Fault tolerant generator systems for wind turbines

The objective of this paper is to review the possibilities of applying fault tolerance in generator systems for wind turbines based on what has been presented in the literature. In order to make generator systems fault tolerant in a suitable way, it is necessary to gain insight into the probability of different failures, so that suitable measures can be taken. Therefore, a literature survey of reliability of wind turbines, electrical machines and power electronic converters is given. Five different ways of achieving fault tolerance identified in the literature are discussed together with their applicability for wind turbines: (1) converters with redundant semiconductors, (2) fault tolerant converter topologies, (3) fault tolerance by increasing the number of phases, (4) fault tolerance of switched reluctance machines, and (5) design for fault tolerance of PM machines and converters. Because converters fail more often than machines, it makes sense to use of fault tolerant converter topologies. Increasing the number of phases is a useful form of fault tolerance because it can be achieved without increasing the cost significantly.

Research on a Tubular Longitudinal Flux PM Linear Generator Used for Free-Piston Energy Converter

The free-piston energy converter (FPEC) is used for a series hybrid electric vehicle scheme, which may have the advantages of high efficiency, compact structure, and reliable operation. The linear generator is an important part in the FPEC, and a tubular longitudinal flux permanent-magnet (PM) linear generator scheme is investigated in this paper. The electromagnetic and thermal properties of the generator are analyzed with commercial software Flux 2D. The generator is optimized from the aspects of material selection and structure optimization. Rare-earth PM material VACODYM 655HR is selected for the permanent magnets, and low-performance low-loss material M235-35A is chosen for the stator lamination. The pole number and machine size are optimized, and the optimum scheme with efficiency of 0.935 and specific power of 1.49 kW/kg is obtained, which meets the requirements of FPEC application

Analytical model of multiphase permanent magnet synchronous machines for energy and transportation applications

Multiphase (more than three) electric machines and drives were used first to limit current amplitude and later reduce torque ripple with early, six-step converters. More recently, the potential benefits of multiphase drives for electric ship propulsion include improved fault tolerance, improved torque density by harmonic injecting, reduced per-phase power rating, reduced ripple torque, reduced rotor loss, and reduced noise. Since the mid 1990s, R&D effort in the area of multiphase machines and drives has accelerated for new aerospace, electric vehicles, and renewable energy applications, especially off-shore wind because of reliability and size. Among the many multiphase options, N×3-phase systems can be split from existing 3-phase windings and still use conventional 3-phase inverters. This paper provides the analytical synchronous-frame d-q model of the 2×3-phase permanent magnet synchronous machine (PMSM), similar to the conventional three phase d-q model, for the purpose of control development. The developed model is verified with FEA simulation results. This paper also presents how to calculate and scale the parameters of the 2×3-phase PMSM from the baseline 3-phase PMSM. Motor drive engineers seeking the same level of 2×3-phase and 3×3-phase PMSM model as the conventional 3-phase PMSM for control research and development can benefit from this study.

End-Winding Inductances of MVA Machines Through FEM Computations and IEC-Specified Measurements

Correct finite-element method (FEM) computations of end-winding leakage inductances of MVA-range machines are discussed. This is shown through validations against total stator reactance measurements of 11 air-suspended stators and a reasonable agreement of FEM computations with the International Electrotechnical Commission 60034-4 standard. To achieve this goal, the need to have several 2-D and 3-D FEM models, including those representing the cooling ducts, is emphasized. Steps to utilize the validated stator FEM models for direct computation of end-winding leakage inductances are also listed. It is also shown that measurements of zero-sequence reactances cannot help the FEM validations and do not provide a reliable estimation of the leakage inductances.

On finding compact motor solutions for transient applications

Applications such as emergency breakers, protective devices in explosive environments, emergency exit openings, etc., fall into a broad category that can be grouped under the general term transient applications. There are cases in which a motor is to be operated only for a short duration. The use of standard procedures to select or design a motor for these transient applications, will most probably result in an over dimensioned and an over-sized motor. An alternate approach is hence required. This paper provides guidelines on how to select an off-the-shelf motor and when necessary, on how to design a new motor for such applications. A design example is also provided, and the analysis is verified by building and testing a prototype. It is found that with some derating calculations, an off-the-shelf selection can yield an induction machine solution. However, there may be cases where the selection is not compact enough. In such cases a new permanent magnet motor design is required and a BLDC motor is a realistic choice from a cost perspective. It is found that for such applications, the most critical design factor is to avoid magnet demagnetisation. The thermal loading is not a design factor owing to the very small application times involved. However, a thermal check on the obtained design is always advisable.

Current observer based position estimation using the stationary frame equivalent of a synchronous frame PI regulator for PMSM sensorless drives

This paper introduces a new current observer-based position estimation method using the stationary frame equivalent of a synchronous frame PI regulator for a 3-phase PMSM. The introduced method eliminates a positive feedback type of interdependency between the performance of the position estimation and the synchronous reference frame transformation by implementing the position estimation method in the stationary frame. In the proposed method, the major drawbacks of steady-state error caused by regulating ac signals with conventional PI control and high frequency ripple in a state variable caused by sliding mode control of the current observer based estimation approach in the stationary frame, are overcome by using the stationary frame equivalent of a synchronous frame PI regulator. The concept and implementation are simple, straightforward, and intuitive. Simulated and experimental results are provided to confirm the attractive performance characteristics of the resulting position estimation using the proposed method.

Medium–Large Induction Machines

In this article, the accuracy levels of different scaling procedures for starting current measurements are addressed through the statistical analysis of locked-rotor tests involving hundreds of medium-large induction machines. This is performed to ascertain the confidence level of the predicted full-voltage starting currents from the reduced-voltage factory measurements, a necessity for motors with a strict tolerance to the starting current level. It is shown that, for traditional scaling methods employing only measurements, this accuracy level is 88-90% for scaling step from 0.6 to 1.0 p.u. voltage. This figure could be raised to 95-97% through a novel method that also uses finite element method (FEM) simulations for each individual test voltage. The differences between FEM and voltage measurements are documented and used for correcting the full-voltage FEM simulation. Further, the use of FEM is shown to help in the understanding of the causes and locations of measured voltage-dependent saturation uncertainties. The region (end-core, overhang, or main core) that contributes the most to saturation uncertainties is shown to be identifiable through origin/parametric dependencies, leading to a better product understanding and more reliable scaling methods in future.