Sam J Williamson

Impact of slot shape on loss and thermal behaviour of open-slot modular stator windings

This paper presents results from an investigation into the optimal design of an open-slot, modular stator winding. The impact of the stator slot shape on the winding temperature rise is explored, taking account the distribution of loss that occurs in the open slot winding, including ac effects, and the heat transfer characteristics from the winding assembly into the stator core pack. The application focus is a single-layer, concentrated wound brushless PM machine, however the work is applicable to other machine formats. Alternative stator lamination profiles are compared; the commonly used parallel sided tooth with a trapezoidal slot, and a parallel sided slot with a trapezoidal tooth. The investigation includes the development of a reduced order thermal model representation of the stator. This model is employed to undertake coupled loss and thermal analyses to provide more accurate comparisons of the designs accounting for ac and temperature effects. The experimental and theoretical findings indicate the parallel sided slot design will result in a 37°C lower winding temperature or an 11% increase in torque at the intended machine operation point.

Characterising the performance of selected electrical machine insulation systems

This paper presents results from an analysis of alternative slot liner materials used in the construction of electrical machines. The slot liner material has a vital safety critical function within a machine assembly, providing electrical insulation between the winding body and stator core pack. Performance measures for the slot liner material include the dielectric breakdown voltage, tensile strength, thermal conductivity and thermal class, amongst others. There is a large variety of slot liner materials available on the market with the material properties altered to suit a particular application. Some of these material properties are strongly dependent on the components and processes employed in construction of the complete winding assembly e.g. type of the winding impregnation and/or method used in impregnation of the stator/winding assembly. Consequently, the manufacturer provided data is usually inadequate when comparing various insulation systems and their individual elements for a particular machine construction. This research is focused on the conductive heat transfer phenomenon from the winding body into the machine periphery in context of the slot liner material used, for a given impregnation type and method. The repeatability of the winding manufacture process is also investigated. Three alternative slot liner materials with different thermal conductivity and ability of absorbing varnish impregnation have been chosen for prototyping of representative stator/winding hardware exemplars. This has been supplemented with a batch manufacture of the stator-winding hardware exemplar for a selected slot liner material. The proposed experimental approach allows for the complete insulation system to be evaluated accounting for the assembly and manufacture nuances. The results suggest that the use of a particular slot liner has an impact on the winding heat transfer and also implications regarding appropriate manufacture and assembly processes used, i.e. some of the materials require special handling. The experimental work has been supplemented with theoretical analysis to provide a more comprehensive insight into the winding heat transfer phenomena.

Characterizing the in situ Thermal Behavior of Selected Electrical Machine Insulation and Impregnation Materials

This paper presents results from an analysis of thermal behavior for alternative slot liner insulation and varnish impregnation materials used in the construction of electrical machines. These materials are typically characterized by the individual material properties altered to suit a particular application. However, the manufacturer provided material data are usually inadequate when comparing the complete insulation systems. This research is focused on the conductive heat transfer from the winding body into the machine periphery in context of the complete insulation system. An experimental approach using the stator-winding segment subassembly has been adopted here, for the representative in situ heat transfer measurements. The effects of impregnation “goodness,” in volume manufacture repeatability, and individual versus in situ material physical properties on the stator-winding thermal behavior are discussed in detail. The results suggest that the use of a particular slot liner insulation and varnish impregnation has a prominent impact on the winding heat transfer, as well as appropriate manufacture and assembly processes used. The experimental work has been supplemented with theoretical analysis to provide a more comprehensive insight into the winding heat transfer phenomena; in particular the winding-to-slot contact thermal resistance.

Control of parallel single-phase inverters in a low-head pico-hydro off-grid network

In rural locations, where the national power grid is not available, pico hydropower (<;5kW) is typically used in stand-alone configurations, with one unit supplying a single house or a small community. This paper investigates the interconnection of several off-grid pico-hydro generation units with differing power ratings and unequal distances from the load. Each unit is identical, employing a single-phase power electronic converter interface, which comprises a rectifier, a DC-DC converter, and a single-phase inverter. There is no communication between the units, each unit using local measurements to control its output with the ability to form and support an off-grid network. The inverter regulates the output voltage and frequency using droop control. The output impedance is fixed using a virtual resistance, decoupling the droop equations. The voltage control loop has addition harmonic loops to improve voltage quality. The droop coefficients and virtual resistance are adjusted according to the power available at the turbine to ensure that the power sharing is proportional to the available turbine power. Experimental results show that the harmonic voltage control loops improve the voltage quality significantly, with the total harmonic distortion reducing from 9.53% to 1.93% when a non-linear load is connected to a single inverter. The transient response of two parallel inverters to the switching in of a non-linear load, and the response of the power sharing ratio to the hydraulic head at one turbine unit being reduced are demonstrated and shown to be stable.

Experimental optimisation of a low-head pico hydro turgo turbine

Rural electrification can help to alleviate poverty, providing energy for daily household and income generation tasks. Where appropriate resources exist, pico hydropower is a cost effective method of electrical generation, especially in a rural context. For pico hydropower at low heads, propeller turbines are traditionally used. However, these are very site specific and the efficiency dramatically reduces when not operated at rated conditions. This paper derives six key experimental parameters for a Turgo turbine using two-dimensional fluid equations, which are experimentally varied to find the affect on the efficiency on a scale prototype, obtaining a design that is optimised for efficiency. Test results are presented up to heads of 3.5 m. It is found that the turbine efficiency is very sensitive to jet vertical aim position, with the optimum aim at 12 mm from the top of the cup with a jet inclination angle of 20°. The nozzle diameter is found to have a critical value of 20 mm, above and below which the turbine efficiency reduces. The turbine wheel diameter is reduced from 7.5 times to 5 times the nozzle diameter with minimal effect on the maximum efficiency, however, this provides an increase in the rotational speed from 490 to 750 RPM. The maximum experimental jet-to-mechanical efficiency is found to be over 90% for the scaled turbine, with lower sensitivity to head and flow rate than propeller turbines. This demonstrates that Turgo turbines can be used efficiently at low heads over a range of site conditions.

A photovoltaic panel modelling method for flexible implementation in Matlab/Simulink using datasheet quantities

This paper presents a detailed method for creating an embedded Matlab model in Simulink for any solar photovoltaic panel starting with its datasheet values. It links extrinsic functions to the Simulink embedded model to provide fast and simple iterative solving of non-linear equations. It also provides a method sufficiently flexible to produce a model output based on panel current or voltage such that it can be cascaded with different Simulink elements.

Effective practices for the concept design of electromechanical systems

Purpose Concept design practices in engineering are not common across industry or academia. There are a number of well-known tools and methods acknowledged as useful in facilitating concept designing, that is, to assist idea generation, aid evaluation and final selection of one winning concept from many. Combinations of these popular concept design tools and methods provide various systematic methodologies by which practitioners propose to conduct or teach concept designing. In this paper, effective practices and trends are observed through the application of a specific concept design methodology over a range of different projects in electromechanical systems design. Design/methodology/approach The concept design methodology utilised in this study has been developed through the adoption of various tools and methods shown to be beneficial to concept designing, supported by previous positive experiences and successful utilisation associated with electromechanical systems research projects in academia. Each stage of the methodology is discussed and six case studies are presented, which are used to explore effective practices for concept designing. Findings Analysis of the case study data reveals the most popular criteria for the selection of concepts in electromechanical systems design, the number of selection criteria and number of initial concepts ideally required to converge on a final winning concept more efficiently, that is without the need for a more detailed second stage of selection using performance metrics. Originality/value Rarely are detailed studies undertaken in concept design, first, to address the justification for the concept design methodology adopted and, second, to show how effective practices emerge through the analysis of non-subjective data over a number of concept design projects. Although the paper uses only six case studies in electromechanical systems design, it is hoped that the approach presented promotes the possible future development of a framework for verification of concept design methodologies across different products, sectors and user groups.

Test characterization of a high performance fault tolerant permanenet magnet machine

It is well understood that an electric machines output performance is limited by its losses and thermal behavior. For novel prototype machines, hardware testing processes are an important part of quantifying these parameters. For some machines, effective characterization may be accomplished using a series of static and simple prime-mover tests. The resulting data permits calibration of loss- and thermal-models. These can then be used to predict on-load performance. Fault-tolerant machines based on single layer winding arrangements are designed to minimize interaction between windings or module-groups. This paper demonstrates that, for such a machine, the losses measured during simple DC and primer-mover tests may be used to infer performance during both ‘healthy’ and ‘faulted’ operating modes. Under faulted conditions the total machine loss is expected to be a combination of module-specific and common losses, which can be directly deduced from hardware tests. This paper discusses the accuracy of loss superposition when applied to a 180 kW multi-channel, fault-tolerant aerospace machine. From observations following faulted and healthy dynamometry tests, there exists close correlation between full-load performance and estimates made from the superposition of losses under discrete operating modes.

Investigation of equivalent stator-winding thermal resistance during insulation system ageing

High performance electrical machine operation is limited by the losses generated in the machine and how well the heat developed by these losses is extracted. For conductive heat transfer from the main winding body, the insulation materials provide the main heat transfer pathway. As a machine ages through its lifetime, these materials will change their properties, and therefore these heat transfer characteristics will also alter. This paper describes the methodology and measurements on a set of motorettes undergoing an insulation ageing process to identify the change in heat transfer characteristics. The motorettes are subjected to a combination of thermal, electrical, mechanical and humidity stresses, before their thermal performance is measured. It has been found that the equivalent thermal resistance of the motorettes over the nominal lifetime of the insulation increases by over 100%, with the current carrying capability of the machine reducing by over 30% across the machines lifetime. It is therefore recommended that this is taken into account during the initial thermal design of the machine to ensure it remains fit for purpose during its complete life.

Technical and economic prospects for the site implementation of a gravitational water vortex power plant in Nepal

Micro hydropower is a very promising renewable energy source for off-grid energy generation in remote areas where the resource exists. This study focuses on integrating an innovative new design of micro hydropower system called the Gravitational Water Vortex Power Plant into existing water infrastructure. Three types of existing water infrastructure are considered applicable for hydropower integration: irrigation canals, reservoirs and weirs. The theoretical designs for the civil works for the low-head gravitational water vortex power plant integrated into these structures are given and the costs of construction are estimated. A scalable system of 1.6kW is also designed and integrated in an existing irrigation canal for the technical performance evaluation and validate the theoretical economical study. The study concludes that the performance of turbine is mostly suitable for rural electrification and to integrate in cost-effective way, the civil works and installation cost should be greatly reduced.