Although the concept of variable-speed drives, based on utilization of multiphase machines, dates back to the late 1960s, it was not until the mid- to late 1990s that multiphase drives became serious contenders for various applications. These include electric ship propulsion, locomotive traction, electric and hybrid electric vehicles, ldquomore-electricrdquo aircraft, and high-power industrial applications. As a consequence, there has been a substantial increase in the interest for such drive systems worldwide, resulting in a huge volume of work published during the last ten years. An attempt is made in this paper to provide a brief review of the current state of the art in the area. After addressing the reasons for potential use of multiphase rather than three-phase drives and the available approaches to multiphase machine designs, various control schemes are surveyed. This is followed by a discussion of the multiphase voltage source inverter control. Various possibilities for the use of additional degrees of freedom that exist in multiphase machines are further elaborated. Finally, multiphase machine applications in electric energy generation are addressed.

Multiphase Electric Machines for Variable-Speed Applications

The area of multiphase variable-speed motor drives in general and multiphase induction motor drives in particular has experienced a substantial growth since the beginning of this century. Research has been conducted worldwide and numerous interesting developments have been reported in the literature. An attempt is made to provide a detailed overview of the current state-of-the-art in this area. The elaborated aspects include advantages of multiphase induction machines, modelling of multiphase induction machines, basic vector control and direct torque control schemes and PWM control of multiphase voltage source inverters. The authors also provide a detailed survey of the control strategies for five-phase and asymmetrical six-phase induction motor drives, as well as an overview of the approaches to the design of fault tolerant strategies for post-fault drive operation, and a discussion of multiphase multi-motor drives with single inverter supply. Experimental results, collected from various multiphase induction motor drive laboratory rigs, are also included to facilitate the understanding of the drive operation.

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Multiphase induction motor drives - : a technology status review

This paper addresses to some of the latest contributions on the application of Finite Control Set Model Predictive Control (FCS-MPC) in Power Electronics. In FCS-MPC , the switching states are directly applied to the power converter, without the need of an additional modulation stage. The paper shows how the use of FCS-MPC provides a simple and efficient computational realization for different control objectives in Power Electronics. Some applications of this technology in drives, active filters, power conditioning, distributed generation and renewable energy are covered. Finally, attention is paid to the discussion of new trends in this technology and to the identification of open questions and future research topics.

State of the Art of Finite Control Set Model Predictive Control in Power Electronics

This paper presents an overview of motor drive technologies used for safety-critical aerospace applications, with a particular focus placed on the choice of candidate machines and their drive topologies. Aircraft applications demand high reliability, high availability, and high power density while aiming to reduce weight, complexity, fuel consumption, operational costs, and environmental impact. New electric driven systems can meet these requirements and also provide significant technical and economic improvements over conventional mechanical, hydraulic, or pneumatic systems. Fault-tolerant motor drives can be achieved by partitioning and redundancy through the use of multichannel three-phase systems or multiple single-phase modules. Analytical methods are adopted to compare caged induction, reluctance, and PM motor technologies and their relative merits. The analysis suggests that the dual (or triple) three-phase PMAC motor drive may be a favored choice for general aerospace applications, striking a balance between necessary redundancy and undue complexity, while maintaining a balanced operation following a failure. The modular single-phase approach offers a good compromise between size and complexity but suffers from high total harmonic distortion of the supply and high torque ripple when faulted. For each specific aircraft application, a parametrical optimization of the suitable motor configuration is needed through a coupled electromagnetic and thermal analysis, and should be verified by finite-element analysis.

Overview of Electric Motor Technologies Used for More Electric Aircraft (MEA)

This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

Since control of a three-phase machine requires only two currents, it is possible to independently control a number of three-phase machines using reduced-switch-count voltage source inverter (VSI) topologies. One such configuration consists of n three-phase machines, supplied by a (2n+1)-leg inverter, where one inverter leg is common for all machines. This paper deals with a particular case of this topology, namely a four-motor drive supplied from a nine-leg (triple three-phase) VSI. A PWM method, developed recently for two-motor and three-motor drives supplied from a five-leg and a seven-leg VSI, respectively, is extended to the four-motor drive for this purpose. Its main characteristic is that the available dc bus voltage is arbitrarily distributable between the machines within the limits prescribed by the dc bus. The PWM method has been implemented and operation of the four-motor drive, supplied from a custom-designed nine-leg VSI, has been tested. Experimental results are reported for open-loop V/f control and closed-loop current-controlled V/f mode of operation. It is believed that the four-motor drive has a good potential for certain real-world industrial applications, since it may enable a considerable saving in the required number of switches and installed inverter power.

A four-motor drive supplied from a triple three-phase voltage source inverter

It has been shown recently that a number of multi-phase machines can be controlled independently, with the supply coming from a single multi-phase inverter, provided that the stator windings are connected in series in an appropriate manner. One such drive system is a two-motor drive, consisting of a symmetrical six-phase machine connected in series with a three-phase machine. The supply is provided from a six-phase current-controlled voltage source inverter (VSI). This paper examines operation of such a drive system when a six-phase symmetrical induction machine is connected in series with a three-phase permanent magnet synchronous machine (PMSM). Basic operating principles of the drive system are at first reviewed. This is followed by a brief description of the experimental system. An extensive presentation of experimental results, collected from a laboratory setup, is then provided. It is shown that a truly independent and decoupled vector control of the two machines results, although a single supply source is used. The two-motor drive system of the proposed structure is seen as a potentially viable industrial solution for applications requiring one high-power and one low-power machine

Combining Induction and Permanent Magnet Synchronous Machines in a Series-Connected Six-Phase Vector-Controlled Two-Motor Drive

Two multilevel five-phase open-end winding drive topologies are analysed and compared in this paper. The first topology consists of a five-phase machine and two two-level five-phase inverters, supplied with isolated dc-link voltages. In the second topology three-level and twolevel five-phase inverters are used to supply a five-phase machine. Variation of two dc-link's voltage ratio can lead to different overall number of phase voltage levels in both considered topologies. Dc-link voltage ratio influence on performance of drive topologies is analysed and experimental verification is provided for the four most suitable drive configurations.

http://ieeexplore.ieee.org/iel7/7148816/7148817/07149077.pdf

On the five-phase open-end winding drives performance

Multiphase (n-phase) machines are often designed with l sub-windings on stator, each having k phases, and the machine is typically operated with l isolated neutral points. However, such a machine can also operate with a single neutral point, which improves the fault-tolerant properties. When a machine is inverter supplied, low order harmonics may be present due to the low switching frequency and nonlinear inverter properties. Moreover, low order zero-sequence harmonics can be deliberately injected to increase dc bus voltage utilisation. This paper investigates a phenomenon that has not been reported so far in relation to asymmetrical multiphase machines with a single neutral point, namely that the presence of balanced low order harmonics in leg voltages produces unbalanced phase voltage harmonics and consequently unbalanced phase current harmonics. By analysing the neutral point (common mode) voltage harmonics, imbalance in the phase voltage harmonics is explained. Analytical expressions for neutral point voltage harmonics and phase voltage harmonics are provided for asymmetrical machine configurations with a single neutral point having arbitrary numbers of sub-windings and sub-winding phases. Theoretical considerations are verified using simulations and experiments with asymmetrical twelve- and nine-phase loads with a single neutral point, respectively.

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Phase voltage harmonic imbalance in asymmetrical multiphase machines with single neutral point

This paper deals with flux harmonic distortion factor (HDF) and current ripple analysis for various discontinuous space vector PWM (SVPWM) techniques, applicable in conjunction with five-phase two-level inverter-fed ac motor drives. The analysis is based on the application of the recently developed polygon approach for multiphase systems. The results are compared throughout with the corresponding continuous SVPWM techniques. Two different SVPWM methods, based on different selections of the active vectors, are encompassed by the analysis. The analytical study is supported with simulation and experimental result, obtained using a five-phase induction motor.

http://ieeexplore.ieee.org/iel5/6014782/6020091/06020530.pdf

Martin Jones

Papers

A four-motor drive supplied from a triple three-phase voltage source inverter

Combining Induction and Permanent Magnet Synchronous Machines in a Series-Connected Six-Phase Vector-Controlled Two-Motor Drive

On the five-phase open-end winding drives performance

Phase voltage harmonic imbalance in asymmetrical multiphase machines with single neutral point

Analytical evaluation of switching characteristics in five-phase drives with discontinuous space vector pulse width modulation techniques