Symmetrical/Asymmetrical Winding Reconfiguration in Multiphase Machines

This paper investigates multiphase drives in which winding configuration (symmetrical or asymmetrical) can be easily obtained by only rearranging voltage source inverter (VSI) power supply cables at the machine’s terminal box. The type of the machines where this is possible is identified and the examples of reconfiguration are given and explained. Following from the examples (for nine- and fifteen-phase cases), a general reconfiguration algorithm is introduced. As shown, changing asymmetrical to symmetrical winding configuration (and vice versa) means that just another mimic diagram needs to be placed over the existing one on the machine’s terminal box. Possible reconfigurations of a six-phase machine, which do not follow the same pattern, are also addressed. Differences caused by different winding configuration are identified and experimentally confirmed using a nine-phase surface mounted permanent magnet synchronous machine (PMSM) and a nine-phase induction machine (IM)

Multiphase induction motor drives - a technology status review

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 operatio

Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase Permanent Magnet Synchronous Machines

This article proposes a modeling approach and an optimization strategy to exploit a third-harmonic current injection for the torque enhancement in multiphase isotropic permanent magnet synchronous machines with nonsinusoidal back electromotive forces. The modeling approach is based on a proper vector space decomposition and on the associated rotational transformation, aimed to properly select a set of stator current space vectors to be controlled. It is presented for a generic (i.e., asymmetrical, with an arbitrary angular shift) winding configuration. The injection strategy is related to the choice of a constant synchronous current set aimed at minimizing the average stator winding losses for a given reference torque by using the first and the third spatial harmonics of the air-gap flux density. The optimal solution has been found analytically and has been developed in detail for a selected set of asymmetrical winding configurations. Both the numerical and experimental results are in good agreement with the theoretical analysis

Control of a nine-phase symmetrical PMSM with reduced rare earth material

The rising demand for high-power fault-tolerant applications such as wind generators and electric vehicles, alongside the desire to achieve better performance, have directed the interests of many research centres around the world towards electric drive configurations comprising AC machines with more than three stator phases. These so-called multiphase machines have become well recognized as an attractive alternative to the conventional three-phase machines and are used when the three-phase counterpart cannot provide a drive system with the desired performance. The Thesis examines advanced control possibilities for multiphase surface-mounted permanent magnet synchronous machines (PMSMs). Although it is well-known that permanent magnet machines are today the first choice in many applications and that their market is anticipated to catch up with the induction machines market in the near future, the main drawbacks of this machine type are the relatively high capital costs, the security of magnet supply and the environmental costs associated with the rear-earth magnet materials used in the rotor construction. This has motivated researchers to investigate methods to reduce the amount of rare earth material used in the construction of these machines. If the amount of permanent magnet material is reduced, this will inevitably result in a machine which produces lower electromagnetic tor que. On the other hand, the additional degrees of freedom, present in multiphase systems, can be exploited to inject, into the stator windings, harmonic current(s) to enhance the developed torque. This work analyses a new nine-phase symmetrical PMSM with two surface mounted magnet poles on the rotor with a shortened span. This simple design produces a highly non-sinusoidal back-electromotive force (back-EMF) comprising high third and fifth harmonic components. It is shown that these harmonic components can be utilised to boost the torque to near the value obtainable with full span magnets, provided a suitable control system is developed. The developed control algorithm is based on the well-known vector space decomposition (VSD) and classic field-oriented control methods. To test the developed control algorithm, phase domain machine model is presented first, for both sinusoidal and non-sinusoidal back-EMF distributions. To transform variables from one reference frame to another, the VSD and rotational transformations are used. The optimal ratios between fundamental and other harmonic current components are derived using the maximal torque-per-Ampere (MTPA) theory. It is shown that, by using optimal current injection, the electromagnetic torque can be improved by 36% with third harmonic only, and, up to 45% with a combination of the fundamental, the third and the fifth harmonics. Simulation results are validated in finite element method software and afterwards verified experimentally using an experimental prototype. Control of the PMSM is next expanded with position sensor fault-tolerant capability. For this purpose, the same EMF spectrum is used. When harmonic current elimination is performed in x-y subspace, remaining hth harmonic order back-EMF can be efficiently used for position angle and speed estimation. For the estimation purpose, phase-locked-loop method is employed. With estimated position/speed, a new control algorithm is devised, which combines control in two auxiliary subspaces with the control of the first plane. The third harmonic is, in combination with the fifth, used for the torque boost prior to the fault, while afterwards, the fifth EMF harmonic enables position estimation for position-sensorless control. Hence, previously stated maximal torque improvement is preserved until position sensor fault is detected, while afterwards machine continues to operate in position-sensorless mode still with partial enhancement of the torque. Control is verified experimentally. Finally, operation in the flux-weakening region is investigated. Because finding sets of multiple harmonic current references which maximize torque by taking into account voltage and current limits leads to a difficult problem to formulate, which is often impossible to solve analytically, the work presented here builds on (offline) numerical optimisation procedure. To obtain best performance, harmonics up to the (and including) fifth are considered. Limitation of voltage is achieved by comparing measured phase-to-phase voltage with maximal dc-link voltage, while thermal (RMS) constraint and inverter switch (peak) current constraint are taken into account by limiting the current. In such scenario, maximal reachable speed is much higher than the base speed, while respecting at the same time both machine and inverter constraints

General Torque Enhancement Approach for a Nine-Phase Surface PMSM with Built-in Fault Tolerance

The paper investigates maximum possible torque improvement in a two-pole surface permanent magnet synchronous machine (PMSM) with a reduced magnet span, which causes production of highly non-sinusoidal back-EMF. It contains a high third and fifth harmonics, which can be used for the torque enhancement, using stator current harmonic injection. Optimal magnet span is studied first and it is shown that with such a value the machine would be able to develop an insignificantly lower maximum torque than with the full magnet span. Next, field-oriented control (FOC) algorithm, which considers all non-fundamental EMF components lower than the machine phase number, is devised. Using maximum-torque per Ampere (MTPA) principles, optimal ratios between fundamental and all other injected components are calculated and then used in the drive control. The output torque can be in this way increased up to 45% with respect to the one obtainable with fundamental current only. Alternatively, for the same load torque, stator current RMS value can be reduced by 45%. Last but not least, a method for position sensor fault mitigation is introduced. It is based on the alternative use of a back-EMF harmonic for rotor position estimation, instead of the torque enhancement. Experimental verification is provided throughout for all the relevant aspects

EFFICIENCY AND RELIABILITY ENHANCEMENT OF MULTIPHASE SYNCHRONOUS MOTOR DRIVES

Multiphase electric machines are attractive in comparison with three-phase ones due to advantages such as fault-tolerant nature, smaller rating per phase and lower torque ripple. More specifically, the machines with multiple three-phase windings are particularly convenient, because they are suitable for standard off-the-shelf three-phase dc/ac converter modules. For instance, they are becoming a serious option for applications such as electric vehicles and wind turbines. On the other hand, in these applications, operation at low power is often required for long time intervals; hence, improving the efficiency under such conditions is highly desired and could save a significant amount of energy in the long term. This dissertation proposes a method to enhance the efficiency of electric drives based on multiple three-phase windings at light load. The number of active legs is selected depending on the required torque at each instant. To ensure that the overall efficiency is effectively optimized, not only the converter losses, but also the stator copper losses, are taken into account. Experimental results verify the theoretical outcomes. Surface-mounted permanent-magnet synchronous motors (SPMSMs) require a position measurement to ensure a high-performance control. To avoid the cost and maintenance associated to position sensors, sensorless methods are often preferred. The approaches based on high-frequency signal injection are currently a well-established solution to obtain an accurate position estimation in SPMSMs. These techniques can be roughly divided into two groups: those based on sinusoidal or on square-wave high-frequency signals. The main drawback of the former is the limitation on the response speed, due to the presence of several low-pass filters (LPFs). On the other hand, the latter methods are sensitive to deadtime effects, and high-frequency closed-loop current control is required to overcome it. This dissertation proposes to improve the sensorless strategies based on sinusoidal high-frequency injection by simplifying the scheme employed to extract the information about the position error. Namely, two LPFs and several multiplications are removed. Such simplification does not only reduce the computational complexity, but also permits to obtain a faster response to the changes in the angle/speed, and hence, a faster closed-loop control. Experimental results based on a SPMSM prove the enhanced functionality of the proposed method with respect to the previous ones based on high-frequency sinusoidal signal injection

General Torque Enhancement Approach for a Nine-Phase Surface PMSM with Built-in Fault Tolerance

The paper investigates maximum possible torque improvement in a two-pole surface permanent magnet synchronous machine (PMSM) with a reduced magnet span, which causes production of highly non-sinusoidal back-EMF. It contains a high third and fifth harmonics, which can be used for the torque enhancement, using stator current harmonic injection. Optimal magnet span is studied first and it is shown that with such a value the machine would be able to develop an insignificantly lower maximum torque than with the full magnet span. Next, field-oriented control (FOC) algorithm, which considers all non-fundamental EMF components lower than the machine phase number, is devised. Using maximum-torque per Ampere (MTPA) principles, optimal ratios between fundamental and all other injected components are calculated and then used in the drive control. The output torque can be in this way increased up to 45% with respect to the one obtainable with fundamental current only. Alternatively, for the same load torque, stator current RMS value can be reduced by 45%. Last but not least, a method for position sensor fault mitigation is introduced. It is based on the alternative use of a back-EMF harmonic for rotor position estimation, instead of the torque enhancement. Experimental verification is provided throughout for all the relevant aspects

Optimal torque/speed characteristics of a Five-Phase Synchronous Machine under Peak or RMS current control strategies

Torque density is usually improved by injecting the third current harmonic for five-phase permanent magnet synchronous machine (PMSM). It increases the degrees of freedom of a multiphase drive. However, it also separates the current limitations of the motor and the transistors, respectively related to the RMS and peak values of the currents. These two constraints are represented by Maximum Torque Per Ampere (MTPA) strategy and Maximum Torque Per Peak Current (MTPPC) strategy. In this paper, these two strategies are studied and analyzed in order to optimize the generated torque with injection of the third current harmonic. Torque improvement principle and the optimizing algorithm considering two constraints are illustrated. Then, the analytical results of these two strategies are compared and discussed. It is shown that injecting the third current harmonic can improve the torque especially at flux-weakening region. Besides, compared with MTPA, MTPPC could produce higher torque for the same inverter current limit.CE2I- Project, Hauts de France, FEDE

Control solutions for multiphase permanent magnet synchronous machine drives applied to electric vehicles

207 p.En esta tesis se estudia la utilización de un accionamiento eléctrico basado en una máquina simétrica dual trifásica aplicada al sistema de propulsión de un vehículo eléctrico. Dicho accionamiento está basado en una máquina síncrona de imanes permanentes interiores. Además, dispone de un bus CC con una configuración en cascada. Por otra parte, se incorpora un convertidor CC/CC entre el módulo de baterías y el inversor de seis fases para proveer el vehículo con capacidades de carga rápida, y evitando al mismo tiempo la utilización de semiconductores de potencia con altas tensiones nominales. En este escenario, el algoritmo de control debe hacer frente a las no linealidades de la máquina, proporcionando un comando de consigna preciso para todo el rango de par y velocidad del convertidor. Por lo tanto, deben tenerse en cuenta los efectos de acoplamiento cruzado entre los devanados, y la tensión de los condensadores de enlace en cascada debe controlarse y equilibrarse activamente. En vista de ello, los autores proponen un novedoso enfoque de control que proporciona todas estas funcionalidades. La propuesta se ha validado experimentalmente en un prototipo a escala real de accionamiento eléctrico de 70 kW, probado en un laboratorio y en un vehículo eléctrico en condiciones reales de conducción.Tecnali

Modelling and control techniques for multiphase electric drives: a phase variable approach

Multiphase electric drives are today one of the most relevant research topics for the electrical engineering scientific community, thanks to the many advantages they offer over standard three-phase solutions (e.g., power segmentation, fault-tolerance, optimized performances, torque/power sharing strategies, etc...). They are considered promising solutions in many application areas, like industry, traction and renewable energy integration, and especially in presence of high-power or high-reliability requirements. However, contrarily to the three-phase counterparts, multiphase drives can assume a wider variety of different configurations, concerning both the electrical machine (e.g., symmetrical/asymmetrical windings disposition, concentrated/distributed windings, etc...) and the overall drive topology (e.g., single-star configuration, multiple-star configuration, open-end windings, etc…). This aspect, together with the higher number of variables of the system, can make their analysis and control more challenging, especially when dealing with reconfigurable systems (e.g., in post-fault scenarios). This Ph.D. thesis is focused on the mathematical modelling and on the control of multiphase electric drives. The aim of this research is to develop a generalized model-based approach that can be used in multiple configurations and scenarios, requiring minimal reconfigurations to deal with different machine designs and/or different converter topologies, and suitable both in healthy and in faulty operating conditions. Standard field-oriented approaches for the analysis and control of multiphase drives, directly derived as extensions of the three-phase equivalents, despite being relatively easy and convenient solutions to deal with symmetrical machines, may suffer some hurdles when applied to some asymmetrical configurations, including post-fault layouts. To address these issues, a different approach, completely derived in the phase variable domain, is here developed. The method does not require any vector space decomposition or rotational transformation but instead explicitly considers the mathematical properties of the multiphase machine and the effects of the drive topology (which typically introduces some constraints on the system variables). In this thesis work, the proposed approach is particularized for multiphase permanent magnet synchronous machines and for multiphase synchronous reluctance machines. All the results are obtained through rigorous mathematical derivations, and are supported and validated by both numerical analysis and experimental tests. As proven considering many different configurations and scenarios, the main benefits of the proposed methodology are its generality and flexibility, which make it a viable alternative to standard modelling and control algorithms