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

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 Third-Harmonic Current Injection for an Asymmetrical Nine-phase PMSM with Non-Sinusoidal back-EMF

The paper investigates an optimal strategy to exploit the third harmonic current injection for the torque enhancement in a nine-phase permanent magnet synchronous machine (PMSM). The machine is with asymmetrical winding configuration and has a single isolated neutral point. The optimization follows the minimization of the average power losses for a given reference torque or, equivalently, the maximization of the developed torque for a given current RMS. It is shown that, in contrast to the situation for a symmetrical configuration, the optimal ratio between the fundamental and the third harmonic components does not correspond to the ratio between the corresponding back-EMF components. It is demonstrated that this is due to the fact that the phase currents have to sum to zero; consequently, the third harmonic current injection in different three-phase sets has to be different with regard to the magnitude and phase shift. The strategy is introduced using an entirely analytical approach and its effectiveness has been successfully validated through numerical simulations

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

Electromagnetic Performance Comparison between 12-Phase Switched Flux and Surface-Mounted PM Machines for Direct-Drive Wind Power Generation

In this article, the 12-phase switched flux permanent magnet (PM) (SFPM) machine and three surface-mounted PM (SPM) machines designed for direct-drive wind power generation are comparatively analyzed. First, feasible stator-slot/rotor-pole combinations for symmetrical 12-phase winding layout are investigated for both machine topologies. Second, the key design parameters of the PM generators including the split ratio and stator teeth width ratio are optimized by finite element analysis to achieve a high phase fundamental EMF per turn and a low cogging torque, both of which are desired by the direct-drive wind power generator. Third, electromagnetic performances including air-gap field, cogging torque, static torque, inductance, output voltage and its regulation factor, output power, and efficiency of the generators are compared. A 10-kW 24-slot/22-pole SFPM prototype is built and tested to validate the FE predicted results.</p

Optimal Third-Harmonic Current Injection for Asymmetrical Multiphase PMSMs

The paper proposes a modelling approach and an optimization strategy to exploit a third harmonic current injection for the torque enhancement in multiphase isotropic PMSMs with non-sinusoidal back-EMFs. The modelling 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 1st and the 3rd 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

High efficiency sensorless fault tolerant control of permanent magnet assisted synchronous reluctance motor

In the last decades, the development trends of high efficiency and compact electric drives on the motor side focused on Permanent Magnet Synchronous Machines (PMSMs) equipped with magnets based on the rare-earth elements. The permanent magnet components, however, dramatically impact the overall bill of materials of motor construction. This aspect has become even more critical due to the price instability of the rare-earth elements. This is why the Permanent Magnet Assisted Synchronous Reluctance Motor (PMaSynRM) concept was brought to the spotlight as it gives comparable torque density and similar efficiencies as PMSM although at a lower price accredited for the use of magnets built with ferrite composites. Despite these advantages, PMaSynRM drive design is much more challenging because of nonlinear inductances resulting from deep cross saturation effects. It is also true for multi-phase PMSM motors that have gained a lot of attention as they proportionally split power by the increased number of phases. Furthermore, they offer fault-tolerant operation while one or more phases are down due to machine, inverter, or sensor fault. The number of phases further increases the overall complexity for modeling and control design. It is clear then that a combination of multi-phase with PMaSynRM concept brings potential benefits but confronts standard modeling methods and drive development techniques. This Thesis consists of detailed modeling, control design, and implementation of a five-phase PMaSynRM drive for normal healthy and open phase fault-tolerant applications. Special emphasis is put on motor modeling that comprises saturation and space harmonics together with axial asymmetry introduced by rotor skewing. Control strategies focused on high efficiency are developed and the position estimation based on the observer technique is derived. The proposed models are validated through Finite Element Analysis (FEA) and experimental campaign. The results show the effectiveness of the elaborated algorithms and methods that are viable for further industrialization in PMaSynRM drives with fault-tolerant capabilities.En últimas décadas, las tendencias de desarrollo de accionamientos eléctricos compactos y de alta eficiencia en el lado del motor se centraron en las maquinas síncronas de imanes permanentes (PMSM) equipadas con imanes basados en elementos de tierras raras. Sin embargo, los componentes de imán permanente impactan dramáticamente en el coste de construcción del motor. Este aspecto se ha vuelto aún más crítico debido a la inestabilidad de precios de los elementos de tierras raras. Esta es la razón por la que el concepto de motor de reluctancia síncrona asistido por imán permanente (PMaSynRM) se ha tomado en consideración, ya que ofrece una densidad de par comparable y eficiencias similares a las de PMSM, aunque a un precio más bajo acreditado para el uso de imanes construidos con compuestos de ferritas. A pesar de drive PMaSynRM resulta muy complejo debido a las inductancias no lineales que resultan de los efectos de saturación cruzada profunda. Esto también es cierto para los motores PMSM polifásicos que han ganado mucha atención en los últimos años, en los que se divide proporcionalmente la potencia por el mayor número de fases. Además, ofrecen operación tolerante a fallas mientras una o más fases están inactivas debido a fallas en la máquina, el inversor o el sensor. Sin embargo, el número de fases aumenta aún más la complejidad general del diseño de modelado y control. Está claro entonces que una combinación de multifase con el concepto PMaSynRM tiene beneficios potenciales, pero dificulta los métodos de modelado estándar y las técnicas de desarrollo del sistema de accionamiento. Esta tesis consiste en el modelado detallado, el diseño de control y la implementación de un drive PMaSynRM de cinco fases para aplicaciones normales en buen estado y tolerantes a fallas de fase abierta. Se pone especial énfasis en el modelado del motor que comprende la saturación y los armónicos espaciales junto con la asimetría axial introducida por la inclinación del rotor. Se desarrollan estrategias de control enfocadas a la alta eficiencia y se deriva la estimación de posición basada en la técnica del observador. Los modelos propuestos se validan mediante Análisis de Elementos Finitos (FEA) y resultados experimentales. Los resultados muestran la efectividad de los algoritmos y métodos elaborados, que resultan viables para la industrialización de unidades PMaSynRM con capacidades tolerantes a fallas.Postprint (published version

On multi-phase machines and current harmonic injection for torque capability improvement

The increased energy demand and the need for electrical machines capable to deliver high torque and power in small volumes is pushing the research community to identify suitable solutions for this target. Nowadays, electrical machines are deeply used also in applications where the weight containment is important such as automotive and aerospace. Multi-phase electrical machines are a suitable candidate to help to get this goal. They present different advantages with respect to classical three-phase machines for example an increased machine torque capability and more tolerance to sustain fault conditions. The average torque is increased thanks to an improved winding factor whereas the fault tolerance improvement is due to the higher number of machine phases. In addition, the torque ripple is lower thanks to an improved magneto-motive force distribution. Moreover, the voltage on the single converter is lower, supplying the machine with the same current of a three-phase system. Another important advantage for the multi-phase arrangement is the possibility to control more harmonics of magnetic field independently thanks to the more degrees of freedom. It means new possibilities to implement various control techniques for improving the machine performance by the injection of additional harmonics higher than the fundamental. This thesis describes the work which has been carried out in the past three years, during the Ph.D program with the results achieved by analytical model implementations, finite element analysis simulations and experimental tests. The main target is to improve the machine torque capability and/or reduce its permanent magnet content. To reach this goal, the multi-phase re-arrangement of three-phase machines and current harmonic injection techniques are proposed for different machine topologies. An analytical model is implemented to reduce the magnet content in surface permanent magnet machines without affecting Joule losses and the average torque. The analytical model is validated via FEA. A model-free technique to improve the torque capability by current harmonic injection is proposed and its concept is validated experimentally on a V-Shape interior permanent magnet machine. Sensitivity analyses are carried out to optimise the V-Shape rotor configuration to improve the torque under fifth current harmonic injection. Studying the flux density in the stator core on a classical three-phase surface permanent magnet machine with a distributed winding layout, it is possible to highlight another advantage of multi-phase machines which consists in a better flux density distribution. The proposed work gives a contribution to the research community in terms of new solutions for increasing the torque capability and/or reducing the permanent magnet content in the machine without affecting its efficiency for different rotor topologies. Moreover, the proposed stator flux density analysis can give important information about the electromagnetic behaviour in three-phase distributed winding surface permanent magnet machines

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

Torque capability enhancement of dual three-phase PMSM drive with fifth and seventh current harmonics injection

© 2017 IEEE. A method for enhancing torque capability of a dual three-phase permanent magnet synchronous machine (PMSM) based on conventional dual three-phase drive system by injecting the fifth and seventh current harmonics without any hardware re-configuration is proposed in this paper. Compared with the third current harmonic injection, which is commonly used to enhance the torque capability of dual three-phase machine, the two isolated neutral points of each set of single three-phase windings do not need to be reconnected to the middle point of dc-link capacitors or an additional power switching bridge to provide flowing path for zero-sequence current. Further, no additional current sensors are required to obtain the feedback of zero-sequence current to regulate it effectively. For a prototype dual three-phase PMSM, the average torque increases approximately by 9% at the cost of 0.56% increase in the 12th harmonic torque ripple. The effectiveness of the torque capability enhancement is confirmed by experiments