DC-link current harmonics and common mode voltage (CMV) are key design challenges for inverter-based power electronic systems. Addressing them collectively without additional hardware and/or complexity has promising advantages. This article investigates the interleaved utilization of modified space vector PWM schemes for parallel-inverter systems, targeting a combined and simultaneous reduction of these quality concerns. A time-domain analytical dc-link current formulation and a new application of sequence-based interleaving are combined in an offline numerical optimization algorithm that locates optimal interleaving shifts synced with the PWM sequence. With simulation and experimental validation, the corresponding numerical results ascertain an effective suppression of dc-link current harmonics alongside CMV reduction. In addition, this article extends the proposed idea to a special application for 3x multiphase machines through experimental validation.

This work proposes a variable switching frequency scheme for a flying capacitor multilevel (FCML) converter in dc-ac operation to better utilize the output inductor for its rated peak-to-peak current ripple and substantially reduce converter losses over the ac line cycle. The proposed technique holds the inductor current ripple constant by leveraging the duty-cycle dependent inductor current ripple, and variably changes the switching frequency to hold this constant at its rated design value. Practical lower switching limits are taken into consideration in developing this approach, and experimental results validate the feasibility in a common micro-controller framework. Hardware results are provided for a 400 Vdc to 240 Vrms 6-level FCML inverter and showcase loss reductions on the order of 10 − 35% across the measured power range, compared to conventional control with fixed frequency operation.

Magnet-free variable phase-pole machines are competitive alternatives in electric vehicles where torque-speed operating region, reliability, cost, and energy efficiency are key metrics. However, their modeling and control have so far relied on existing fixed-phase and pole-symmetrical models, limiting their drive capabilities especially when switching the number of poles on the fly. This paper establishes the harmonic plane decomposition theory as a space-discrete Fourier transformation interpretation of the Clarke transformation, decomposing all pole-pair fields into a fixed number of orthogonal subspaces with invariant parameters. The model remains unaltered for all phase-pole configurations, guaranteeing continuity even under phase-pole transitions. Relations of the state and input space vectors, and model parameters to those of the vector space decomposition theory used for multiphase machines are established via the use of the complex winding factor. Experiments confirm the modeling theory and demonstrate its practical usefulness by performing a field-oriented-controlled phase-pole transition. Non-trivial configurations with more than one slot/pole/phase and a fractional phase number are also demonstrated.

A multiphase induction machine model using vector space decomposition provides insights into many space harmonics through decoupled reference frames. These decoupled reference frames host specific space vectors related to particular space harmonics. Based on the physical winding configuration, these vector spaces can be excited independently and simultaneously for the production of torque. However, this approach may result in beat oscillations due to interference between excited vector spaces if proper synchronization of vector spaces is not maintained. This paper describes this phenomenon through experimental tests. Furthermore, a solution eliminating the beat oscillations is proposed while optimizing the stator current or rotor flux linkage peaks. The effectiveness of the solution is experimentally verified on a 9-phase  induction machine. 

With falling semiconductor prices, multiphase electrical machines are gaining more attention from academic and industrial research. So-called variable-pole machines are multiphase induction machines with squirrel cage rotors with the ability to be excited with a different number of magnetic pole pairs. Using this kind of machines can reduce the total cost of ownership of an electrical drive, mainly through reduction of the cost of running and the cost of NOT running. The omission or reduction of the mechanical gear has the potential to increase the overall system efficiency. Moreover, the additional degrees of freedom allow for true fault tolerance.

This work first introduces the harmonic plane decomposition theory for a unified machine model independent of the excitation of the variable-pole machine. This allows continuous modeling in any condition without model discontinuity. Based on the harmonic plane decomposition, this work presents vector control schemes. Subsequently, it presents pole transition strategies generating the control references for minimum torque dip pole transitions. Moreover, flux weakening during the pole transition maintains the voltage within the drive’s limits.

Next, this work presents the harmonic-plane-decomposition fault detection, a fast, non-invasive, and computational lean fault detection for variable pole machines. By leveraging the additional degrees of freedom due to the increased amount of independent currents, the fault detection shifts partially from information over time to information over space. It continues by demonstrating a pole transition under faulty conditions by adapting a minimum stator copper losses current injection post fault control such that it can carry out a pole transition. In this way, true fault tolerance is achieved.

Lastly, this work also shows a sensorless operation of a multiphase electrical machine capable of operation at low and zero speeds. This is achieved by separating the torque generation and the sensorless observer in different harmonic planes as well as introducing feedback for all estimated state variables.

This work shows technical solutions to take advantage of all independent currents in a variable-pole machines. Applying these techniques in a drive system poses electrical drives with such machines as a viable option for industrial and traction applications.

Med sjunkande priser på halvledare får flerfasiga elektriska maskiner mer uppmärksamhet inom akademisk och industriell forskning. Så kallade variabelpoliga maskiner är flerfasiga induktionsmaskiner med burrotorer som kan exciteras med olika antal av magnetiska polpar. Användning av denna typ av maskiner kan minska den totala ägandekostnaden för en elektrisk drivenhet, främst genom minskning av kostnaden för att köra och kostnaden för att INTE köra. Utelämnandet eller minskningen av den mekaniska växeln har potential att öka den totala systemeffektiviteten. Dessutom möjliggör de ytterligare frihetsgrader en sann feltolerans.

Detta arbete introducerar först teorin om harmonisk planedekomposition för en enhetlig maskinmodell oberoende av variabel-polmaskinens excitation. Detta möjliggör en kontinuerlig modellering under alla förhållanden utan modelldiskontinuitet. Baserat på harmonisk planedekomposition presenterar detta arbete vektorstyrningsmetoder. Därefter presenteras strategier för polövergång som genererar styrreferenser för minimala momentdipolövergångar. Dessutom bibehåller fältsförsvagning under polövergången spänningen inom driftens gränser.

Därefter presenterar detta arbete feldetektering med harmonisk plan dekomposition feldetektering, en snabb, icke-invasiv och beräkningsmässigt enkel feldetektering för variabelpoliga maskiner. Genom att utnyttja de ytterligare frihetsgraderna på grund av den ökade antalet oberoende strömmar, skiftar feldetekteringen delvis från information över tid till information över rum. Arbetet fortsätter med att demonstrera en polövergång under felaktiga förhållanden genom att anpassa en ströminjektion för minimala kopparförluster i statorn, så att den kan utföra en polövergång. På så sätt uppnås sann feltolerans.

Slutligen visar detta arbete också en sensorlös drift av en flerfasig elektrisk maskin som kan arbeta vid låga hastigheter och nollhastigheter. Detta uppnås genom att separera momentgenereringen och den sensorlösa observatören i olika harmoniska plan samt genom att införa återkoppling för alla uppskattade tillståndsvariabler.

Detta arbete presenterar tekniska lösningar för att dra nytta av alla oberoende strömmar i variabel-polmaskiner. Att tillämpa dessa tekniker i ett drivsystem gör elektriska drivsystem med sådana maskiner till ett livskraftigt alternativ för industriella och dragapplikationer.

The Clarke transformation is mathematically a Vandermonde matrix of a discrete Fourier transformation. Its purpose is to transform space-vector quantities in the space domain. One necessary condition is that the base function forms an n-th root of unity. This condition translates into requiring all sampling points to be equally spaced. However, manufacturing constraints and imperfections in electric machines may result in non-equally spaced magnetic axes, especially in the case of multiphase electrical machines, making the conventional inverse Clarke transformation used in classic vector control inaccurate. The mathematical inverse Clarke transformation solves this issue, and the existence of this matrix is always ensured. This avoids additional control action and for model predictive controllers, an accurate model is crucial. Analytical results and a simple experimental validation on a multiphase induction machine show the improvements introduced by changing the transformation matrix.

The Clarke transformation is mathematically a Vandermonde matrix of a discrete Fourier transformation. Its purpose is to transform space-vector quantities in the space domain. One necessary condition is that the base function forms an n-th root of unity. This condition translates into requiring all sampling points to be equally spaced. However, manufacturing constraints and imperfections in electric machines may result in non-equally spaced magnetic axes, especially in the case of multiphase electrical machines, making the conventional inverse Clarke transformation used in classic vector control inaccurate. The mathematical inverse Clarke transformation solves this issue, and the existence of this matrix is always ensured. This avoids additional control action and for model predictive controllers, an accurate model is crucial. Analytical results and a simple experimental validation on a multiphase induction machine show the improvements introduced by changing the transformation matrix.

Multiphase induction machines are investigated as competitive magnet-free candidates for high torque-density, overload-capable, and wide torque-speed range applications. These benefits are enabled by torque-enhancing harmonic injection and phase-pole changes. However, such operational modes may subject the different harmonic planes to cross-saturation. Consequently, a well-performing drive requires an accurate model. This paper models the inter-plane cross saturation between the excited harmonic planes in the main flux path and the rotor slot-bridges. It is valid even for unsynchronized harmonic magnetic fields manifesting themselves during current control as low-frequency beats in the voltage amplitude. An advanced Gamma-model also including the skin effect of the rotor is proposed and validated against experiments. The results indicate that the proposed model represents the behavior of a 15-kW variable phase-pole machine more accurately than a constant parameter Gamma-model. This warrants better field orientation and torque output predictions in future drives.

Multiphase electrical machines are inherently fault tolerant due to the higher number of phases. An important step in achieving a fault tolerant control is the ability to detect and identify the fault. In variable phase-pole drives, which are a class of multiphase machines that change the number of pole pairs during real-time operation, there are further unexplored possibilities. Based on the harmonic plane decomposition theory for variable phase-pole machines, a fault detection and identification algorithm is proposed, which analyzes the spatial harmonics of the current distribution. The fault detection is fast, operation independent, non-invasive, parameter-insensitive, and computationally simple. Experimental tests validate the proposed method.

Automotive applications are revisiting the use of induction machines as magnet-free propulsive solutions due to theirintrinsic robustness and reliability. Special multiphase configurations are under investigation to reduce the losses further andfulfill the stringent energy-efficiency and compactness requirements of the automotive industry. One of these configurations isknown as variable-pole machines, which allows the number ofmagnetic pole pairs to change on the fly. These machines canstretch the torque-speed operating region, exploit the maximumtorque capability, and exhibit competitive efficiency. Althoughfault tolerance has been widely explored for multiphase machines,the same cannot be said for variable-pole machines because,until recently, complete models to describe their dynamics under any condition, including magnetic pole changing and faultoccurrences, were unavailable. This paper presents a post-faultcontrol strategy for variable-pole machines with an open-phasefault, which can operate during pole changing and addressesthe issue of fault-tolerant operation. The effectiveness of thecontrol system is verified by experimental tests carried out withan eighteen-phase variable-pole induction machine prototype.