Emil Levi

Multiphase Electric Machines for Variable-Speed Applications

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.

A review of RFO induction motor parameter estimation techniques

An induction motor is the most frequently used electric machine in high-performance drive applications. Control schemes of such drives require an exact knowledge of at least some of the induction motor parameters. Any mismatch between the parameter values used within the controller and actual parameter values in the motor leads to a deterioration in the drive performance. Numerous methods for induction machine on-line and off-line parameter estimation have been developed exclusively for application in high-performance drives. This paper aims at providing a review of the major techniques used for the induction motor parameter estimation. The paper is illustrated throughout with experimental and simulation examples related to various parameter estimation techniques.

A novel concept of a multiphase, multimotor vector controlled drive system supplied from a single voltage source inverter

Since variable speed electric drive systems are supplied from power electronic converters, it is possible to utilize ac machines with a phase number higher than three. It is shown in the paper, using general theory of electrical machines, that an increase of the stator phase number to at least five (or more) enables completely independent vector control of two (or more) multiphase machines that are supplied from a single current-controlled voltage source inverter. In order to achieve such an independent control it is necessary to connect multiphase stator windings of the machines in series and perform an appropriate phase sequence transposition. The concept is equally applicable to any multiphase ac machine type and its major advantage, compared to an equivalent multimotor three-phase drive system, is the saving of a certain number of inverter legs. The actual saving depends on the number of phases. The concept is introduced using an n-phase induction machine as the starting point and further analysis is restricted to an odd number of phases, for the reason explained in the paper. Classification of all the possible cases that may arise is given, followed by presentation of connection diagrams for selected phase numbers. Detailed verification of the proposed concept is provided by simulating the operation of a seven-phase three-motor drive system, controlled using indirect rotor flux oriented control principles. Some preliminary experimental results, which confirm the feasibility of a two-motor series-connected drive system, are included as well. The main advantages and drawbacks of the concept, when compared with an equivalent three-phase multimotor drive system, are finally addressed.

A unified approach to main flux saturation modelling in D-Q axis models of induction machines

Main flux saturation is most frequently modelled by selecting either stator and rotor d-q axis currents or stator and rotor d-q axis flux linkages as state-space variables. This paper attempts to unify main flux saturation modelling in d-q axis models of induction machines by presenting a general method of saturation modelling. Selection of state-space variables in the saturated machine model is arbitrary and appropriate models in terms of different state-space variables result by application of the method. A couple of models, obtainable with different selection of state-space variables, are presented. The cross-saturation effect is explicitly present in all the models, except for the one with stator and rotor flux linkage d-q axis components as state-space variables. The models are verified by simulation and experimental investigation of induction generator self-excitation. >

Identification of complex systems based on neural and Takagi-Sugeno fuzzy model

The paper describes a neuro-fuzzy identification approach, which uses numerical data as a starting point. The proposed method generates a Takagi-Sugeno fuzzy model, characterized with transparency, high accuracy and a small number of rules. The process of self-organizing of the identification model consists of three phases: clustering of the input-output space using a self-organized neural network; determination of the parameters of the consequent part of a rule from over-determined batch least-squares formulation of the problem, using singular value decomposition algorithm; and on-line adaptation of these parameters using recursive least-squares method. The verification of the proposed identification approach is provided using four different problems: two benchmark identification problems, speed estimation for a DC motor drive, and estimation of the temperature in a tunnel furnace for clay baking.

Advances in Converter Control and Innovative Exploitation of Additional Degrees of Freedom for Multiphase Machines

Multiphase variable-speed drives and generation systems (systems with more than three phases) have become one of the mainstream research areas during the last decade. The main driving forces are the specific applications, predominantly related to the green agenda, such as electric and hybrid electric vehicles (EVs), locomotive traction, ship propulsion, “more-electric” aircraft, remote offshore wind farms for electric energy generation, and general high-power industrial applications. As a result, produced body of significant work is substantial, making it impossible to review all the major developments in a single paper. This paper therefore surveys the recent progress in two specific areas associated with multiphase systems, namely power electronic supply control and innovative ways of using the additional degrees of freedom in multiphase machines for various nontraditional purposes.

Space vector modulation schemes for a five-phase voltage source inverter

Application of power electronics in electric drives enables utilisation of AC machines with a phase number higher than three. Such multiphase motor drives are nowadays considered for various applications. Multiphase drives are invariably supplied from multiphase voltage source inverters (VSIs) and adequate methods for VSI pulse width modulation (PWM) are therefore required. This paper analyses different space vector PWM (SVPWM) schemes for a five-phase VSI, which can be used for five-phase motor drives. A detailed model of a five-phase VSI is presented first in terms of space vectors. Next, the existing technique of utilising only large space vectors is elaborated. It is shown that this SVPWM method leads to generation of high amounts of low-order output voltage harmonics. Finally, a novel SVPWM method is introduced, which enables operation with pure sinusoidal output voltages up to a certain reference voltage value, which is smaller than the maximum achievable with the given DC link voltage. To enable full utilisation of the DC bus voltage, this SVPWM scheme is complemented with another one, which unavoidably does lead to generation of some low order harmonics. These harmonics are however of significantly lower values than when only large vectors are used. A detailed performance evaluation of the existing and newly developed schemes is done and it is presented in terms of quality of the output voltage waveforms. Simulation results are included throughout the paper to illustrate and verify the theoretical considerations

A stator resistance estimation scheme for speed sensorless rotor flux oriented induction motor drives

Accurate knowledge of stator resistance is of utmost importance for correct operation of a number of speed sensorless induction motor control schemes in the low speed region. Since stator resistance inevitably varies with operating conditions, stable and accurate operation at near-zero speed requires an appropriate online identification algorithm for the stator resistance. The paper proposes such an identification algorithm, which is developed for the rotor flux-based model reference adaptive system (MRAS) type of the speed estimator in conjunction with a rotor flux oriented control scheme. In this speed estimation method, only one degree of freedom (out of the two available) is utilized for speed estimation. It is proposed to utilize the second available degree of freedom as a means for adapting the stator resistance online. The parallel stator resistance and rotor speed identification algorithm is developed in a systematic manner, using Popovs hyperstability theory. It increases the complexity of the overall control system insignificantly and enables correct speed estimation and stable drive operation at near-zero speeds. The proposed speed estimator with parallel stator resistance identification is at first verified by simulation. Extensive experimentation is conducted next at low speeds of rotation and successful stator resistance identification is achieved down to 0.5-Hz frequency of rotation.

Iron loss in rotor-flux-oriented induction machines: identification, assessment of detuning, and compensation

Iron loss, traditionally ignored in vector control schemes, has recently attracted more attention as a cause of detuned operation of rotor-flux-oriented induction machines. Appropriate mathematical tools, that enable evaluation of detuning due to iron loss, have become available, and these have been used so far only in assessment of detuning for rated speed operation in the constant flux region. The available studies are based on the measurement of iron losses with voltage supply of rated frequency. This paper attempts to provide a more detailed treatment of iron loss induced detuning in rotor-flux-oriented induction machines by presenting at first an experimental method of iron loss identification over the entire frequency (speed) range of interest. The experimental results enable calculation of the equivalent iron loss resistance that is subsequently used in evaluation of detuning. The regimes dealt with encompass motoring and braking operation in the base speed range and motoring in the field-weakening region up to the five times rated speed. It is shown that detuning in the base speed range will be the highest at rated speed operation and will exhibit opposite trends in motoring and braking regions. Detuning in the field-weakening region is found to be significantly in excess of the one at rated speed, provided that the machine operates at high speeds with relatively light loads. As compensation of iron loss seems to be necessary in this case, the concluding part of the paper presents a novel rotor flux estimator that utilizes experimentally identified equivalent iron loss resistance values and enables elimination of detuning that is otherwise present. The estimator is a modified version of the well-known scheme that operates on the basis of measurement of stator currents and rotor speed (position). Its ability to compensate for iron loss is verified by simulation.

Generalised Sinusoidal PWM with Harmonic Injection for Multi-Phase VSIs

Application of power electronics in electric drives enables utilisation of ac machines with a phase number higher than three. Such multi-phase motor drives are nowadays considered for various applications. Multi-phase drives are invariably supplied from multi-phase voltage source inverters (VSIs) and adequate methods for VSI pulse width modulation (PWM) are therefore required. This paper deals with carrier-based PWM, where harmonic injection enables an improvement in the dc bus utilisation without moving into the over-modulation region, while still providing sinusoidal output phase voltages. A generalised concept of n th harmonic injection for n-phase inverters is presented. The viability of the proposed concept is proved by simulation taking five-phase VSI as an example. The method of fifth-harmonic injection in five-phase VSIs enables operation with pure sinusoidal voltages up to a certain reference voltage value which is smaller than the maximum achievable with the given dc link voltage. Sinusoidal PWM is further modified, by injecting other harmonics, in order to enable full utilisation of the dc bus voltage. The correlation of this PWM scheme with space vector PWM, based on utilisation of large vectors only, is established in this way. Some experimental results, illustrating application of the sinusoidal PWM in series-connected five-phase two-motor drives, are included.