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This article constitutes an introductory part of the special section on Intelligent Fault Monitoring and Fault-Tolerant Control in Power Electronics, Drives and Renewable Energy Systems. In the current issue of the journal, the first part of this section is published. Accepted articles are focussed mainly on the sensor-fault diagnosis methods for T-type inverter-fed dual-three phase PMSM drives, partial demagnetization, faults of the permanent magnet synchronous generator (PMSG) and online open phase fault detection (FD) in the sensorless five-phase induction motor drive implemented with an inverter output LC filter and third harmonic injection. Also, neural networks (NN) application in the detection of stator and rotor electrical faults of induction motors has been proposed in one of the papers, and the observer-based FD concept for unknown systems using input–output measurements was applied to a brushless direct current motor drive with unknown parameters.

Muitiphase variable-speed drives, supplied from two-level voltage source inverters (VSIs), are nowadays considered for various industrial applications. Although numerous pulsewidth modulation (PWM) schemes for multiphase VSIs, aimed at sinusoidal output voltage generation, have been developed, no detailed analysis of the impact of these modulation schemes on the output current ripple has ever been reported. This paper presents a comprehensive analytical analysis and comparison of the output current ripple caused by the application of three different continuous PWM schemes, using a five-phase VSI as an example. Main properties of sinusoidal PWM, fifth harmonic injection PWM, and space vector PWM are elaborated and analyzed using the harmonic flux concept. Space vector theory is applied in the analysis. As a result, harmonic distortion factors are obtained for each PWM scheme. Theoretical considerations are verified by simulations and experimental investigation using a custom-designed five-phase VSI-fed induction motor drive. Index Terms--Harmonic distortion factor, multiphase drives, output current ripple, pulsewidth modulation (PWM), voltage source inverters.

Since independent control of an ac machine requires only two currents, additional degrees of freedom that exist in multiphase (n > 3) machines can be used for other purposes. One possibility is the series connection of the stator windings, in an appropriate manner, which enables independent control of a set of machines although a single muitiphase voltage-source inverter (VSI) is used as the supply. The concept has been explored recently in a considerable depth. This paper examines an alternative to the series connection, parallel connection of a multitude of multiphase machines supplied from a single VSI. This eliminates a shortcoming of the series arrangement, the requirement to have open-end stator winding machines. The general principles of paralleling the multiphase machines to a single multiphase VSI are at first set forth, with the subsequent development concentrating on the two-motor five-phase drive system. Dynamic and steady-state models of the two-motor drive are developed, and the vector control scheme is explained. Theoretical findings are verified by extensive experimental investigation on a laboratory setup. It is shown that parallel-connected multiphase multimotor drives represent an advanced example of the well-known analogy between series and parallel connection in electrical engineering. The quality of dynamic performance is the same as with series-connected drive systems. However, parallel connection suffers from a number of serious shortcomings that will prevent its application in industry. Index Terms--Dynamic and steady-state modeling, independent vector control, multiphase ac drives, parallel connection.

This paper develops a seven-level inverter structure for open-end winding induction motor drives. The inverter supply is realized by cascading four two-level and two three-level neutral-point-clamped inverters. The inverter control is designed in such a way that the common-mode voltage (CMV) is eliminated. DC-link capacitor voltage balancing is also achieved by using only the switching-state redundancies. The proposed power circuit structure is modular and therefore suitable for fault-tolerant applications. By appropriately isolating some of the inverters, the drive can be operated during fault conditions in a five-level or a three-level inverter mode, with preserved CMV elimination and dc-link capacitor voltage balancing, within a reduced modulation range. Index Terms--Capacitor voltage balancing, common-mode voltage (CMV) elimination, multilevel inverter, open-end winding induction motor.

Two series-connected two-motor drive systems with a single inverter supply have been proposed recently. The first one consists of two five-phase machines supplied from a single five-phase inverter, whereas the second one comprises one symmetrical six-phase machine, a three-phase machine, and a single six-phase inverter. It has been verified experimentally that, by introducing an appropriate phase transposition in the series connection, completely independent dynamic control of the two machines can be realized using principles of vector control. Detailed dynamic models in the stationary common reference frame have also been reported for the two drive systems. These are taken here as the starting point, and equivalent circuit steady-state representation of the series-connected two-motor drive systems is developed. The equivalent circuits can then be used to evaluate steady-state characteristics of the drives in the same manner as it is done for three-phase induction machines, as well as for the inverter design. The dc-link voltage required for operation of the two series-connected machines is investigated next. It is shown that the specific winding connections lead to a dc voltage requirement that is smaller than it would have been had the windings of the two machines been connected directly in series. Experimental results, obtained during steady-state operation of the series-connected five-phase and six-phase two-motor drives, are presented. Developed equivalent circuit representation is finally verified by comparing calculated and experimentally obtained values for various voltages of the two-motor drives. Index Terms--Equivalent circuits, five-phase, induction motors, multimotor drives, multiphase machines, six-phase, vector control.

Two-level multiphase voltage source inverters (VSIs) are typically used as the supply for multiphase machines. Such machines are often with concentrated winding in which case, certain low-order stator current harmonics are injected to provide torque enhancement. Also, a multimotor drive system can be realized by connecting a number of multiphase machines in series, using phase transposition while supplying the whole system from a single multiphase VSI. In both situations, it is important to know the limits of the inverter operation in the linear modulation region. This paper develops a simple method that enables analytical determination of the boundaries of the linear modulation region for all multiphase inverters with a prime number of phases. The limits are independent of the actual pulse width modulation (PWM) method utilized, and are equally applicable to both carrier-based and space vector PWM techniques. Theoretical considerations are verified experimentally using the five-phase and seven-phase VSI rigs. Index Terms--Dc-bus utilization, multiphase ac drives, multiphase voltage source inverters (VSIs), pulse width modulation (PWM).

The paper presents a five-level inverter scheme with reduced power circuit complexity for an induction motor drive. The scheme is realized by cascading conventional two-level and three-level neutral point clamped inverters in conjunction with an open-end winding three-phase induction motor drive. An inverter control scheme with common-mode voltage (CMV) elimination, along with a simple dc link voltage control, is developed by using only switching states with zero CMV for the entire modulation range. Theoretical considerations are experimentally verified for a variety of operating conditions. Index Terms--Common-mode voltage (CMV) elimination, induction motor drives, multilevel inverter, open-end winding structure.

This paper analyzes a recently introduced two-motor five-phase drive system with series connection of stator windings. It has been shown, using physical reasoning, that the introduction of an appropriate phase transposition in the series connection of two machines leads to a complete decoupling of the flux/torque-producing currents of one machine from the flux/torque-producing currents of the second machine. Consequently, independent vector control of the two machines becomes possible while using a single current-controlled five-phase voltage source inverter as the supply. The drive system modeling and control are first elaborated in this paper by taking both machines as induction motors. It is shown, using rigorous mathematical derivations, that the independent control of the two machines results due to the placement of machines in two different subspaces of the five-dimensional space. This is enabled by phase transposition in series connection. The models of the complete drive in the stationary common reference frame and in the rotor-flux-oriented reference frames of the two machines are developed. An associated vector control scheme for the two-motor drive is presented next. The second part of this paper describes an experimental setup used further on to evaluate the dynamic behavior of the two-motor drive. Performance is investigated by extensive experimentation for various transients (acceleration, deceleration, reversing, and disturbance rejection). Excellent decoupling of control of the two machines is achieved. Both the concept of the drive and the approach to modeling and control are thus fully verified experimentally. Finally, the advantages and shortcomings of the series-connected five-phase two-motor drive are discussed, and potential application areas are highlighted. Index Terms--Experimental investigation, five-phase machines, two-motor drives, variable-speed drives, vector control.

Multimotor drives, based on utilization of multiphase machines, have been proposed recently. Independent vector control of all the machines of the group can be achieved while using a single multiphase voltage source inverter supply, provided that the stator windings are connected in series in an appropriate manner. The concept is applicable to any system phase number greater than or equal to five. Detailed theoretical and simulation studies have been reported for multiphase multimotor drive configurations of this type and it has been suggested that the concept is valid regardless of the type of ac machines. This paper examines operation of such a drive system when a five-phase induction machine is connected in series with a five-phase synchronous reluctance machine and provides detailed experimental verification of the possibility of independent control of the two-motor drive comprising five-phase motors of different types. The operating principles of the drive system are at first reviewed and a brief description of the experimental system is then given. An extensive presentation of experimental results, collected from a laboratory setup, is further provided. It is shown that fully independent and decoupled vector control of the two machines results, although a single supply source is used. Index Terms--Experimental verification, five-phase, multimotor drives, multiphase machines, vector control.

Vector control accuracy of induction motor drives is affected by variations of motor parameters that are treated in the control algorithm as constant values, and by the phenomena that are not modeled at all and are therefore unaccounted for in the controller. Detuning sources of the first type include variations of rotor and stator resistance, mutual inductance, and leakage inductances, while the second category includes stator iron (core) losses. All these sources of detuned operation have been studied in a considerable depth in the past. It appears that the only potential source of detuned operation, which has never been studied before, is the stray load loss (SLL), which belongs to the category of unmodeled phenomena. This paper develops an analytical model that characterizes detuning due to SLLs in indirect rotor-flux-oriented (RFO) current-fed induction motor drives in steady-state operation by means of the orientation angle error, actual to reference rotor-flux ratio, and actual to reference-torque ratio. A quantitative assessment of the impact of SLL on accuracy of rotor-flux-oriented control is performed, with the necessary motor parameters obtained from IEEE 112-B standard measurements and subsequent equivalent circuit parameter fitting. Detuning is also examined in transient operation. It is shown that, although SLLs are comparable to the iron losses in the studied machine (of approximately the same value in the rated operating point), their impact on accuracy of vector control is much smaller when compared to the iron loss induced detuning. Index Terms--Induction machines, stray load losses, vector control.

A novel concept for multimotor drives, based on utilization of multiphase machines, has been proposed recently. Independent vector control of all the machines of the group can be achieved while using a single multiphase inverter supply, provided that the stator windings are connected in series in an appropriate manner. One specific case of such a drive system is a two-motor drive, comprising a symmetrical six-phase machine connected in series with a three-phase machine. The group is supplied from a six-phase current-controlled voltage source inverter (VSI). The available considerations related to this two-motor drive system, consisting of two induction motors, are currently of purely theoretical nature, with the proof-of-concept provided by simulation only. This paper analyzes series-connected two-motor six-phase drive, comprising a six-phase symmetrical induction machine and a three-phase permanent magnet synchronous machine (PMSM) and provides, for the first time, detailed experimental verification of the possibility of independent control of the two motors. The operating principles of the drive system are at first reviewed and a brief description of the experimental system is then given. The emphasis is placed on an extensive presentation of experimental results, collected from a laboratory setup. 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. Index Terms--Experimental verification, multimotor drives, multiphase machines, six-phase, vector control.

This paper analyzes a six-phase two-motor drive, consisting of a six-phase voltage-source inverter (VSI), a six-phase induction machine, and a three-phase induction machine. Stator windings of the two machines are connected in series in an appropriate manner. This enables full decoupling of the dynamics of the two machines by means of vector control. Detailed mathematical modeling of the system is performed and the set of d-q equations, describing the two machines connected in series to a six-phase VSI, is developed. The resulting model clearly shows the possibility of independent vector control of the two machines, although a single VSI is used as the supply. An experimental rig is constructed and existence of the decoupled dynamic control of the two machines supplied from a single VSI is fully verified by extensive experimental testing. Index Terms--Multimotor drives, multiphase machines, six-phase, vector control.

Independent flux and torque control of an ac machine can be achieved by means of vector control, utilizing only two stator d-q current components. Consequently, in ac machines with a phase number greater than three, there exist additional degrees of freedom. Although they can be used to enhance the torque production of a multiphase machine through injection of higher stator current harmonics, an entirely different purpose is possible as well. The additional degrees of freedom can be utilized to control independently other machines within a multimotor drive system. In order to do so, it is necessary to connect stator windings of all the multiphase machines in series, with an appropriate phase transposition, apply a vector control algorithm to each machine separately, and supply the stator windings of the multi-machine system from a single current controlled voltage source inverter (VSI). Inverter current control is performed in the stationary reference frame, using inverter phase currents. The foundations of the concept are set forth in the paper, for an arbitrary odd n-phase case, using the general theory of electrical machines. Further analysis is performed for all the theoretically possible odd phase numbers and it is shown that the number of machines connectable in series depends on the properties of the phase number. Connection diagrams are illustrated next for some selected phase numbers and vector control, including the inverter reference current generation, is detailed for the multimotor drive system. The main advantages and drawbacks of the concept are discussed and verification is provided by simulation of a nine-phase four-motor drive system. Index Terms--Multimotor drives, multiphase machines, vector control.

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. Index Terms--Multimotor drives, multiphase machines, series connection, vector control.

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. Index Terms--Competitive neural network, fuzzy model, process industry modeling, speed estimation of motor drives, system identification.

Optimal efficiency control of induction motor drives implies operation at reduced flux levels with light loads. Two problems in light load operation are a large speed drop after sudden load torque increase and slow acceleration. In order to improve response in these transients, an algorithm for optimum dynamic distribution of the available maximum inverter current into the flux-producing and the torque-producing stator current components is developed in this paper. The proposed algorithm accounts for the main flux saturation effect in the machine and the dynamics of the flux variation. Its performance is illustrated by means of simulation and experimental results. Superiority of the developed algorithm over some of the existing methods is proved by comparing the speed drops, which result after sudden load torque increase during operation at light load, and by examining an acceleration transient under light load condition. Index Terms-Current limit, disturbance rejection, induction motor drives, main flux saturation, vector control.

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 Popov's 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. Index Terms-MRAS estimator, sensorless vector control, stator Resistance estimation.

This paper proposes a practical digital-signal-processor (DSP)-based algorithm for the online efficiency optimization of induction motor drives. The algorithm combines good features of the search control and loss-model-based control, while simultaneously eliminating their major drawbacks. It ensures very fast convergence without the need for any a priori knowledge of the motor loss-related parameters. The minimum loss operating point is decided from a functional approximation of the motor and the power converter losses, in the form of a suitably defined loss function. The loss function parameters are obtained online from input power measurement and a dedicated identification routine acting in conjunction with the common drive control functions. Proposed online identification of the loss function parameters relies upon the signals and states readily available within the drive controller memory and does not involve the user entry nor the initialization procedure. The proposed controller is coded into a conventional low-cost 16-bit DSP and verified on a 2.2-kW induction motor drive prototype. Index Terms--Digital signal processors (DSPs), induction motor drives (IMDs), minimum energy control, power conversion.

Mutual inductance of an induction machine may vary considerably when the flux reference varies. An important and frequent application of a variable flux reference is the operation in the field-weakening region. Standard assumption of constant mutual inductance is no longer valid and it becomes necessary to compensate for the mutual inductance variation. The paper proposes a novel method for online mutual inductance identification in vector controlled induction machines. The method is characterized with very simple structure, ease of implementation, very low parameter sensitivity, and capability to provide an accurate estimate under both transient and steady state operating conditions. Full experimental verification of the proposed scheme is provided and a number of potential applications in a vector controlled induction motor drive are discussed. Index Terms--Induction motor drives, main flux saturation, mutual inductance identification, vector control.

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 online and offline 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. Index Terms--Induction motor drives, parameter offline identification, parameter online estimation, vector control.

One of the frequently discussed applications of artificial intelligence in motion control is the replacement of a standard proportional pins integral (PI) speed controller with a fuzzy logic (FL) speed controller. Regardless of all the work, it appears that a thorough comparison of the drive behavior under PI and FL speed control is still missing. This paper attempts to fill in this gap, by providing an in-depth comparison of operation of a vector-controlled permanent-magnet synchronous motor, using at first an experimental rig. Speed responses, obtained under PI and FL speed control, are recorded and compared for a variety of operating conditions. The transients studied include response to large step speed command from standstill with nominal inertia and an increased inertia, response to small step speed reference change and response to step load torque application. The transient behavior is examined for various initial speed settings, so that a thorough comparison is enabled. Experimental results are further supplemented with a set of simulation results, obtained using a different permanent-magnet machine and a different FL controller. Better generalization of the results is enabled in this way. It is shown that superiority of the FL speed control is less pronounced than it is often portrayed in the literature on the basis of limited comparisons. Indeed, in a number of cases, PI speed control provided a superior speed response. Index Terms--Comparative analysis, fuzzy logic speed control, porportional plus integral speed control, vector-controlled ac drives.

The paper proposes a modified version of the model reference adaptive system (MRAS) based speed estimator, whose outputs of the reference and the adjustable model are rotor flux space vectors. The estimator is modified in such a way that the variation in the instantaneous level of the main flux saturation during operation in the field weakening is recognized and properly compensated at all times. The speed estimation scheme is equally applicable to both vector controlled and direct torque controlled induction machines, since it operates in the stationary reference frame and requires measurement of only stator voltages and currents. Verification of the proposed scheme is provided by simulation and by experimentation on an indirect feed-forward rotor flux oriented induction machine for speed references of up to twice the base speed. Index Terms--Field weakening operation, induction motor drives, main flux saturation, sensorless control, speed estimation.

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.

Vector control principles are derived under the assumption that iron core loss may be neglected. The paper discusses effects of this simplification on performance of vector controlled induction machines. Indirect stator, air gap and rotor flux oriented control of a current fed machine are dealt with, with the emphasis on indirect rotor flux oriented control. Models of vector controlled induction machines that account for the iron loss are derived, and the mechanism of core loss influence on behavior of the drives is highlighted. It is shown that detuning caused by neglecting the core loss will inevitably take place. The amount of detuning is investigated for steady-state operation, for all three types of vector control. Some insight into detuning during transient operation is provided for the case of indirect rotor flux oriented induction machine. The concluding part of the paper presents a new decoupling circuit for indirect rotor flux oriented control. The circuit accounts for iron loss and thus eliminates detuning which is otherwise present.