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.

Multiphase Electric Machines for Variable-Speed Applications

The recently introduced proportional-resonant (PR) controllers and filters, and their suitability for current/voltage control of grid-connected converters, are described. Using the PR controllers, the converter reference tracking performance can be enhanced and previously known shortcomings associated with conventional PI controllers can be alleviated. These shortcomings include steady-state errors in single-phase systems and the need for synchronous d-q transformation in three-phase systems. Based on similar control theory, PR filters can also be used for generating the harmonic command reference precisely in an active power filter, especially for single-phase systems, where d-q transformation theory is not directly applicable. Another advantage associated with the PR controllers and filters is the possibility of implementing selective harmonic compensation without requiring excessive computational resources. Given these advantages and the belief that PR control will find wide-ranging applications in grid-interfaced converters, PR control theory is revised in detail with a number of practical cases that have been implemented previously, described clearly to give a comprehensive reference on PR control and filtering.

Proportional-resonant controllers and filters for grid-connected voltage-source converters

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 operation.

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Multiphase induction motor drives - : a technology status review

Voltage source inverters connected to the grid in applications such as active rectifiers, active filters, uninterruptible power supplies, and distributed generation systems need an optimal ac current control. To obtain zero steady-state error at the fundamental frequency (i.e., unity power factor), the use of a standard integrator in a rotating frame is as effective as the use of a resonant controller in a stationary frame. However, the grid voltage harmonics influence the current controller and generate current harmonics unless several integrators in multiple rotating frames or resonant compensators in a stationary frame are adopted. In this letter, a hybrid system consisting of a proportional integral (PI) controller plus a generic hth harmonic resonant controller implemented in a frame rotating at the n th harmonic frequency is discussed in detail. The hth harmonic controller is able to decrease both the (h - n)th and (h + n)th harmonics, while the PI controller is able to decrease other harmonics if the synchronization phase signal adopted for the frame transformation is unfiltered. It is demonstrated that the use of a PI and sixth harmonic resonant compensator is effective for both positive and negative sequence fifth and seventh harmonics; hence, four harmonics are compensated with the proportional integral-resonant (PI-RES) controller implemented in a synchronous frame. Simulation and experimental tests validate the proposed analysis

Multiple harmonics control for three-phase grid converter systems with the use of PI-RES current controller in a rotating frame

Review of multilevel voltage source inverter topologies and control schemes

In this paper, a current control scheme, based on proportional-integral regulators using sinusoidal signal integrators (SSIs), is proposed for shunt type power conditioners. The aim is to simplify the implementation of SSI-based current harmonic compensation for industrial implementations where strict limitations on the harmonic distortion of the mains' currents are required. To compensate current harmonics, the SSIs are implemented to operate both on positive and negative sequence signals. One regulator, for the fundamental current component, is implemented in the stationary reference frame. The other regulators, for the current harmonics, are all implemented in a synchronous reference frame rotating at the fundamental frequency. This allows the simultaneous compensation of two current harmonics with just one regulator, yielding a significant reduction of the computational effort compared with other current control methods employing sinusoidal signal integrators implemented in stationary reference frame. A simple and robust voltage filter is also proposed by the authors to obtain a smooth and accurate position estimation of the voltage vector at the point of common coupling (PCC) under distorted mains' voltages. The whole control algorithm has been implemented on a 16-b, fixed-point digital signal processor (DSP) platform controlling a 20-kVA power conditioner prototype. The experimental results presented in this paper for inductive and capacitive loads show the validity of the proposed solutions.

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

A direct torque control (DTC) strategy for dual-three phase induction motor drives is discussed in this paper. The induction machine has two sets of stator three-phase windings spatially shifted by 30 electrical degrees. The DTC strategy is based on a predictive algorithm and it is implemented in a synchronous reference frame aligned with the machine stator flux vector. The advantages of the discussed control strategy are: constant inverter switching frequency, good transient and steady-state performance and low distortion of machine phase currents respect to direct self control (DSC) and other DTC schemes with variable switching frequency. Experimental results are presented for a 10 kW DTC dual three-phase induction motor drive prototype.

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Direct torque control for dual-three phase induction motor drives

The paper aims to perform an overview on the state-of-the-art in the control of multi-phase drives employing dual-three phase induction machines. In particular, the paper is focused on modeling aspects, Pulse-Width Modulation (PWM) techniques for Voltage Source Inverters (VSI), Field Oriented Control (FOC) and Direct Torque Control (DTC) strategies for dual-three phase induction machines. Furthermore, the paper briefly presents the advantages of dual-three phase induction motor drives over the conventional three-phase drives and the different applications reported in the literature.

/pdf/dual-three-phase-induction-machine-drives-control-a-survey-4hb97f7k6l.pdf

Dual-Three Phase Induction Machine Drives Control-A Survey

Vector control of a dual three-phase induction machine, with two sets of three-phase stator windings spatially shifted by 30 electrical degrees, is elaborated in the paper. The stator windings are fed from a current-controlled PWM six-phase IGBT voltage source inverter (VSI). The main problem in realisation of such a drive is an adequate current control algorithm, which has to cope with the inherent asymmetries of the drive. Furthermore, the digital implementation has to ensure zero steady state current error for the whole frequency range of the drive. After reviewing the existing solutions, the paper proposes a current control scheme that is based on four digital current controllers in the stationary reference frame. A design procedure for the current controllers is presented and current controllers are implemented in a laboratory test rig, in conjunction with a previously developed PWM scheme that provides operation with low values of low-order output voltage harmonics. Experimental tests are conducted and the results are given for a direct rotor field oriented control (DRFOC) of a 10 kW dual three-phase induction motor drive prototype. The results confirm the validity of the control scheme.

Dual three-phase induction motor drive with digital current control in the stationary reference frame

The paper deals with the problem of the unbalanced current and power sharing in the dual three-phase induction machine supplied by a sinusoidal voltage source. In particular, the case discussed has two three-phase sets of full-pitch windings, spatially shifted by 30 electrical degrees with isolated neutrals. The unbalanced current sharing between the two winding sets is due to the not eliminable small system asymmetries. First, the machine model to compute the unbalances is presented; second, the analysis of the system asymmetries and their effects on the machine operation is discussed. Simulated and experimentally tested sinusoidal supply conditions are presented for a 10 kW dual-three phase induction machine prototype.

F. Farina

Papers

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

Direct torque control for dual-three phase induction motor drives

Dual-Three Phase Induction Machine Drives Control-A Survey

Dual three-phase induction motor drive with digital current control in the stationary reference frame

Analysis of the asymmetrical operation of dual three-phase induction machines