A. Tani

FOC and DTC: two viable schemes for induction motors torque control

Field-oriented control and direct torque control are becoming the industrial standards for induction motors torque control. This paper is aimed at giving a contribution for a detailed comparison between the two control techniques, emphasizing advantages and disadvantages. The performance of the two control schemes is evaluated in terms of torque and current ripple, and transient response to step variations of the torque command. The analysis has been carried out on the basis of the results obtained by numerical simulations, where secondary effects introduced by hardware implementation are not present.

Matrix converter modulation strategies: a new general approach based on space-vector representation of the switch state

In this paper, a novel representation of the switch state of a three-phase to three-phase matrix converter is presented. This approach, based on space-vector notation, simplifies the study of the modulation strategies, leading to a complete general solution of the problem and providing a very useful unitary point of view. The already established strategies can be considered as particular cases of the proposed general solution. Using this approach, it can be verified that the space-vector modulation technique, owing to the intrinsic two degrees of freedom, represents the general solution of the modulation problem of matrix converters. This technique can be considered the best solution for the possibility to achieve the highest voltage transfer ratio and to optimize the switching pattern through a suitable use of the zero configurations.

The use of matrix converters in direct torque control of induction machines

In this paper, a new control method for matrix converters is proposed which allows, under the constraint of unity input power factor, the generation of the voltage vectors required to implement the direct torque control (DTC) of induction machines. Using this control method, it is possible to combine the advantages of matrix converters with the advantages of the DTC schemes. Some numerical simulations are carried out, showing the effectiveness of the proposed method in steady-state and transient conditions. Some experimental tests were also carried out demonstrating the practical feasibility of this control scheme.

Control of Multiphase Induction Motors With an Odd Number of Phases Under Open-Circuit Phase Faults

In this paper, a control scheme for multiphase induction motor drives is presented and assessed. This control scheme ensures the operation of a multiphase drive under an open-circuit phase fault that can occur in one or more phases at the same time. A fault-tolerant control strategy is developed. The mathematical analysis is based on the space vector representation of the multiphase system and is valid either in transient or steady-state operating conditions. The feasibility of the drive is verified by means of experimental tests carried out on a prototype of seven-phase induction motor.

Effects of flux and torque hysteresis band amplitude in direct torque control of induction machines

Direct torque control of induction machines allows high dynamic performance to be obtained utilising a simple signal processing method. Furthermore, this control technique does not require current regulators so reducing the hardware requirements. In this paper, the influence of the amplitude of flux and torque hysteresis bands on switching frequency, torque and flux ripple, current distortion and drive losses is investigated. The advantages of a careful choice of hysteresis band amplitudes are emphasised. The numerical results are verified by experimental tests carried out on a DSP based processing system.<<ETX>>

Performance analysis of a speed-sensorless induction motor drive based on a constant-switching-frequency DTC scheme

A control technique which utilizes the stator flux components as control variables has been applied to a speed sensorless induction motor drive. The scheme may be regarded as a development of a DTC scheme, aimed to achieve a constant switching frequency operation. At each sampling period, the required voltage vector is calculated on the basis of the error between the reference and the estimated stator flux vector. The problems related to the VSI dead time and the stator flux estimation at low speed have been analyzed, and an efficient solution has been proposed. The performance of the drive system has been verified by experimental tests, and good results have been achieved in both steady state and transient operating conditions.

Large-Signal Model for the Stability Analysis of Matrix Converters

The interest in using the matrix converter (MC) for motor drive applications and energy conversion systems is steadily increasing due to its main advantage of performing a direct coupling between two three-phase alternating current sources without the need of an intermediate direct current bus. This characteristic, together with the presence of inductance-capacitance input filters and the feedforward compensation of the input voltage variations, might yield unstable operation in electrical drives. In this paper, a theoretical analysis of MCs based on a large-signal model is presented with the aim to show which parameters may affect the stability and to explain the reason of this phenomenon. The theoretical analysis is supported by several experimental tests carried out on an MC prototype

Analytical investigation of torque and flux ripple in DTC schemes for induction motors

AC motor drives based on the direct torque control (DTC) of induction machines allow high dynamic performance to be obtained with very simple control schemes. The drive behaviour, in terms of flux and torque response, is dependent on the voltage vector selection strategy utilised and the operating conditions. In this paper, a detailed discussion of DTC basic principles is carried out in order to determine analytically the relationships between the applied voltage vector and the corresponding torque and flux variations. The analysis allows the flux and the torque ripple to be easily determined. It has been observed that the effects produced by a voltage vector are strongly dependent on both the rotor speed and the voltage vector direction relative to the rotor flux. Some significant diagrams are presented showing the theoretical results obtained. The analysis can be useful in defining new voltage vector selection criteria and in optimising the dynamic behaviour with minimal torque and flux ripple.

Improvement of direct torque control performance by using a discrete SVM technique

The basic concept of direct torque control of induction machines is investigated in order to emphasise the effects produced by a given voltage vector on stator flux and torque variations. The low number of voltage vectors which can be applied to the machine using the basic DTC scheme may cause undesired torque and current ripple. An improvement of the drive performance can be obtained using a new DTC technique based on the application of space vector modulation for prefixed time intervals. In this way, a sort of discrete space vector modulation is introduced. Numerical simulations and experimental tests have been carried out to validate the proposed method.

Optimal Use of Zero Vectors for Minimizing the Output Current Distortion in Matrix Converters

This paper is focused on the current quality of matrix converter controlled by space vector modulation (SVM) feeding inductive loads. The analysis carried out in this paper leads to the determination of the optimal use of zero vectors, i.e., the switching pattern leading to the minimum rms value of the load current ripple. The optimization of the switching pattern is based on the graphical analysis of the loci described by the ripple of the current vector in the alpha- beta reference frame. As a result, a set of analytical relationships that allow the online calculation of the duty cycles of the zero vectors is presented. Finally, simulation and experimental results confirm that the current ripple of the proposed modulation strategy is lower than that of traditional SVM strategies and, in some cases, with a reduced number of commutations.