Giovanni Serra

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.

Implementation of a direct control algorithm for induction motors based on discrete space vector modulation

The basic concept of direct torque control of induction machines is investigated in order to emphasize 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 algorithm based on the application of the space vector modulation (SVM) for prefixed time intervals. In this way a sort of discrete space vector modulation (DSVM) is introduced. Numerical simulations and experimental tests have been carried out to validate the proposed method.

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.

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.

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.

Direct torque control of induction motor drives

Direct torque control (DTC) is an emerging technique for controlling PWM inverter-fed induction motor (IM) drives. It allows the precise and quick control of the IM flux and torque without calling for complex control algorithms. In principle, moreover, it requires only the knowledge of the stator resistance. The tutorial starts by reviewing the basic operation of an IM and of a PWM inverter using the space vector theory. The field-oriented (FO) control of an IM drive is also reviewed. Then the concept of DTC is illustrated and three DTC-based strategies [i.e. switching table (ST), direct self control (DCS), space vector modulation (SVIM)] are described. The ST strategy is dealt with in detail, discussing the results which can be obtained with different choices of the switching table. Problems associated with the selection of the amplitude of the hysteresis bands of the flux and torque controllers are illustrated by means of experimental data. Merits and limits of DTC are reported and a comparison with FO control is given.

A general approach for the analysis of the input power quality in matrix converters

In this paper, a general approach based on a linearized analysis is employed in order to determine the matrix converter performance in terms of input-current quality. Considering input disturbances such as voltage unbalance and voltage distortion as a linear deviation from the fundamental harmonic component, it is possible to evaluate analytically the input-current harmonic content. It is emphasized that the input-current frequency spectrum is affected by the input-current modulation strategy employed. By using an optimal dynamic modulation strategy, it is possible to minimize the input-current harmonic content. Some numerical results are given to confirm the analytical solutions.

Evaluation of the input current quality by three different modulation strategies for SVM controlled matrix converters with input voltage unbalance

A new input current modulation strategy for a matrix power converter is presented. The performance of the new strategy during input voltage unbalance is compared to two other strategies. The difference of the three modulations only concerns the detection of the reference angle for the input current vector. The input current quality is evaluated using four different criteria. It is concluded that depending upon the modulation strategy, the distortion of the input current may appear either as a series of positive sequence harmonics or a negative sequence fundamental component or a positive and a negative series of harmonic components with reduced amplitudes. Good accordance is found between the analytical solutions and numerical simulations.