Report and Abstract for Direct Torque Control

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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.

Keywords—AC–AC Power Conversion, Induction Motor Drives, Direct Torque Control

I. Introduction

THREE-PHASE matrix converters have received considerable attention in recent years because they may become a good alternative to voltage-source inverter pulse width-modulation (VSI-PWM) converters [1]–[6]. In fact, the matrix converter provides bidirectional power flow, sinusoidal input/output waveforms, and controllable input power factor. Furthermore, the matrix converter allows a compact design due to the lack of dc-link capacitors for energy storage. With reference to the control methods, two approaches are widely used. The first one is based on transfer function analysis and has been proposed in [1]. The second one is based on space-vector modulation (SVM) technique, which has some advantages, such as immediate comprehension of the required commutation processes, simplified control algorithm, and maximum voltage transfer ratio without adding third harmonic components [5], [7]–[9].

The direct torque control (DTC) technique for induction motors was initially proposed as DTC [10] or direct self-control [11], then the method was generalized to current-source-inverter-fed induction motors and to VSI-fed and current-source-inverter-fed synchronous machines [12]. The main advantages of DTC are robust and fast torque response, no requirements for coordinate transformation, no requirements for PWM pulse generation and current regulators. In [13] and [14], a control scheme for induction motors based on DTC has been analyzed, but the rotor flux is assumed as reference, instead of stator flux, in order to achieve the highest pull-out torque. Using a VSI, different vector selection criteria can be employed to control the torque and the flux leading to different switching strategies. Each strategy affects the drive behavior in terms of torque and current ripple, switching frequency, and two- or four-quadrant operation capability [15]–[17]. In [18], a speed-dependent switching strategy has been proposed in order to achieve fast torque response in a wide speed range.

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 DTC of induction machines. The appropriate switching configuration of the matrix converter is directly selected, at each sampling period, using an opportune switching table. The table is entered by the outputs of three hysteresis controllers applied to the errors of stator flux, electromagnetic torque, and input power factor, respectively. Using this control method, it is possible to combine the advantages of matrix converters with the advantages of DTC schemes.

The good performance of the proposed scheme has been tested using a realistic numerical simulation of the whole drive. The steady-state and the transient behavior have been investigated. In both cases, the results obtained emphasize the effectiveness of the proposed drive system.

II. Direct Torque Control by Matrix Converter

A. Matrix Converter Theory

In three-phase/three-phase matrix converters, the nine bidirectional switches allow any output phase to be connected to any input phase as schematically represented in Figure. 1.

There are 27 possible switching configurations; among these, only 21 can be usefully employed in the DTC algorithm. These configurations are summarized in Table I. The first 18 switching configurations (named ±1, ±2 … ±9) have the common feature of connecting two output phases to the same input phase. The corresponding output line-to-neutral voltage vector and input line current vector, have fixed directions, as represented in Figure. 2 and 3, and will be named “active configurations.” The magnitude of these vectors depends upon the instantaneous values of the input line-to-neutral voltages and output line currents respectively as shown in Table I. Three switching configurations determine zero input current and output voltage vectors and will be named “zero configurations.”

The remaining six switching configurations have the three output phases connected to a different input phase. In this case, the output voltage and input current vectors have variable direction and cannot be usefully used.

It should be noted that the voltage vectors produced by a matrix converter can be utilized using the SVM technique to synthesize the instantaneous voltage vector required by field-oriented control of induction motors [5]–[9].

B. Basic DTC Principles

In principle, the DTC is a hysteresis stator flux and torque control that directly selects one of the six nonzero and two zero voltage vectors generated by a VSI (Figure. 4), in order to maintain the estimated stator flux and torque within the hysteresis bands. In particular, the stator flux is controlled by a two-level hysteresis comparator, whereas the torque by a three-level hysteresis comparator, as shown in Figure. 5 and 6, respectively. On the basis of the hysteresis comparator outputs and the stator flux sector number, the most opportune VSI voltage vector is selected at each sampling period, according to the switching table given in Table II.

As an example, considering the stator flux vector lying in sector-1, the voltage vectors V2 and V6 can be selected in order to increase the flux while V3 and V5 can be applied to decrease the flux. Among these, V2 and V3 determine a torque increase, while V5 and V6 a torque decrease. The zero-voltage vectors are selected when the output of the torque comparator is zero, irrespective of the stator flux condition. Using the switching table given in Table II, it is possible to implement DTC schemes having good performance.

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