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.

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

This paper focuses on direct torque control (DTC) for three-phase AC electric drives. A novel model predictive control scheme is proposed that keeps the motor torque, the stator flux, and (if present) the inverter's neutral point potential within given hysteresis bounds while minimizing the switching frequency of the inverter. Based on an internal model of the drive, the controller predicts several future switch transitions, extrapolates the output trajectories, and chooses the sequence of inverter switch positions (voltage vectors) that minimizes the switching frequency. The advantages of the proposed controller are twofold. First, as underlined by the experimental results in the second part of this paper, it yields a superior performance with respect to the industrial state of the art. Specifically, the switching frequency is reduced by up to 50% while the torque and flux are kept more accurately within their bounds. Moreover, the fast dynamic torque response is inherited from standard DTC. Second, the scheme is applicable to a large class of (three-phase) AC electric machines driven by inverters.

Model Predictive Direct Torque Control—Part I: Concept, Algorithm, and Analysis

In this paper, direct power control (DPC) of three-phase pulsewidth-modulated rectifiers without line voltage sensors is presented. The new system is based on virtual flux (VF) estimation. Theoretical principles of this method are discussed. The steady-state and dynamic behavior of VF-DPC are presented that illustrate the operation and performance of the proposed system compared to a conventional DPC method. Both strategies are also investigated under unbalance and predistorted grid. It is shown that the VF-DPC exhibits several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal. Test results show the excellent performance of the proposed system.

Virtual flux based direct power control of three-phase PWM rectifiers

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.

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

Four control techniques for pulse-width modulation (PWM) rectifiers in AC adjustable speed drives are presented. In particular, the so-called virtual-flux oriented control (VFOC) and virtual-flux based direct power control (VF-DPC) schemes are described and compared with their voltage based counterparts. These are the voltage oriented control (VOC) and voltage-based direct power control (V-DPC) techniques. Theoretical background is provided, and results of computer simulations and laboratory experiments are given, documenting advantages and disadvantages of the individual control strategies.

A comparative study of control techniques for PWM rectifiers in AC adjustable speed drives

A novel direct torque control method for an induction motor is presented which is quite different from field-oriented control. Improving the torque response of a large-capacity induction motor using two sets of three-phase inverters and an open-data induction motor is of special concern. Instantaneous voltage vectors applied by an inverter have redundancy characteristics which provide some flexibility for selecting the inverter switching modes. By using this switching freedom, control is achieved according to the following priorities: (1) high-speed torque control; (2) regulation of the primary flux; (3) decreasing the zero phase sequence current; and (4) minimization of the inverter switching frequency. Simulations and experiments have been carried out to verify the feasibility of this priority control, accompanied by comparisons with another control scheme. Torque frequency-response corner frequencies above 2000 Hz have been experimentally measured, and time constants of 4 ms have been achieved for rotor-speed step responses from -500 to 500 r/min. The peak transient torque during the step change is about 20 times the rate torque. >

High-performance direct torque control of an induction motor

A wireless power transfer (WPT) system has been considered to be applied to various charging applications. Conversion efficiency is one of the important factors and is in need of improvement in various approaches. This paper proposes a WPT system consisted of a synchronous rectification without transfer side signals. The proposed rectifier is applied to a magnetic-field WPT system with a series-series topology. This system is characterized by the use of the voltage sensor at the secondary resonant capacitor to achieve the synchronous rectification. The theoretical analysis concludes by the secondary resonant capacitor voltage being 90 degrees forward from the rectifier input voltage. A synchronous rectifier is designed, constructed, and tested to verify the principles of operation. The conversion efficiency of the proposed synchronous system from dc to dc is 91.8% achieved at 85-kHz 3-kW output power and 1. 6pt higher than the diode rectifier method. The efficiency distribution results from the loss of the synchronous rectifier is 53.2 W lower than that of the diode rectifier.

Y. Ohmori

Papers

High-performance direct torque control of an induction motor

Resonant Capacitor Voltage Based 85-kHz 3-kW Synchronous Rectification on Wireless Power Transfer System