L. Zhong

Analysis of direct torque control in permanent magnet synchronous motor drives

This paper describes an investigation of direct torque control (DTC) for permanent magnet synchronous motor (PMSM) drives. It is mathematically proven that the increase of electromagnetic torque in a permanent magnet motor is proportional to the increase of the angle between the stator and rotor flux linkages, and, therefore, the fast torque response can be obtained by adjusting the rotating speed of the stator flux linkage as fast as possible. It is also shown that the zero voltage vectors should not be used, and stator flux linkage should be kept moving with respect to the rotor flux linkage all the time. The implementation of DTC in the permanent magnet motor is discussed, and it is found that for DTC using available digital signal processors (DSPs), it is advantageous to have a motor with a high ratio of the rated stator flux linkage to stator voltage. The simulation results verify the proposed control and also show that the torque response under DTC is much faster than the one under current control.

A novel direct torque controlled interior permanent magnet synchronous machine drive with low ripple in flux and torque and fixed switching frequency

A modified direct torque control (DTC) scheme for interior permanent magnet synchronous machine (IPMSM) is investigated in this paper, which features in very low flux and torque ripple and almost fixed switching frequency. It is based on the compensation of the error flux linkage vector by means of space vector modulation. Modeling and experimental results show that the flux and torque ripples are greatly reduced when compared with those of the basic DTC. With the new scheme, very short sampling time is not essential. All the advantages of the basic DTC are still retained. In addition, fixed switching frequency at different operating conditions becomes possible. The field-weakening control of this drive is also studied; an IPM DTC drive with a wider operation range and lower flux and torque ripple has been achieved experimentally.

A direct torque controlled interior permanent magnet synchronous motor drive incorporating field weakening

This paper presents a new control scheme for the wide speed range operation of interior permanent magnet synchronous motor (PMSM) drives, where both torque and stator flux linkage are directly controlled. The proposed scheme possesses some attractive features when compared to conventional current controlled drives. Current controllers followed by PWM or hysteresis comparators and coordinate transformation are not used. This eliminates the delays through these networks and offers the possibility of dispensing with the rotor position sensor for the electronic commutator, if the initial rotor position is known only approximately. The scheme incorporates all the usual control regimes such as the maximum torque per ampere operation in constant torque region, the flux weakening region, and operates the drive within the voltage and current limits of the motor/inverter. The control scheme has been verified by simulation and experimental tests with a prototype interior magnet motor. This paper describes the scheme in detail, followed by results of its implementation.

A direct torque controller for permanent magnet synchronous motor drives

This paper describes an investigation of direct torque control (DTC) for permanent magnet synchronous motor (PMSM) drives. The analysis of PMSMs shows that the increase of electromagnetic torque is proportional to the increase of the angle between the stator and rotor flux linkages and therefore fast torque response can be obtained by increasing the rotating speed of the stator flux linkage as fast as possible. The implementation of DTC in PMSM drives is discussed and the switching table specific for an interior PMSM is derived. The proposed control is implemented on a prototype PMSM, which has a standard induction motor stator, and the experimental results show that the torque response is extremely fast. It is also demonstrated that the position sensor is not essential for the inner torque control loop of PMSM drives with DTC.

A novel direct torque control for interior permanent-magnet synchronous machine drive with low ripple in torque and flux-a speed-sensorless approach

A novel direct torque control (DTC) scheme for interior permanent magnet synchronous machine is proposed in this paper, which features low torque and flux ripple and almost fixed switching frequency. The torque and flux ripples have been significantly reduced if compared with those of the basic DTC reported in the literature. A speed estimation scheme is integrated with the proposed DTC scheme.

A direct torque-controlled interior permanent-magnet synchronous motor drive without a speed sensor

This paper reports results of further investigation of the so-called direct torque control (DTC) technique to an interior permanent magnet (1PM) synchronous motor drive. This torque control technique for IPM motors requires no dq-axes current controllers and coordinate transformnation networks. A completely sensorless 1PM motor drive with DTC, which uses a new speed estimator from the stator flux linkage vector and the torque angle, is presented. It is shown that including the torque angle in the estimation process results in a far more accurate transient speed estimator than what is reported in the existing literature.

Problems associated with the direct torque control of an interior permanent-magnet synchronous motor drive and their remedies

This paper investigates problems associated with the implementation of a direct torque control (DTC) strategy for an interior permanent-magnet synchronous motor drive. The DTC technique is increasingly drawing attention because of elimination of current controllers and, hence, their inherent delays, and elimination of the rotor position sensor. The latter advantage perhaps is the main impetus for considering this new approach of torque control. Problems associated with this controller, namely, the offset in the current measurements, the stator resistance variation, and the requirement of initial rotor position are addressed in this paper. Ways of mitigating of these problems are also investigated in this paper. These are evaluated with modeling and experimental studies, results of which are also presented.

An investigation of a modified direct torque control strategy for flux and torque ripple reduction for induction machine drive system with fixed switching frequency

A novel direct torque control (DTC) control scheme for induction motor drives is proposed in this paper, which features low torque ripple, low flux ripple and almost fixed switching frequency. Simulation and preliminary experiment comparisons show the flux and torque ripples are greatly reduced if compared with the basic DTC scheme, while the switching frequency is fixed, all the advantages of the basic DTC are still retained with the new scheme.

A fuzzy observer for induction motor stator resistance for application in direct torque control

The direct torque control (DTC) system is one of the favourable control schemes for AC motor drives since it has the important advantage that system performance is not dependent on the motor parameters except the stator resistance. However, if the stator resistance varies due to heating, the performance of the system will suffer if the stator resistance value used in calculating the stator flux does not match the actual one. The compensation for the effect of the variation of stator resistance then becomes necessary. This paper describes a fuzzy observer, which can estimate the stator resistance online, according to the actual stator current, motor speed and operation time. The fuzzy observer for stator resistance proposed in this paper is applicable not only in the DTC systems of induction and permanent magnet motors but also in other type of motor drive systems.

A single/two-phase, regenerative, variable speed, induction motor drive with sinusoidal input current

The single-phase induction motor with two windings, main and auxiliary, is probably the most widely used motor in the world. The mains operated single-phase motor usually operates at low power factor, low efficiency and at fixed speed. At most, two or three fixed speeds are provided when required, through manual intervention. Such fixed-speed operation hinders product designers from incorporating many interesting and useful features in their products. The present concern on harmonic pollution of the supply and low power factor operation, as embodied in the recent IEC555-2 standard, also calls for power factor correction measures to be included in applications where a single-phase motor is used. This paper presents a variable speed single-phase motor (with two windings) drive that utilises just six switches as found in the emerging intelligent power modules. Just one integrated module with six switches serves to implement the input rectifier with sinusoidal input current, and the two-phase VSI or CSI inverter to drive the two phases of the motor with balanced ampere-turns. The input rectifier is also reversible, so that the motor can be braked with energy return to the mains, thus operating with high efficiency at all times.