Francesco Profumo

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.

Improved current control strategy for power conditioners using sinusoidal signal integrators in synchronous reference frame

In this paper, an improved current control scheme for shunt type power conditioners is proposed, with the aim of simplifying current harmonic compensation for industrial implementations where strict limitations on the harmonic distortion of the mains currents are required. To compensate current harmonics, the proposed scheme is based on proportional-integral regulators using sinusoidal signal integrators, implemented to operate both on positive and negative sequence signals. One regulator, for the fundamental current component, is implemented in the stationary reference frame. The other regulators, for the current harmonics, are implemented in a synchronous reference frame rotating at the fundamental frequency. This allows to compensate simultaneously two current harmonics with just one regulator. The whole control algorithm has been implemented on a 16-bit, Fixed-point digital signal processor (DSP) platform controlling a 20 kVA prototype. Experimental results are presented to show the validity of the proposed solutions.

A comparison between the axial flux and the radial flux structures for PM synchronous motors

The aim of the paper is the comparison of the axial flux (AF) structures versus the conventional radial flux (RF) structures for PM synchronous motors. The comparison procedure is based on simple thermal considerations. Two motor typologies are chosen and compared in terms of delivered electromagnetic torque. The comparison is developed for different motor dimensions and the pole number influence is put into evidence. The paper reports the complete comparison procedure and the related results analysis. The obtained results show that, when the axial length is very short and the pole number is high, axial flux motors can be an attractive alternative to conventional radial flux solutions.

Digital field-oriented control for dual three-phase induction motor drives

A direct rotor-field-oriented control of a dual-three phase induction motor drive is described in this paper. The induction machine has two sets of stator three-phase windings spatially shifted by 30 electrical degrees. The stator windings are fed by a current-controlled pulsewidth-modulation (PWM) six-phase voltage-source inverter. Three key issues are discussed: (1) the machine dynamic model is based on the vector space decomposition theory; (2) the PWM strategy uses the double zero-sequence injection modulation technique which gives good results with low computational and hardware requirements; and (3) to eliminate the inherent asymmetries of the drive power section, a new current control scheme is proposed. Experimental results are presented for a 10-kW dual three-phase induction motor drive prototype.

Zero-speed tacholess IM torque control: simply a matter of stator voltage integration

Most industry experts agree that the next generation of commercial drives will include some sort of sensorless torque control. Achieving a modest level of control in the very-low-speed range greatly increases the competitive value of the drive and expands its range of applications. After reviewing many of the previously presented sensorless control methods, this paper shows that the simple method of stator-flux orientation can provide zero-speed torque control equally as well as more complex approaches, which rely on elaborate mathematical models to improve the operating range. This paper experimentally demonstrates that, by using only a slight amount of low-pass filtering in the integration of the stator voltage, a reasonable flux estimate can be obtained for stator-flux-field-oriented torque control. With this simple method, adequate torque control has been demonstrated over a wide range of speed and load conditions, and even at zero rotor speed with moderate or heavy load torque.

Axial flux machines drives: a new viable solution for electric cars

For electric car propulsion systems, the wheel motor is an application that requires the electrical machine has shape flexibility, compactness, robustness, high efficiency, and high torque. Axial flux machines are an interesting solution, where the motor is directly coupled to, or inside, the drive wheels. In this paper, axial flux induction and synchronous machines as wheel motor applications are presented and some considerations for each motor type are drawn by the authors. The structure with two rotors seems to be a very promising solution for both induction and synchronous machines. In the induction motor case, the two rotors can rotate at different speeds, thus the motor can act as a mechanical differential. The axial flux permanent magnet motor with two rotors is very compact and can be integrated inside the wheel.

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.

Stator resistance tuning in a stator-flux field-oriented drive using an instantaneous hybrid flux estimator

A self-tuning control scheme for stator-flux field-oriented induction machine drives in electric vehicles operating over a wide speed range is discussed in this paper. The stator flux can be determined accurately from the terminal voltage when the machine is operating at high speed. However, at low speed, the stator resistance must be known to calculate the stator flux. The problem of calculating the stator flux accurately over the entire speed range is addressed. The rotor flux can be found from the machine speed and rotor time constant. The stator flux, at low speed, is then calculated directly from the rotor flux. By alternating between these two methods of determining the stator flux, a self-tuning operation is achieved, wherein the stator and rotor resistances are periodically updated. Since both methods of determining the stator flux are forced to track one another, a smooth transition between flux estimators is obtained. The torque and flux are then controlled in a deadbeat fashion. Good torque control over a wide speed range can therefore be obtained. With the proposed scheme, the advantages of direct torque control are obtained over the entire speed range with the addition of a speed sensor.

Complete analysis and comparative study of digital modulation techniques for dual three-phase AC motor drives

The main goal of the paper is to perform a complete analysis and a comparative study of different digital pulsewidth modulation (PWM) techniques for dual three-phase induction machine drives. Six different digital PWM strategies are considered: four of them are present in the literature, the other two have been introduced by the authors in this paper. The comparison between the modulation strategies is based on several criteria: current harmonic minimisation, hardware and software implementation complexity with low cost fixed-point DSP platforms. Simulation results are provided to emphasize the advantages and disadvantages of each method. Experimental tests have been carried out to validate the most promising strategy which gives satisfactory results in terms of current harmonic minimisation and low implementation complexity.

Direct torque control for dual-three phase induction motor drives

A direct torque control (DTC) strategy for dual-three phase induction motor drives is discussed in this paper. The induction machine has two sets of stator three-phase windings spatially shifted by 30 electrical degrees. The DTC strategy is based on a predictive algorithm and it is implemented in a synchronous reference frame aligned with the machine stator flux vector. The advantages of the discussed control strategy are: constant inverter switching frequency, good transient and steady-state performance and low distortion of machine phase currents respect to direct self control (DSC) and other DTC schemes with variable switching frequency. Experimental results are presented for a 10 kW DTC dual three-phase induction motor drive prototype.