Analytical analysis of the harmonic effects of a PWM AC drive

A novel analysis is presented of the harmonic content of current, torque pulsations, and harmonic copper losses of a three-phase induction machine fed by a two-phase pulsewidth modulation (PWM) inverter. The purely analytical results are based on the assumption that the switching frequency is high compared with the fundamental frequency. It is shown that the results hold accurately for frequency ratios f_s /f₁>9     

Implementation of a PWM regular sampling strategy for AC drives

The regular sampling technique offers a feasible basis for the microprocessor control of inverters in pulse width modulated (PWM) AC drives. An implementation of this technique is presented for the speed control of an induction motor in the frequency range 5-105 Hz. Hardware and software design principles are outlined and discussed for an 8 b microprocessor. Spectral analyses of the sampling technique with a sinusoidal modulating function and with a third harmonic added to this function are derived, with emphasis on the fundamental and dominant components. Experimental and computed results are presented for the motor control in steady-state, and for transient current and speed in closed-loop operation, with excellent correlation     

Fuzzy-tuning current-vector control of a three-phase PWM inverterfor high-performance AC drives

This paper proposes a new discrete fuzzy-tuning current-vector control (FTC) scheme for three-phase pulsewidth modulation (PWM) inverters. The proposed current control scheme can achieve fast transient responses and, at the same time, have very low total harmonic distortion in output current during steady-state operation. The proposed FTC scheme generates quasi-optimum PWM patterns by using a closed-loop control technique with instantaneous current feedback. The proposed FTC scheme has been realized using a single-chip digital signal processor (TMS320C14) from Texas Instruments. Experimental results are given to verify the proposed fuzzy-tuning current control strategy for three-phase PWM inverters     

Novel switching techniques for reducing the speed ripple of AC drives with direct torque control

The main theme of this paper is to present novel switching techniques, which insert zero-voltage vectors and/or more nonzero-voltage vectors to the conventional switching table, for AC drives with direct torque control. For the same sampling frequency of a drive controller, the proposed techniques are quite effective in reducing the torque pulsation and the speed ripples of the motors, as demonstrated in several experimental results. Moreover, the experimental confirmations have been made not only on an induction machine but also on a permanent-magnet synchronous machine.     

Improved direct torque and flux vector control of PWM inverter-fedinduction motor drives

In this paper, a direct torque and stator flux vector control system is presented. The principle of this method was proposed by Takahashi and Noguchi in 1985. In contrast to the field oriented control, no coordinate transformation and current control loop is required. In practical application, however, problems occur with starting and operation in the zero speed region. This paper shows how, by introducing an additional carrier signal to the torque controller input, a robust start and improved operation in the low speed region can be achieved. The simulation and experimental results which illustrate the performances of the proposed system are presented. Also, nomograms for controller design are given     

Asymmetrical PWM technique with harmonic elimination and powerfactor control in AC choppers

This paper describes the asymmetrical pulse width modulated (APWM) control technique for single phase AC choppers, which improves the input power factor and eliminates the harmonics of the output voltage up to a specified order. This technique also enables linear control of the fundamental component of the output voltage. The APWM switching patterns at the specified phase angle are obtained by the Newton-Raphson method and can be implemented by a one-chip microprocessor. Theoretical comparisons are made with conventional PWM technique and the computed performance indicates the superiority of the proposed APWM technique. Practical verification of the theoretical predictions is presented to conform the capabilities of the new technique     

An overvoltage suppression scheme for AC motor drives using a halfDC-link voltage level at each PWM transition

Passive filters are conventionally used to suppress overvoltage in the motor terminal, either by reducing the voltage rise rate at the inverter output, or by decreasing the motor terminal impedance. We propose an overvoltage suppression scheme that renders the use of passive filters unnecessary. This approach differs from general filter methods, in that it is independent,of dυ/dt and does not try to reduce dυ/dt. Our scheme utilizes the middle voltage level V_DC /2 at each pulsewidth modulation voltage transition, where V_DC represents the DC-link voltage. The duration of the middle voltage level is controlled in such a way that reflected voltages are cancelled out at the motor terminal. Optimal cancellation is achieved when the duration of V_DC/2 is equal to twice the transport delay of the cable. Further, if reflection coefficients at the motor terminal and the inverter output are equal to ±1, no overvoltage takes place. The proposed scheme requires the use of six auxiliary insulated gate bipolar transistor switches. Simulation as well as experimental results are presented here     

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

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.     

Minimization of torque ripple in SRM drives

The torque pulsations in switched reluctance motors (SRMs) are relatively higher compared to sinusoidal machines due to the doubly salient structure of the motor. The magnetization pattern of the individual phases together with the T-i-&thetas; characteristics of the motor dictate the amount of torque ripple during operation. Both machine design and electronic control approaches have been used to minimize the torque ripple in SRMs. This paper presents an extensive review of the origin of torque ripple and the approaches adopted over the past decade to minimize the torque ripple. A hybrid torque-ripple-minimizing controller that incorporates the attractive features of some of the techniques developed in the past decade is presented along with simulation and experimental results     

Acoustic noise reduction in sinusoidal PWM drives using a randomlymodulated carrier

Acoustic noise in an inverter-driven electric machine can be reduced by avoiding the concentration of harmonic energy in distinct tones. One method to spread out the harmonic spectrum without the use of programmed PWM (pulse width modulation) is to make the switching pattern random. It is proposed that the switching pattern can be randomized by modulating the triangle carrier in sinusoidal PWM with bandlimited white noise. All of the advantages of sinusoidal PWM are preserved with this technique. These include, real-time control, linear operation, good transient response, and a constant average switching frequency. By controlling the bandwidth and RMS value of the bandwidth limited noise modulation, it is shown that the instantaneous variation in switching frequency and the bandwidth of the energy spectrum in the machine can be specified within predetermined limits. Experimental results show the absence of acoustic noise concentrated at specific tones, which is present in conventional sinusoidal modulation     

Sensorless speed measurement using current harmonic spectralestimation in induction machine drives

This paper proposes a sensorless speed measurement scheme that improves the performance of transducerless induction machine drives, especially for low-frequency operation. Speed-related harmonics that arise from rotor slotting and eccentricity are analyzed using digital signal processing. These current harmonics exist at any nonzero speed and are independent of time-varying parameters, such as stator winding resistance. A spectral estimation technique combines multiple current harmonics to determine the rotor speed with more accuracy and less sensitivity to noise than analog filtering methods or the fast Fourier transform. An on-line initialization routine determines machine-specific parameters required for slot harmonic calculations. This speed detector, which has been verified at frequencies as low as 1 Hz, can provide robust, parameter-independent information for parameter tuning or as an input to a sensorless flux observer for a field-oriented drive. The performance of the algorithm is demonstrated over a wide range of inverter frequencies and load conditions     

Implementation and control of distributed PWM cascaded multilevel inverters with minimal harmonic distortion and common-mode voltage

Cascaded multilevel inverters can be implemented through the series connection of single-phase modular power bridges. This work presents details on how these bridges should be implemented and operated to synchronize their pulse-width-modulation (PWM) carriers, fundamental references and sampling instances to implement a network-controlled cascaded inverter with distributed PWM computation and overall optimal system performance. The paper begins by detailing the development and control of an integrated power bridge, designed with its own digital signal processor and associated control circuitry. Details describing the networked control algorithm and signal protocol needed for synchronizing the multiple power bridges through a dynamically fast data communication network, are then presented to achieve optimum harmonic cancellation and reduced common-mode voltage. The practicality and performance of the presented modular implementation concepts have been confirmed through the close match between simulation and experimental results obtained using a modular cascaded five-level inverter prototype.     

Commutation torque ripple reduction in brushless DC motor drives using a single DC current sensor

This paper presents a comprehensive study on reducing commutation torque ripples generated in brushless DC motor drives with only a single DC-link current sensor provided. In such drives, commutation torque ripple suppression techniques that are practically effective in low speed as well as high speed regions are scarcely found. The commutation compensation technique proposed here is based on a strategy that the current slopes of the incoming and the outgoing phases during the commutation interval can be equalized by a proper duty-ratio control. Being directly linked with deadbeat current control scheme, the proposed control method accomplishes suppression of the spikes and dips superimposed on the current and torque responses during the commutation intervals of the inverter. Effectiveness of the proposed control method is verified through simulations and experiments.     

Measurement of harmonic distortion in microwave photodetectors

The use of an offset phase-lock heterodyne system with a large dynamic range (>110 dB) for measuring photodetector (PD) harmonic distortion out to microwave frequencies is discussed. Up to 40 harmonics and second-harmonic levels only 17.8 dB below the fundamental were observed in high-speed PDs with only 1.7 mW incident. Measurements of harmonic power versus incident optical power are reported, along with associated PD bandwidth reduction     

Identification and compensation of torque ripple in high-precisionpermanent magnet motor drives

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments     

On the harmonic distortion coefficients of active-loaded amplifiers

Starting from the modified Ebers-Moll model which represents the Early effect more accurately than conventional models with the Early voltage, new expressions to calculate the harmonic distortion coefficients of active-loaded BJT amplifiers are derived and applied to the determination of the optimum operating point, where the total harmonic distortion is at a minimum. The results are not obtainable with SPICE, therefore derived expressions provide the possibility of completing the simulation with manual calculations.     

Torque and velocity ripple elimination of AC permanent magnet motor control systems using the internal model principle

This paper addresses the problem of torque and velocity ripple elimination in AC permanent magnet (PM) motor control systems. The torque ripples caused by DC offsets that are present in the current sensors of the motor driver and the digital-to-analog converters of the motion controller are studied and formulated mathematically. These torque ripples eventually generate velocity ripples at the speed output and degrade the system performance. In this paper the torque ripples are modeled as a sinusoidal function with a frequency depending on the motor speed. The internal model principle (IMP) is then used to design a controller to eliminate the torque and velocity ripples without estimating the amplitude and the phase values of the sinusoidal disturbance. A gain scheduled (GS) robust two degree of freedom (2DOF) speed regulator based on the IMP and the pole-zero placement is developed to eliminate the torque and velocity ripples and achieve a desirable tracking response. Simulation and experimental results reveal that the proposed GS robust 2DOF speed regulator can effectively eliminate the torque ripples generated by DC current offsets, and produce a velocity ripple-free output response.     

Recent progress in the development in solid-state AC motor drives

The author summarizes important developments in AC drive design that have occurred in the past several years. He discusses: converter technology, covering the matrix converter, PWM voltage and current link converters; and resonant link converters; AC motor technology, covering stepping motor drives, DC brushless motor drives, and synchronous reluctance motors; and control technology for AC drives, covering online and offline parameter identification and efficiency-maximizing control     

A new dynamic model of hysteresis in harmonic drives

In this paper, we propose a new dynamic model to describe the hysteresis phenomenon in harmonic drives. The experimental observation of the dynamic torque-displacement relationship for a harmonic drive shows a hysteresis characteristic indicating the simultaneous presence of energy storage and energy dissipation mechanisms. To completely characterize these mechanisms and yet have a simple representation for control, we develop a new hysteresis model using the heredity concept of dynamic systems. This model represents the hysteresis phenomenon by a combination of a nonlinear stiffness component and a nonlinear damping component leading to a mathematically well-posed nonlinear differential equation. The parameters of the model are identified using optimization techniques. We present some important mathematical properties of the model that give insight into model behavior and thus establish a mathematical basis for control. Numerical simulations in comparison with experimental data using our Harmonic Drive Test Apparatus verify the accuracy of the proposed model to represent the complex hysteresis dynamics of harmonic drives.