Digital quasi-random modulated SFAVM PWM in an AC-drive system

A main research topic in PWM-VSI inverter-driven electrical machines is to reduce the generated acoustic noise which often is dominated by a multiple of the switching frequency in the inverter. This paper proposes a modulation scheme for reducing the acoustic noise effect from an AC machine which can be implemented digitally for low and high performance systems. The scheme is based on a stator flux asynchronous vector modulation (SFAVM) imposed by a digital band limited PWM white noise generator for varying the switching frequency randomly. The white noise generator can be used for 8, 16 and 32 bit microcontrollers. The modulation strategy is tested in a 1 kVA high performance 16 bit microprocessor controlled AC drive system. Voltage-spectra on the line-to-line voltage and the acoustic noise spectra are presented and show that the new modulation strategy can decrease the noise effect. The stator-flux-polygon and the line current are measured and demonstrate the random modulation strategy. Finally, the total sound pressure level from the AC machine is investigated with fixed switching frequencies and with different randomly modulated frequency spans. It is concluded that a properly chosen fixed switching frequency has the lowest total sound pressure level. However, the random modulation strategy distributes the noise frequencies and the noise is more comfortable and less annoying     

EMI reduction in switched power converters using frequency Modulation techniques

Frequency-modulation techniques have been used to reduce electromagnetic interference (EMI) produced by the clock of digital systems working in the range of hundreds of megahertz. The working principle consists of modulating the original constant clock frequency in order to spread the energy of each single harmonic into a certain frequency band, thus reducing the peak amplitude of EMI at harmonic frequencies. Nowadays, the switching frequency of power converters has increased up to values that make interesting the application of such techniques to reduce EMI emissions due to switching of power circuits. This paper presents the theoretical principles of frequency modulation using deterministic profiles for the modulating function. It shows the effectiveness of such methods in terms of EMI reduction for different modulation profiles and other parameters. The method is compared with other methods using random modulation. Tests carried out on a buck converter are presented for experimental validation of the method. A short discussion on optimal modulation profiles and parameters is also included.     

Conducted EMI Reduction in Power Converters by Means of Periodic Switching Frequency Modulation

Spread spectrum clock generation techniques were originally developed to reduce electromagnetic interference (EMI) in communications and microprocessor systems working in the range of hundreds of megahertz. Nowadays, the switching frequency of power converters has been increasing up to values that make worthy the application of such switching frequency modulation techniques to reduce EMI emissions in power converters. Although random modulations have been applied before to power converters, periodic patterns can provide some advantages. First, theoretical principles of frequency modulation using three periodic patterns for the modulating function are presented. The influence of some important modulation parameters on the EMI reduction is analyzed and some considerations about the EMI filters design are also presented. The effectiveness of such methods in terms of EMI reduction is demonstrated theoretically and confirmed with experimental results obtained from tests carried out on two converters. The first one is a 2.5 W buck converter that can be switched up to 1 MHz and the second one is a 600 W boost converter switching at 40 kHz. In both cases, attenuations obtained in conducted EMI are evaluated. Finally, special attention has been paid to input current and output voltage ripple in order to evaluate possible undesired side-effects produced by this technique.     

Random pulse-width modulation technique for voltage-controlled power inverters

As an alternative to the existing deterministic pulse-width modulation methods, a random pulse-width modulation technique for voltage-controlled power inverters is proposed. Advantages of the technique include non-repetitiveness of the switching pattern, simplicity of the hardware, and uncommonly high limits of switching frequency. A prototype random pulse-width modulator is described and experimental results are presented.     

Random switching techniques for inverter control

New techniques which dramatically reduce acoustic noise by randomising the inverter switching frequency for inverter control are presented. It is shown that the proposed techniques combine the advantages of conventional pulsewidth modulation (PWM) techniques based on regular-sampling techniques and random PWM techniques     

Improved modulation techniques for PWM-VSI drives

PWM-VSI based AC motor drives have two main problems. The inverter is nonlinear which causes instability problems in some specific working points of the AC machine and it emits acoustic noise due to the switching frequency. Nonlinearities like dead-time in the inverter, load dependent DC-link voltage ripple and the voltage drop across the switches are modeled and compensated by improved modulation techniques in order to obtain an almost ideal inverter. Different feedback and feedforward techniques are proposed. The acoustic noise is reduced by using a random modulation strategy. Measurements show a significant improvement by using feedforward and feedback techniques for linearizing the inverter. An improvement in reduction of the acoustic noise emission is also achieved by using random modulation. It is concluded that a combination of a random modulation strategy and feedforward/feedback techniques gives an almost ideal AC motor drive system     

A new digital zero average current error control algorithm for inverters

Inverter performance is typically measured against its ability to produce a high fidelity output sinusoidal waveform with narrow band switching frequency and good transient response. For this purpose, pulse-width modulation techniques are used to generate the required control signals for the inverter semiconductor switches. Current control voltage-source inverters can be realized using one of the recently proposed zero average current error (ZACE) techniques based on analogue technology. A new and improved algorithm that belongs to the family of ZACE methods suitable for digital technology is proposed in this paper. Advantages include a more robust algorithm with quicker computation allowing fast response and higher switching frequency from a digital signal processor. The effectiveness of the new current control algorithm is demonstrated through simulations first. Experimental results taken from a low power laboratory prototype are also presented to verify the theoretical considerations.     

A methodology for true comparison of analytical and measuredfrequency domain spectra in random PWM converters

The analysis of random pulsewidth modulation (PWM) techniques has matured into a state where analytical expressions have been derived to aid in understanding the frequency domain characteristics. Derivations of the formulae require a lot of algebra, and the expressions must be verified by laboratory measurements. It is shown, however, that factors originating from digital signal processing techniques make the comparison difficult if proper measures against misinterpretation are not taken. A methodology to overcome the problems is presented. Novel expressions for the random lead-lag pulse position technique and the random switching frequency technique are also presented as well as their verifications by laboratory measurements on a full-bridge DC/DC power converter     

A high-performance vector control of an 11-level inverter

This paper presents a switching strategy for multilevel cascade inverters, based on the space-vector theory. The proposed high-performance strategy generates a voltage vector across the load with minimum error with respect to the sinusoidal reference. In addition, it generates very low harmonic distortion operating with reduced switching frequency, without the use of traditional sinusoidal pulsewidth modulation techniques or more sophisticated vector modulation methods.     

Characteristic Analysis for Multisampled Digital Implementation of Fixed-Switching-Frequency Closed-Loop Modulation of Voltage-Source Inverter

In this paper, a fixed-switching-frequency closed-loop modulation of a voltage-source inverter (VSI), upon the digital implementation of the modulation process, is analyzed and characterized. The sampling frequency of the digital processor is considered as an integer multiple of the modulation switching frequency. An expression for the determination of the modulation design parameter is developed for smooth modulation at a fixed switching frequency. The variation of the sampling frequency, switching frequency, and modulation index has been analyzed for the determination of the switching condition under closed loop. It is shown that the switching condition determined based on the continuous-time analysis of the closed-loop modulation will ensure smooth modulation upon the digital implementation of the modulation process. However, the stability properties need to be tested prior to digital implementation as they get deteriorated at smaller sampling frequencies. The closed-loop modulation index needs to be considered maximum while determining the design parameters for smooth modulation. In particular, a detailed analysis has been carried out by varying the control gain in the sliding-mode control of a two-level VSI. The proposed analysis of the closed-loop modulation of the VSI has been verified for the operation of a distribution static compensator. The theoretical results are validated experimentally on both single- and three-phase systems.      

Prediction of 2-level PWM inverter efficiency using MATLAB/Simulink

This article proposes a direct approach for the prediction of inverter efficiency using MATLAB/Simulink, instead of an indirect loss calculation approach based on analytical models. In analytical approach, efficiency is obtained by calculating individual losses separately, such as switching losses, conduction losses and harmonic losses using analytical models. However, this approach requires accurate analytical models and complicated calculations, due to the variation in the switching frequency, switching transient and modulation techniques. In the proposed approach, the actual waveform of the inverter system is directly generated using MATLAB/Simulink. The instantaneous voltage and current waveform including switching transients are generated. Thus, the proposed approach is very simple and convenient for efficiency prediction. The proposed approach also works for any system parameters or control methods, such as various pulse-width modulation (PWM) techniques, different switching frequencies, switching devices and load types. The proposed approach can be adopted for the efficiency prediction of any switching strategies and any types of inverters such as neutral-point-clamped (NPC) inverters, H bridge inverters and H5 topology, since the topologies are modelled as circuits in the MATLAB/Simulink program and no analytical model is required for the proposed approach. Furthermore, the proposed approach can provide operation techniques and conditions such as PWM techniques and switching frequency that offer high efficiency. In this article, inverter performance is evaluated for various PWM techniques and switching frequencies. The PWM technique and switching frequency that offer high efficiency is obtained. Finally, the proposed approach is verified by experimental results.     

Experimental investigation of a naval propulsion drive model with the PWM-based attenuation of the acoustic and electromagnetic noise

An extensive experimental investigation of a 40-hp ac drive was conducted with the focus on mitigation of the acoustic and electromagnetic noise, and vibration, by means of random pulsewidth modulation (RPWM) employed in the drive's inverter. The drive was a laboratory model of an electric propulsion system for naval vessels, particularly electric submarines, in which the noise mitigation is crucial for survivability. Three PWM methods were compared: 1) the classic deterministic PWM, characterized by a constant switching period equal to the sampling period of the digital modulator; 2) the known RPWM technique, referred to as RPWM I, in which the switching and sampling periods are varied simultaneously in a random manner; and 3) a novel RPWM method, referred to as RPWM II, with a constant sampling period and the switching periods randomly varied around an average value equal to the sampling period. The experimental results have confirmed the mitigating properties of RPWM with respect to the acoustic and electromagnetic noise, and vibration. Because of the fixed sampling frequency, the RPWM II technique is technically more convenient than the classic RPWM I method and only marginally less effective in flattening the peaks of noise spectra. Importantly, conclusions drawn from the described study are valid for ac drives in general.     

Design and implementation of vector-controlled induction motordrives using random switching technique with constant sampling frequency

Recent results have shown that random switching techniques reduce electromagnetic interference, annoying acoustic noise, and other undesirable effects. However, to incorporate random switching techniques into digital-controlled induction motor drives, it requires dynamic adjustment of the gains of controllers. This paper presents details of the design and implementation of induction motor drives using a new random switching technique. It is shown, and confirmed by experimental results in this paper, that the sampling frequency of inverter control is constant, and therefore it is not required to adjust the gains of controllers dynamically. The details of controller design of the random switching inverter-controlled vector drives are fully explored, including the controller design in the discrete-time domain and the effect of the random technique on the speed response. Moreover, the advantages and disadvantages of inverter-controlled vector drives using random switching techniques are highlighted by experimental results     

无电压传感器的三电平PWM整流器定频直接功率控制

给出了三电平PWM整流器的数学模型,在传统的直接功率控制和电压定向控制的基础上,结合虚拟磁链控制的优点,提出了一种新的三电平PWM整流器定频直接功率控制方法。该控制方法省略了电网电压传感器,实现了有功功率和无功功率的动态解耦,调制环节采用空间电压矢量调制,开关频率固定。仿真结果表明,该控制方法实现了单位功率因数运行,网侧电流谐波小,具有良好的动静态性能,保证了中点电位的平衡。     

A quadratic buck converter with lossless commutation

High switching frequency associated with soft commutation techniques is a new trend in switching converters. Following this trend, the authors present a buck pulsewidth modulation converter, where the DC voltage conversion ratio has a quadratic dependence on duty cycle, providing a large step-down. By introducing two resonant networks, soft switching is attained, providing highly efficient operating conditions for a wide load range at high switching frequency. Contrary to most of the converters that apply soft-switching techniques, the switches presented are not subjected to high switch voltage or current stresses and, consequently, present low conduction losses. The authors present, for this converter, the principle of operation, theoretical analysis, relevant equations and simulation and experimental results     

Modified Phase-Shifted PWM Control for Flying Capacitor Multilevel Converters

The issue of voltage imbalance remains a challenge for the flying capacitor multilevel converter. The phase-shifted pulsewidth modulation (PS-PWM) method has a certain degree of self-balancing properties. However, the method alone is not sufficient to maintain balanced capacitor voltages in practical applications. The paper proposes a closed-loop modified PS-PWM control method by incorporating a novel balancing algorithm. The algorithm takes advantage of switching redundancies to adjust the switching times of selected switching states and thus maintaining the capacitor voltages balanced without adversely affecting the system's performance. Key techniques of the proposed control method, including selection of switching states, calculation of adjusting times for the selected states, and determination of new switching instants of the modified PS-PWM are described and analyzed. Simulation and experimental results are presented to confirm the feasibility of the proposed method     

改进型PWM灰度调制技术在FED中的应用研究

根据FED显示器响应特性,设计了改进型PWM灰度调制方法.通过确定驱动脉冲顺序,结合人眼的视觉特性设计了最优PWM调制技术.通过调整驱动脉冲增量,实现了单位时间亮暗转换次数最大化和亮暗转换频率均匀化的优化目标.采用FPGA控制技术实现了改进型PWM灰度调制,减少了FED图像灰阶损失,提高了图像显示质量.     

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 comparative study of carrier-frequency modulation techniques forconducted EMI suppression in PWM converters

A rigorous mathematical analysis and a comparative study of carrier-frequency modulation (CFM) techniques for the conducted electromagnetic interference (EMI) suppression in pulsewidth-modulated converters is presented. CFM techniques dither the switching period with a small amplitude variation around the nominal value, so that the harmonic power is redistributed over the spectrum of concern. Two types of dithering signals, including the periodic and random signals, are investigated in this paper. The operational characteristics as well as the input and output power spectra of the converters with the two modulating signals are compared. In particular, their characteristics in the low- and high-frequency harmonic power redistribution will be depicted. It is shown that random CFM (RCFM) gives a more effective way to disperse the harmonics around the switching frequency than the periodic CFM (PCFM) with the same frequency deviation. However, RCFM introduces higher low-frequency harmonics than the PCFM at the converter output. Furthermore, effects of the resolution filter bandwidth in the electromagnetic compatibility analyzer on conducted EMI measurement is discussed. The validity of the analyses is confirmed experimentally by using a dc/dc buck converter operating in continuous conduction mode     

A novel random PWM technique with low computational overhead and constant sampling frequency for high-volume, low-cost applications

A novel random pulse-width modulation (PWM) technique for three-phase voltage-source inverters, characterized by low computational overhead, a variable switching frequency, and a constant sampling frequency, is presented. The technique is based on two strategies: 1) the so-called arithmetic PWM (APWM), which yields the same switching patterns as the classic space-vector modulation, but with minimal computational effort and 2) randomization of switching periods by varying the delay of switching cycles with respect to corresponding sampling cycles. Simplicity of the technique, named a variable-delay random PWM (VDRPWM) method, allows its implementation in cheap, low-end processors. It makes the VDRPWM the best choice for high-volume, low-cost applications, such as domestic and automotive ac drives and UPSs. The random aspect of the technique has a mitigating effect on the acoustic and electromagnetic noise emitted by the supplied system. This feature has been confirmed by experiments with a 40-hp induction motor drive.