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Shunt active power filters (APF) are commonly used for the reduction of current harmonics and improvement of the power factor in power systems with nonlinear loads, such as diode rectifiers. A pulsewidth modulation (PWM) power converter constitutes the main component of the APF. The low-order harmonics of the line current are attenuated, but the switch-mode operation of the converter results in electromagnetic interference (EMI) spreading to the grid. Specifically, clusters of harmonics appear in the frequency spectra of voltages and currents of the converter at multiples of the switching frequency. In this paper, transferring the discrete spectral power of those harmonics to the continuous spectral power density is proposed as means for mitigation of the EMI. It is accomplished by randomization of the switching periods using a novel random PWM method (RPWM II). In contrast to the existing random PWM methods, in RPWM II the sampling frequency of the digital modulator is constant and equal to the average switching frequency. Computer simulations and experimental investigation of an APF designed for shipboard power systems are described, and the results are presented. They demonstrate significant reduction of the EMI, a feat achieved at practically no expense. Index Terms--Active power filters (APFs), electromagnetic interference (EMI) mitigation, random pulsewidth modulation (RPWM).

Electromagnetic noise, typical for systems with switching power converters, is especially troublesome in automotive electric drive systems because of the multitude of sensitive electronic equipment onboard of modern cars. To satisfy the relevant engineering norms, passive radio-frequency (RF) filters must be installed in the power electronic part of a drive. The RF filters add to the cost and size of the drive, which car designers strive to minimize to increase the passenger space. In this paper, active attenuation of the electromagnetic noise by means of the so-called random-delay pulse width modulation is described in application to an inverter-fed automotive ac drive. Based on computer simulations and experimental investigation, this simple and inexpensive method is shown to be highly effective. Index Terms--Automotive drives, electromagnetic noise, filters, pulse width modulation (PWM).

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. Index Terms--Acoustic- and electromagnetic-noise mitigation, naval ac drives, random pulsewidth modulation (RPWM).

The tandem converter is an arrangement of two inverters connected in parallel: a high-power pulse-amplitude modulated current-source inverter and a low-power pulse-width modulated voltage-source inverter. In comparison with an equivalent single voltage-source inverter, this structure offers several advantages, significant reduction of switching losses in particular. Results of experiments with a tandem inverter, and those of computer simulations of induction motor drives with that converter, are presented. Three vector control schemes for such drives were investigated: a) rotor-flux orientation with a voltage-controlled tandem converter, b) rotor-flux orientation with a current-controlled tandem converter, and c) stator-flux orientation with a voltage-controlled tandem converter. Performance characteristics of the described control schemes are discussed. Index Terms--Field-oriented control, frequency converters, induction motor drives, pulse-width modulation (PWM).

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. Index Terms--Noise and electromagnetic interference (EMI) mitigation, power inverters, random pulse-width modulation (RPWM).

A sensorless hybrid direct torque control (DTC) drive for high-volume low-cost industrial, domestic, and automotive applications has been developed using the inexpensive TMS329F243 digital signal processor controller. Under transient operating conditions, the drive is controlled using the classic bang-bang DTC. In the steady state, using linear torque and flux controllers, the control system generates a reference voltage vector for the inverter feeding the induction motor. The vector is then realized using the well-known space-vector pulsewidth modulation (SVPWM) technique. As a result, the irregular switching pattern typical for the classic DTC strategy is replaced with a quasi-optimal one, typical for the SVPWM. Description of the drive, including a novel motor state observer, is followed by selected experimental results confirming its excellent operating characteristics. Index Terms--Direct torque control (DTC), observers, sensorless ac drives, space-vector pulsewidth modulation (SVPWM).

A sensorless induction motor drive is presented, in which the principles of sliding-mode control, direct torque control (DTC), and space-vector modulation are combined to ensure high-performance operation, both in the steady state and under transient conditions. Merits of the classic DTC transient behavior are preserved, while the steady-state operation is significantly improved. The torque and flux controllers, and motor state observer are of the sliding-mode type. The inverter is directly controlled on the basis of torque and flux errors, using space-vector pulsewidth modulation. Computer simulations and experimental results presented demonstrate the robustness, accuracy, quickness, and low-chattering, wide-speed-range operation of the drive. Index Terms--Induction motor drives, sliding-mode observers (SMOs), variable-speed drives, variable-structure systems.

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. Index Terms--Direct power control, PWM rectifiers, vertical flux.

Results of a comparative investigation of deterministic and random pulsewidth modulation (PWM) techniques to be employed in a new ac drive for electric vehicles are presented. The study, employing the advanced simulation package SABER from Avant! Corporation, was focused on the reduction of electromagnetic interference (EMI) when a deterministic PWM strategy is replaced with an RPWM method. To assess the impact of the PWM technique on the drive performance, the drive efficiency, torque ripple, and dynamic response of the current control system were also investigated. Two RPWM techniques characterized by random variations of switching frequency of the inverter were considered: one with the sampling frequency of the modulator varying in step with the switching frequency, and one with a fixed sampling frequency equal to the average switching frequency. The study has demonstrated good EMI-mitigating potentials of RPWM techniques, whose use results in EMI reduction by 10 dB and more, while the drive performance is barely affected. Index Terms--AC drives, electric cars, electromagnetic interference (EMI), inverter control, random pulsewidth modulation (RPWM).

A novel method for accurate calculation of power spectra of the pulsewidth modulation (PWM) voltage-source inverter with randomized switching frequency is presented. The case of a limited pool of switching frequencies, convenient and adequate for technical purposes, is considered, and a mathematical background for this type of random PWM (RPWM) is provided. It is shown that the limited-pool RPWM gives rise to not only a continuous spectrum, but, also, under certain circumstances, pure power spectral components (harmonics), typical for the deterministic PWM. Criteria for the existence of harmonics are given, and formulas for both the power and the power spectral density are derived and verified experimentally. Index Terms--Inverters, random pulsewidth modulation, spectral analysis.

A combination of a large current-source inverter and a small voltage-source inverter circuits is analyzed. The resultant hybrid inverter inherits certain operating advantages from both the constituent converters. In comparison with the popular voltage-source inverter, these advantages include reduced switching losses, improved quality of output current waveforms, and faster dynamic response to current control commands. Description of operating principles and characteristics of the hybrid inverter is illustrated with results of experimental investigation of a laboratory model. Index Terms--Control, hybrid circuits, inverters.

A space vector PWM technique for inverter fed ac drives which, in contrast with known random modulation methods, is characterized by a very small pool of randomly used switching frequencies, is presented. The limited size of the pool facilitates design of digitally implemented subsystems of the drive, whose settings depend on the length of sampling interval. A method for optimal selection of pool frequencies is described and validated by experimental evidence, including EMI measurements. Index Terms--Inverters, optimization, random PWM.

A novel pulsewidth modulation (PWM) technique for voltage-source inverters, based on the idea of space vectors of switching integrals is presented. In contrast to most existing PWM methods, no explicit timing is required, smooth transition from the PWM mode to the square-wave mode is ensured, and minimum allowable widths of pulses and notches are guaranteed. The control algorithm is conceptually simple and easily implementable in a digital signal processor. Four parameters of the modulation strategy are adjustable, allowing tuning of the modulator to requirements imposed by the application. Voltage spectra have a low harmonic content, most of the power being dispersed as the continuous power density. A prototype modulator is described and illustrated by the results of laboratory experiments. Index Terms: Digital signal processors, pulsewidth modulation, voltage-source inverters

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 formulas 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 converter. Index Terms - DC/DC converters, random pulsewidth modulation, spectral analysis.

Analysis of spectral effects of random pulsewidth modulation (RPWM) in voltage-source inverters is presented. The randomized parameter is the width of individual switching intervals, and it is shown that the optimal solution consists in random selection of only two width values whose ratio is not an integer. For the optimal randomization scheme, powers in the de and fundamental components of the switching function are determined, while the whole remaining power appears as noise. A closed-form expression for the noise spectral density is given. The developed theory provides better understanding of effects of random-width modulation on the power spectrum of inverter output voltage, and allows explicit design of RPWM strategies satisfying specified criteria of spectral optimization. Theoretical considerations are illustrated by computational examples. Index Terms - Inverters, optimization, random PWM, voltage control.

A novel randomized control strategy for three-phase voltage source inverters, based on the voltage space vectors, is described. An implicit asymmetrical modulating function results in switching losses in the inverter being reduced by about half in comparison with those using the classic space vector pulsewidth modulation (PWM) method. The pulse rate is varied within individual 60 [degrees] sectors of the vector plane, so that the power spectra of the output voltage are spread over a wide frequency range and acquire a continuous part. The relevant theoretical analysis, computer simulations, and experimental results are presented. Index Terms - Inverters, random PWM, space vectors, switching losses.