Analysis of power and power spectral density in PWM inverters withrandomized switching frequency

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 formulae for both the power and the power spectral density are derived and verified experimentally     

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.     

Energy optimization of a certain class of incremental motion DCdrives

A joint approach to the design and control of energy-optimal incremental motion, load-invariant DC drives is presented. The drive is to rotate a given load through a definite angle in limited time, at minimum energy dissipation in the motor windings. It is required that the motor be of the minimum possible rating. An energy-optimal multimode control strategy for the drive is developed, and its microprocessor-based implementation is proposed. The motor and the so-called key parameters of the drive are determined by graphical solution of a nonlinear constrained optimization problem. A design example is included to illustrate the theoretical considerations     

An integral space-vector PWM technique for DSP-controlledvoltage-source inverters

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     

Power spectra of a PWM inverter with randomized pulse position

Random pulse width modulation in static power converters results in the partial transfer of power from the discrete spectrum of the output voltage to the continuous spectrum, with advantageous effects on the operation of the supplied electromechanical systems. In this paper, a random PWM technique with randomized pulse position for three-phase voltage-controlled inverters is analyzed. Closed-form equations for the discrete and continuous power spectra of the line-to-line and line-to-neutral voltages of the inverter have been derived and confirmed by experiments. Presented theory opens the way to numerical optimization of the voltage spectra of randomly modulated inverters     

Space vector PWM technique with minimum switching losses and avariable pulse rate [for VSI]

A novel randomized control strategy for three-phase voltage source inverters, based on 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 PWM method. The pulse rate is varied within individual 60° 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. Relevant theoretical analyses, computer simulations and experimental results are presented     

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.     

Can soft-starters help save energy?

This article describes results of an experimental evaluation of seven commercial soft-starters used with three motors of different power ratings. Several performance indicators have been measured and compared to illustrate operation of soft-starters and assess their energy-saving capabilities. Three standard squirrel-cage induction motors, rated at 3 kW, 7.5 kW, and 22 kW, were used. Several European manufacturers provided soft-starters for the investigation. Tests were performed under the steady-state and transient operating conditions