International Research Journal of Applied and Basic Sciences 2013 Available online at www.irjabs.com ISSN 2251-838X / Vol, 6 (9): 1243-1248 Science Explorer Publications Research and Development Random pulse width modulation Abdolreza Esmaeli 1 andmohsen Mobini 2 1.Plasma Physics and Nuclear Fusion Research School, Nuclear Science and Technology Research Institute, Tehran, Iran. 2. Faculty of Electrical Engineering, Maziar University, Nour, Iran *Corresponding Author email: aesmaeli@aeoi.org.ir ABSTRACT: Random pulse width modulation (RPWM) approaches can make the harmonic spectrum of inverter output voltage be continuously distributed without affecting the fundamental frequency component, and thus the acoustic noise and mechanical vibration of an inverter-fed ac motor drive are greatly reduced. First, the effects of the attributes of random signal on the inverter output harmonic spectrum distribution characteristics are analyzed using intuitive concept. Then based on which, the quantitative design, Simulink simulation and implementation of the proposed RFPWM inverter are introduced. The proposed RFPWM inverter is employed to power an indirect field-oriented induction motor drive. The simulated and measured results indicate that uniform random distribution of inverter output harmonic spectrum and thus smaller acoustic noise and mechanical vibration are obtained by the proposed RFPWM scheme. Keywords: Induction motor drive, intuitive analysis, quantitative design, random frequency pwm. INTRODUCTION The research documented in this thesis addresses random pulse-width modulation (PWM) and its applications in hard-switched power electronic converters. This emerging PWM technology has gained considerable interest in academia during the last decade, although industrial applications are still few in number (Cichowlas et al., 2005).The key property that differentiates random PWM from classic PWM, which generates time-periodic switching functions, is that random PWM produces switching functions that have a nondeterministic (random) component. For a well-designed random PWM technique it is, nonetheless, still possible to get an accurate synthesis of the commanded reference waveform, \ie random PWM behaves exactly as its deterministic counterpart with respect to generating a switching function that allows the reference signal to be extracted by, typically, low-pass filtering (Esmaeli, 2010).As a consequence of the non-repetitive switching functions, the frequency-domain spectra for randomized modulators are very different from the spectra caused by a deterministic modulation strategy. Essentially, the spectrum for classic PWM consists of discrete frequency components clustered around multiples of the PWM carrier frequency, whereas random PWM --- as least partially --- transfers the power carried by the harmonics into the continuous density spectrum (Azcondo et al. 2005, Jovanovic and Jang 2005).This spectral spreading property is the key to understand the current interest in random PWM. In particular, it has been demonstrated by experiments in the literature that the otherwise annoying tonal acoustic noise emitted from ac motors and other magnetic components in converterbased systems operating with a carrier frequency in the audible range may be alleviated substantially from a psychoacoustic point of view in an inexpensive manner by using random PWM instead of deterministic PWM. Other investigations have shown that random PWM may also be used to obtain compliance with standards for electro-magnetic compatibility with less filtering/shielding efforts, because the spectral peaks are reduced compared to deterministic PWM operation.deterministic modulators are well understood, but many details relating to random PWM have not yet been properly analyzed from a theoretical point of view. This research attempts to narrow this gap by providing thorough analyses of different random PWM schemes that are considered possible candidatesfor industrial applications. The main emphasis is put on the standard threephase voltage-source converter which is widely used for ac drives, active mains rectifiers, uninterruptible power supplies, etc, although much of the work is of general validity in power electronics. Prior to giving a detailed analysis of specific random PWM strategies, the basic principles for random PWM are discussed (Tse et al., 2010).The random pulse width modulation (RPWM) technique is a new and effective method to let the inverterfed ac motor drive possess low acoustic noise and mechanical vibration when lower switching frequency is chosen. So it is increasingly attracted much attention in practical applications. Generally speaking, the key spirit
of RPWM is to let the time positions of switching signalsfor inverter switches be randomly varied (Tse et al., 2010). Although many RPWM schemes and their successful applications have been reported (Satyanarayana et. al., 2010), their analyses and designs are not easy to be understood by practical power electronic engineers. In addition, the closed form of harmonic spectrum analysis and the quantitative design procedure of a RPWM inverter are difficult to be derived. Thus the major purpose of this paper is to present a random frequency PWM (RFPWM) scheme and its application for an inverter-fed induction motor drive. The proposed RFPWM inverter is employed in an indirect field-oriented induction motor drive with its effectiveness being demonstrated by some simulation and measured results. THE PROPOSED RFPWM SCHEME An indirect field-oriented induction motor drive with the proposed RFPWM current-controlled voltage source inverter is shown in Fig. 1. Figure1. Indirect field-oriented induction motor drive with the proposed RFPWM current-controlled voltage source inverter. It consists of a voltage source inverter,an induction motor with its rotor mechanically coupled to adc generator, which uses switched resistance as dynamicload, a current-controlled PWM switching scheme, an indirectfield-orientation mechanism, a speed feedback control loop anda random frequency triangular wave generator. It is known that since the frequency of the triangular waveused in a standard SPWM inverter is constant, the harmonicspectrums of inverter output voltages and asshown in Fig. 2 are fixedly located.
Figure 2. Harmonic spectrums of conventional SPWM inverter output voltages This will lead to the annoyed acoustic noise and undesired mechanical vibration problems. Fortunately, the harmonic spectrum can be made randomly distributed by using the random frequency triangular wave generator shown in Fig. 1. In this section, the effects of random signal magnitude and bandwidth on the resulted harmonic spectrum distribution characteristic are first analyzed detailedly. Then according to the results, a quantitative approach is proposed to design the proposed RFPWM scheme. SIMULATIONRESULTS The current-controlled RFPWM inverter-fed induction motor drive in Fig. 1 with the motor parameters listed in (1) is simulated using Simulink. Only the detailed simulink simulation block diagrams of the look-up table based random signal generator and the VCO for yielding the random frequency triangular wave are shown in Fig. 3(a) and (b).
Figure 3. Simulink simulation block diagrams: (a) look-up table based random signal generator and (b) VCO. In the PRBS ROM-based RSPWM inverter, we could change the clock rate to control the speed of reading the ROM and thus the varying speed of the random signal. The results indicate that when clock frequency is set as Hz, the spectrum of triangular wave is not distributed uniformly but concentrates at several frequencies discretely. Fig. 4(a) (d) show, respectively, the simulated total control voltage, the spectrum of, the triangular wave and the spectrum of. The results in Fig. 4(c) and (d) clearly indicate that the frequency of the resulted triangular wave has been randomly varied from 1.5 khz to 4.5 khz with the mean value of 3 khz. Figure 4. Simulated results of the IFO induction motor drive with the designed RFPWM inverter at half load Figure 5 shows the measured results at full load ( rpm, ), satisfactory results by the designed inverter can also be observed. The measured acceleration of vibration ( m/s ) and acoustic noise spectra of the motor drive with the standard inverter and proposed RFPWM inverter at half load ( rpm, ) are compared in Fig. 5(a) and (b). Great reductions in mechanical vibration and acoustic noise of the motor drive by employing the proposed inverter are clearly seen from the results.
Figure 5. Measured acceleration and acoustic noise spectra of the IFO induction motor drive at half load (! = 1000 rpm, R = 42:8 ): (a) with the standard SPWM inverter and (b) with the proposed RFPWM inverter. CONCLUSION A RFPWM inverter for induction motor drive has been presented in this paper. The effects of random signal features on the harmonic spectrum of inverter output are first analyzed intuitively. Then accordingly, the quantitative design, the Simulink simulations and the hardware implementation of the proposed RFPWM inverter are introduced in detail. The proposed RFPWM scheme is very easy to be comprehended for practical engineers. Some simulation and experimental results have demonstrated effectiveness of the proposed RFPWM inverter. REFERENCES
Azcondo, et al. 2005. Power-modecontrolledpower-factor corrector for electronic ballast, IEEE Trans. Ind.Electron., Vol. 52, No. 1, pp. 56 65, Feb. 2005. Cichowlas, et al. 2005. Active filtering function of three-phase PWM boost rectifier under different line voltage conditions, IEEE Trans. Ind. Electron., vol. 52, no. 2, pp. 410 419, Apr. 2005. Esmaeli. 2010.EMC Aspects of PWM Inverter Fed AC Motor Drive System, Lap Lambert Academic Publishing, Germany. Jovanovic and Jang.2005. State-of-the-art, single-phase, active power-factor-correction techniques for high-power applications An overview, IEEE Trans. Ind. Electron., vol. 52, no. 3, pp. 701 708, Jun. 2005 Satyanarayana, et al. 2010. Random pwm algorithms for VSI fed induction motor drives With fixed switching Frequency, International Journal of Engineering Science and Technology,Vol. 2(12), 2010, 6968-6975 Tse, et al. 2010. A Comparative Investigation on the Use of RandomModulation Schemes for DC/DC Converters, IEEE transactions on industrial electronics, Vol. 47, No. 2