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 Esmaeli1 andMohsen Mobini2
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
Intl. Res. J. Appl. Basic. Sci. Vol., 6 (9), 1243-1248, 2013
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.
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Intl. Res. J. Appl. Basic. Sci. Vol., 6 (9), 1243-1248, 2013
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).
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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.
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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
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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
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