ACOUSTIC RESONANCE REJECTION VIA VOLTAGE MODULATION METHOD FOR HPS LAMPS L.M.F. MORAIS, P.F. DONOSO-GARCIA, S.I.SELEME JR, P. C. CORTIZO, F.N.A. SILVA Universidade Federal de Minas Gerais Departamento de Engenharia Eletronicaˆ 31270-901 Belo Horizonte, MG - BRASIL Emails: [email protected], [email protected], [email protected], porfi[email protected], [email protected] Abstract— A voltage modulation method in order to prevent the resonance evolutes with some drastic consequence [7], acoustic resonance (AR)inHPS lamps is presented. Two voltage [8], [9]; PWM modulation techniques via inverter are proposed. The first 4) Using square current waves in low frequency, this method one injects harmonic components in the lamp voltage reference. The second one consist of high frequency voltage modulated by avoids the AR because the instantaneous power of a a low frequency waveform. These techniques allow the control square wave is theoretically constant [10], [11], [12]; of crest factor other than rejecting the AR. 5) Driving the lamp with DC current also avoids acoustic Keywords - Acoustic resonance avoidance, HPS lamps, har- resonance [13]; monics injection, PWM, modulation low frequency. 6) Using square current waves of high frequency also pre- vents the acoustic resonance but this approach implies I. INTRODUCTION in high levels of electromagnetic interference (EMI), specially radio frequency emitted by the discharge itself; HPS High pressure sodium ( ) lamps present some nice 7) A way of reducing the interference levels of the approach characteristics, such as high light efficiency, long lifetime 6) above and yet, preventing the occurrence of acoustic (around 24,000 hours) and pleasant colors, which justify its resonance is to superimpose some harmonics (the third large scale use in public lighting. The main challenge when and the fifth, for instance) to the high frequency sinu- HPS designing electronic ballasts for lampsistoavoid soidal fundamental wave. The resulting wave would be acoustic resonance, which consists on gas pressure fluctuation approximately square [14], [15] in terms of spreading the inside the discharge tube of the lamp when supplied by high frequency modes, without the inconvenience of high EMI kHz frequency power source (from a few to hundreds of levels; kHz ). The most relevant effects of the acoustic resonance are 8) Modulating the voltage applied to the lamp, i.e., a high the light movement and fluctuation, usually called flicker, the frequency current modulated by a low frequency wave- light arc extinction due to its lengthening, the destruction of form [16]. the discharge tube owing to overheating and, even when the The present paper studies both the harmonic injection arc is not extinguished, there are, eventually, temperature and technique and the modulated high frequency voltage by a Ra variations [1]. low frequency waveform technique in a PWM inverter-based There are ways to avoid the occurrence of the acoustic reso- ballast, aiming at the rejection o acoustic resonance. nance and its nuisance, that consist basically in three strategies: This paper has the following structure: First, the introduc- first, avoid frequencies where the acoustic resonance occurs; tion discusses the acoustic resonance phenomena and reviews second, to switch frequencies whenever the acoustic resonance some approaches proposed in the literature to prevent it. is detected which implies in methods for detecting it; third, Following the introduction, Section II presents the proposed even when operating the ballast in frequencies where acoustic harmonic injection technique and voltage modulation applied resonance occurs, to avoid to excite it through spreading the to the lamp. Section III discusses the LC filter design in frequency spectrum. There are several methods in literature terms of the constraints imposed by the presence of the (third) dealing with this problem: harmonic. Section IV presents and discusses the experimental 1) Choosing a constant frequency from (20 − 200)kHz results obtained with these techniques, while in the conclu- where, specifically, in a small range around the chosen sions, the impact of the method proposed on the acoustic frequency the acoustic resonance does not occur [2], [3], resonance rejection is emphasized. [4]; 2) Using very high frequencies, avoiding the whole range II. VOLTAGE WAV E F O R M STUDY where acoustic resonance occurs, normally higher than As it was mentioned before, using a constant frequency in 500kHz [5], [6]; the (20 − 200)kHz range in a specific value where acoustic 3) Operating with circuits, that somehow detect the occur- resonance does not occur is not an efficient strategy due to rence of the acoustic resonance and switch, through some dependency of this phenomenon to the lamp power, manufac- modulation strategy, the frequency of the inverter before turer, shape and also with the aging of the lamp. f (kHz) (Hz) GE 70W - MV PHILIPS 70W - MV injection. On the other hand, changes on the ballast operating 160 f SON 150W SON-S 150W (Imn) (Imn) frequency would demand a complete redesign of the filters. 140 6 220 510 The proposed method consists in injecting harmonics 10 120 120 310 020 020 through their addition them to the fundamental reference signal 210 of the pulse width modulation inverter which will synthesize 100 110 110 the voltage applied to the resonant filter feeding the stationary 0014 80 0012 0010 voltage to the lamp. The main feature of this method is its 4 008 10 modularity since the harmonics are injected via software. 60 006 004 The reference voltage signal of the PWM is given as: 40 002 002 20 n 3 vref = ai sin(2iπffundt) (1) 0 10 i=1 a) b) Fig. 1. a)AR free frequency bands for lamps of same power and where ffund is fundamental frequency of the supply voltage. different manufacturers [17] and b) same manufacturer and different An immediate advantage of this approach is the need of type or power [1] a single output LC filter no matter how many harmonics are injected. Another important feature is the simplicity with Figure 1 a) presents acoustic resonance (from now on named which reference signals are generated without the burden of as AR) free frequency bands for two lamps of the same power calculating the phase shift φ between gate signals of each and different manufacturers [17] whereas, Figure 1 b) shows inverter leg. it for two other lamps of different type or power and the same manufacturer. It is quite clear for these two examples that the Another feasible approach to avoid acoustic resonance in HPS occurrence of AR happens in a large spectrum of frequencies lamps consists in applying to the lamp a high frequency and does not show a predictable pattern [1]. voltage modulated by a low frequency waveform. This low The present paper deals with this problem, investigating a frequency modulation avoids the acoustic resonance [16]. This new approach that consists first, in injecting harmonics to is implemented in the present work generating a modulated the supply voltage of the lamp through a PWM inverter signal as the reference to the PWM. This reference signal (as mentioned in Section I Method 7, above) and second, is constituted by a fundamental frequency multiplied by the using a frequency operation range not frequently used in modulating frequency as: electronic ballasts which is (1 − 10)kHz. In order to validate the proposed method, the influence of the third harmonic amplitude in the rejection of the AR is studied. vref = sin(2aπffundt) ∗ sin(2aπfmodt). (2) The harmonic injection approach is based on the spread of the frequency spectrum of the power delivered to the lamp In order to assure a crest factor less than 1.8, the reference thus reducing the power associated to each frequency. The idea signal is saturated and thus, limiting the crest factor within behind the method is that if any of the frequencies applied to the norm values (American National Standard ANSI C78.42, the lamp corresponds to an AR frequency, its intensity (power) Part IV ”relevant lamp data sheets”, sets its maximum value would not be enough to excite this phenomenon. at 1.8). Reference [18] asserts that ballasts with higher crest The approach presented by Alonso et al. [14] is based on factors may result in depreciation of lumen output or reduced the harmonic injection where the fundamental frequency is lamp life. supplied by one of the inverter legs switching with frequency f LC An important issue to be analyzed is the ration between through an filter and a third harmonic frequency is then f the fundamental and the modulating frequencies, a = mod , supplied by the second leg switching at frequency 3f and ffund phase delay through a second LC filter. which guarantees acoustic resonance rejection. In [16] a ratio a =0, 0017 = 120 The method presented in [14] is based on the independent 68k is proposed. In the present work a funda- control of each of the inverter legs and on the design of mental frequency of 12kHz has been chose and modulating separate filters for the desired harmonics. The main charac- frequencies of 150Hz and 300Hz have been tested in simu- teristics of this approach consist on the need of one filter for lation. In the experimental setup a modulating frequency of each harmonic injected, increasing the number of components 300Hz was used due to a faster variation in the instantaneous of the ballast, and, the need of one inverter leg for each power. harmonic injected, that implies the need of extra inverter The considered technique allows to limit the voltage peaks, legs in the case of injecting harmonics higher than the third, in such a way that the crest factor is (also limited) set in 1.8. amounting thus to the complexity of the inverter in such case.