Fig.3 shows the experimental setup. The Osram MH lamp (Powerball HCI-TT) operates at high frequency. (>10 kHz). The function generator supplies the driving.
MODELING AND CHARACTERIZATION OF ACOUSTIC RESONANCE IN METAL HALIDE LAMPS Fang Lei1, 2 , Pascal Dupuis 1 , Olivier Durrieu1 , Georges Zissis 1 , Pascal Maussion1 1 LAPLACE, Université de Toulouse, CNRS, INPT, UPS, Toulouse, France 2 Xi’an Shiyou University, School of Electronic Engineering, Xi’an, China Email: {flei & dupuis & durrieu & zissis & maussion}@laplace.univ-tlse.fr Abstract – This paper presents a modeling of the lamp voltage with acoustic resonance in metal halide lamps. The modeling is implemented in PSIM based on the signal with amplitude modulation. A lookup table of the lamp voltage is then proposed. Both periodic and stochastic signals appear in the voltage envelope when acoustic resonance occurs in MH lamps. Thus, a very simple circuit of the voltage envelope for acoustic resonance detection is also given. Simulation and experimental results of acoustic resonance detection show that the designed circuit can detect acoustic resonance well. Additionally, voltage envelope characterizations are analyzed by fast Fourier transform. Keywords -- Acoustic resonance (AR), electronic ballast, envelope detection, modeling, metal halide (MH) lamps.
1.
INTRODUCTION
Acoustic resonance (AR) occurs in high intensity discharge (HID) lamps and can cause plasma instability, lamp extinction or destruction [1], [2], [3], [4]. Since the 1960s, the AR phenomenon has been studied by researchers. However, AR is a complicated problem which refers to several disciplines, electrical, electronics, electromagnetic, acoustic and so on. Thus, AR is still not completely understood until now. The research achievements about AR can be grouped into several categories: a) AR characterization AR not only causes arc fluctuations, deformations and light flicker, but also changes color temperature and chromaticity coordinate significantly [5]. As presented in [1], AR can change electrical parameters , increasing the lamp voltage, decreasing the current. In [6], AR can cause noisy sounds in the arc tube of HID lamps . Overall, there is no doubt that AR hampers the development of HID lamps and reduces lamps’ lifetime. b) AR mechanism and theoretical frequencies In [7], AR mechanism has been defined. Alternative electric current represents a periodic heat source in the arc tube. The resulting temperature fluctuations can cause pressure oscillations. The ballasts operating at an eigenfrequency produce standing waves which affect the velocity field of the plasma and cause arc flicker. According to the solution of the simplified acoustic wave equation, three kinds of AR frequency modes can
be calculated in HID lamps: the longitudinal mode, the radial mode and the azimuth mode [8]. c) AR Detection AR detection methods can be classified depending on how the AR is detected: measuring lamp electrical parameters (voltage, current, or relative impedance) [1], [3], [9], or observing lamp physical parameters (sound emission, thermal parameters, or optical parameters) [6], [10], [11]. d) AR avoidance In order to avoid AR in HID lamps, s ome methods have been proposed, such as square wave supply [12], pulse operation [13], extra-high frequency [14], frequency or amplitude modulation [15], injection of fundamental and the third harmonic [16], and injection of several adjacent frequency signals [17]. The main ideas are to reduce the power harmonic below the AR excitatio n threshold. Although certain free AR windows exist in HID lamps, due to the production tolerance or ageing time of lamps, these free windows remain unpredicted. f) Model of Acoustic streaming Acoustic streaming in HID lamps become a hot research topic in recent years. In [10], [18], the numerical model, including acoustic streaming using 2D axisymmetric geometry was implemented in an HID lamp. In [19], 3-D multi-physics mode of high intensity discharge mode has been set up to calculate the acoustic streaming velocity field inside the arc tube of HID lamps. It has recently been discovered that the acoustic streaming phenomenon is the link of the high-frequency
resonances to the low-frequency light flicker. The highfrequency sound wave causes a low-frequency movement of the plasma arc which is as light flicker. As presented in [20], there are low frequencies of discharge arc flicker and arc fluctuations, due to the excitation of AR in MH lamp. The reason of the low frequency flicker with AR can be identified as acoustic streaming effects. However, no paper focuses on the modeling of the lamp voltage. To our knowledge, this is the first paper to propose the lamp voltage modeling with AR in MH lamps. AR causes a low-frequency movement on plasma arc and results in fluctuations of the voltage envelope. Thus, a simple low cost circuit is used to detect fluctuations of the voltage envelope. This paper is organized as follows. The principle of high-frequency ballast and the experimental setup are presented in section 2. Section 3 shows the modeling of lamp voltage with AR occurrence. The results of AR detection are presented and analyzed in section 4. Section 5 gives conclusions.
HID lamps can be regarded as a resistance. The transfer function of LCC circuit can be expressed by (1). V𝑙𝑎𝑚𝑝 V𝑖𝑛
=
1 𝑆 𝐿𝑠 𝐶𝑝 𝐶 +𝐶 𝑝 𝑠 1 1 𝑆3 + 𝑆2 + 𝑆+ 𝑅 𝑙𝑎𝑚𝑝𝐶𝑝 𝐿𝑠 𝐶𝑠𝐶𝑝 𝑅 𝑙𝑎𝑚𝑝𝐿𝑠𝐶𝑠 𝐶𝑝
(1)
2.2 EXPERIMENTAL S ETUP Fig.3 shows the experimental setup. The Osram MH lamp (Powerball HCI-TT) operates at high frequency (>10 kHz). The function generator supplies the driving signals of the electronic ballast. A digital power meter is used to monitor the power and the power factor. Arc images are captured by a digital camera. A digital oscilloscope is used to get the lamp voltage, current, power. As presented in [23], low-frequency fluctuations exist in the lamp voltage. Thus, a rectifier circuit is applied to obtain the voltage envelope for AR detection. FFT is used to analyze characterizations of lowfrequency fluctuations of the voltage envelope. Function Generator Yokogawa FN120
2. OPERATION PRINCIPLE OF HIGHFREQUENCY B ALLAS T AND EXPERIMENT
Electronic Ballast High Frequency
MH Lamps
Envelope Detection Rectifier Circuit
Osram
S ETUP Power Source
2.1 CIRCUIT OF HIGH-FREQUENCY ELECTRONIC BALLAST The high-frequency electronic ballast is supplied by an LCC half-bridge inverter [21], [22], which is shown in Fig.1. The ballast consists of the DC supply Vdc, two switches Q1 and Q2 , the coupling capacitor Cs , the resonant inductor Ls , the resonant capacitor Cp and lamps. The LCC resonant circuit generates the high voltage required to strike the arc and maintains a stable arc after ignition.
Q1
Resonant Tank Cs Ls
Vdc Q2
Cp
Vlamp
Fig.1. Circuit of high-frequency electronic ballas t Ls
VAB1
Kikusui PCR500M
Digital Power Meter Yokogawa WT210
Digital Camera Nikon D3300
Digital Oscilloscope
FFT Analysis
TDS2024
Matlab
Fig.3. Experimental setup
3.
M ODELING OF LAMP VOLTAGE WITH AR OCCURRENCE
3.1 LAMP VOLTAGE AND VOLTAGE ENVELOPE WITH AR In our experiments, two Osram 150W MH lamps are tested. In this kind of lamps, AR phenomenon is very intensive and cause extinguished. In [15], it has already been shown that AR level increases with the rise of the operating power in HID lamps. To make the lamp work stable and avoid lamp explosion, the lamp operating power is lower than the nominal power in our experiment. Fig.4 shows the lamp voltage and the voltage envelope without AR and with AR occurrence.
Cs
Cp
Rlamp
Fig.2. Simplified circuit using fundamental approximation The simplified circuit of the LCC circuit using fundamental approximation is shown in Fig.2 in which
(a)
Without AR (f=18.0 kHz and Plamp =64 W)
(b) (b)
Periodic AR signal (f=17.0 kHz and Plamp =65 W)
(c)
Stochastic AR signal (f=16.9 kHz and Plamp =65 W)
AR detection circuit
Fig.4. Lamp voltage and envelope without/with AR (c)
In Fig.4.a, without AR, the lamp voltage amplitude is constant and the voltage envelope is flat. With AR, two kinds of signals are observed. One is the periodic signal in Fig.4.b and the other one is the stochastic signal in Fig.4.c. These low-frequency fluctuations are most likely due to temperature changes in the arc. As can be seen From Fig.4, the lamp voltage waveform is very similar to the signal with an amplitude modulation. 3.2 MODELING OF ACOUSTIC S IGNALS AND AR DETECTION This simulation is based on the principle of amplitud e modulation. The result when the carrier c(t) is multiplied by the positive quantity (1+m(t)) is 𝑦(𝑡): 𝑦(𝑡) = (1 + 𝑚(𝑡)) ∗ 𝑐(𝑡) (2) Hence, lamp voltage with AR occurrence can be modeled by equation 3: 𝑅 𝑉𝑙𝑎𝑚𝑝 = (𝑉𝑚 + 𝑉𝑑𝑐 + 𝑉𝑟𝑎𝑛𝑑 ) ∗ 𝑉𝑐 ∗ 2 (3) 𝑅1+𝑅2
Vm , Vdc, Vrand and Vc represent the information signal, positive quantity, environment noise, carrier signal respectively. Our models are built in PSIM according to equation 3. Fig.5 and Fig.6 show the lamp voltage modeling with AR and AR detection.
Lamp voltage with AR
(a)
Clamper
AR detection
As can be seen from Fig.5.a, the lamp voltage modelin g is composed of four main parts: the lamp voltage with AR, the clamper, the AR detection and the low-pass filter. The clamper is used to overcome the interference caused by noise fluctuations. The aim of the low-pass filter is to get low-frequency variations in the voltage envelope. The amplitude and the frequency of Vm depend on AR intensity and what kind of AR signal is detected. Vdc is added in order to ensure that the (Vm +Vdc) is positive. Vrand is the random noise from outside environment. The amplitude and the frequency of Vc depend on the operating state of the lamp. In our case, all of these parameters are set based on Fig.4. In [23], there are low-frequency fluctuations of current or voltage with AR occurrence in the MH lamps. In [24], a periodic or stochastic flicker is observed, when AR occurs in the MH lamps. These findings are consistent with our experimental results in Fig.4. Thus, the periodic and stochastic fluctuations of the lamp voltage are simulated in Fig.5 and Fig.6 respectively. In Fig.6, a digital filter is added to fit with the real AR signal. The AR detection circuit is as same as in Fig.5.b.
Lamp voltage with AR
Low pass filter
Lamp voltage model with periodic AR and AR detection
Simulated lamp voltage and envelope/periodic AR
Fig.5. Modeling of lamp voltage with periodic AR and AR detection
(a)
Clamper
AR detection
Low pass filter
Lamp voltage model with stochastic AR and AR detection
(b)
Simulated lamp voltage and envelope/stochastic AR
Fig.6. Modeling of lamp voltage with stochastic AR signal and AR detection Comparing Fig.4 with Fig.5 and Fig.6, the periodic and stochastic signal of the lamp voltage envelope can be simulated. It means that our modeling is very close to the lamp voltage with AR occurrence. Simulatio n results also show that our detection circuit can extract the lamp voltage envelope well. 3.3 LOOKUP TABLE OF LAMP VOLTAGE WITH AR In order to extract the real voltage envelope and verify our detection circuit, the real lamp voltage from a lookup table in AR situation is shown in Fig.7. The schematic is composed of four parts, the lookup table of lamp voltage, clamper, AR detection and low pass filter.
As PSIM just can read limited data (maximum 30000 points). Thus, two lookup tables are used to import the data of real lamp voltage from experimental results. Firstly, data of LKUP1 are imported and then data of LKUP2 are imported. The two intervals of simulation results are merged into Fig.7.c. In this simulation, the stochastic AR signal is simulated. The simulation of the periodic AR signal is similar to the stochastic signal.
Lamp voltage with AR
(a)
Clamper
AR detection Low pass filter
Schematic of lamp voltage using look-up table
(c)
Lamp voltage and voltage envelope
Fig.7. Lookup table of lamp voltage and voltage envelope detection with AR The AR detection circuit in Fig.7 is as same as in Fig.4.b. As can be seen from Fig.7.c, the designed detection circuit can follow the voltage envelope well.
4.
RES ULTS OF AR DETECTION
4.1 S CHEMATIC OF AR ENVELOPE DETECTION In Fig.7 and Fig.8, simulation results shows that our AR detection circuit can detect AR well. Consequently, the schematic of real AR detection circuit is shown in Fig.8.
Fig.8. Schematic of AR envelope detection In Fig.8, U1 is the amplifier AD804. U1A and U1B are voltage follower and inverter. The principle of the circuit in Fig.8 can be regards as a rectifier. When the input signal is negative, the U1B works and the output signal is positive. When the input signal is positive, the U1A works and the output is positive as well. Finally , the low-pass filter is used to extract low-frequency fluctuations in the voltage envelope. This filter is composed of resistor R2 and capacitor C1 , the cutoff frequency is 33Hz. 4.2 REAL TIME OF AR DETECTION When MH lamps work at normal operation, there is no flicker and the lamp arcs are straight. The arc image of MH lamps at normal situation is shown in Fig.9.a. Conversely, the arcs bend and the light flashes due to the AR phenomenon. Arc discharge deformations can
(b)
The real data of lamp voltage in Fig.4 (b)
be clearly observed in Fig.9.b. The detection results of voltage envelope are presented in Fig.10.
when AR occurs at f=17.0 kHz, the periodic the lowfrequency fluctuation is detected in the lamp voltage. In Fig.10.c, at f=16.9 kHz, the stochastic signal is detected in the voltage envelope. Overall, the designed circuit can distinguish AR an AR-free cases well. 4.3 FFT ANALYSIS OF VOLTAGE ENVELOPE In order to characterize AR, FFT is used to analyze the voltage envelope in the Osram MH lamp. FFT analysis of voltage envelopes is shown in Fig.11.
(a)
Without AR
(b)
With AR
Fig.9. Lamp arc shape
(a) (a)
(b)
FFT analy sis of the periodic signal with AR (∆f=0.05 Hz) in Fig.4.a and Fig.4.b
Without AR f=18.0 kHz
Periodic AR signal f=17.0 kHz
(b)
FFT analy sis of the stochastic signal with AR (∆f=0.5 Hz) in Fig.4.a and Fig.4.c
Fig.11. FFT analysis of voltage envelopes
(c)
Stochastic AR signal f=16.9 kHz
Fig.10. Voltage envelopes in Osram MH lamp As can be seen in Fig.10.a, no AR occurs and the voltage envelope is constant. However, in Fig.10.b ,
As seen from Fig.11.a, a ten Hz low-frequency fluctuation in the voltage envelope is observed, when AR occurs at f=17.0 kHz in the MH lamp. In Fig.11.b , it is obvious that the stochastic signal is detected in AR situation at f=16.9 kHz. In low-frequency domain (050 Hz), the amplitude of the voltage envelope with AR occurrence is much higher than without AR, about 100 times. FFT analysis of the voltage envelope shows that AR can cause the periodic or stochastic signal in the
lamp voltage envelope. These low-frequency fluctuations are correlated with arc motion, which are most likely caused by temperature changes in arc tube.
5.
[9]
CONCLUS IONS
The modelings of the lamp voltage and AR characterizations are presented in this paper. Two kinds of AR signals, the periodic and stochastic signal, are observed and analyzed in Osram MH lamps. The simulation results of the AM modulation modelin g show that our modeling is close to the real lamp voltage. Additionally, regardless of the simulation or the experimental results of the AR detection show that the proposed rectifier circuit is robust to detect AR. Voltage envelope spectra in the low-frequency domain further verify that two kind of signals (the periodic and stochastic signal) are detected in the voltage envelope when AR occurs in MH lamps. The modeling of the lamp voltage with AR will be helpful for researchers to achieve AR detection and propose avoidance methods for high-frequency electronic ballasts .
[10]
ACKNOWLEDGMENTS
[15]
This design and basic experiment has been achieved in Laboratory on Plasma and Conversion of Energy (LAPLACE) in Toulouse, France. This work is also supported by the Chinese Scholarship Council (CSC).
[16]
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