AU J.T. 11(1): 42-47 (Jul. 2007) The Case Study of 5 kHz – 25 kHz High Frequency Adjustment in Converter Circuit to Generate Ozone Gas Siseerot Ketkaew Faculty of Engineering, Ramkhamhaeng University Bangkok, Thailand <siseerot@eng.ru.ac.th> Abstract This paper presents a case study on changing the switching frequency in a converter circuit to generate ozone gas using a high-voltage, high-frequency, switching power supply. This supply uses a flyback converter of 100 VA (Output 3 kV), the operational frequencies are from 5 kHz to 25 kHz, and the control circuit uses a pulsewidth-modulation (PWM) technique. Under the one-hour test of the ozone generator at wind velocity of 0.259 x 10-6 m3/sec, the switching frequency of 5 – 25 kHz can generate ozone gas of 99.6 – 120.5 mgO3/hr. Keywords: switching frequency, flyback converter, ozone gas, control circuit. ozone gas. Therefore, both the voltage level and the applied frequency have to be controlled during the generation of ozone gas (Dalarat et al. 2004). The energy range being used to produce ozone gas from chemical components is from 493 kJ/mol to 762.23 kJ/mol. The adapting unit has the required energy within the range from 5.583 kWh/m3 to 8.631 kWh/ m3. Since there is only 21% oxygen in the air, the required energy should be within the range from 1.17243 kWh/m3 to 1.620 kWh/m3. It is enough to generate ozone gas in the gap of two-layer electrodes connected in series. (Dalarat et al. 2004). Introduction Nowadays, the ozone gas is widely brought to use for living needs such as using ozone to clean vegetables instead of using manganese, to kill diseases and to reduce the quantity of chlorine in water. However, bringing ozone to clean the air has some drawbacks. For example, if it is used at high concentration, it can irritate the body. It is health effective only if its quantity is properly controlled for a given application. Therefore, the control of the quantity of ozone gas should match some required technical specifications. This is the main reason to study the artificial generation of ozone gas. This paper presents an evaluation of the effect of changing the switching frequency in a converter circuit to generate a particular ozone quantity. The highvoltage high-frequency circuit is constructed by using the principle of switching the ac power supply. A high-voltage high-frequency signal is supplied to the load consisting of twolayer electrodes connected in series to produce ozone gas. The generation of ozone gas is based on the principle of spreading molecules of oxygen. One can produce ozone gas on the basis of the equation O2 + O = O3. The ozone gas can withstand a high voltage level. Also, the heat affects the quantity of the occurring Technical Report The Process of Generating Ozone Gas The air comprises mainly of 79% nitrogen (N2) and 21% oxygen (O2). The rest are inert gases and steam. The ozone gas is a gas consisting of 3 oxygen atoms under unstable status with easy dispersion that depends on both the environment and the density of the produced quantity. The production procedure relies on the process of generation of a free oxygen atom from an oxygen molecule in the air. After that the free oxygen atom is combined together with an oxygen molecule to obtain ozone gas (O3) 42 AU J.T. 11(1): 42-47 (Jul. 2007) which is brought to use in both industrial and health systems. The occurring process of ozone gas generation forms from two sub-processes – ionization process and dissociation process. The ionization, spreading of gas, is based on the increment of electron avalanche leading to an insulator breakdown where the electric current flows through the border line of the insulator. This follows to the occurrence of heat due to the current flow of the insulator breakdown which causes the ozone gas to disintegrate due to its lower energy. Therefore, the ozone gas production should not result in a breakdown, i.e., the electron energy from the electric field should be lower than the ionization energy, but it should be high enough to separate the oxygen atoms (Ketkaew 2005). Fig. 1. Co-core structure. cylinder: Ozone tube’s Co-core Cylinder: Ozone Tube’s Structure Fig. 2. Block-diagram of ozone generator. The Gauss's law for the electric field says that the electric flux through any closed surface is proportional to the amount of electric charge Block-Diagram of Ozone Gas Generator → → contained within that surface, Q = ∫ D⋅ d A . Fig. 2 shows the ozone gas generator, which is constructed with the use of the highvoltage, high-frequency, switching power supply. The ac input voltage is 180 V, 50 Hz, needed by the rectifier circuit to produce the dc voltage of 255 V. The dc voltage signal is delivered to the inverter controlled by PulseWidth Modulation (PWM) in order to obtain the operational frequencies within the range 5 kHz – 25 kHz. The low voltage signal of the inverter of the primary circuit is amplified to the 3 kV High-Frequency High-Voltage (HFHV) signal of the secondary circuit which is fed into the electrode tube for the generation of the ozone gas. S The electric field of a co-core cylinder of length l and radius r, r1 ≤ r ≤ r2, (see Fig. 1) is given by (Dalarat et al. 2004): E (r ) = Q 1 . 2πε l r (1) The voltage, V, across between both cylinders is given by: re r2 r Q dr Q = ln 2 . 2πε l r 2πε l r1 r1 V = ∫ E (r )dr = ∫ r1 (2) Therefore: E (r ) = V , r1 ≤ r ≤ r2 . (3) r2 r ln r1 The maximum electric field stress occurs on the inside of cylinder’s surface and is given by: E MAX = E (r1 ) = V r1 ln Technical Report r2 r1 . Electrode Tube Design and Energy Use The principle of ozone tube design relies on an unsmooth electric field for the generation of the ozone gas quantity. Therefore, a twolayer electric insulator is chosen for the electrode design due to the permittivity (ε) (4) 43 AU J.T. 11(1): 42-47 (Jul. 2007) differences of the electric insulator. It is suitable for the generation of a non-uniform electric field to have variable but close to ε values of each layer under electric field stress. As shown in Fig. 3, a two-layer co-core cylinder for ozone tube design is chosen under the following conditions (Ketkaew 2005): - Silica is chosen for the 1st layer electric insulator due to its effectiveness in generating ozone gas, where ε1 = 8, the diameter is 2.9 cm and the length is 18 cm. - Air is chosen for the 2nd layer electric insulator, where ε2 = 1. - Cathode frilled aluminum (for rubbing pots) in filament coil inside of the silica’s electric insulator is used. The reason is that aluminum has a high conductivity. - The anode is a stainless steel cylinder, where the diameter is 3.3 cm and the length is 18 cm. Fig. 3. The structure of electrode tube. Calculation of Electric Field (E) and Voltage (V) of Ozone Tube Design of HF-HV Switching Power Supply In Fig.3: r1 = 1.35 cm, r2 = 1.45 cm, r3 = 1.59 cm, l = 30 cm. For energies from 5.58 kWh/m3 to 7.73 kWh/m3, if the air is composed of 21% oxygen (O2), the chosen energy range is 1.172 – 1.620 kWh/m3 (Ketkaew 2005). As The high-frequency, high-voltage, (HFHV) switching power supply of high-ripple voltage is controlled by IC LM555. Switching devices, power MOSFETs IRFP460, are used in the flyback converter controlled by the PWM strategy from IC LM555 (National Semiconductor 2006). The switching frequencies range from 5 kHz to 25 kHz. The energy from the converter is transferred through the HF-HV transformer to produce the HF-HV high-ripple voltage supplying the electrode tube. The structure and the circuit of this supply are shown in Figs. 4 and 5. Air volume = π(r3 – r2)2 x l = π(1.59–1.45)2 x 30 = 1.846 cm3, (5) with maximum energy per volume (Wmax) of 1.620 kWh /m3 and minimum energy per volume (Wmin) of 1.172 kWh/m3, then: Wmax = 1.620 x 103 x 1.846 x 10-6 = 0.00299 Wh, Wmin = 1.172 x 103 x 1.846 x 10-6 = 0.00216 Wh. Emin and Emax are obtained from Eq. (6) below: 1 W = ∫ εE 2 dv , (6) 2 Vol 2Wmin 2 × 0.00265 = ε Vol 8.854 × 10 −12 × 2.262 = 16.273 kV/cm, Emin = 2Wmax 2 × 0.00366 = ε Vol 8.854 × 10 −12 × 2.262 = 19.129 kV/cm. Emax = Technical Report Fig. 4. The structure of HF-HV converter circuit. 44 AU J.T. 11(1): 42-47 (Jul. 2007) Fig. 5. High-frequency, high-voltage, (HF-HV) converter circuit. Experimental Results Table 1. Results of testing the breakdown voltage of the electrode tube. Results of Measurements of Signals of the IC LM555 and the Output Voltage of the HF-HV Transformer Order 1 2 3 4 5 6 7 8 9 10 The switching frequency is 25 kHz. The results of testing the breakdown voltage of the electrode tube are shown in Table 1. The average value of the breakdown voltage is VBreakdown (avg.) = 35.38 / 10 = 3.538 kV. One should use 3 kV because it is lower than the breakdown voltage (see Fig. 6). The experimental set-up is shown in Figs. 7 and 8. Technical Report 45 Breakdown voltage kV 3.51 3.53 3.51 3.52 3.58 3.53 3.52 3.59 3.53 3.56 AU J.T. 11(1): 42-47 (Jul. 2007) Results of the Used Switching Frequency Adjustment and the Ozone Gas Quantity (a) The tests are conducted at 3 kV constant output. The experimental results are shown in Table 2 and the meaning of each parameter is explained below: - f (kHz) is the frequency of the converter; - Vin(rms) (V) is the input voltage of the converter; - Iin(rms) (A) is the input current of the converter; - Pin (W) is the input power of the converter; - PF is the power factor of the converter; - Vout (kVdc) is the output voltage of the converter; - Ozone quantity (mgO3/hr) is the ozone gas generated by the ozone generator. (b) Fig. 6. (a) VGS and VDS signals of the power MOSFETs; (b) Output voltage of the HF-HV transformer at 3 kV. (a) (b) Fig. 7. (a) The equipment for testing the breakdown voltage; (b) The testing of the breakdown voltage of the electrode tube (Ketkaew 2005). Conclusion From the experiments for the study of the effect of changing the switching frequency, one can evaluate the process of ozone gas production. The experimental results in Table 2 demonstrate the relationship between the switching frequency and the quantity of generated ozone gas. With the increase of the switching frequency, increased quantities of ozone gas are generated because the shifting of the frequency level in the converter circuit has an effect on the production resonance at the ozone tube. Therefore, the quantity of generated ozone gas changes accordingly. Fig. 8. HF-HV transformer. Table 2. Results of the used adjustment of the switching frequency and its effect on the ozone gas quantity of the ozone generator (the constant output voltage is set to 3 kV). f (kHz) 5 10 15 20 25 Technical Report Vin (rms) (V) 180 180 180 180 180 Iin (rms) (A) 0.63 0.63 0.63 0.63 0.63 Pin (W) 79.38 79.38 79.38 79.38 79.38 46 PF 0.6 0.6 0.6 0.6 0.6 Vout (kVdc) 3 3 3 3 3 Ozone quantity (mgO3 /hr) 99.6 104.2 110.8 116.1 120.5 AU J.T. 11(1): 42-47 (Jul. 2007) References Fig. 9. The chemical components equipment being used during the tests. Chryssis, G. 1989. High frequency switching power supply. McGraw Hill, New York, NY, USA. Dalarat, J.; Sreeuthaiporn, T.; Namkratok, Y.; and Rooptong, W. 2004. A study and construction of high voltage high frequency full bridge inverter for ozonizer. Master Thesis in the Department of Electrical Engineering, Faculty of Engineering, Mahanakorn University of Technology, Bangkok, Thailand. Ketkaew, S. 2002. Air cleaner by using high voltage electrostatic. Proc. IEEE Int. Conf. on Power System Technology. Kunming, China, 13-17 October, pp.1611-1614. Ketkaew, S. 2005. The study of ozone gas generating technique using high frequency, high voltage dc switching power supply of high ripple voltage (in Thai). Journal of King Mongkut’s Institute of Technology Lat Krabang 22 (2): 1-6. Mohan, N.; Undeland, T.M.; and Robbins, W.P. 1989. Power electronics: Converters, applications, and design. John Wiley & Sons, New York, NY, USA. National Semiconductor. 2006. LM555 Datasheet, pp. 1-12. Rattanawichain, P. 2002. Ozone generator for solar energy. Master Thesis in Department of Electrical Engineering. Faculty of Engineering, King Mongkut’s Institute of Technology Lat Krabang, Bangkok, Thailand. Trerutpicharn, S.; Deeon, S.; and Potivejkul, S. 1996. High voltage high frequency transformer for testing insulator. 19th Electrical Engineering Conference, Bangkok, Thailand, 7-8 November. and The ozone gas quantity, which the ozone gas generator produces, can be used widely and applied for solving the environmental problems. Fig. 9 shows the quantity of ozone gas occurring after using chemicals for testing. The chemicals in this figure include also potassium iodine (KI). The ozone gas is initially put into KI. This affects the change of the solution color from transparent to yellow. Starch liquid pours into this solution that makes the solution to change its color again. The color will be dark blue. Then, sodium thiosulphate is used in tritratation in order to change the color from dark blue to transparent in the end. This method can detect and test the ozone gas quantity. Acknowledgements The author wishes to thank the Department of Environment, Faculty of Engineering, Ramkhamhaeng University, for the support in conducting the experiments. The assistance of Ms. Munlica Kanjan is also appreciated. Technical Report 47