2 t. The output voltage of the converter can be calculated with (2) by assuming that i a is zero when ωt =

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1 Volume 119 No , ISSN: (on-line version) url: ijpam.eu AN IMPROVED POWER FACTOR CORRECTION SYSTEM FOR THREE PHASE DIODE RECTIFIER 1 S.AARTHISURIYA, 2 S.SHRINE, 1,2, Assistant Professor, Department of Electrical Engineering, BIST, BIHER, Bharath Univeristy Chennai-73. aarthisuriya.eee@bharathuniv.ac.in Abstract - This work describes a new technique for shaping the line current and improving the total harmonic distortion (THD) of the input current as well as the power factor of a threephase suppressed-link rectifier inverter. The method proposed in this paper makes use of only three bi-directional low power static switches with a relatively simple gating circuit and the switches are gated at line frequency. The main feature of the topology is low cost, small size, 80% of efficiency and simplicity, and is excellent for retro-fitting front-end rectifier Index terms-drives, power factor, three-phase rectifier, total harmonic distortion (THD). 1. INTRODUCTION In the context of the global economy we envisage today, it becomes important to keep the manufacturing process efficient to minimize the cost of production. In the past years, the power quality issues such as the line current harmonics and consequent voltage distortions have been the source of annoyance for the industry[1-6]. A conventional inverter for three phase induction motor drives uses a diode bridge rectifier to provide the dc link voltage. However, this kind of topology injects a large number of current harmonics into ac mains. In the recent years, a number of new techniques have been proposed for improving the power factor of threephase rectifiers. A dc dc boost topology with a threephase rectifier and three boost inductors at the ac side. Although the circuit draws a high-quality current from the ac source[7-11], with almost unity power factor, in high-power application the boost switch stands as a challenging design problem. Other proposals utilize special magnetic devices to achieve a high-power factor without active switches[12-19], like interphase transformers. In such cases, the cost, volume, weight, and additional power losses on the magnetic components withstand as a major limitation for high-power applications. Three-phase PWM Rectifiers are increasingly used for industrial applications. They have the characteristics of nearly sinusoidal input current waveform with unity displacement power factor and regeneration capability. However, due to the high switching losses, PWM rectifier cannot be used in high power conversion. Moreover, filters are still required to suppress high switching frequency harmonics. Complicated gating circuit also adds to its higher cost. The aim of the present work is to find a technique to improve the power factor of the extensively used three-phase uncontrolled rectifier, at power levels in the range of several kilowatts. The approach used for the improvement of power factor also helps to enhance the system efficiency. Va Vb Vc La Lb Lc Fig.1. Front end Rectifier D1 D3 D5 D4 D6 D2 Sa Sb Sc Ca Cb Ro 7299

2 D+ M D- D- D+ Fig.2. Bidirectional Switch II. CIRCUIT DESCRIPTION ANALYSIS AND PRINCIPLE OF OPERATION OF FRONT END RECTIFIER The voltage sources, V a, V b, and V c in Fig. 1 denote the three-phase ac system. Diodes D1 to D6 are low-frequency rectifiers, with load Ro and input inductors L a, L b, and L c. Two identical capacitors C a, C b helps to provide a balanced central node between the positive and negative output terminals. Each of the bidirectional switches S a, S b, and S c were assembled with a low-power MOSFET connected between the dc nodes of a diode bridge, as in Fig. 2. The diodes D+ conducts during positive input voltage and D- conducts during negative input voltage. In the proposed converter, the switches operate at low frequency and the gating circuit is a very simple one. Fig.3 Switching pattern of Bidirectional switchess a, S b,and S c Fig. 3 shows the gate scheme for the bidirectional switches on the proposed converter. In a conventional three-phase diode rectifier with capacitive load, the line current shows a delay of approximately 30 relative to the line voltage, resulting in periodical intervals where such current is null. The effect is a low power factor and high harmonic distortion of the input current. On the proposed rectifier, each of the bidirectional switches S a, S b, and S c is gated on during an appropriate interval, providing an alternative path for the input current. Each gate pulse starts when the corresponding phase voltage is null, with a pulse width denoted by α. On a first approach, was set to 1/12 of the voltage period, or 30, for all the load conditions of the converter. These switches operate at only two times the line frequency and gating circuit is relatively simple. Using a suchlike switching pattern, the bidirectional switches operate at low frequency, with reduced power losses, and use lowcost devices. Along with this feature, the input inductors can be assembled with ordinary core materials, resulting in a low-cost ac-to-dc converter with a high power factor, suitable to high-power applications[20-26]. For the circuit analysis, six topological stages are presented, corresponding to the halfperiod shown in Fig.4. In a three-phase rectifier only two diodes conduct at any given time. As a result, the current in the third phase is zero. However, in this approach, the switch connecting to the third phase is turned on during that particular duration. Hence, the input current rises and is governed by[27-35], Vi 2 ia t 1 cos t (1) 2.. f. L 3 The output voltage of the converter can be calculated with (2) by assuming that i a is zero when ωt = Vi Vo 7 3 (2) Vo V i where, V o is the rated output voltage and V i is the input line to line voltage. Assuming that the input current supplies the load during the 90 to 120 interval, the critical input inductance can be determined by using 2 2 Vi 2 Vi L (3) fp fp Where, o o 7300

3 f is the ac system frequency and P o is the rectifier rated output power. Fig 4. Topological stages for the half period of Input Voltage III. SIMULATION RESULTS The complete diagram of the proposed method with front end PF rectifier and rear-end suppressed dc-link [36-45],inverter set is shown in Fig. 5. For the purpose of illustrating the design procedure, a converter with the following specifications is chosen 1) input ac line to line voltage 230 V; 2) ac frequency 50 Hz; Fig.5. Complete Schematic of Proposed scheme. 7301

4 The critical input inductance of La,Lb,Lc at the rectifier input side is The input current waveform for a conventional converter is shown in Figs = I n 100% I1 n 2 (12) The power factor and input current THD of the frontend rectifier are calculated by using (11) and (12), respectively, with and without the improvement scheme. The rectifier input current THD and power factor are then calculated over the entire operating range of the proposed rectifier inverter structure Fig. 7. Input Current of the conventional rectifier Fig. 9 Input Current waveform of Front end rectifier with Bidirectional Switch Fig. 8 Input Current and voltage waveform of Front rectifier without Bidirectional Switch end Fig.10 rotor current waveform of the induction motor The input power factor is defined as I1 Input power factor = cos I s (11) where I 1 - magnitude of the fundamental component of the rectifier input current Is rms value of the rectifier input current; phase difference between the input current and voltage. THD of input current in percentage 7302

5 Fig 11 sator current wave form of the induction motor value of mh. Hence, precision control is not required for the input inductance in order to obtain a good input power factor. It has been found that the capacitances of the output dc link capacitors have negligible effect on the performance of the rectifier, this is because the capacitors are only used to provide a balance dc central node for the power factor correction scheme to work. This simply means that the value of the capacitance is irrelevant in the selection requirement. In fact, the capacitors should be chosen based on their ripple current specifications. The converter output power is directly proportional to the rms value of its load current. Fig 12 Torque wave form of the induction motor Fig.11 Rectifier input power factor with variations in inverter output power Fig 13 Inverter output phase voltage The relationship between the input power factor and the input line inductance is also obtained as shown in Fig. 10. This shows that the proposed PF improvement scheme is still quite effective even with a large 30% variation in output power. It can also be noticed that both the power factor and THD deteriorate at the low-inverter output power region. IV. CONCLUSION Fig.10 Relationship between input power factor and input inductance Notice that a relatively good input power factor (above 0.8) can still be obtained even with large variations of from 10 to 40 mh, from its rated For mid to high power ac motor drives, traditional converter input power factor can be poor especially at low motor speeds. Traditional passive filters or 12-pulse topology for the front-end rectifier only adds to heavy weight and complexity. Based on some recent proposed rectifier power factor correction technique, a novel converter ac drive is proposed and is verified to be efficient and functional over a wide operating range. In this method a highly efficient and low cost ac drive can be easily built to operate within a wide load range. The proposed bidirectional switch along with its controller is also a good power factor correction in the front-end 7303

6 converters of the existing ac drives. With these features, the proposed converter will be an excellent energy saver and a power factor enhancer required in a clean power environment. REFERENCES 1. Nimal, R.J.G.R., Hussain, J.H., Effect of deep cryogenic treatment on EN24 steel, Mathematics, V-116, I-17, PP , Parameswari, D., Khanaa, V., Deploying lamport clocks and linked lists, International Journal of Pharmacy and Technology, V-8, I-3, PP , Parameswari, D., Khanaa, V., Case for massive multiplayer online role-playing games, International Journal of Pharmacy and Technology, V-8, I-3, PP , Parameswari, D., Khanaa, V., Deconstructing model checking with hueddot, International Journal of Pharmacy and Technology, V-8, I-3, PP , Parameswari, D., Khanaa, V., The effect of self-learning epistemologies on theory, International Journal of Pharmacy and Technology, V-8, I-3, PP , Pavithra, J., Peter, M., Gowtham Aashirwad, K., A study on business process in IT and systems through extranet, International Journal of Pure and Applied Mathematics, V-116, I-19, PP , Pavithra, J., Ramamoorthy, R., Satyapira Das, S., A report on evaluating the effectiveness of working capital management in googolsoft technologies, Chennai, International Journal of Pure and Applied Mathematics, V-116, I-14, PP , Pavithra, J., Thooyamani, K.P., A cram on consumer behaviour on Mahindra two wheelers in Chennai, International Journal of Pure and Applied Mathematics, V-116, I-18, PP-55-57, Pavithra, J., Thooyamani, K.P., Dkhar, K., A study on the air freight customer satisfaction, International Journal of Pure and Applied Mathematics, V-116, I-14, PP , Pavithra, J., Thooyamani, K.P., Dkhar, K., A study on the working capital management of TVS credit services limited, International Journal of Pure and Applied Mathematics, V-116, I-14, PP , Pavithra, J., Thooyamani, K.P., Dkhar, K., A study on the analysis of financial performance with reference to Jeppiaar Cements Pvt Ltd, International Journal of 14, PP , Peter, M., Dayakar, P., Gupta, C., A study on employee motivation at Banalari World Cars Pvt Ltd Shillong, Mathematics, V-116, I-18, PP , Peter, M., Kausalya, R., A study on capital budgeting with reference to signware technologies, International Journal of Pure and Applied Mathematics, V-116, I-18, PP-71-74, Peter, M., Kausalya, R., Akash, R., A study on career development with reference to prem heera surgicals, Mathematics, V-116, I-14, PP ,

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