Int. J. Engg. Res. & Sci. & Tech. 2015 Rozer Franklin J and S Thirunavukkarasu, 2015 Research Paper ISSN 2319-5991 www.ijerst.com Special Issue, Vol. 1, No. 3, May 2015 International Conference on Advance Research and Innovation in Engineering, Science, Technology and Management ICARSM 15 2015 IJERST. All Rights Reserved DIODE CLAMPED MULTILEVEL INVERTER BASED POWER FACTOR CORRECTION WITH FED BLDC MOTOR DRIVE Rozer Franklin J 1 * and S Thirunavukkarasu 1 *Corresponding Author: Rozer Franklin J The active control of reactive power is indispensable to stabilize the power systems and to maintain the supply voltage. A Static synchronous compensator (STATCOM) using the voltage source inverters (VSIs) have been widely accepted as the next generation of the reactive power controllers of power system. Recently, power quality and custom power have wide- spread use of nonlinear electronic equipment and the power quality requirements of sensitive loads. A Power Factor Correction (PFC)-based Canonical Switching Cell (CSC) converter-fed Brushless DC Motor (BLDCM) drive for low-power house hold applications. The BLDCM is electronically commutated for reduced switching losses in VSI due to low-frequency switching. A prototype of the proposed configuration is developed, and its performance is validated with test results for the control of speed over a wide range with a unity power factor at universal ac mains. The simulation result shows the proposed system withstand at high voltage using MATLAB/simulink. Keywords: STATCOM, Power factor correction (PFC), BLDCM, Diode clamped multilevel inverter INTRODUCTION Traditionally, a multi pulse inverter consisting of several voltage source inverters connected together through zigzag arrangement transformers is used for var compensation. This transformer are most expensive equipment in the system, produce about 50% of the total losses of the system and causes difficulties in control due to dc magnetizing and surge over voltage problems resulting from saturation of the transformers. The new cascaded inverter eliminates the bulk of transformers required by static var compensators that employ the multiples inverter and that can respond much faster. This inverter gene- rates almost sinusoidal staircase voltage with only one time switching per line cycle. Brushless DC electric motor (BLDC motors, BL motors) also known as electronically commutated motors (ECMs, EC motors) are synchronous motors that are powered by a DC electric source via an integrated inverter/switching power supply, which produces an AC electric signal to drive the motor[1]. 1 PG Scholar, Professor, Department of Electrical and Electronics Engineering, Annai Mathammal Sheela Engineering College, Namakkal, India. 56
Due to the oscillating motion of waves, the ocean environment can require bidirectional and variable speed operation of the generator. In this research, the efficiency of a set of small brushed dc, induction, brushless dc, and synchronous reluctance drives and machines were compared in constant and oscillating operation. For application considerations, these two motor drives have to be differentiated on the basis of known engineering criteria. Some of the criteria used to assess these two machines include power density, torque per unit current, speed range, feedback devices, inverter rating, cogging torque, ripple torque, and parameter sensitivity [2], [3]. In proposed system, a multi phase inverter consisting of several voltage source inverters connected together zigzag arrangement transformers is used for var compensation. The stator of the BLDCM consists of three-phase concentrated windings and rotor has permanent magnets [4], [5]. It is also known as an electronically commutated motor (ECM) since an electronic commutation based on rotor position via a three-phase voltage source inverter (VSI) is used [8], [9]. Therefore, the problems associated with brushes, such as sparking, and wear and tear of the commutator assembly are eliminated. Permanent-magnet brushless dc and ac machines and drives are compared in terms of their constant torque and constant power capabilities, and various PM machine topologies and their performance are reviewed. Finally, methods for enhancing the PM excitation torque and reluctance torque components and, thereby, improving the torque and power capability [7]. To overcome such problems a single phase single switch power factor correction AC-DC converter topology based on a Cuk converter is proposed to feed voltage source inverters based PMBLDCM. ROLE OF DIODE CLAMPED INVERTER The schematic of inverter system is as shown in Fig. 1, in which the battery or rectifier provides the dc supply to the inverter. The inverter is used to control the fundamental voltage magnitude and the frequency of the ac output voltage. AC loads may require constant or adjustable voltage at their input terminals, when such loads are fed by inverters, it is essential that the output voltage of the inverters is so controlled as to fulfill the requirement of the loads. For example if the inverter supplies power to a magnetic circuit, such as a induction motor, the voltage to frequency ratio at the inverter output terminals must be kept constant. This avoids saturation in the magnetic circuit of the device fed by the inverter. Figure 1: Schematic for Inverter System As in the single phase voltage source inverters PWM technique can be used in three-phase inverters, in which three sine waves phase shifted by 120 with the frequency of the desired output voltage is compared with a very high frequency carrier triangle, the two signals are mixed in a comparator whose output is high when the sine wave is greater than the triangle and the comparator output is low when the sine wave or 57
typically called the modulation signal is smaller than the triangle. This phenomenon is explained the output voltage from the inverter is not smooth but is a discrete waveform and so it is more likely than the output wave consists of harmonics, which are not usually desirable since they deteriorate the performance of the load, to which these voltages are applied. It is evident that the multilevel concept will be a prominent choice for power electronic systems in future years, especially for medium-voltage operation. Multi-level inverters are the modification of basic bridge inverters. They are normally connected in series to form stacks of level. The number of levels in an inverter bridge defines the number of direct current (DC) voltage steps that are required by the inverter bridge in order to achieve a certain voltage level at its output. CONTROL IMPLEMENTATION Because the controller must direct the rotor rotation, the controller requires some means of determining the rotor s orientation/position (relative to the stator coils.) Some designs use Hall Effect sensors or a rotary encoder to directly measure the rotor s position. Others measure the back EMF in the undriven coils to infer the rotor position, eliminating the need for separate Hall Effect sensors, and therefore are often called sensorless controllers. A typical controller contains 3 bi-directional outputs, which are controlled by a logic circuit. Simple controllers employ comparators to determine when the output phase should be advanced, while more advanced controllers employ a microcontroller to manage acceleration, control speed and fine-tune efficiency. Controllers that sense rotor position based on back-emf have extra challenges in initiating motion because no back-emf is produced when the rotor is stationary. This is usually accomplished by beginning rotation from an arbitrary phase, and then skipping to the correct phase if it is found to be wrong. This can cause the motor to run briefly backwards, adding even more complexity to the startup sequence. Other sensorless controllers are capable of measuring winding saturation caused by the position of the magnets to infer the rotor position. POWER FACTOR CORRECTION Power factor is simply a name given to the ratio of actual power (active power) being used in a circuit, expressed in watts or more commonly kilowatts (kw), to the power which is apparently being drawn from the mains, expressed in voltampere or more commonly kilo volt-ampere (kva). P.F. = Active Power (kw) Apparent Power (kva): All modern industries utilize electrical energy in some form or other. Two basic categories of load are encountered in alternate current (AC) networks. 1. Resistive Loads: Devices containing only resistance e.g. incandescent lamps, heaters, soldering irons, ovens, etc. The current drawn from the supply is directly converted into heat or light. Since the voltage is assumed to be constant, the actual power (kw) being used is identical to the apparent power (kva) being drawn from the line. The power factor is therefore unity or 1. In these purely resistive circuits, the current and voltage sine wave peaks occur simultaneously and are said to be in phase. 2. Inductive Loads: All motors and transformers depend on magnetism as the basis of their 58
operation. Magnetism is a force and in the physical sense is not consumed. In AC motors and transformers, magnetic forces are only required periodically. Consequently, a permanent magnet cannot be used and the necessary magnetism is produced by electrical means. The electrical current needed for this purpose is not fully utilized. The most practical and economical power factor improvement device is the capacitor. As stated previously, all inductive loads produce inductive reactive power. Capacitors on the other hand produce capacitive reactive power, which is the exact opposite of inductive reactive power. In this instance, the current peak occurs before the voltage peak, leading by a phase angle of 90. By careful selection of capacitance required, it is possible totally cancel out the inductive reactive power when placed in circuit together. To prevent the continual flow of reactive current back and forth between the load and power station, a capacitor, which is in effect a reactive current storage device, is connected in parallel with the load. The reactive current supplied by the power station and used for the magnetic force when the load is switched on does not now return to the power station but instead flows into the capacitor and merely circulates between the latter and the load. Consequently the distribution lines from the power station are relieved of the reactive current. Individual Motor Compensation: Most effective correction is obtained by connecting individual capacitors directly to the terminals of each motor. The motor and capacitor can be controlled jointly by the motor switchgear. The capacitor rating should be matched as closely as possible so that the power factor of the entire plant can be corrected to the optimum value, irrespective of the number of motors switched on. The size of capacitor required may be determined from Table 3 by taking the motor kw and speed into consideration. Table 3 is a guide only and no guarantee of correct power factor. The correct method of maximum capacitor rating can be determined by using the following formula: QC = 0.9Io V 3 Where Io = motor magnetizing current Qc = capacitor power in VAr If the magnetizing current is not known, 95% of the motor no-load current can be used as an approximate value. Care should be taken not to exceed the value calculated to avoid dangerous over voltages and possible self excitation of motors at switch-off. Over compensation can cause higher supply voltages which can cause consequent break down of motor insulation and flashover at motor terminals. To be safe, rather use standard capacitor sizes (as indicated below). For this reason, individual motor compensation is not recommended for motors which are rapidly reversed e.g. cranes, hoists, etc. Centralized Compensation (Automatic Power Factor Correction): In large industrial plants where many motors are generally in use or, when the main reason for power factor is to obtain lower electricity bills, then centralized compensation is far more practical and economical than individual motor compensation. In this instance, large banks or racks of capacitors are installed at the main incoming distribution boards of the plant and are sub-divided into steps which are automatically switched in or out depending on specific load requirements by means of an automatic control system, improving the overall power factor of the network. 59
Substantiating Power Factor Correction Costs: Assuming a plant has a total load of 500 kw and a power factor (cos ) of say 0.75 lagging. Supply authorities kva demand charge is approximately R40.00 per kva (actually above R50.00 in most areas. Johannesburg is currently R53.10). kw/ PF= kva/500 kw /0.75 PF= 666 kva Total costs @ R40.00/kVA = R26,640.00/month By installing capacitors to improve power factor (cos ) to 0.98 lagging new costs are; 500 kw/0.98 PF= 510 kva Total costs @ R40.00/kVA = R20,400.00/month Therefore savings monthly = R6,240.00 A complete system required to effect power factor from 0.75 to 0.98 (as in above example) would require a system of 360 kvar which would currently cost approximately R43,000.00. Power factor correction usually pays for itself well within 12 months of the initial purchase (7 months in above example) and continues saving indefinitely. It therefore stands to reason that more significant savings can be anticipated with the ever increasing escalation costs of electricity in the future. correct capacitor size can be calculated by multiplying the factor when crossing the horizontal and vertical columns in the table below by kw. PROPOSED MODEL OF BLDC MOTOR BLDC motor is widely used in many type of applications like industry, home appliances, electronics equipments etc. it has many advantages than other motors like dc motor and induction motor. It does not require any type of brushes for commutation. It has better speed torque characteristics than other motors. It offers long operating life, noiseless operation, high efficiency & high dynamic response. The stator consists of stacked steel laminations. It has stator windings connecting in star phase. The rotor is made of permanent magnet & can vary from two to eight pole pairs with north & south poles. Brushless dc motor defined as permanent magnet synchronous motor. The motor require sensor for rotor position. Figure 2: Proposed model of BLDC Motor Calculating Capacitor Requirements: It is imperative that correct capacitor sizes be selected when calculating capacitor requirements. In the case of centralized compensation, it is recommended that the first capacitor step be equal to half the value of the following steps, to allow a smooth overall linear correction system. Table 4 (right) will assist in calculating capacitor values in specific applications. Prior knowledge of the following is required, Power factor before applying capacitors (left vertical column), required power factor (top horizontal row) and Total consumption in kw. The 60
By connecting the DC source to the AC output by different combinations of the four switches, S1, S2, S3 and S4, three different voltage output levels can be generated for each DC source. To obtain +Vdc, switches S1 and S4 are turned on, whereas Vdc can be obtained by turning on switches S2 and S3. By turning on S1 and S2 or S3 and S4, the output voltage is 0. The ac outputs of each of the different full-bridge inverter levels are connected in series such that the synthesized voltage waveform is the sum of the inverter outputs. A cascaded inverter with N input sources will provide (2N+1) levels to synthesize the AC output waveform. If one assumes that the rating of the diodes is much more the same as the rating of the main switches in the converter, a problem will arises with the diode clamped converter and the number of diodes required in the circuit will increases with the number of levels. SIMULATION AND RESULTS The proposed system is implemented in simulation software platform. The work is done in MATLAB Version 7.12.0(R2011a). The Image processing toolbox is made use of for the work. The work is executed in Graphic User Interface (GUI). The high dynamic range image is produced using the software Adobe Photoshop CS. MATLAB is a high-performance language for technical computing. It integrates computation, visualization, and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation. Let us consider that the switch chain from the positive top rail, to the output connection. Having half of the total number switches exist either in the top or bottom half of the switch chain as this will corresponds to m-1 switches. The simulation result of three phase cascaded output is shown in Figure 3. Figure 3: Three Phase Cascaded Output MATLAB provides a large number of standard elementary mathematical functions, including abs, sqrt, exp, and sin. Taking the square root or logarithm of a negative number is not an error; the appropriate complex result is produced automatically. MATLAB also provides many more advanced mathematical functions, including Bessel and gamma functions.. It includes highlevel commands for two-dimensional and threedimensional data visualization, image processing, animation, and presentation graphics. It also includes low-level commands that allow you to fully customize the appearance of graphics as well as to build complete graphical user interfaces on your MATLAB applications. The Fig.4. Shows the simulation waveform of BLDC motor speed and torque. Figure 4: BLDC Motor Speed and Torque 61
CONCLUSION A PFC-based CSC converter-fed BLDCM drive has been proposed for targeting low-power household applications. A variable voltage of dc bus has been used for controlling the speed of BLDCM which eventually has given the freedom to operate VSI in low-frequency switching mode for reduced switching losses. A front-end CSC converter operating in DICM has been used for dual objectives of dc-link voltage control and achieving a unity power factor at ac mains. The performance of the proposed drive has been found quite well for its operation at variation of speed over a wide range. A prototype of the CSCbased BLDCM drive has been implemented with satisfactory test results for its operation over complete speed range and its operation at universal ac mains. REFERENCES 1. Chan T F, Yan L T, and Fang S Y (2002), Inwheel permanent-magnet brushless DC motor drive for an electric bicycle, IEEE Trans. Energy Convers., Vol. 17, No. 2, pp. 229 233. 2. Brekken T K A, Hapke H M, Stillinger C, and Prudell J (2010), Machines and drives comparison for low-power renewable energy and oscillating applications, IEEE Trans. Energy Convers., Vol. 25, No. 4, pp. 1162 170. 3. Zhu Z Q and Howe D (2007), Electrical machines and drives for electric, hybrid, and fuel cell vehicles, IEEE Proc., Vol. 95, No. 4, pp. 746 765. 4. Pillay P and Krishnan R (1991), Application characteristics ofpermanent magnet synchronous and brushless DC motors for servo drives, IEEE Trans. Ind. Appl., Vol. 27, No. 5, pp. 986 996. 5. Xia C L (2012), Permanent Magnet Brushless DC Motor Drives and Controls, Hoboken, NJ, USA: Wiley. 6. Kenjo T and Nagamori S (1985), Permanent Magnet Brushless DC Motors, Oxford, U.K.: Clarendon. 7. Chan T F, Yan L T, and Fang S Y (2002), Inwheel permanent-magnet brushless DC motor drive for an electric bicycle, IEEE Trans. Energy Convers., Vol. 17, No. 2, pp. 229 233. 8. Brekken T K A, Hapke H M, Stillinger C and Prudell J (2010), Machines and drives comparison for low-power renewable energy and oscillating applica- tions, IEEETrans. Energy Convers., Vol. 25, No. 4, pp. 1162 1170. 9. Zhu Z Q and Howe D (2007), Electrical machines and drives for electric, hybrid, and fuel cell vehicles, IEEE Proc., Vol. 95, No. 4, pp. 746 765. 10. Zeraoulia M, Benbouzid M E H, and Diallo D (2006), Electric motor drive selection issues for HEV propulsion systems: A comparative study, IEEE Trans. Veh. Technol., Vol. 55, No. 6, pp. 1756 1764. 62