COMMUTATION TORQUE RIPPLE REDUCTION IN THE LCL FILTER BASED BLDC MOTOR USING MODIFIED SEPIC AND THREE-LEVEL NPC INVERTER S.NAVEENA 1, Dr.S.SIVA PRASAD 2 1 PG Scholar, Vidya Jyoti Institute of Technology, Hyderabad, TS, India. 2 Professor&HOD of EEE Dept., Vidya Jyoti Institute of Technology, Hyderabad, TS, India ABSTRACT In this paper presents LCL filter with power converter topology to effectively reduce the torque ripple due to the phase current commutation of a brushless dc motor (BLDCM) drive system. A combination of a 3-level DCMLI, a modified SEPIC, LCL filter and a dc-bus voltage selector circuit is employed in the proposed torque ripple suppression circuit. For pure reduction of torque pulsation, the dcbus voltage selector circuit and LCL filter is used to apply the regulated dc-bus voltage from the modified SEPIC during the commutation interval. In order to further mitigate the torque ripple pulsation, the 3- level DCMLI is used in the proposed circuit. Finally, simulation results show that the proposed topology is an attractive option to reduce the commutation torque ripple significantly at low and high-speed applications. Index Terms Brushless direct current motor (BLDCM), dc-bus voltage control, modified singleended primary-inductor converter, three-level diode clamped multilevel inverter (3-level DCMLI), torque ripple, LCL filter. I. INTRODUCTION TO BLDC MOTORS tools, and vehicles ranging from model aircraft to automobile. BRUSHLESS VS BRUSHED MOTORS Brushed DC motors were invented in the 19th century and are common. Brushless DC motors were made possible by the development of solid state electronics in the 1960s. An electric motor develops torque by alternating the polarity of rotating magnets attached to the rotor, the turning part of the machine, and stationary magnets on the stator which surrounds the rotor. One or both sets of magnets are electromagnets, made of a coil of wire wound around an iron core. DC running through the wire winding creates the magnetic field, providing the power which runs the motor. However, each time the rotor rotates by 180 (a half-turn), the position of the north and south poles on the rotor are reversed. Brushless DC electric motor (BLDC motors, ) also known as electronically commutated motors (ECMs, EC motors), or synchronous DC motors, are synchronous motors powered by DC electricity via an inverter or switching power supply which produces an AC electric current to drive each phase of the motor via a closed loop controller. The controller provides pulses of current to the motor windings that control the speed and torque of the motor. The construction of a brushless motor system is typically similar to a permanent magnet synchronous motor (PMSM), but can also be a switched reluctance motor, or an induction (asynchronous) motor. The advantages of a brushless motor over brushed motors are high power to weight ratio, high speed, and electronic control. Brushless motors find applications in such places as computer peripherals(disk drives, printers), hand-held power Fig1 illustrates the general principle of the brushed motor. If the magnetic field of the poles remained the same, this would cause a reversal of the torque on the rotor each half-turn, and so the average torque would be zero and the rotor would not turn. Therefore, in a DC motor, in order to create torque in one direction, the direction of electric current through the windings must be reversed with every 180 turn of the rotor (or turned off during the time that it is in the wrong direction). This reverses the direction of the magnetic field as the rotor turns, so the torque on the rotor is always in the same direction. Page No:709
COMMUTATOR In brushed motors, invented in the 19th century, this is done with a rotary switch on the motor's shaft called a commutator. It consists of a rotating cylinder divided into multiple metal contact segments on the rotor. The segments are connected to wire electromagnet windings on the rotor. Two or more stationary contacts called "brushes", made of a soft conductor like graphite press against the commutator, making sliding electrical contact with successive segments as the rotor turns, providing electric current to the windings. Each time the rotor rotates by 180 the commutator reverses the direction of the electric current applied to a given winding, so the magnetic field creates a torque in one direction..commutation TORQUE RIPPLES IN BLDC MOTORS In conventional DC motors with brushes, the field winding is on the stator and armature winding is on the rotor. The motor is expensive and requires maintenance due to the brushes and accumulation of brush debris, dust, commutator surface wear, and arcing. The brushless DC (BLDC) motor could overcome this issue by replacing the mechanical switching components (commutator and brushes) using electronic semiconductor switches. The BLDC motor has a permanent magnet rotor and a wound field stator, which is connected to a power electronic switching circuit BLDC motor drives have high efficiency, low maintenance and long life, low noise, control simplicity, low weight, and compact construction. Due to these features, the BLDC motor has become a very popular and viable product in the market. In fact, the BLDC motor has more advantages compared with other types of AC motors in the market. Fig.2 BLDC motor with hall effect sensors II.WORKING PRINCIPLE AND OPERATION OF BLDC MOTOR BLDC motor works on the principle similar to that of a conventional DC motor, i.e., the Lorentz force law which states that whenever a current carrying conductor placed in a magnetic field it experiences a force. As a consequence of reaction force, the magnet will experience an equal and opposite force. In case BLDC motor, the current carrying conductor is stationary while the permanent magnet moves. Based on the shape of the BEMF (Fig.1), brushless motors can be trapezoidal or sinusoidal. In the BLDC motor, permanent magnets produce an air gap flux density distribution, which is trapezoidal, resulting in trapezoidal BEMF waveforms. Torque pulsations in BLDC motors brought about by the deviation from ideal conditions are either related to the design factors of the motor or to the power inverter supply, thereby resulting in nonideal current waveforms. Undesirable torque pulsation in the BLDC motor drive causes speed oscillations and excitation of resonances in mechanical portions of the drive, leading to acoustic noise and visible vibration patterns in high-precision machines. BLDC motor torque pulsations produce noise and vibration in the system. Therefore, minimization or elimination of noise and vibration is a considerable issue in BLDC drive. Fig 4 Sinusoidal Induced EMF fig 3 bldc motor construction Page No:710
Fig 5 Trapezoidal Induced EMF III.SINGLE-ENDED CONVERTER (SEPIC) PRIMARY-INDUCTOR The single-ended primary-inductor converter (SEPIC) is a type of DC/DC converter that allows the electrical potential (voltage) at its output to be greater than, less than, or equal to that at its input. The output of the SEPIC is controlled by the duty cycle of the control transistor. A SEPIC is essentially a boost converter followed by a buck-boost converter, therefore it is similar to a traditional buck-boost converter, but has advantages of having non-inverted output (the output has the same voltage polarity as the input), using a series capacitor to couple energy from the input to the output (and thus can respond more gracefully to a short-circuit output), and being capable of true shutdown: when the switch is turned off, its output drops to 0 V, following a fairly hefty transient dump of charge. SEPICs are useful in applications in which a battery voltage can be above and below that of the regulator's intended output. For example, a single lithium ion battery typically discharges from 4.2 volts to 3 volts; if other components require 3.3 volts, then the SEPIC would be effective. Figure.6: Schematic of SEPIC Circuit operation As with other switched mode power supplies (specifically DC-to-DC converters), the SEPIC exchanges energy between the capacitors and inductors in order to convert from one voltage to another. The amount of energy exchanged is controlled by switch S1, which is typically a transistor such as a MOSFET. MOSFETs offer much higher input impedance and lower voltage drop than bipolar junction transistors (BJTs), and do not require biasing resistors as MOSFET switching is controlled by differences in voltage rather than a current, as with BJTs. When switch S1 is turned off, the current I C1 becomes the same as the current I L1, since inductors do not allow instantaneous changes in current. The current I L2 will continue in the negative direction, in fact it never reverses direction. It can be seen from the diagram that a negative I L2 will add to the current I L1 to increase the current delivered to the load. Using Kirchhoff's Current Law, it can be shown that I D1 = I C1 - I L2. It can then be concluded, that while S1 is off, power is delivered to the load from both L2 and L1. C1, however is being charged by L1 during this off cycle, and will in turn recharge L2 during the on cycle. Fig7: With S1 closed current increases through L1 (green) and C1 discharges increasing current in L2 (red) Because the potential (voltage) across capacitor C1 may reverse direction every cycle, a non-polarized capacitor should be used. However, a polarized tantalum or electrolytic capacitor may be used in some cases, [2] because the potential (voltage) across capacitor C1 will not change unless the switch is closed long enough for a half cycle of resonance with inductor L2, and by this time the current in inductor L1 could be quite large. Page No:711
+ For a stepped waveform such as the one depicted in Fig 17 with s steps, the Fourier Transform for this waveform follows Figure 8: With S1 open current through L1 (green) and current through L2 (red) produce current through the load INVERTER An inverter is an electrical device that converts direct current (DC) to alternating current (AC) the converted AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits. Static inverters have no moving parts and are used in a wide range of applications, from small switching power supplies in computers, to large electric utility high voltage direct current applications that transport bulk power. Inverters are commonly used to supply AC power from DC sources such as solar panels or batteries. The electrical inverter is a high power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. (4.2) Fig.9 Single-phase structure of a multilevel cascaded H-bridges inverter Cascaded H-Bridges inverter A single phase structure of an m-level cascaded inverter is illustrated in Figure 16. Each separate DC source (SDCS) is connected to a single phase full bridge, or H-bridge, inverter. Each inverter level can generate three different voltage outputs, +V dc, 0, and V dc by connecting the DC source to the ac output by different combinations of the four switches, S 1, S 2, S 3, and S 4. To obtain +V dc, switches S 1 and S 4 are turned on, whereas V dc can be obtained by turning on switches S 2 and S 3. By turning on S 1 and S 2 or S 3 and S 4, 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. The number of output phase voltage levels m in a cascade inverter is defined by m = 2s+1, where s is the number of separate DC sources. An example phase voltage waveform for an 11 level cascaded H-bridge inverter with 5 SDCSs and 5 full bridges is shown in Fig 4.2. The phase voltage Fig.10 Output phase voltage waveform of an 11 level cascade inverter with 5 separate dc sources. ANALYSIS OF TORQUE RIPPLE IN A BLDCM DRIVE SYSTEM The mathematical model of BLDCM is described as follows: The electromagnetic torque produced by the BLDCM is expressed as Page No:712
EMF (Em ), rate of change of phase current during commutation period is expressed by where R: phase resistance, v1, v2, v3 : phase voltages of threephase stator windings, L: armature inductance, i1, i2, i3 : phase currents of three-phase stator windings, un : neutral point where Vdc is the dc bus voltage. The torque equation before the commutation is expressed as follows: The outgoing phase current(i1 ) becomes zero from its steadystate value (Im ) during the time interval tf, which can be expressed as Fig11. Commutation current transition from phase v1 to v2. (a) Before commutation. (b) At commutation. (c) After commutation. to ground voltage, ωm : angular velocity of BLDCM, and e1, e2, e3 : phase back EMFs. In order to minimize the commutation torque ripple of BLDCM, the influence of phase current slew rates of rising phase and decaying phase during the commutation period is analyzed. A sixstep voltage source inverter is employed for the control of BLDCM. For the torque ripple analysis, the current transition from phase v1 to v2 during commutation period is considered. At the beginning of commutation period, MOSFET T1 is turned OFF to de-energize the phase v1 and MOSFET T2 is turned ON to energize the phase v2, with phase v3 remaining in the conduction state. In 120 degree conduction method, two power MOSFETs conduct at each 60 electrical degrees, one MOSFET from the upper arm and other MOSFET from the lower arm. Before the commutation period, the MOSFETs T1 and T2 are turned ON and current through the circuit builds up. At the start of commutation period, T1 is switched OFF, and then freewheeling diode D4 starts to conduct due to stored energy in the inductor. After the commutation process, the MOSFETs T2 and T3 continue to conduct. The difference in current slew rates between the incoming phase and outgoing phase generate torque ripple. Assuming negligible resistance and constant back Fig12. Proposedconverter topology with a dc-bus voltage selector circuit for BLDCM. IV. NOVEL TOPOLOGY FOR THE BLDC MOTOR DRIVE SYSTEM A system diagram of a proposed new converter topology for the BLDCM drive system based on a 3-level DCMLI and a modified SEPIC is shown in Fig. 5. In this topology, the 3- level DCMLI is proposed to reduce current ripple, and modified SEPIC is included to adjust the dc-bus voltage based on the rotational speed of the BLDCM. The dc-bus voltage selector circuit is constructed with power MOSFETs (S1, S2, S3, and S4). It is used to select the desired dc-bus voltage for signifi- cant torque ripple reduction during commutation interval. The MOSFET-based 3-level DCMLI is operated at a switching of 80 khz, which provides Page No:713
significant torque ripple suppression than the conventional 2-level inverter. In this 3-level DCMLI, the dc-bus voltage is divided into 3 levels by the capacitors C5 and C6. To obtain the desired commutation voltage, the duty cycle of the modified SEPIC can be adjusted during the non commutation period to maintain Vdc = 8Em. At the start of commutation period, the regulated voltage from the modified SEPIC is instantly applied by voltage selector circuit for significant torque ripple suppression The commutation path of three-level DCMLI leg A is depicted in Fig. 6. The following modes of operation of the 3-level DCMLI are discussed based on the polarity of the voltage at the inverter output terminals and direction of the load current. Operating mode 1: The inverter output voltage as well as load current (i1 ) both are positive. The power MOSFETs QA1, QA2 and clamping diode DM1 are active in this operating mode. The commutation current alternates between the MOSFET QA1 and clamping diode DM1 during the commutation process. The current (i1 ) flows from the positive terminal of the power supply through the MOSFETs QA1 and QA2 as long as MOSFET QA1 is switched ON. If MOSFET QA1 is turned OFF, load current transfers from MOSFET QA1 to clamping diode DM1. The current now flows from the neutral point (N) to inverter output terminal through the clamping diode DM1 and MOSFET QA2. The MOSFET QA2 remains conducting at all times. Operating mode 2: In this operating mode, the inverter load current (i1 ) remains positive but the inverter output voltage is negative. The commutation of current goes back and forth between clamping diode DM1/MOSFET QA2 and the diodes DA3/DA4 Operating mode 3: The inverter output voltage as well as load current (i1 ) both are negative. In this operating mode, the commutation current goes back and forth between clamping diode DN1 and MOSFET QA4. When MOSFET QA4 is switched ON, the load current (i1 ) passes through MOSFETs QA3 and QA4 from the inverter output terminal. If MOSFET QA4 is turned OFF, load current transfers from MOSFET QA4 to clamping diode DN1. As a result, the load current now passes through MOSFET QA3 and clamping diode DN1 from the inverter output terminal A to the neutral point (N). The MOSFET QA3 remains conducting at all times. Operating mode 4: In this operating mode, the inverter load current becomes negative and the output voltage is still positive. The commutation of current goes back and forth between clamping diode DN1/MOSFET QA3 and the diodes DA1/DA. The mathematical expression for output voltage of the modified SEPIC is given as where D is the duty-ratio of the modified SEPIC. The back EMF (Em ) is proportional to motor speed, i.e., where Ke is the back EMF coefficient. The following expression can be used to estimate the duty cycle of MOSFET M based on the measured motor speed: Fig.13 Block diagram of the PWM controller for 3- level DCMLI. In practice, the commutation period of the BLDCM is much shorter compared to the time taken by the modified SEPIC for dc-link voltage adjustment close to 8Em. Hence, MOSFET-based voltage selector circuit has been used, which instantly applies the regulated dc-bus voltage from the modified SEPIC for torque ripple suppression during commutation period. Equation (5) is used to estimate the real commutation period t1. To compensate load or speed changes, the commutation period T is kept always more than t1 and the corresponding relationship commutation period. Fig. 8 shows the flow chart of proposed voltage control method for the proposed topology. Page No:714
TABLE I PARAMETERS OF BLDC MOTOR Rated Voltage (V) 200 Rated Power (W) 518 Rated Speed (r/min) 6000 Rated Torque (Nm) 0.825 Pole Pairs 4 Phase Resistance (Ω) 3.10 Phase Inductance (mh) 3.09 Back EMF Coefficient [V/(rad/s)] 0.227 V. SIMULATION RESULTS The MATLAB/ Simulink model of the BLDCM drive fed with a conventional 2-level inverter, 3-level DCMLI, 2-level inverter with SEPIC and a switch selection circuit, and the proposed converter are built and simulations are carried out under different switching frequency to investigate the torque ripple pulsation. The simulations are done in the MATLAB/Simulink R2012a software environment. The rated parameters of the BLDCM are listed in Table I. The SEPIC is used to regulate the dc-bus voltage based on the rotational speed of the BLDCM. A dc-link voltage selection and control strategy has been proposed for the commutation torque ripple suppression in BLDCM using MOSFET-based switch selection circuit. FIG14: Current and torque waveforms of 3 BLDCM fed by a 2-level inverter with SEPIC and a switch selection circuit at 1000 r/min and 0.825Nm with 5 khz switching frequency. BLDCM fed by proposed topology The control scheme of the 3-level DCMLI, consists of an outer speed control loop and an inner current control loop. A speed controller takes inputs from the measured speed (ωm) and reference speed (ωm+). The error (ωe ) in reference speed and measured speed is amplified by the proportionalintegral (PI) controller. The reference current signal generated by the speed controller is compared with 0.the measured current signals and the errors are fed through the PI current controller. The resultant control voltage signals generated by the current controller are compared with positive and negative triangular waveforms to generate PWM signals. SIMULATION RESULTS: BLDCM fed by a 2-level inverter with SEPIC and a switch selection circuit. FIG15: Current and torque waveforms of 3 BLDCM fed by proposed topology at 1000 r/min and 0.825Nm with 5 khz switching frequency Page No:715
VI. CONCLUSION In this project, a commutation torque ripple reduction circuit has been proposed using 3-level DCMLI with modified SEPIC and a dc-bus voltage selector circuit. The suggested dc-bus voltage control strategy is more effective in torque ripple reduction in the commutation interval. The proposed topology accomplishes the successful reduction of torque ripple in the commutation period and results are presented to compare the performance of the proposed control technique with the conventional 2- level inverter, 3-level DCMLI, 2-level inverter with SEPIC and the switch selection circuit-fed BLDCM. In order to obtain significant torque ripple suppression, quietness, and higher efficiency, 3-level DCMLI with modified SEPIC and the voltage selector circuit is the most suitable choice to obtain high-performance operation of BLDCM. The proposed topology may be used for the torque ripple suppression of BLDCM with the very low stator winding inductance. REFERENCES [1] N. Milivojevic, M. Krishnamurthy, Y. Gurkaynak, A. Sathyan, Y.-J. Lee, and A. Emadi, Stability analysis of FPGA-based control of brushless dc motors and generators using digital PWM technique, IEEE Trans. Ind. Electron., vol. 59, no. 1, pp. 343 351, Jan. 2012. [2] X. Huang, A. Goodman, C. Gerada, Y. Fang, and Q. Lu, A single sided matrix converter drive for a brushless dc motor in aerospace applications, IEEE Trans. Ind. Electron., vol. 59, no. 9, pp. 3542 3552, Sep. 2012. [3] X. Huang, A. Goodman, C. Gerada, Y. Fang, and Q. Lu, Design of a five-phase brushless dc motor for a safety critical aerospace application, IEEE Trans. Ind. Electron., vol. 59, no. 9, pp. 3532 3541, Sep. 2012. [4] J.-G. Lee, C.-S. Park, J.-J. Lee, G. H. Lee, H.-I. Cho, and J.-P. Hong, Characteristic analysis of brushless motor considering drive type, in Proc. Korean Inst. Electr. Eng., Jul. 2002, pp. 589 591. [5] T. H. Kim and M. Ehsani, Sensorless control of BLDC motors from near-zero to high speeds, IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1635 1645, Nov. 2004. [6] T. J. E. Miller, Switched Reluctance Motor and Their Control. London, U.K.: Clarendon, 1993. [7] K. Ilhwan, N. Nobuaki, K. Sungsoo, P. Chanwon, and Chansu Yu, Compensation of torque ripple in high performance BLDC motor drives, Control Eng. Pract., vol. 18, pp. 1166 1172, Oct. 2010. [8] S. S. Bharatkar, R. Yanamshetti, D. Chatterjee, and A. K. Ganguli, Reduction of commutation torque ripple in a brushless dc motor drive, in Proc. 2008 IEEE 2nd Int. Power Energy Conf., 2008, pp. 289 294. [9] K.-Y. Nam, W.-T. Lee, C.-M. Lee, and J.-P. Hong, Reducing torque ripple of brushless dc motor by varying input voltage, IEEE Trans. Magn., vol. 42, no. 4, pp. 1307 1310, Apr. 2006. [10] J. H. Song and I. Choy, Commutation torque reduction in brushless dc motor drives using a single dc current sensor, IEEE Trans. Power Electron., vol. 19, no. 2, pp. 312 319, Mar. 2004. [11] X. F. Zhang and Z. Y. Lu, A new BLDC motor drives method based on BUCK converter for torque ripple reduction, Proc. IEEE Power Electron. Motion Control Conf., 2006, pp. 1 4. [12] W. Chen, C. Xia, and M. Xue, A torque ripple suppression circuit for brushless dc motors based on power dc/dc converters, in Proc. IEEE Ind. Electron. Appl. Conf., 2008, pp. 1453 1457. Author's Profile: S.NAVEENA, P pursuing M.Tech with specialization of Power Electronics & Electrical Drives from Vidya Jyothi Institute of Technology Autonomous, Hyderabad. She received B.Tech degree in Electrical and Electronics Engineering from Vardhaman College of Engineering Shamshabad in 2015.Her area of interest includes Power Electronics, E-mail id:navi.sudamalla@gmail.com Dr.S.SIVA PRASAD, Professor & HOD of EEE Dept. has awarded Ph.D. Electrical Engineering in 2012 (February) from J.N.T.U HYDERABAD and had his M.Tech with specialization of Power Electronics in 2003.He has obtained his B.Tech Degree in Electrical and Electronics Engineering from S V University. He is having 20 years of Experience and currently working as Professor & HOD of EEE Dept. of Vidya Jyoti Institute of Technology, Aziz Nagar, Hyderabad, India. He received Bharat Vibhushan Samman Puraskarǁ from The Economic and Human Resource Development Associationǁ in 2013 and received Young Investigator Award in 2012. He has published about 60 technical papers in International and National Journals and Conferences and filed one patent. He is Life member of ISTE and member of IEEE. His Research areas include Power Electronics & Drives, PSD&FACTS Controllers. E-mail id:eeehod@vjit.ac.in. Page No:716