Shravani Mathur, Prof. Vikas Kumar / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.493-496493 | P a g eCalculation of total Current harmonic distortion by PWM methodfor permanent magnet synchronous motors*Shravani Mathur, **Prof. Vikas Kumar*M.Tech scholar, OIST, bhopal**Asst.Prof.,OIST,BhopalAbstractAdjustable speed drive are becoming asignificant load component for power distribution.It involves the use of permanent magnetsynchronous motors This paper involves analysis,control and modeling of permanent magnetsynchronous motors and also investigates itseffects on Total current harmonic distortion onan adjustable speed by the use of PWMgenerator and Vector control method .By usingPark’s transformation ,we can convert threephase system to two phase and then convert it tostationery two axes system (d, q) By varying twovectors of flux and torque,(one on quadrature andother on direct axis) we can control the rotorcurrent and therefore, speed and THD forcurrent and voltage can be calculated. Harmonicsare lesser in amplitude and more in frequency ascompared to the fundamental.Index Terms—Adjustable speed drives. Fieldoriented control, PWM generator,MATLAB/Simulink , PMSM ,three phase statorcurrentsI. ADJUSTABLE SPEED DRIVESAdjustable Speed drives are required inmany applications. The use of ASDs leads to currentharmonics pollution in the power grid and toelectromagnetic interference (EMI) with theenvironment. Power quality and EMI are theconstraints on electric induction motor drives. Theseloads are distributed all over an Electric networkHarmonics are undesirable as this interference withpower apparatus can result in unacceptable levels ofvoltage distortion. Harmonic measurements requirespecial equipment, which is quite expensive and notalways available. This paper investigates themodeling and control of a synchronous motor for anadjustable speed drive and harmonic analysis wascarried out. Using ac drives and ac motors have theadvantages of lower maintenance, higher speeds andsmaller size. Compared to dc drives, the higher costof ac drives is in part compensated by a lower acmachine cost because of presence of brushes andcommutator in the dc machines. Compared touncontrolled ac motors, supplied by a power grid, theefficiency of inverter controlled drives can be vastlyincreased by, e.g. flux optimizationII. Permanent Magnet Synchronous MotorsAn electric drive may be operated in onedirection of rotation or both directions of rotationsdepending upon requirements. Per-phase equivalentcircuits widely used in steady-state analysis anddesign of ac machines, are not appropriate to predictthe dynamic performance of the motor. A dynamicmodel is necessary to understand and analyze vectorcontrol of ac motor drives. A significantbreakthrough was achieved in the analysis ofthree-phase ac machines through the development ofthe reference frame theory. The machine model canbe transformed to another reference frame to usingthese techniques. It is possible to simplify thecomplexity of the mathematical machine model byjudicious choice of the reference frame. Initiallythese techniques were developed for the analysis andsimulation of ac machines, but are now beingextensively used in the digital control of suchmachines. The need for compact and accuratemachine models is obvious, as digital controltechniques are being extended to control current,torque and flux of the ac machines.Permanent magnet (PM) synchronousmotors are widely used in low and mid powerapplications such as computer peripheralequipments, robotics, adjustable speed drives andelectric vehicles.The growth in the market of PM motordrives has demanded the need of simulation toolscapable of handling motor drive simulations.Simulations have helped the process of developingnew systems including motor drives, by reducingcost and time. Simulation tools have the capabilitiesof performing dynamic simulations of motor drivesin a visual environment so as to facilitate thedevelopment of new systems.In this work, the simulation of a fieldoriented controlled PM motor drive system isdeveloped using Simulink. The simulation circuitwill include all realistic components of the drive
Shravani Mathur, Prof. Vikas Kumar / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.493-496494 | P a g esystem. This enables the calculation of currents andvoltages in different parts of the inverter and motorunder transient and steady conditions. The losses indifferent parts can be calculated facilitating thedesign of the inverter. A closed loop control systemwith a PI controller in the speed loop has beendesigned to operate in constant torque and fluxweakening regions. Implementation has been donein Simulink.A comparative study of hysteresis andPWM control schemes associated with currentcontrollers has been made in terms of harmonicspectrum and total harmonic distortion. Simulationresults are given for two speeds of operation, onebelow rated and another above rated speed A motorof 3Hp,220V, 60Hz,1800 rpm or 188.5 rad/sec isused.III. Detailed Modeling of PMSMDetailed modeling of PM motor drivesystem is required for proper simulation of thesystem. The d-q model has been developed on rotorreference frame At any time t, the rotating rotord-axis makes an angle θr with the fixed stator phaseaxis and rotating stator mmf makes an angle α withthe rotor d-axis. Stator mmf rotates at the same speedas that of rotorFig. depicts self control permanent magnet motorsField Oriented Control of PM MotorsThe PMSM control is equivalent to that ofthe dc motor by a decoupling control known as fieldoriented control or vector control. The vector controlseparates the torque component of current and fluxchannels in the motor through its stator excitation.The vector control of the PM synchronous motor isderived from its dynamic model.Considering the currents as inputs, the three currentsare:ia = Im sin(ωrt +α )ib =Im sin (wt+α -2pi/3)ic=Im sin(wt+α-4pi/3)Constant torque operationConstant torque control strategy is derivedfrom field oriented control, where the maximumpossible torque is desired at all times like the dcmotor. This is performed by making the torqueproducing current iq equal to the supply current Im.That results in selecting the α angle to be 90 º degreesFig.1 Matlab simulink of PWM generator fedpermanent magnet synchronousmotorDiscrete,Ts = 5e-005 s.powerguiv+-Voltage Measurement1v+-Voltage MeasurementgABC+-ABC+-ABCScope4Scope2Scope1ScopeTmmABCPermanent MagnetSynchronous Machine mis_abcwmTe3.34e-4*u^2FcnPulsesDiscretePWM GeneratorDC Voltage Sourcei+ -0.75 0.8 0.85 0.9 0.95 1-10-505100.75 0.8 0.85 0.9 0.95 180901001101200.75 0.8 0.85 0.9 0.95 1-50510TimeFig.1.a waveforms of harmonic component of statorcurrent,speed,mechanical torqueFig.2 Matlab simulink by Field oriented controlIV. Mathematical EquationsThese are the instantaneous values of three phasestator currents displaced by 120 degree eachΨrd=Lm isd/1+sTrWhere Lm =Mutual inductanceIsd =direct axis stator currents =Laplace TransformTr =Rotor Time costant, Lr/RrTm =3/2Lm/Lrzp Ψrd isqWhere Lr= Rotor resistanceZp =number of pole pairsI sq= quadrature axis stator currentTm = motor torqueV. DRIVE SYSTEM SIMULATION INSIMULINKThis chapter describes different toolsavailable for electrical and electronic systemssimulation and then justification is given forselecting Simulink for the PMSM system.
Shravani Mathur, Prof. Vikas Kumar / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.493-496495 | P a g eBlock by block an explanation is given for Simulinksimulation of the drive system.Distortion IndexThe most commonly used index formeasuring the harmonic content of a waveform is thetotal harmonic distortion (THD). It is a measure ofthe effective value of a waveform and may be appliedto either voltage or current.Total harmonic distortion is thecontribution of all the harmonic frequency currentsto the fundamental. Just as waveforms can be addedto produce distorted waves, distorted waves may bedecomposed into fundamental and harmoniccomponents.Total Current Harmonic Distortion (THDi)The THDi is a measure of the effective valueof the harmonic components of a distorted waveform.This index can be calculated for either voltage orcurrent. The following equation gives THD forcurrent.where Ih is the rms value of harmoniccomponent h of the current. The THD provides agood idea of how much extra heat will be realizedwhen a distorted voltage is applied across a resistiveload. Likewise, it can give an indication of theadditional losses caused by the current flowingthrough a conductor.VI. Simulation ToolsStudy of electric motor drives needs theproper selection of a simulation tool. Their complexmodels need computing tools capable of performingdynamic simulations. Today with the growth incomputational power there is a wide selection ofsoftware titles available for electrical simulationssuch as ACSL, ESL, EASY5, and PSCSP are forgeneral systems and SPICE2, EMTP, and ATOSEC5for simulating electrical and electronic circuits. IESEand SABER are examples of general-purposeelectrical network simulation programs that haveprovisions for handling user-defined modules.SIMULINK® is a toolbox extension of theMATLAB program. It is a program for simulatingdynamic systemsSimulink has the advantages of beingcapable of complex dynamic system simulations,graphical environment with visual real timeprogramming and broad selection of tool boxes Thesimulation environment of Simulink has a highflexibility and expandability which allows thepossibility of development of a set of functions for adetailed analysis of the electrical drive . Its graphicalinterface allows selection of functional blocks, theirplacement on a worksheet, selection of theirfunctional parameters interactively ,and descriptionof signal flow by connecting their data lines using amouse device.Tables and figuresPhase PWMInverter(%)A 20.89B 21.40C 23.77Vector controlled ASDBy evaluation of the harmonic distortion spectra ofPWM inverter fed ASD and Vector controlled ASDit is clear that harmonic distortion is very less invector control rather in PWM schemeThe following table gives the total current harmonicdistortion levels for all the phases A, B and C forPWM inverter fed permanent magnet synchronousmotors.VII. ConclusionThe issue of power quality has become moreapparent due to the ever increasing number ofnon-linear loads. The low frequency harmoniccurrent produced by these loads can cause damage tothe power system equipment. Adjustable speed drive(ASD) is becoming a significant load component forpower distribution.Designing of these loads to reduce theproblems arising from harmonic pollution is done inthis paper. This paper involves simulation of ASDsuch that power quality is significantly improved byreducing the harmonics to a significant levelSimulation ResultsThe system built in Simulink for a PMSMdrive system has been tested with the two currentcontrol methods, Hysteresis and PWM, at theconstant torque and flux-weakening regions ofoperation. The parameters were taken from reference. IGBT (Insulated gate Bipolar Junction Transistor)parameters of the inverter are giveninAppendix1.The motor is operated with constanttorque up to its rated speed and beyond that ratedspeed flux-weakening mode is adopted. Simulationresults are given at electrical speeds of 188.5 radiansper second (31 Hz) and 600 radians per second (95
Shravani Mathur, Prof. Vikas Kumar / International Journal of Engineering Research andApplications (IJERA) ISSN: 2248-9622 www.ijera.comVol. 3, Issue 3, May-Jun 2013, pp.493-496496 | P a g eHz). The above speeds represent below and aboverated speed of the motor.REFERENCES M.Farhney ,"Design Considerations whenApplying various ASD topologies to meet Harmonic Compilance", Paper No. PCIC -2010- 15 Copyright material IEEE. Mukhtar Ahmed,"High Performance ACdrives: Modeling, Analysis and Control",Springer Publications, London, 2010. Yu Yu; Yang Zhao;" H armonic andInterharmonic currents generated by theVSI fed Adjustable Speed Drives", IEEEInternational Conference IPMEC09, 2009,pp. 2464-2467. Li Zhang," Power Converter Circuits",Marcel Dekker Inc., New York, 2004. Sumitha Mohan, Henry GCuldner RalfBriest Henrik Wolf Dresden University ofTechnology , “Analysis and controlaspects of harmonic distortion in thefront-end threephase four-wire PWM boostrectffier “EPE 2005 - Ditsdcn “Simulation Study of AC Motor SpeedSensorless Vector Control System Based onSVPWM “ JIN Hui, ZHAO Yue-lingCollege of Electric Engineering LiaoningUniversity of Technology ,Jinzhou, China,2009 Ninth International Conference onHybrid Intelligent Systems” A Proof of Concept Study of PredictiveCurrent Control for VSI-DrivenAsymmetrical Dual Three-Phase ACMachines Federico Barrero, SeniorMember, IEEE, Manuel R. Arahal,Member, IEEE, “Pulse Multiplication in AC–DCConverters for Harmonic Mitigation inVector-Controlled Induction Motor Drives“,Bhim Singh, Senior Member, IEEE, G.Bhuvaneswari, Senior Member, IEEE, Motores Sinchrones de Imanes Permanents,Manuel Pérez Donsión, Manuel A.Fernandéz Ferro, 1990Universidade deSantiago de Compostela Elektrické pohony pro dynamicky náročnéaplikace, Martin Diblík, 2006 Technickáuniverzita v Liberci Permanent Magnet Synchronous MachineModel for Real-time SimulationA. B.Dehkordi, A. M. Gole, T. L. Maguire 2005,International Conference on Power SystemTransients in Montreal Direct Torque Controlof a PermanentMagnet Synchronous Motor,David Ocen,2005 Stockholm