Low Cost Microcontroller Based Implementation of Modulation Techniques for Three-Phase Inverter Applications

Transcription

1 Low Cost Microcontroller Bsed Implementtion of Modultion Techniques for Three-Phse Inverter Applictions T. Erfidn, S. Urgun, Student Member, IEEE, nd B. Hekimoglu, Student Member, IEEE Abstrct: Sinusoidl nd Spce Vector Pulse Width Modultion (SPWM-SVPWM) techniques hve been extensively used to build inverters, which generte AC voltge from voltge. The min ppliction of this type of inverters is for vrible speed induction motor drives. With the populrity of microprocessors, the implementtions of these techniques re being used widely in inverters. This pper describes the procedures for the implementtion of these two modultion techniques on low-cost high speed microcontroller. Index Terms: PWM, SVPWM, SPWM, driver, three-phse inverter nd microcontroller. I. INTRODUCTION The lternting power supply is usully obtined from electric rotting mchines clled genertors, lterntors, or synchronous genertors nd these mchines operte t fixed speed to generte three-phse power t desired frequency. By using threephse trnsmission lines gret mount of power tht is produced t generting sttion, trnsmitted over long distnces. In plces where three-phse or single-phse power supply cn power mchine, there my not be ny need to obtin it from dc source, however there re situtions where the conversion of dc supply to time-vrying supply becomes necessity such s when there is power outge nd no remote power-generting system is vilble. The conversion to time-vrying (c) voltge my lso be necessry where the single or three-phse power is beyond rech such s vehicles nd houses in rurl res. Most of the power is generted t 50-Hz or 60-Hz depending on ntionl stndrds in different countries. However, we my lso need power source t some other frequency, such s 400 Hz s in ircrfts or 20 khz s in industril heters. To obtin the power t frequency other thn the stndrd frequency is to first convert c power to dc nd then reconvert to c t the desired frequency. The power conversion from dc to c mkes use of power electronics switches such s trnsistors, SCRs, MOSFETs nd IGBTs. However SCRs re usully being voided for inverter ppliction becuse it requires circuits not only for gting but lso for its commuttion, which simply mens switching the SCR off fter it hs been conducting for predetermined time. On the other hnd, MOSFETs nd IGBTs re good cndidtes for the switches in dc-to-c conversion pplictions. The min drwbck of dc-to-c conversion is tht the output voltge is not sinusoid. Beside the fundmentl component, it my include hrmonic components. These hrmonic components cn be filtered out by using high-frequency filters on the output side. To void the hrmonic content there re mny different techniques hve been proposed nd put into prctice. These techniques require sophisticted gting circuits or controls. Sinusoidl pulse-width modultion (SPWM) nd spce vector modultion (SVPWM) re two of these techniques. During the pst decdes, PWM hs been studied extensively [1]-[2]. In literture, mny different PWM techniques hve been developed nd with these techniques; wide liner modultion rnge, less switching loss, less totl hrmonic distortion (THD) in the spectrum of switching wveform, esy implementtion nd less computtion time hve been chieved. In the pst, in most pplictions crrier-bsed PWM methods were widely used. The erliest modultion signls for crrierbsed PWM re sinusoidl. However, with sinusoidl threephse PWM, the liner modultion rnge cn not be extended for line-to-line voltges. To overcome this limittion, the nonsinusoidl crrier-bsed PWM methods, which use zerosequence signls, hs emerged. SVM, s non-sinusoidl crrier-bsed PWM method, uses the spce-vector concept to compute the duty cycle of the switches nd is one of the most importnt PWM method for three-phse converters [3]-[4]. With the development of microprocessors, esy digitl implementtion nd wide liner modultion rnge for output line-to-line voltges re the notble fetures of spce vector modultion. T. Erfidn is with the Deprtment of Electricl Engineering, Koceli University, Koceli, Veziroglu Cmpus Turkey (e-mil: trik@kou.edu.tr). S.Urgun is with the Civil Avition College, Koceli University, Koceli, Turkey (e-mil: urgun@ieee.org). B.Hekimoglu is with the Civil Avition College, Koceli University, Koceli, Turkey (e-mil: hekimoglu@ieee.org) /08/ IEEE 541

2 Microcontrollers nd Digitl Signl Processors (DSPs) re incresingly being used in pplictions previously ssigned to the nlogue domin. As their functionlity continues to increse by integrting more peripherls into single pckge, they re becoming vible option to replce nlogue designs. Applictions previously controlled using nlogue techniques were single-phse inverters, but tody there is shift towrds the digitl due to the benefits microcontrollers or DSPs cn offer [5-6], s shown in Tble 1. TABLE 1 Comprison of nlogue nd digitl control [6]. Anlogue Control Digitl Control Advntges High bndwidth High resolution Ese of design Disdvntge Component ging Temperture drift Hrdwired design Progrmmble solution Insensitive to environment Cretes numericl Shows precise problem behvior difficult to design Advnced lgorithm Cpble of dditionl functions II. THE POWER CIRCUIT The three-phse power converter shown in Figure 1 is commonly used in industril pplictions. The opertions of the power converters re bsed minly on the switching of power semiconductor devices nd s result, the converters introduce current nd voltge hrmonics into supply system nd on the output of the converters. At the inverter side, there re six power switches, which re connected in nti-prllel with six diodes. At side, the cpcitor is used to crete voltge t the converter end of the inductors to oppose the voltge t the utility end [3]. The AC voltge produced by the modultion hs peks restricted to less thn the voltge. Fig. 1. Three-phse inverter circuit. Both modultion techniques, SPWM nd SVPWM, will be used in this circuit. A. SPWM Signl Genertion PWM hs been studied extensively during the pst decdes. Mny different PWM methods hve been developed to chieve the following ims: wide liner modultion rnge; less switching loss; less totl hrmonic distortion (THD) in the spectrum of switching wveform; nd esy implementtion nd less computtion time. SPWM modultor is comprised of modultion signls nd crrier signl. The opertion of PWM cn be divided into two modes [7], 1) Liner Mode: In the liner mode, the pek of modultion signl is less thn or equl to the pek of the crrier signl. When the crrier frequency is greter thn 20 modultion signl frequency, the gin of SPWM G=1.. 2) Nonliner Mode: When the pek of modultion signl is greter thn the pek of the crrier signl, over modultion occurs with G<1. The six-step mode mrks the end of the nonliner mode. The THD of output switched wveforms increses. Figure 1 shows the circuit of three-phse PWM inverter. It is usully controlled in bipolr mnner. The PWM of phse A, B, nd C is usully jointly controlled by tringulr wve crrier V, nd the phse differences between modultion c signlsv, V, nd V of the three phses re lwys 120. s1 s 2 s3 The power switch components of phse A, B, nd C re controlled in the sme wy. Suppose the switch is n idel component, nd tke phse A s n exmple. If V > V s1 c, cut-off signl is sent to upper bridge rm power switch S1, nd cut-on signl is sent to lower bridge rm power switch S4, then the output voltge for phse A, V, is equl to Vd/2 with A0 respect to the current hypotheticl neutrl point O. If V < V s1 c, cut-on signl is sent to S1, nd connection signl is sent to S4, then the output voltge for phse A, V, A0 is equl to -Vd/2. S1 nd D1, s well s S4 nd D4, motion signls or sttes re lwys complementry. If connection signl is sent to S1(S4), either S1(S4) or the diode D1(D4) continuous flow will be connected; this will be determined by the direction nd mgnitude of the current initilly found in electric inductive lod, nd it is similr to the condition wherein the single-phse, two-level PWM bipolr circuit is controlled. The control methods of phse B nd C re the sme s tht of phse A. Reference, crrier nd produced inverter signls re shown in Figure

3 eqution, the vector V cn be decomposed into v 0, v 1,.. v 7 in infinite number of wys. However, in order to reduce the number of switching ctions nd mke full use of ctive turn on time, the vector is commonly split into two nerest djcent vectors nd zero vectors in n rbitrry sector. Fig. 2. Inverter reference nd crrier voltge, switching signl, inverter outputs. B. SVPWM Signl Genertion With the development of microprocessors, spce-vector modultion hs become one of the most importnt PWM methods for three-phse converters. It uses the spce-vector concept to compute the duty cycle of the switches. It is simply the digitl implementtion of PWM modultors. An ptitude for esy digitl implementtion nd wide liner modultion rnge for output line-to-line voltges re the notble fetures of spce vector modultion. The opertion of spce vector modulted voltge source inverter is revisited. The three phse two level inverter is shown in Fig. 1. Its switching opertion is chrcterized by eiglht switch sttes i = (SW, SWb, SWC), i= 0, The output voltges of the inverter re controlled by these eight switching sttes. Let the inverter voltge vectors, v (000),.., v (111) correspond 0 7 to the eight switching sttes. These vectors form the voltge vector spce s shown in the Fig. 3. The three phse reference voltges cn be represented by spce vector V with the mgnitude v nd phse ngle θ [8]. In smpling intervl, the output voltge vector is expressed s t t t V = v + v v (1) T T T s s s where t, t,.. t re the turn on time of the vectors v , v 1,.. v 7 respectively nd T s is the smpling time. From the bove Fig. 3. Voltge vector spce The equtions of the effective time of the inverter switching sttes cn be given s [8], t t b 0 = KV = 2 sin( π /3 α ) KV /2) ( + ) 2 sin( α ) t = ( T t t s b K = 3 T / 4V s Where V Mgnitude of commnd or reference voltge vector t Time period of switching vector tht lgs V tb Time period of switching vector tht leds V t0 Time period of zero switching vector Ts Smpling time = (1/ f ) s α Angle of V in 60 sector f Switching frequency s V link voltge The time periods need to be distributed such tht symmetricl PWM pulses re produced. To produce such pulses, the instnt of switching on for ech phse nd ech sector is clculted. The generlized eqution for turn on instnt clcultion for phse A is given below [8]. T = T v T g α (3) ( / 4) ( ) A_ ON s s (2) 543

4 Where g ( α ) = 3 π S = 4V 3 π + S = 4V 4V sin( / 3 α ) sin( α ), 1, 6 sin( / 3 α ) sin( α ), 2 3 sin( π / 3 α ) + sin( α ), = 3, 4 3 sin( π / 3 α ) sin( α ), = 5 4V S S (4) ( ) g α is defined s the turn on pulse width function. To mintin the symmetry of switching, the turn off instnt T is clculted nd given below. A_ OFF T = T T (5) A_ OFF s A_ ON For phses B nd C, the switching instnts re sme but phse shifted by 120. III. IMPLEMENTATION For the implementtion of fore-mentioned techniques Microchip s dspic30f4011 microcontroller is used, which hs six motor control PWM chnnels nd cn run up to 120MHz [9]. The block digrm nd picture of the implemented system re shown in Figures 4 nd 5, respectively. Fig. 5. The picture of the implemented system. dspic microcontroller is progrmmed by ICD2 for SPWM nd SVPWM techniques. For sine nd cosine vlues look-up tble with 64 word size is plced in progrm memory of microcontroller. For specified ngle, liner interpoltion will be used on this tble [9]. IV. RESULTS The results of implemented system for the cse of 25V supply voltge, 20kHz PWM nd SVPWM switching frequency with str connected RL lod (R=47ohm, L=1.2mH) re given in Figures 6 nd 7. () ICD2 PWM dspic Microcontroller Circuit Driver Gte Pulse 3-phse A.C. V + - Power Circuit RL Lod (b) Fig. 4. The block digrm of the implemented system. (c) Fig. 6. SPWM signls t 20kHz () switching signls of S1, (b) phse voltge nd phse current, (c) phse-phse voltge. 544

5 Rectifier", Power Engineering, Energy nd Electricl Drives, POWERENG, pp , () [5] T. Abeyseker, C. M. Johnson, D. J. Atkinson, nd M. Armstrong, "Elimintion of sub-hrmonics in direct look-up tble (DLT) sine wve reference genertors for low-cost microprocessor-controlled inverters," IEEE Trnsctions on Power Electronics, vol. 18, pp , [6] M. C Trigg, H. Dehbonei, C. V. Nyr, "Digitl Sinusoidl PWM Genertion using Low-cost Micro-controller Bsed Single-Phse Inverter" Emerging Technologies nd Fctory Automtion, 2005., Volume 1, Sept (b) [7] T. M. Rown, R. J. Kermn, nd T. A. Lipo, Opertion of nturlly smpled current regultors in trnsition mode, IEEE Trns. Ind. Applict., vol. 23, pp , July/Aug [8] A.Muthurmlingm, D. Sivrnjni nd S. Himvthi, Spce Vector Modultion of Voltge fed Inverter Using Artificil Neurl Networks, IEEE Indicon 2005 Conference, pp Chenni, Indi, Dec (c) Figure 7. SVPWM signls t 20kHz () switching signls of S1, (b) phse voltge nd phse current, (c) phse-phse voltge. V. CONCLUSION Inverters, which generte AC voltge from voltge, re being widely used in motor control driver, UPS nd solr system pplictions. The success of output AC voltge genertion highly depends on the switching techniques used in inverters such s SPWM nd SVPWM. This pper presented low-cost high speed microcontroller bsed system for implementtion of these two modultion techniques nd the results re tken successfully. VI. REFERENCES [1] Lowery, T.F.; Petro, D.W., "Appliction considertions for PWM inverter-fed low-voltge induction motors", Industry Applictions, vol. 30, pp , [2] Heng Deng; Orugnti, R.; Srinivsn, D., "PWM methods to hndle time dely in digitl control of UPS inverter", Power Electronics Letters, vol., pp. 1 6, [3] Kun Xing; Lee, F.C.; Borojevic, D.; Zhihong Ye; Mzumder, S., "Interleved PWM with discontinuous spcevector modultion", Power Electronics, vol. 14, pp , [9] Using the dspic30f for Vector Control of n ACIM Microchip Technology Inc., [10] "PICREF- 1: Uninterruptible Power Supply Reference Design," vol. 2004: Microchip Technology, [11] I. Ahmed, "Implementtion of PID nd Dedbet Controllers with the TMS320 Fmily," vol. 2004: Texs Instruments, [12] Kerkmn, R.J.; Leggte, D.; Seibel, B.J.; Rown, T.M., "Opertion of PWM voltge source-inverters in the overmodultion region", Industril Electronics, vol. 43,pp , [13] Bowes, S.R.; Grewl, S., "Novel hrmonic elimintion PWM control strtegies for three-phse PWM inverters using spce vector techniques", Electric Power Applictions, vol. 146, Issue 5, pp , [14] Keling Zhou; Dnwei Wng, "Reltionship between spce-vector modultion nd three-phse crrier-bsed PWM: comprehensive nlysis [three-phse inverters]", Industril Electronics, vol. 49, pp , [15] Kwsinski, A.; Krein, P.T.; Chpmn, P.L. "Time domin comprison of pulse-width modultion schemes", Power Electronics Letters, vol. 1, Issue 3, pp , [16] Mlinowski, M.; Jsinski, M.; Kzmierkowski, M.P., "Simple direct power control of three-phse PWM rectifier using spce-vector modultion (DPC-SVM)". Industril Electronics, vol. 51, pp , [4] Belhdji, L.; Alioune, K.; Ghennm, T.; Kheloui, A., "A New Spce Vector Current Control of the Three Phse PWM 545

6 Brn Hekimoglu (M 2000) ws born in Diyrbkir, TURKEY, on September 11, He grduted from İstnbul Technicl University (Electricl Engineering) nd Florid Stte University (Mthemtics Eduction), respectively. He is currently reserch ssistnt t Koceli University Civil Avition College nd studying for PhD degree in Electricl Engineering Deprtment. Stilmis Urgun (M 2008) ws born in Tokt, TURKEY on April 1, He grduted from Koceli University. He is currently n instructor t Koceli Univerity Civil Avition College nd studying for PhD degree in Electricl Engineering Deprtment. Trik Erfidn ws born in Stuttgrt, GERMANY on April 19, He is currently n ssistnt professor t Koceli Univerity Electricl Engineering Deprtment. 546