Low-Cost PWM Drive for an Electric Mini-Baja Type Vehicle

Transcription

1 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering Low-Cost PWM Drive for n Eletri Mini-Bj Type Vehile Smuel E. de Luen UNESP - São Pulo Stte University, Gurtinguetá, Brzil luen@feg.unesp.br Frniso Jose Grndinetti UNITAU - University of Tubté, Tubté, SP, Brzil grndi@unitu.br Mrio Abud Mrelino UNESP - São Pulo Stte University, Gurtinguetá, SP, Brzil bud@feg.unesp.br Abstrt This pperwork presents Pulse Width Modultion (PWM) drive for n eletri mini-bj-type r. The eletri vehile trtion is provided by bttery-fed 1-kW three-phse indution motor. The open-loop speed ontrol is implemented with n equl voltge/frequeny rtio, in order to mintin onstnt mount of torque on ll veloities. The PWM is implemented by low-ost 8-bit miroontroller provided with optimized ROM hrts for distint speed vlue implementtions, synhronized trnsition between different hrts nd redued odd hrmonis genertion. This tehnique ws implemented using single pssenger minibj vehile, nd the essys hve shown tht its pplition resulted on redued urrent onsumption, besides eliminting mehnil prts. Keywords: indution motor, eletri vehile, speed drive, PWM, miroontroller 1. Introdution The min purpose for using eletri vehiles (EVs), s opposed to internl-ombustion rs, is tht they re zeroemission vehiles. Besides not giving off rbon dioxide to the tmosphere, whih is one of the min uses for ir pollution in metropolitn res, EVs require virtully zero-mintenne, no periodi oil hnges, mufflers tht rust out, burst hoses, nd so on, besides being tremendously silent. Motor nd ontrol tehnology for EVs re vilble tody. The min problem found on vehiles supported by btteries is (bttery) ost nd, to some extent, the smller rnge nd the reltively time-onsuming refueling, s ompred to gsoline-powered rs. Fortuntely, bttery tehnology nd osts re improving rpidly, nd dvned nikel-hydride btteries re getting into volume prodution (Riezenmn, 1998; Cooni nd Gge, 2002; Cooper, 2005; Lm et ll, 2005). Initilly, min EVs produers deided to produe n EV with hrteristis losest to gsoline-powered internlombustion engines (ICEs), rossing distnes of up to 240 km wy with no bttery re-hrge nd hieving speeds s high s 192 km/h. However, given tht 40% of vehiles in megities drive less thn 60 km dy, n EV with rnge of 150 km should be tully muh ttrtive (Riezenmn, 1998; Nunim, 1996; Min, 2005; Fiz, 2005). As of now, there is not onsensus mong utomkers in regrds to wht motor, trnsmission, bttery, nd speed ontroller to use in EVs, reting mny different ombintions found on EVs in the mrketple tody (Sduto nd Fenton, 1993). As environmentlists, government nd utomkers ontinue their struggle to mke onsumers onfident in EVs, the following onepts on EV must be highlighted, if mnkind is to breth len ir in megities, nd slow down plnet wrming: 1) The EV must be smll vehile, providing trnsporttion for four pssengers t most. It nnot be the fmily s only r, but it should be the dily drive one, insmuh s huge mount of fmilies hve two or three rs. It should be used for distnes of 200 km t the most. Therefore, it is n extremely ttrtive vehile for going to work. 2) ICEs nnot be ompred to EVs, for the resons given bove. Although EVs n elerte s fst s ICEs, tht would put the bttery utonomy in jeoprdy. EVs must hve optimized elertion slopes, whih must be somewht independent of the driver s need for elertion. As result, utomkers nnot use trditionl mrketing onepts with potentil buyers, if EVs re to be mss-produed nd mrketed worldwide. Shortly, the EV must be lightweight, with the lest number of moving prts, nd with limited number of eletril essories (in tropil ountries t lest). This pperwork ims the study nd implementtion of n experimentl EV, nd emphsizes the use of low-ost PWM drive. 2. Min Vehile Fetures The Mini-Bj is n idel vehile for sportive ontests nd edutive gols. It is somewht rugged nd ble to drive on lmost ny soil ondition. It fetures rer wheel drive, whih is powered by four-yle, eight horsepower, gsoline, ir-ooled engine produed by Briggs & Strtton Corportion. It n hieve top speed of 60 km/h. The onepssenger-set r is mde of tubulr 1020 steel, nd it is pproximtely 2.0 m x 1.0 m x 1.55 m, between xle distne of 1.70 m, nd 205 kg mss, inluding five liters of gsoline in the fuel tnk. It lso fetures onil

2 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering trnsmission with two stges of redution by hins, with no reverse ger. There were not mny mehnil prts dpted to the eletril Mini-Bj used on the essys for this speifi reserh. The struture ws mounted in luminum, nd the trnsmission is powered by belt nd pulleys diretly onneted to the motor in 5:1 rtio. In spite of the muh greter effiieny of eletril motors in reltion to internl ombustion motors, EVs still need drsti redutions in erodynmi drg nd in rolling resistne, in order to inrese their rnge. This relies on new design for the tires nd vehile shpe. Aerodynmi drg, the fore tht opposes movement, is inresed by the qudrti power of the veloity, nd n be redued in two different wys. First, the overll speed of the vehile n be redued, but this mkes the vehile less ttrtive to onsumers. Seond, by diminishing the erodynmi drg oeffiient tht tody vries from 0.3 to 0.4. Speil grphi designs for the EV n deliver 0.2 oeffiient vlue. Another redution n be hieved by using more rigid nd nrrower tires, lthough omfort ould be dversely ffeted. Doubling pressure for ommon tires, the 0.01 to 0.02 verge rnge for the rolling resistne n be lowered to the to rnge (Sduto nd Fenton, 1993). An intelligent method to inrese EV effiieny is through the use of regenertive brking, insted of ommon frition brkes tht remin on the vehile for osions in whih very fst stop is demnded. This pperwork disusses the implementtion of low-ost, open-loop PWM inverter using three-phse indution motor, speifilly built for the mini-bj EV. The option for the development of n EV equipped with three-phse indution motor is bsed on the ese of implementtion of speed drive, no strt-up iruit, nd esiness of movement reversion. 3. The Speed Control Indution motors re essentilly onstnt speed motors, if onneted to onstnt voltge nd frequeny soure. The opertion speed for indution motor is diretly relted to the synhronous speed. If lod torque inreses, the drop in speed will be very smll, whih is onvenient for onstnt speed pplitions. Currently, the min speed ontrol tehniques for three-phse indution motors re vetor ontrol, whih employs digitl signl proessors, nd PWM (Anonymous, 1994; Rshid, 1993). By fr, the open-loop PWM is the esiest tehnique to implement, if motor speed does not need to be very urte, s is the se on EVs, where the driver ontrols the speed through the elertor pedl The Constnt Flux Control Method The output torque of three-phse indution motor, M d, n be lulted through (1), where P is the number of poles, V m is the ounter eletromotive fore (CEMF), w s is the ngulr frequeny of the sttor urrent, w r is the ngulr frequeny of the rotor urrent, R r is the rotor resistne, nd L lr is the loked rotor indutne (Anonymous, 1994; Fitzgerld et llii, 1992). M d = 3 (P/2) (V m /w s ) 2 [w r /R r 2 + (w r L lr ) 2 ] R r. (1) Eqution (1) implies tht the output torque of the mhine will depend only on w r, irrespetive of the sttor frequeny, w s, provided the rtio V m /w s is kept onstnt. This rtio is the pek mplitude of the irgp flux, ψ m: ψ m = V m /w s. (2) By mintining irgp flux onstnt, nd vrying the sttor frequeny w s, fmily of torque versus speed urves n be hieved for the motor, s shown in figure 1. This method of speed ontrol is known s onstnt flux ontrol (Anonymous, 1994). M d-mx Torque, M d 0 Sttor frequeny, w s (p.u.) 1 Figure 1. Constnt-flux torque versus frequeny urves.

3 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering Figure 1 implies tht this tehnique n meet ny lod torque, within the pbility of the motor, t ll speeds, up to the rted speed. Furthermore, it is possible to obtin high torque for strting the motor, by operting it t low frequeny. Nevertheless, the implementtion of the bove method is not strightforwrd, s the CEMF nnot be mesured diretly. And lthough it n be lulted by mesuring the motor terminl voltges nd urrents, this implies on inresing ontrol iruit omplexity nd ost (sensors, onditioning iruits, nlog to digitl onverters, DSPs, et). It must be tken into ount tht seemingly smll ost inrese in r omponent (s for exmple, in the integrted iruit utilized in the ontrol iruitry) leds to onsiderble inome lost, in seril prodution of the r. Fortuntely, for prtil purposes, the bove tehnique n be implemented in n pproximte mnner, by keeping the V s /w s rtio onstnt, s the sttor terminl voltge, V s, nd the CEMF re resonbly lose in mgnitude t lmost ll, but t low speeds. At low speeds, keeping V s /w s onstnt is not equivlent to keep the flux onstnt, resulting in muh lower torque. In this work, the torque derese t low speed hs been overome by giving boost on sttor voltge, V s, bove the onstnt V s /w s, s shown in figure 2. Inverter Btteries A B C /A /B /C 3-phse indution motor A /A B /B C /C gte drivers gte drivers gte drivers w * V * Slow strt V s w s Optimized PWM Look-Up Tbles 8051 miroontroller-bsed ontroller 3.2. Pulse Width Modultion Purpose nd Inverter Figure 2. Blok digrm for the ontroller. Adjustble speed drives hve undergone gret struturl modifitions. At first, their working priniple ws nlogil, but nowdys it hs beome digitl, thnks to the stonishing tehnologil dvne on solid-stte power swithes nd miroproessors. The speed drive n be divided into two prts: the inverter (or d- onverter) nd PWM iruit. A bttery feeds the inverter, whih is gted by the PWM iruit, in order to feed the three-phse indution motor, in ordne with the ommnded speed, w*, s illustrted in figure 2. The bsi purpose of the PWM method is to produe the required mplitude nd frequeny of the fundmentl, while moving the energy in the hrmonis to higher rnge in the frequeny spetrum. At suh high frequenies, the motor indutne will exhibit substntil retne, therefore limiting the hrmoni urrents drwn from the inverter nd reduing the resulting torque pulstions reted by these high-frequeny hrmonis. The inerti of the mehnil system lso helps filtering out high frequeny torque pulstions, llowing the motor to run smoothly Digitl Projet Diffiulties Sinusoidl urrent genertion by using PWM nd indutive lods is ommonple. However, its digitl implementtion with low-ost miroontroller is not strightforwrd, minly if it is to drive three-phse indution motor in wide speed rnge, nd yet mintining onstnt torque, s is the se with the EV. In bttery-fed inverter, with tringulr rrier wve nd sinusoidl modulting signl, the modultion index must be vried, in order to keep the flux onstnt (nd so the motor s output torque), wheres different speeds re imposed to the motor. Furthermore, high frequeny rrier, sy 1.5 khz, is needed to ese filtering of high-frequeny urrent hrmonis. Also, hrmoni ontent n be redued by using rrier wve whose frequeny is muh higher thn the modulting signl frequeny (Rshid, 1993). In miroontroller-bsed implementtion, however, this hs two importnt implitions: 1) The softwre must ompre the two wves gret number of times in every rrier period, in order to generte the PWM signl. Moreover, for three-phse inverter, the omputtionl lod beomes three times

4 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering greter. 2) To detet the ext instnt where the tringulr rrier wve rosses the three referene sinusoids, the system s smpling frequeny must be high enough. For the ske of illustrtion, 30-kHz smpling frequeny (tht would result in modest 20 smples per yle of 1.5-kHz rrier wve), would leve the miroontroller only 33.3 µs between smples to do ll the job (typilly brnhing to timer interrupt servie routine, ompring the tringulr rrier wve with the three sinusoidl referenes, rrying out onditionl jumps, updting the three PWM ontrol signls, nd returning from interrupt, besides reding the speed referene signl nd lulting the suitble modultion index). On-line omputed swithing instnts ould be muh diffiult, if not impossible, tsk to low-ost 8-bit miroontroller. Pre-lulted PWM ontrol signls burned s tbles into the miroontroller s Red Only Memory (ROM) n be n ppeling solution, sine it voids muh of the omputtionl lod lredy mentioned (Simrd et lli, 1991). Nevertheless, s in the EV pplition frequeny s low s 5 Hz must be generted, n unrel mount of ROM would be required to sve PWM ontrol signls of, sy, dozen different frequenies. Using 30-kHz smpling frequeny, PWM ontrol signls for yle of the 5-Hz modulting signl would lone oupy 6,000 ROM lotions, wheres the sme PWM signls for 60-Hz modulting sinusoid would oupy only 500 ROM positions. In this work, tehnique hs been employed whih besides solving the problem of the size of the tbles, for low frequenies, synhronizes the modultion, voiding pulsting torques nd low frequeny hrmonis Optimiztion of PWM Tbles In this work, simple tehnique hs been devised to redue the PWM tbles size to be burned into the miroontroller s ROM, whih dereses the need for fst miroontroller s well s for lrge ROM size, mking it possible to use stndrd industry-proven, low-ost 8-bit miroontroller s the entrl ontrol blok of n experimentl- EV PWM drive (Mrelino nd Fiorotto, 1997). A first step to generte PWM tble for given modulting sinusoid onsists on hoosing rrier wve whose frequeny is n ext multiple of the modulting signl. Furthermore, the tringle rrier is fored to hve fixed number of yles within one modulting yle, no mtter the modulting frequeny, so tht the number of pulses in eh yle of the PWM output voltge is fixed (nd does not depend on the output frequeny). In other words, the resulting tble size for prtiulr output frequeny depends only on the smpling frequeny. The riteri to selet the number of pulses nd the smpling frequeny will be disussed in the next subsetion. Tble 1 illustrtes the working priniple of the optimiztion tehnique. The left hnd olumn ontins the first 18 non-optimized smples for fititious three-phse PWM ontrol signls, wheres the right hnd olumn ontins the 12 optimized smples for the sme PWM signls. As Tble 1 shows, redution in tble (olumn) size is obtined by sving the smples (bytes) representing the desired PWM signls nd their repetition numbers. Therefore, the greter the smples repetition times, the shorter the optimized hrt. Tble 1. Norml nd Optimized Speed Chrts (smple number redution: 18 to 12),b,: distint byte vlues NORMAL OPTIMIZED TABLE TABLE A A 4 b 3 b b 2 b 5 1 3

5 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering 3.5. Prtil Considertions The rtio, n, between the frequeny of the tringle rrier, f, nd the frequeny of the modulting sinusoids, f m, must fulfill the riteri summrized bellow (Anonymous, 1994). 1) n must be n integer number, in order to synhronize tringle rrier nd modulting sinusoids, so tht the output voltge is periodi nd free from sub-hrmonis. 2) n must be multiple of 3, to ensure tht the output voltge wveforms produed by eh inverter phse is the sme. 3) n must be n odd number, in order to get the output voltge rid of even hrmonis. 4) n must be the gretest possible number, to ese high-frequeny hrmonis filtering by motor s indutne. 5) n must be the smllest possible number, so tht the requirement for fst-response PWM ontrol iruit re loose. In this work, the hosen vlue for n ws 21 for it fulfilled ll of these reommendtions. On PWM tble lultions, whih hve been done by progrm written in C lnguge, by hoosing smpling frequeny whih is multiple of four of the tringle rrier s frequeny eses synhroniztion of the rrier with the three sinusoidl referenes. For desired output frequeny, f s, the number of smples, N, per rrier period is lulted through (3): N = 1 / (f s n t s ) (3) where t s is the smpling period, i.e., the intervl between two onseutive smples of the PWM signls sved on ROM. The smpling period is lso the ritil time the miroontroller hs to rry out ll tsks nd, for the Intel 80C31miroontroller employed in this work, this time intervl is ir 22 µs, for rystl osilltor of 12 MHz. Therefore, for f s = 60 Hz, by mens of (3), one gets N = As the result must be integer nd multiple of four, N is pproximted to 36 nd, using (3) gin, the rel sttor frequeny is Hz. The smll devition between the desired nd the tul sttor frequeny is no problem for the EV pplition. To generte 60-Hz sttor frequeny, the number of PWM smples in the ROM tble is 756 (i.e., n x N = 21 x 36). In this work, PWM tbles hve been reted for twelve distint sttor frequenies, from 5 Hz to 60 Hz, every 5 Hz. In eh frequeny, suitble smpling frequeny hs been employed, in order to keep the number of PWM smples in ROM equl to 756. The smpling periods, for some of the frequenies, hve resulted in rel, but not integer, numbers. However, s the miroontroller must be interrupted in integer intervls of miroseonds ( 12 MHz rystl osilltor hs been used), in those ses, the required sttor frequenies hve suffered smll djustments (e.g., Hz in lieu of 55 Hz), ll of them negligible from the prtil point of view. To the so generted PWM tbles hs been pplied the tble redution tehnique lredy desribed, before burning the smples into ROM. The optimiztion nd the synhroniztion tehniques hve llowed tht the tble for 5 Hz, initilly oupying 6000 ROM lotions, ould oupy only 756 ROM lotions, resulting in smpling frequeny round 4000 Hz. Although this frequeny is inside of the udio rnge, the noise is minimum sine the mplitude is well redued by the onstnt V/f requirement. 4. Vehile Test nd Results For the Mini-Bj EV prototype, motor mnufturer EBERLE do Brsil hs dpted squirrel-ge three-phse indution motor, whih hs 2 poles nd 1120 W rted power t 110 V nd 60 Hz. During the tests, the EV hs been fed through 100-V mins retified d-power supply, in lieu of btteries. An extension ord hs been employed tht llows the vehile to go round. The mximum speed obtined in the ssys hs been 18 km/h. Controlled elertion nd regenertive breking hs been employed. The inverter iruitry inludes 6 N-hnnel power MOSTETs (IRF450 Insulted-Gte Bipolr Trnsistors) nd their gte drives (IR2110), ll mnuftured by Interntionl Retifier In Mesurements of Motor Current Eletri urrent mesurements hve been effeted in one of the mhine s phse, by using 160 mω resistor s urrent sensor nd onneting it in series with the phse winding. Current wveforms hve been reorded through hndheld digitl osillosope. Figure 3 shows the phse urrent following step hnge, from 0 to 60 Hz, in the frequeny referene, whih orresponds to the phse urrent due to speed step requirement. In this essy, the reording time (ir 250 ms) hs been hosen to fous the exessive phse urrent resulting from the sudden frequeny hnge, whih ultimtely is used by the high rotor slip level during motor strt up. Using suitble snning speed, the lmost sinusoidl nture of the urrent wveform hs been observed. Figure 4 shows the phse urrent wveform for the sme hnge in frequeny requirement (0 to 60 Hz) s in the lst se. This time, however, the ontrol lgorithm limits the motor s elertion, imposing four intermedite frequenies to the inverter s output voltge. This results in lower slip of the rotor nd thus in smller trnsient urrents. From the EV driver s viewpoint, however, the elertion senstion is norml, beuse eh intermedite frequeny lsts only 150 ms. Nevertheless, limiting urrent trnsients,

6 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering besides lowering solid-stte swithes (IGBTs) speifition (whih hve to withstnd smller urrents), lessen sttoroils Joule effet loss (proportionl to urrent squre power), whih ultimtely extends the EV s bttery life. In ddition, unlike most soft strters figure 4 shows tht frequeny trnsitions during elertions re rried out during zero voltge rossing. This diminishes hrmonis nd sub-hrmonis genertion. Figure 3. Phse urrent following sudden hnge of speed. 5. Conlusion Figure 4. Phse urrent following ontrolled hnge of speed. Wheres mss prodution nd worldwide sles of eletri rs depend on lowering bttery pries nd inresed bttery utonomy, tody s EVs re undoubtedly suitble t lest s the fmily s seond vehile nd this ould help to improve ir qulity in big ities, through the redution in rbon dioxide emissions, nd world s dependene on fossil fuel. In this pperwork, simple tehnique to generte three-phse PWM ontrol signls with low-ost 8-bit miroontroller hs been presented tht llowed the reliztion of onstnt-torque vrible speed drive for threephse indution motor. The motor, inverter nd ontroller hve been tilored to modified mini-bj type EV, in whih the essys hve shown tht the speed ontrol lgorithm n signifintly redue the trnsient urrent mplitudes, thus inresing bttery life nd llowing the use of heper semiondutor swithes. 6. Referenes Anonymous, S., 1994, A Tutoril in AC Indution nd Permnent Mgnet Synhronous Motors, Anlog Devies In., Msshusetts, pp Cooper, A., 2005, Develoment of High-Performne Led-Aid for Bttery for New-Genertion Vehides, Journl of Power Soures (June) 144 (2): pp Cooni, A. & T. Gge, 2002, A plug for Plug-in Crs, IEEE Spetrum (April) 2002, pp Fiz J, Moyed-Zdeh K, 2005, Design of Switdhed Relutne Mhine for Strter;Genertor of Hybrid Eletri Vehile, Eletri Power Systems Reserh (August) 75 (2-3): pp

7 Proeedings of COBEM th Interntionl Congress of Mehnil Engineering Fitzgerld, A. E., Kingsley Jr, C., nd Umns, S. D., 1992, Eletri Mhinery, MGrw-Hill, New York. Lm, L.T., High, N.P., Phylnd, C.G., 2005, Novel Tehnique to Ensure Bttery Relibility in 42-V PowerNets for New-Genertion Automobiles, Journl of Power Soures (June) 144 (2): pp Mrelino, M. A. nd Fiorotto, F.A., 1997, Disrete, optimized nd synhronous PWM genertion, Brzilin Ptent PI (in Portuguese). Nunim, D., 1996, Eletri Vehiles, Pro. IEEE Interntionl Symposium on Industril Eletronis, Vol. 1, pp Rshid, M. H., 1993, Power Eletronis Ciruits, Devies, nd Applitions (2nd ed.), Prentie Hll, Upper Sddle River. Riezenmn, M. J., 1998, EVs: The Rod Ahed, IEEE Spetrum (Deember), pp Sduto, T.nd Fenton, B.C., 1993, The EV Revolution. Populr Eletronis, Vol. 10, no 4 (April), pp Simrd, R., A., Cheriti, T. A., Meynrd, K. Al-Hddd, nd Rjgopln, V., 1991, A Eprom-bsed PWM Modultor for Three-phse Soft Commutted Inverter, IEEE Trnstions on Industril Eletronis, Vol. 38, No. 1, pp Responsibility notie The uthors re the only responsible for the printed mteril inluded in this pper.