Impact of the design method of permanent magnets synchronous generators for small direct drive wind turbines for battery operation

Transcription

1 Impact o the degn method o permanent magnet ynchronou generator or mall drect drve wnd turbne or battery operaton Mha PREDESCU, Dr. eng.*, Aurelan CRǍCIUNESCU, Pro. Dr. **, Andre BEJINARIU, Eng.*, Octavan MITROI, Eng.*, Adran NEDELCU, Eng.* *ICPE SICE Centre, Splaul Unr 313, Bucharet,Sector 3, RO-3138, Romana. Emal: predecu@cpe.ro, andre.bejnaru@cpe.ro, octavan.mtro@cpe.ro **Unverty Poltehnca Bucharet, Splaul Independenţe 313, Bucharet, Romana, Emal: acracunecu@netcape.net Keyword : wnd turbne, ynchronou generator, drect drve, battery, arol Abtract. In tand-alone drect drve wnd turbne power generator, worng on battery, the permanent magnet ynchronou generator the mot common oluton. The ytem can be analyzed a the combnaton o three ubytem: wnd turbne, electrc generator and the load, whch the battery or energy torage. For hgh energy traner rom wnd to the battery, thoe component hould be degned n a partcular way. The paper approache the mpact o the component parameter o each ubytem on power traner rom wnd to the load. 1 Introducton The drect couplng o ynchronou generator to the wnd turbne rae ome problem n matchng the mechancal output charactertc o the wnd turbne to the charactertc o the ynchronou generator or maxmum power traner. In th power tranmon chan the ynchronou generator can be omehow eaer to trm or maxmum power traner. The paper decrbe the degn method and expermental wor done on a mall power wnd turbne, 1W rated power, wth a 4Ah battery torage unt. The decrpton o the experment wor gve the reult o tet on rare earth, permanent magnet ynchronou generator, accordng to the degn method. curve, dependng heavly on the degn and worng regme. A wnd ytem degned n uch way to enure an acceptable level o energy avalablty to the load and to capturng and torng the maxmum energy o the wnd, wth mnmum nvetment. Every component o the ytem: turbne, generator and torage unt have an approprate degn method. There are two objectve o the degner: degnng each component or achevng maxmum ecency/cot and matchng the component n uch a way that the maxmum energy tranerred through the ytem. Two o the three wnd ytem component o the Fgure, the turbne and the torage unt nteract wth the envronment: the turbne wth the wnd and the battery wth the load. The degn o thoe two component depend on wnd reource and the load prole, repectvely. The electrc generator ln the turbne wth the battery. Energy traner n a wnd ytem The bloc dagram o a mall tand-alone wnd turbne power ource n Fgure 1. The generaled archtecture o the wnd power ource n Fgure n whch the energy low through the three man component. The netc energy o the wnd converted by the wnd turbne nto mechancal energy, then by the ynchronou generator nto electrc energy whch tored n derent way: electrochemcal, chemcal, netc or mechancal potental energy. From the torage ytem, the load tae the requeted amount o energy, regardle o the wnd avalablty. Each component n Fgure ha t own non lnear ecency Fgure 1-Archtecture o a wnd ytem Thereore, the trmmng o the energy chan or bet overall ecency can be ealy acheved by an approprate degn procedure o the electrc generator.

2 - requency o the nduced voltage. = n p ( n-rotatonal peed; p-number o pole par. Fgure -Energy tranmon chan 3 Optmal degn o PM ynchronou generator A. Output power computaton. Interacton wth battery For mall wnd turbne, drect drve permanent magnet generator a common contructve oluton or horzontal ax wnd turbne. The old oluton wth mechancal peed multpler avourable or the generator that operate at hgh rotatonal peed. The dmenon are maller and ecency hgh. The mechancal multpler, on the other hand, a buly pece o equpment whch requre regular mantenance and t a major ource o alure. The drect drve oluton aembly mpler, more relable, but rae everal degn problem and need a lghtly derent pecc technology. In th cae, the generator ha large number o pole, and larger dameter. The coggng torque ncreae wth the dameter o the generator, thu, pecal meaure or dmnhng th paratc torque become very mportant. The ue o rare earth permanent magnet, due to ther low prce and hgh pecc energy, mprove the weght/power rato o ynchronou generator. Thereore, the method decrbed reer to drect drve PM ynchronou generator. The degn method baed on clac approach o the relatonhp between dmenon and traner uncton o permanent magnet ynchronou generator. The goal o the demontraton below ndng the parameter o the electrc generator on whch the degner can act upon or better matchng the turbne and the load (battery or maxmum power traner. The phae electromotve orce n a generator computed a: π Ue = wwφ (1 U e -electromotve orce; w-number o turn o a phae wndng; w -wndng actor; Φ-pole magnetc lux; From(1 and ( we can dene the voltage actor,, a: U n e = (3 The actor contant parameter o a generator, dependng on dmenon and materal ued or contructng the generator. Accordng to the mpled theory o ynchronou electrc generator, the phae equvalent mpedance : Z = R + X (4 R-wndng phae retance; X -ynchronou reactance. The equvalent crcut : Fgure 3-Equvalent crcut o PM generator U b -output DC voltage(battery voltage d - traner coecent o the recter brdge. The computaton o the power delvered to the battery by a AC generator can be explaned on AC equvalent crcut o the generator-battery aembly, Fgure 4. The power njected nto battery : P Fgure 4 -Mono phae equvalent crcut 3 n U / b d b = m (5 d R + n d U

3 X = (6 n R = R / d R / d (7 In (5-(7 we have: cab + bat P-actve power; m-number o phae o the generator-3; -ynchronou reactance coecent; R cab -Cable retance; R bat -battery nternal retance. B. Input power. Interacton turbne-generator Power curve o the turbne For a degner t very mportant to have a procedure or computng the power curve o the turbne or to have t meaured n wnd tunnel. The computatonal method or output power curve P(n at derent wnd peed n the normal operatng range o the wnd turbne tae nto conderaton the ollowng parameter: a. the number o blade; b. the arol prole o the blade; c. the varaton o the chord length along the blade; d. the varaton o the toron angle o the blade along the length o the blade; e. (the Reynold number at derent rotatonal and wnd peed. A good degn o the electrcal generator hould have the nput mechancal curve that ln the pea o the power curve at derent wnd peed. That the Pc curve n. In uch manner, the wnd turbne wll extract the maxmum avalable energy rom wnd. It evdent that the optmum degned generator hould have the power curve tted to Pc, n the operatonal wnd peed range. Another charactertc that hould be condered n a degn that the generator tart njectng the power nto the battery at a certan peed, n, at whch the output voltage o the generator, eq. (1, equal the battery voltage, ee Fgure 5. In operaton, the wnd turbne peed up untl reache n, durng whch the wnd turbne loaded only by rctonal torque o the generator. Beyond n, the turbne loaded manly by the battery current. The peed n cannot be lowered too much, becaue the number o turn o the wndng wll be very large, ee (1, and t mpoble to mae a generator wth the power curve a decrbed prevouly. 4 Degn method o PM ynchronou generator The power curve Pc n Fgure 5 can be approxmated to a traght lne on the mot mportant regon o the operatng range o the wnd turbne. Around nomnal power, the power curve hould approache the lne: P ( n / U / ( n n = (8 b d Equaton (8 the relatonhp o the power o the turbne wth geometrc charactertc o the generator P[w] 5 Pc P[w] 5 Lnear power curve Wnd peed Wnd peed 15 1 m/ 11m/ 1 m/ 9 m/ 8 m/ 7 m/ 5 m/ 15 1 m/ 11m/ 1 m/ 9 m/ 8 m/ 7 m/ 5 m/ n n[turn/mn] Fgure 5-Sample o computed power curve o a mall wnd For the pecalt n aerodynamc o wnd turbne, the power coecent computed n term o tp peed rato λ. For an electrcal engneer more common to have the power curve dependng on rotatonal peed n whch mae eaer the connecton wth the phenomena wthn electrc generator. In Fgure 5 a computed power curve o a 1 W wnd turbne, ung a developed procedure, whch tae nto conderaton the above decrbed parameter n n[t/mn] Fgure 6-Approxmaton o generator power curve (, and the load (U b. Ung the ame output charactertc o the turbne a n Fgure 5, and the approxmated optmal curve (8 reult the graphc repreentaton n Fgure 6. The number o par o pole o the generator, p, related to the maxmum rotatonal peed o the generator n normal operaton. For buldng the generator, the magnetc materal are the ame a or uual electrc machne. The magnetc loe are low and ecency ept hgh when the output requency not to much beyond 5Hz.

4 p = 3 / n max (9 Accordng to the method, the voltage actor : = U b /( d n (1 I the wndng retance R much lower than the ynchronou mpedance X, the coecent : 1 ( nmax n = (11 nmax Pmaxd 1 3 The pole magnetc lux : Φ = (1 p w The nner dameter o the generator gven by (13:,3 p 6 D = (13 π Bδ A nn n whch: D nner dameter; B δ ar gap magnetc eld denty; A-total current; n N -rated turnng peed(uually n N =n max. w The length o the magnetc core can be computed ung the nown expreon: l p Φ = π D B δ C. Inluence o the wndng (14 Once the overall dmenon, D and l, o the generator, the number o turn o the wndng, w, play a crucal role or reachng the optmal power curve (8, decrbed n Fgure 6. The relatonhp between ynchronou mpedance X and the number o turn o the wndng, w, : X = r w n (15 In whch r a contant dependng only on the geometry o the generator. I the geometry o the cro ecton o the generator etablhed, together wth the length o the magnetc core, then the retance o one phae wndng : ( l + l R( w = ρ w = R w (16 ς p c ρ-copper retvty; l -length o ront ecton o the col; c -area o the tator lot; ς-ll actor o the lot. Gven the cro ecton o the generator, R a contant or the generator, havng the meanng o the retance o a col wth one turn. Alo, the phae retance o the wndng depend on quare o the number o turn (16. Baed on equvalent crcut n Fgure 4, the power delvered to the battery : U b, b w n e P( w,n = U b br w Rb Rcab ( b ( w n e (17 b -1 or hal brdge recter or or ull brdge. Power [w] In Fgure 7 the reult o the computaton o output power o a 3W wnd generator n relaton to the wndng (no o turn and rotatonal peed. It obvou that t an well dened value o w or whch the output power optmum. 5 Expermental wor A. The problem The method decrbed wa ued or degnng a 1W, 3 blade wnd turbne, wth rated wnd peed 9,5 m/. The wnd turbne nject the energy n a 4Ah, 4V battery. The output charactertc o the wnd turbne hould t the lnear approxmaton n Fgure 6. Input data or the degn method: #o turn/wndng Rated power: 1W Rated wnd peed: 9,5m/ n t/mn Input power curve (lnear aprox.: P = 3, 57 ( n n Battery voltage: 4V Recter: 3 phae ull brdge rotatonal peed [turn/mn] Fgure 7-Output power v. rotatonal peed and #turn/wndng

5 B. The reult The geometry o the generator (ee Fgure 8 : Inner dameter: 157mm Outer dameter: 3mm Magnetc core length: 65mm Number o par pole: 8 Rare earth permanent magnet(n7h 16 The Fgure 11 the meaured nput mechancal charactertc o the generator wth load 4V battery. In the rt regon, at low turnng peed, the meaured curve tart earler than the lnear approxmaton. The tartng turnng peed n, depend on tate o charge o the battery. At hgh turnng peed the meaured curve become aturated and devate rom the approxmated traght lne. In Fgure 8 the cro ecton o the generator. Both tator and rotor are made o #,5mm lcon ron ung laer cuttng machne. For prototypng t cheaper to mae the rotor n the ame tme wth tator heet. P[W] 1 8 Lnear aprox n[t/mn] Fgure 11-Meaured output power curve o 1W generator 6 Concluon Fgure 8-Magnetc heet In Fgure 1 t the expermental layout or meaurng the power curve o the generator. The generator wa teted ung 1 battere V, 4Ah een n Fgure 9. Fgure 9-Battere ued a load The developed method or degnng PM ynchronou generator or low power drect drve wnd turbne baed on the dea that the output mechancal charactertc o the turbne and the load hould be matched by the electrc generator. Th concept come rom the act that the turbne and the load (battery are manly condtoned by the envronment (wnd and load and the generator the lnng pece between thoe two component that hould play the role o trmmng. The method gve arly good reult or mot o the operatng range o the generator. Future renement o the method could brng more accuracy n the reult. Wnd tunnel tet on turbne o derent ze could contrbute to the valdaton o the concept. Reerence [1]R.Gach, J.Twele, Wnd Power Plant, Solarprax, Jame&Jame-London []G.L Johnon,Wnd Energy Sytem, Englewood lm NJ 1985 [3]C. Bala, Man electrce, Bucuret, 1979-n Romanan Fgure 1-Expermental layout