Caracterización del sector Acuícola

a l G enerator. T he g en e rato r used is a perm a n en t m agnet, single phase, synchronous A C m ac h in e o f disc geom etry . Such a m a c h in e is also ca lled an "axial flux' generator, as th e d irection o f the m agnetic flux is p arallel to the shaft. A sketch o f a typical axial flux m ach ine is show n in figu re 5.4, ta k e n from reference [58] w h ere an account o f recent dev elop m ents in axial flux generato rs is given. T h e generator in the p resent w ork w a s d ev elo ped at the U niv ersity o f W a rw ick . It is now being p ro du ced by M arlec E ngineering C o . Ltd, fo r use w ith the R u tla n d F urlm atic 1800 w in d turbine m anufactured by th e sam e com pany. It w as gran ted to u s by the d irec tor o f the above m entioned co m p an y . T h e rotor m agnet has 12 poles an d com prises tw o halves that can b e separated (see fig ure 5.4). W hen the tw o halves are c lo se together, they are held in place by m agnetic forces. The stator has a set o f tw elv e coils conn ected in series with

140 lo o p s each. They are m ade o f copper w ire and they do not hav e iro n cores for en h an cin g th e m agnetic flux through them. T h ey are however em bedded in epo xy resin and thu s th e stator is qu ite solid and strong. T h e stator is positioned betw een the two rotor h alv e s and tw o cable leads provide the electrical pow er to th e load. T he stator's rated c u rre n t is 2A (m anufacturer's data). T h e g en erato r is designed to pro vid e about 220 W o f ele ctrica l pow er at a rotational speed o f 6 0 0 rpm when connected to an appropriate load. Its co ld arm ature resistance is 25.7 Q . a t 2 0 ° C. The generator can be connected d irectly to th e prototype turbine, w ithout hav ing to use a gearbox.

b) D escription o f th e test rig, m o tor and torque transducer. T h e test rig to w hich th e generator w as attached com p rises a D C m otor conn ected to a torque tran sd u cer. T h e DC m otor w a s used fo r d riv in g th e generator. B oth its stator and rotor are electrom agnets. Its rotational speed c a n v ary from 0 to 3000 rpm depending on the po w e r fe d to the field and arm ature w indings. T h e m otor was w ired-up as a separately ex cited o n e during the tests. F or exciting the arm ature winding, a variac and a three p hase rectifier unit w as used. F or the fie ld w inding (w hich had lo w er pow er requ irem ents), an o ff-th e-sh e lf variac w ith a b uilt-in rectifier w as used. T h e m otor ro tational speed could be increased either b y increasing the arm ature cu rren t o r by d ec rea sin g th e field cu rren t. T h e m otor w a s linked to the generator v ia a torque transdu cer fo r m easuring the torque. T he tran sducer is a TG-2/B V ibro -m eter slipringless one. Its operating principle as explained in [59] is as follows: T h e centre o f the tran sd uc er shaft is reduced in diam eter (as c a n b e seen in figure 5.5) an d this portion is the m ea su rin g section. T w o flanges fixed o n e ith e r side o f this section h old an inductive co il sy stem and its cores. T he co ils are fix ed o n the larger d iam eter p art o f the shaft, w h ile th e ir cores are fixed o n the sm aller d iam ete r part. W hen a torque (eith er static or dyn am ic) is applied, th e sm aller diam eter p art o f the shaft is tw isted. H ence, th e relative

positio n betw een th e coils an d their co res changes and likew ise d o es th e co il inductance. T h ere are fo u r co ils conn ected in a fo u r arm inductive bridge arran gem en t. At no load, the bridge is b alanced. W hen a torque is applied, the bridge im balance voltage can be u sed fo r m easu rin g the torque. T he b ridge is supplied with p ow er v ia a ro tary transm itter. A n o th er rotary tran sm itter is used fo r picking-up the o utp ut signal. H ence, no sliprings are necessary. T h e torque transd ucer characteristics are:

•M inim um torque: 1.9 Nm. •M axim um torque: 38.0 Nm. •A ccuracy: 0 .2 5-0.5 % o f fsd

(full scale display). •M axim um speed: 22000 rpm.

T o g eth e r with the transducer, a V ibro-m eter 8 -M C A -l/A O m u lti-channel carrier freq u en cy system is used fo r providing the bridge input signal an d pick ing -u p the output. T h e 8 -M C A -l/A O system consists o f a po w er sup ply /o scillato r u n it and a carrier frequency am plifier. As show n in the b lock diagram o f figure 5.6, the oscillato r provides th e signal fo r th e torque tran sducer bridge excitation. T h e b rid ge im balance signal is pick ed -up by the am plifier. It then p asses through a b and-pass filter, a dem odulator, an a m p lifier and tw o low -pass filters, an d is finally directed to w ard s th e o u tp u t term inals. T h e output from th e torque m eter and a m p lifier w as read on a D C voltm eter. The output vo ltag e was in the range o f -2.5V to + 2.5 V , with the torque ra ng e vary in g according to the differen t sensitivity settings o f the "range" rotary sw itch o f the am p lifier. The range s elec ted for the test was "25", the sec o n d m ost sensitive, that d iv id e d the output signal by 5. T h e m ost sensitive setting (that did n't attenuate the signal) failed to prod uce repeatable re su lts and suffered from too m uch noise so it was not used. T h e a m p lifier needed at lea st h a lf an h o u r to heat-up before the tests could start. B efore each ru n , the bridge w as

po sition b etw een the coils an d th eir cores ch ang es an d lik ew ise does the coil inductance. T h ere are fo u r coils co nn ected in a fo u r arm inductive b ridge arrangem ent. A t no load, the b ridg e is balanced. W hen a torque is applied, the bridge im balance voltage can be used fo r m easu rin g th e torque. T he bridge is sup plied w ith p o w e r via a rotary transm itter. A n o th er ro tary tran sm itter is u sed fo r pick ing -u p the ou tp u t signal. Hence, n o sliprings are necessary. T h e to rq u e tran sduc er ch aracteristics are:

•M in im um torque: 1.9 Nm . •M axim um torque: 3 8 .0 Nm . •A ccuracy: 0.25 -0 .5 % o f fsd

(fu ll scale display). •M axim um speed: 220 00 rpm .

T o g eth e r with th e transd ucer, a V ib ro -m eter 8 -M C A -l/A O m ulti-channel carrier freq u en cy system is used fo r p rov idin g the bridge input signal an d picking-up the output. T h e 8 -M C A -l/A O system co n sists o f a po w e r su p p ly /osc illa tor unit and a carrier freq u en cy am plifier. As show n in the block diagram o f figure 5.6, the o scillator provides th e sig nal fo r the torque tran sduc er bridge excitation. T he b ridge im balance signal is pick e d -u p by the am plifier. It then passes through a band-p ass filter, a dem odulator, an am p lifier and tw o low -pass filters, an d is finally d irec ted to w ards the output term inals. T h e o u tp u t from the torque m e te r and am plifier w as re ad on a D C voltm eter. T h e output v o ltag e w as in th e range o f -2 .5 V to + 2.5V , w ith th e torq u e ra ng e varying according to th e d iffe ren t sensitivity settin gs o f the "range" rotary sw itch o f the am plifier. T he range s elec ted fo r the test w as "25”, th e secon d m ost sen sitive, th at d iv id ed th e output signal by 5. T h e m o st sensitive setting (th at didn't attenuate the signal) fa ile d to p roduce repeatable re su lts an d suffered from to o m uch noise so it w as not used. T h e am plifier needed at lea st h a lf an h ou r to heat-up b efore the tests co uld start. B efore each run, the bridge was

balanced at n o lo ad by adjustm ent o f a resistive and a capacitive potentiom eter. T he to rqu e transducer and am plifier w ere calibrated statically: One en d o f th e transdu cer sh aft w a s clam ped on the test rig fram e, the other being connected to a pulley. W eig h ts w ere hung from the pulley and fo r each w eight the am plifier output voltage w as recorded. There w ere tw o m easuring units on the am plifier and they w ere both calibrated. U n its 1 and 2 w ere on the rig h t and left hand side o f the am plifier respectively. T h e to rq ue direction w as clockw ise looking from the m otor towards the torque m eter an d w a s chosen to coincide with the wind turbine direction o f rotation du rin g the p ro to ty p e tests. The calibration curves o f both units are show n in figure 5.7. It is evident th at the linearity o f the transducer is excellent. Unit 2 w as chosen to be used, as a least squ are line could be fitted through the m easurem ent points with less scatter than unit 1. (S ee fig u re 5.7). T he least square line fitted for unit 2, show n in figure 5.7 is:

F = AV out (in K g)

w i t h : A = 10.1 Kg/V

T he torque co u ld th en b e calculated in Nm m ultiplying F with C L: Q = F C L

w ith C a co n v e rsio n factor, C=9.81 N/Kg and L the pulley torque arm , L=0.07m. R epeatability o f the voltage readings was check ed and it w as found that that the d ifference in v o ltag e betw een any tw o repeated readings fo r the sam e torque was ±0 .0 IV a t the m ost. T h is is co n sistent w ith the m anufacturer's data: The torque m eter accuracy is ± 0.5% o f fsd. In o u r ca se, fsd is 2.5V and hence the instrum ent accuracy is ±0.0125V . T he D C vo ltm eter sensitivity w as 0.005V , w hich corresponds to a torque o f 0.03N m , ie 6 3 tim es th e tran sd u c er m inim um torque.

c ) O th er instrum ents used. A Feedback E W 604 electrom agnetic w att

m eter w as used fo r m easuring the g enerator pow er ou tpu t. Its po w e r range lies between 250 m W and 10 kW . Its accuracy is better than 2.0 % o f fsd. T hree v ariable resistors w ere used as loads fo r th e generator, each fo r a different cu rren t range. T h ey con sisted o f w oun d coils. T h eir length co uld vary using a ru n n e r that disconnected th e desirable p ercen tag e o f resisto r length. T hese resistors were not purely ohm ic loads h o w ever, due to th eir co il-lik e shape (as it will be seen later). T h eir characteristics are:

•R esistor 1 : 0-1024 Q , Imtx = 1.0 A •R esistor 2 : 0 -6 1 4 Q , Imax = 1.7 A •R esistor 3 : 0-164 Q , Imw = 3.2 A

A tac hom e ter w as u sed fo r picking-up the rotational sp eed w hen attached to the m otor shaft. F o r reading the g en erator term inal voltage and current, a digital A C v o ltm e ter and an av o m e te r (respectiv ely) w ere used. A sketch o f the experim ental set-up is sh o w n in figure 5.8.