Generator model parameters estimation.

The resu lts o f the tests fo r evaluating the g en erato r m odel p aram eters are p re sen ted in the follow in g section.

• . ai O p en circuit test. T he o pen circuit characteristics o f the g en e rato r are show n in figure 5.9. It is evident th at EA increases linearly with 0) in creasing and eq uation 5 .1.2 holds. T h e slope K w as calcu lated from th e abo ve figure:

K - 0.294V /rpm S ince to (in rpm ) = (60/2 n) to (in rad/sec),

K = 2.81 V sec and since coe = (P/2) co = 6 co.

Ke = 0.4 6 8 V see.

b) S h o n circuit test. T h e short circuit test was then ca rrie d out. The cu rren t flo w ing thro u gh the shorted g en erato r w indings w as m easured for different v alu e s o f to. F o r e a ch value o f to, the cu rrent w as found to decrease w ith tim e, as the stato r w indings w e re heating-up. E no ug h tim e w as allo w ed fo r the cu rren t to stabilize b efo re readings w ere taken. A fter each m easu rem en t o f I, the g enerator w as brought to a halt an d a D C vo ltag e w as applied to its term inals fo r m easuring R A. In figure 5.1 0 the variatio n o f th e sh o rt circuit cu rrent (I) with o> is show n. T he current d o es not increase lin early with to in creasin g, d ue to the chang ing R A as ex plain ed in [4]. From equation 5 .1.4 , it can b e seen that i f the sy nchronous im pedance (Zg = [RA2 + toe2 L S2J,/2) had been constant, th en I(co) w ould have been linear. H ow ever, Z s increases since RA in creases with tem p eratu re. W ith I an d RA know n, L s can be calcu lated from equatio n 5 .1.5 . Ls w as estim ated fo r 11 v alues o f 0) ranging from 135 rpm to 285 rpm . N o re adings w ere tak en beyo nd 285 rpm , as the g enerator w as heating excessively. T h e fo llo w ing table co n ta in s th e calculated Ls values.

TABLE 1 0) (rpm ) 4 <H> 135 0.0465 149 0.0498 161 0.0475 179 0.0459 199 0.0489 204 0.0454 (m in) 216 0.0498 (m ax) 228 0.0458 237 0.0480 244 0.0483 285 0 .0470 L j = 0 .0475 H ( + 4 .8% - 4.5% )

T h e L s v alues d o n o t seem to follow any p artic u la r trend. They are scattered ab o u t the av e rag e value (sh o w n at the foot o f table 1). T h is valu e is com parable with that estim ated

fo r a s im ilar g enerator in (61 J. T he generator exam ined in [61] is a perm anen t m agnet synchronous one o f larg er size and ratings than th e one used here, an d its L s value was foun d to be 0.0 2 7 H, w hich is o f the sam e order o f m agnitude as the L s calcu lated here. T he scatter in the L§ v alues w as caused by the scatter in I and RA.

c) Estim ation o f arm ature resistance. RA was estim ated using the

technique described in section 5.1.1. However, the generator's o p erating conditions are ex pected to vary w idely d u rin g the system prototype test, and use o f a sin g le value o f RA w as not con sidered realistic. In order to m onitor the R A change, a therm o-couple needs to be in stalled close to the stator w indings, so that the stator tem perature is known. H ow ever, at the time th e generator test was carried out, a therm o-couple was not available an d the best alternative w as to m onitor the variation o f RA with I, assum ing that the am bien t tem perature would be the sam e (about 22° C ), so th at the generator w indings w o u ld alw ays settle at the sam e tem perature for a certain operating condition. T h e above assum ption w as thought to be reasonable, as the test was ca rrie d out indoors. T h e v ariation o f RA w ith I is show n in figure 5.11. The RA increase w ith I increasing is not linear, d u e to the no n -linear increase o f the g enerator tem perature. F o r currents up to 1.0 A , the generato r w as m ildly w arm , while for larger currents it heated-up rapidly. A 5th o rder p olynom ial w as found to approxim ate the RA(I) curve best. It is show n in fig u re 5.11.