PHNG PHP IU CH VECTOR KHNG GIANPhng php iu ch vector khng gian xut pht t cc ng dng ca vector khng gian trong my in xoay chiu, sau c m rng trin khai trong cc h thng in ba pha. y l phng php m gii thut ch yu da vo k thut s v l mt trong cc phng php c s dng ph bin nht hin nay trong lnh vc in t cng sut lin quan n iu khin cc i lng xoay chiu ba pha nh iu khin truyn ng in xoay chiu, iu khin cc mch lc tch cc, iu khin cc thit b cng sut trn h thng truyn ti in. 1. Khi nim vector khng gian v php bin hnh vector khng gian: Cho i lng ba pha va, vb, vc cn bng, tc tha mn h thc: va + vb + vc = 0 Php bin hnh t cc i lng ba pha va, vb, vc sang i lng vector theo h thc:

v s = k (v a + a.vb + a .vc )

2

trong :1 3 a = e j 2 = + j 2 2

c gi l php bin hnh vector khng gian v i lng vector v gi l vector khng gian ca i lng ba pha. Hng s k c th chn vi cc gi tr khc nhau.Vi k = 2/3, php bin hnh khng bo ton cng v vi k = 2 / 3 php bin hnh bo ton cng sut. Vd: cc i lng 3 pha dng sin: va = Vmsint vb = Vmsin(tvc = Vmsin(tVector khng gian theo nh ngha:

2 ) 3 4 ) 3

vs =

2 2 2 4 Vm sin( x 0 ) + a sin( x 0 ) + a Vm sin( x 0 )] 3 3 30)

[

= Vm( cos( x 0 ) + j sin( x 0 ) ) = Vm e j(x -

Nh vy, trong h to vung gc - , vector khng gian vs c bin Trang 1

Vm bt u t v tr Vm e j s quay xung quanh trc ta vi tn s gc.0

2. Php bin hnh vector khng gian ngc: Vi h s k = 2/3, php bin hnh ca vector khng gian ngc v s cho ta thu c i lng ba pha t vector khng gian nh sau:

va = Re{ v } vb = Re{ a . v } = 2

1 3 . Re{ v }+ . Im{ v } 2 2 1

vc = Re{ a . v } = - . Re{ v }. Im{ v } 2 2 T hnh 4.1 v cc h thc va dn gii, d suy ra kt qu ca php bin hnh vector khng gian ngc chnh l hnh chiu ca i lng vector v s ln h trc ta lch pha 1200 trong mt phng vector phc.2V j Vector v c biu din di dng tng qut : v = d e 3 3 k

3

Trang 2

2.1 Xt ng c xoay chiu ba pha:

Mi pha ca ng c c hai trng thi:1 (ni vi cc + ca Vd), 0 (ni vi cc - ca Vd). Do c ba pha (ba cp van bn dn) nn s c 8 kh nng ni cc pha ca ng c vi Vd, c m t trong bng:

0 Pha u Pha v Pha w 0 0 0

1 1 0 0

2 1 1 0

3 0 1 0

4 0 1 1

5 0 0 1

6 1 0 1

7 1 1 1

Cc kh nng ni pha ng c vi Vd :+ Vsv Vd cun dy pha u H4.3 Vsu cun dy pha v

vs

cun dy pha w H4.4

Vsw = -2 Vd/3 Vsu = Vsv = 2Vd/3

v s = 2 Vd/3

Trang 3

Xt kh nng th 2 ca bng 1 vi s ni dy hnh 4.3 ta d dng tnh c in p ri trn tng cun dy pha u, v hoc w. Ngoi ra, khi a vo b tr hnh hc ca ba cun dy pha trn mt phng hnh 4.4 th kh nng th 2 s tng ng vi trng hp p t ln ba cun pha vector in p v s vi module 2Vd/3. in p thc s ri trn tng pha chnh l hnh chiu ca v s ln trc ca tng cun dy. Cc vector in p ca cc kh nng cn li u c xy dng tng t nh vy. Cc vector chun c k hiu v 0 , v 1 v 7 nh s th t ca bng. Trong :

v 0 : c ba cun dy pha ni vi cc - v 7 : c ba cun dy pha ni vi cc +

2.2 Phng php iu ch vector khng gian:

Q1, Q2, Q3, Q4 : cc gc phn t S1, S2 S6 : cc gc phn su Hnh 4.5 Biu din v tr ca cc vector chun trong h ta ,. Module ca cc vector chun lun c gi tr 2Vd/3. tng ca phng php iu ch vector khng gian l to nn s dch chuyn lin tc ca vector khng gian tng ng trn qu o ng trn ca vector in p b nghch lu. Vi s dch chuyn u n ca vector khng gian trn qu o trn, cc sng hi bc cao c loi b, quan h gia tn hiu iu khin v bin in p tr nn tuyn tnh. Vector tng ng y chnh l vector trung bnh trong mt thi gian ly mu Tc ca qu trnh iu khin b nghch lu p.

Trang 4

Xt mt gc phn su th nht (S1) hnh 4.6 ca hnh lc gic to thnh bi nh ca 3 vector v 1 , v 2 , v 0 ( v 7 ). Gi s rng trong thi gian ly mu Tc, cho tc dng v 1 trong thi gian T1, v 2 trong thi gian T2 v v 0 trong thi gian cn li (Tc-T1-T2). Vector tng ng c tnh bng vector trung bnh bi chui tc ng lin tip nu trn:

vs =

1 v 1 dt + Tc 0T1 T1

T 1+T 2 T1

v 2 dt +

Tc T 1+T 2

v

0

dt

1 2Vd v s = dt + Tc 0 3

T 1+T 2

T1

Tc 2Vd j / 3 e dt + 0dt 3 T 1+T 2

2Vd T1 2Vd j / 3 T 2 = v1 + 2 v 2 v s = 3 Tc + 3 e 1 Tc 2Vd j / 3 T1 T2 2Vd e vi 1 = ; 2 = ; v1 = ; v2 = Tc Tc 3 3

Nu ton b chu k l chu k c ch, c php dng thc hin vector, khi ny module ti a cng khng th vt qua 2Vd/3. Do vy ta c cng thc sau:

v s max = v1 = ..... = v6 = 2Vd/3

t: v p =t1 v 1 = v 1 ; v t = t2 v 2 = Tc Tc v 2 Suy ra: v s = t1 v 1 + t2 v 2 = v p + v t

T1

T2

Trang 5

T1 =

vp

v s max

Tc

T2 =

vt

v s max

Tc

Mt cch tng qut hn, vector trung bnh c vit di dng :

T1 T 2 Tc (T 1 + T 2) v s = v p + v t + v 0 = Tc v 1 + Tc v 2 + v0 ( v7 ) Tc

T hnh 4.6: vs sin(600) = v p sin(600)

vp =

2 v s sin(600- ) (a) 3 2 v s sin( ) (b) 3

v s sin(600) = vt sin(600)

vt =

Gi N l s xung ct trong mt chu k ca in p t Ts, fx : tn s ct xung (fc) fx = N fs

Tx =

1 Nf s

Da vo nguyn tc iu khin U/f = const vs

fs

= u max f max

vs

u max

=

fs f max

u max : bin thnh phn c bn in p pha ti u max = Vd 3 vs

=

fs f max 3

Vd

2 2 3 vs T v s sin(600- ) T1 = (a) 1 v1 = Tc sin(600- ) Tc 3 3 2Vd T1 = fs f max 1 1 sin(600- ) T1 = sin(600- )(d) Nf s Nf max T2 = 1 (e) Nf max

(b)

2 T2 v2 = v s sin Ts 3

T7 = T0 = Ts + (T1 + T2) Do ta ch xt gc phn su th nht nn : 0< T : chu k cho php.

T max =

2 T cos(30- ) 3

- Khi cc kha bn dn ch t c trng thi ng ngt n nh sau mt thi gian ng ngt nht nh no , cho nn T0 v T7 khng c php b hn thi gian ng ngt ca loi kha bn dn m bin tn s dng. - Vic thc hin trnh t to vector in p theo ng l thuyt nh trn s khng thun li cho vic xy dng phn cng p ng c nhu cu cht ch v ng b, c bit l cc h thng nhiu vi x l. khc phc nhc im trn ta thay i trnh t thc hin vector in p, c th l: v 0 v1 v 2 v7 v7 v 2 v1 v 0

- trnh ngn mch trong nghch lu (kha trn v kha di ca mt nhnh ng thi cng ng), khi chuyn trng thi gia hai kha, ta phi lm tr xung ng kha mt thi gian TD bng tng thi gian ngt cng thm mt on thi gian an ton (TD =1,21,3Toff)

Trang 10

- Bng phng php iu ch vector khng gian ta c th to ra c vector in p v s quay trn vi vn tc gc= 2fs. Tn s fs c gi tr thay i, phng php ny gi l phng php iu ch khng ng b. Tnh khng ng b ny s lm cho mch stator xut hin thm nhiu hi bc cao gy ra tn hao ph v moment lc k sinh. iu ny s nghim trng hn khi fx/fs cng b i. khc phc nhng nhc im ny ta iu ch theo nguyn tc fx/fs =const. Phng php ny c gi l phng php iu ch ng b. *Nu vector trung bnh c iu khim theo qu o ng trn th vector trung bnh s cng pha v c module t l vi vector yu cu. ng trn n tip lc gic l qu o ca vector khng gian ca b nghch lu p 2 bc c th t c trong phm vi iu khin tuyn tnh. Bn knh ng trn ny chnh bng bin thnh phn c bn in p ti Ut1. Ut1 =Vd 3

= Vd cos30 = 0.907 3 2Vd

2 3

Ch s iu ch : m =

Vd

2.2.2 Tnh ton thit k vector khng gian: N l s lng xung ct trong mt chu k Ts. Do tnh cht i xng ca cc th nng pha trn khng gian vector nn N ch c th nhn cc gi tr sau: N=9+6n n=0,1,2,3 Khi N c xc nh, vic thay i fs c thc hin thng qua thay i fx. Khi , trnh fx tr nn qu cao khi tng fs gi tr N phi c thay i ty theo fs. Trong phm vi thit k ti tn s fs c chia thnh 5 khong (010), (1020), (2030), (3045), (4550). Tng ng vi nm khong tn s fs s c nm gi tr N khc nhau. Ngoi vic s dng quy lut iu khin U/f=const. gim tn hao ph v monent lc k sinh cn phi s dng phng php iu ch ng b fx/fs =const. T cc gi tr N, fmax=50Hz, ta tnh c T1 , T2 tng ng vi cc v tr ca v s trong khng gian vector.

fx =010Hz, chn N=60 fx =1020Hz, chn N=48 fx =2030Hz, chn N=36 fx =4550Hz, chn N=12

Trang 11

Bng gi tr Tc (Tc = 1/Nf)f 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 fx0.2 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8 5 5.2 5.4 5.6 5.8 6 6.2 6.4 6.6 6.8 7 7.2 7.4 7.6 7.8 8 8.2 8.4 8.6 8.8 9 9.2 9.4 9.6 9.8 10 N 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 Tc 83333 41667 27778 20883 16667 13889 11905 10417 9259 8333 7576 6944 6410 5952 5556 5208 4902 4630 4386 4167 3968 3788 3623 3472 3333 3205 3086 2976 2874 2778 2688 2604 2525 2451 2381 2315 2252 2193 2137 2083 2033 1984 1938 1894 1852 1812 1773 1736 1701 1667

51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100

10.2 10.4 10.6 10.8 11 11.2 11.4 11.6 11.8 12 12.2 12.4 12.6 12.8 13 13.2 13.4 13.6 13.8 14 14.2 14.4 14.6 14.8 15 15.2 15.4 15.6 15.8 16 16.2 16.4 16.6 16.8 17 17.2 17.4 17.6 17.8 18 18.2 18.4 18.6 18.8 19 19.2 19.4 19.6 19.8 20

60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60

2042 2003 1965 1929 1894 1860 1827 1796 1766 1736 1708 1680 1653 1628 1603 1578 1555 1532 1510 1488 1467 1447 1427 1408 1389 1371 1353 1335 1319 1302 1286 1270 1255 1240 1225 1211 1197 1184 1170 1157 1145 1132 1120 1108 1096 1085 1074 1063 1052 1042

Trang 12