IV
IV. u ng c 3pha s dng li 1pha
ng c 3pha c th lm vic li 1pha nh ng c 1pha khi dng t in m my ng
c c th t n 80% cng sut nh mc. Tuy nhin ngi ta thng p dng vi ng c
3pha cng sut nh di 2KW . Khi mi ng c cn phi chn cho 1 s v tr s t in
cho ph hp .
a. V nguyn tc chuyn i cc cun dy 3pha sang hot ng 1pha
in p nh mc trn cun dy khng i
Phi t 1 trong 2 cun dy pha thnh cun lm vic cun cn li thnh cun
khi ng
Tr s t in phi chn sao cho gc lch pha gia dng in cun lm vic v khi
ng t 900.
b. Cc s nguyn l chuyn i
Theo nguyn tc trn tu theo in p ngun v in p nh mc ca cun dy pha m
ta chn 1 trong 4 s sau:
S hnh 1 v hnh 3 p dng cho trng hp in p li UL =UphaC
S hnh 2 v hnh 4 p dng cho trng hp in p li UL =UdyC
V d : Mt ng c 3pha c nhn hiu (/Y 220/380v
Nu in p ngun cung cp cho ng c l 220v sau khi u thnh 1pha th ta
chn s hnh 1 v hnh 3
Nu in p ngun cung cp cho ng c l 380v sau khi u thnh 1pha th ta
chn s hnh 2 v hnh 4
Hnh 1 Hnh 2
Hnh 3
Hnh 4
c.Tnh chn tr s t in lm vic (t ngm) v t khi ng theo cng thc
Bc 1: Tnh in dung t in lm vic
Trong :
Ipha m l dng in nh mc ca ng c
UL l in p ngun 1pha m ng c s hot ng khi u 1pha
k l h s tnh ton ph thuc vo tng s u dy c th :
- S hnh 1 : k = 4800
- S hnh 2 : k = 2800
- S hnh 3 : k = 1600
- S hnh 4 : k = 2740
Bc 2: Tnh in p t in lm vic
UC ( 1.5 ULBc 3: Tnh in dung t in khi ng
Ck =(2(3)CLV
d. Tnh tr s t in theo kinh nghim
Vi nhng ng c chy li 220v th c 1kw phi c CLV= 65(F
V d : ng c 3pha 220/380v, 0,6kw u li chy 1pha 220v th phi dng t
in c in dung:
CLV = 65x 0,6 = 39 (F
Ck =(2(3)CLV = (78(117)(F
V d : Mt ng c 3pha cng sut 1kw in p 220/380v dng in 4.2/2.4A. Hy
u li s dng mng 1pha 220v.
Gii:
- Nu theo kinh nghim
CLV = 65 (F
Ck =(2(3)CLV = (130(195)(F
Hai t ny l t du c Uc ( 380v
Theo cng thc ta chn s
+ Vi hnh 1
Ck =(2(3)CLV = (184(276)(F
+ Vi hnh 3
Ck =(2(3)CLV = (62(93)(FV. Tnh ton chuyn i cc thng s khi qun
dy
1. Tnh ton thay i dy qun t dy nhm sang dy ng
Thng s c (Al = ?
WAl = ? kiu qun cch u
Khi chuyn sang dy ng
(Al > (Cu
WC khng thay i
2. Tnh ton thay i in p s dng
Thng s c (c = ? Wc = ?
- Thng s c (mi = ? Wmi = ?
Khi tnh ton chuyn sang in p mi s vng dy thay i theo cn kiu dng,
cch qun, cch u cc cun dy vn d nh c
a. Cch tnh theo h s quy i
Nu gi kq l h s quy i in p th
Trong : Uc l in p s dng c ; Um l in p s dng mi
Tnh s vng dy qun mi
Wm = kq . Wc
Tnh ng knh dy qun
(c ng knh dy qun c (mm); (m ng knh dy qun mi (mm)
V d : Qut bn c Uc =127V cun dy c 4 bi mi bi 380 vng
(c = 0,35mm tnh cho chy li in 220V
Gii
H s quy i
=
S vng cho mt bi dy mi
Wm = kq . Wc =1,732 .380 = 658 vng
Tnh ng knh dy qun
b. Cch tnh theo bng tra
Tnh ng knh dy
Tra bng t (c c Sc sau tnh Sm
Sau tra bng c ng knh dy mi
- Tnh s vng dy qun
V d trn :
(c = 0.35Wc = 380 vngUc =1 27 v
Tra bng Sc = 0.0962
Tra bng (m = 0.27mm
vng
Ck
K
A
U~
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Y
Cn
Cn
Y
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Cn
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U~
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Ck
Cn
Y
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U~
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Ck
Cn
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U~
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Ck
_1151938145.unknown
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