Tests on a Single Phase Transformer

Transcription

1 Hong Kong nstitut of ational Education (Tsing Yi) Dpartmnt of Enginring Elctrical Enginring Principls Laboratory Sht: EEE3405/LAB03 Studnt nam: Cours / Yar: Dat: Tsts on a Singl Phas Transformr Objctivs To dtrmin th paramtrs of th quivalnt circuit, fficincy and voltag rgulation of th givn transformr by opn circuit tst and short circuit tst. Apparatus 1 1kA, 300:150 singl-phas transformr 1 1kA, 0/ 0 to 50 variac (China modl LTDGC) 1 lctronic wattmtr (Fdback modl EW604) 1 ammtr 0/5A (BEW modl BP-80) 1 DMMs Componnts Connction wirs EEE3405/LAB03 Pag 1 of 8

2 PART A: BACKGROUND Transformr is usd to stp-up or stp-down voltags and may b dsignd in a numbr of diffrnt forms for various functions. t is thus commonly usd in transmission and distribution systms. A singl-phas transformr (with th xcption of th auto-transformr, which commonly calld variac) consists of two windings, primary and scondary, lctrically insulatd from ach othr but wound on a common laminatd cor. To minimiz magntic lakag, primary and scondary windings ar wound clos to ach othr on both limbs of th iron cor, forming ithr a sandwich or a concntric arrangmnt. Th transformr is a highly fficint machin, with xtrmly small losss. Thrfor, it can b takn that: 1 1 N N 1 To dtrmin th fficincy of th givn transformr, th mthods commonly usd ar th Opn circuit tst and Short circuit tst. Opn Circuit Tst EEE3405/LAB03 Pag of 8

3 Th connction diagram and th quivalnt circuit for th opn circuit tst ar shown in Figur 1 and Figur rspctivly. Opn circuit tst is usd to find th magntizing branch of th quivalnt circuit of th givn transformr. For thn supply voltag supply currnt Supply powr P th powr factor of th opn circuit tst is : cos φ P and p m cosφ sinφ Hnc, R p (magntizing rsistanc accountd for iron loss) p and X m (magntizing rsistanc) m EEE3405/LAB03 Pag 3 of 8

4 Short Circuit Tst Th connction diagram and th quivalnt circuit for th short circuit tst ar shown in Figur 3 and Figur 4 rspctivly. Short circuit tst is usd to find th lakag branch of th quivalnt circuit of th givn transformr. For thn supply voltag supply currnt Supply powr P th powr factor of th opn circuit tst is : cos φ P Hnc Z R X P Z R whr Z is quivalnt lakag impdanc; R is lakag branch rsistanc accountd for coppr loss and X is lakag branch ractanc. EEE3405/LAB03 Pag 4 of 8

5 Aftr w carrid out th opn circuit tst and short circuit tst of th givn transformr, w can find out th paramtrs of th quivalnt circuit, which is Th voltag rgulation of th givn transformr can also b known at any givn valu of powr factor, lagging or lading.. R. 1 ( pr unit valu ) or 1( R cosφ ± X sinφ). R. ( pr unit valu) whr + for lagging powr factor - for lading powr factor φ is th powr factor angl of th load Th fficincy of th givn transformr is Efficincy Powr output Powr input 1 - ron loss + Coppr loss Powr output + ron loss + Coppr loss EEE3405/LAB03 Pag 5 of 8

6 PART B: PROCEDURE 1. Connct th circuit as shown in Figur 1.. Connct th trminals for th 150 at th primary sid of th transformr undr tst (stp up connction). Do not switch on th powr supply. 3. Chck th circuit connction at last by two group mmbrs. 4. Ask th tchnician to chck th circuit 5. St th output of th variac to zro bfor switching on th supply. 6. ncras th output of th variac to 70 and tak th wattmtr, ammtr and voltag radings. 7. Rpat stp (6) at an intrval of 10 ach up to 150 with all mtr radings bing takn. nput supply voltag () Ammtr (A) oltmtr () Wattmtr (W) Dcras th input voltag of th variac to zro and switch off th powr supply. 9. Connct th circuit as shown in Figur 3 (stp down connction). Do not switch on th powr supply. 10 Short circuit of th scondary trminals of th transformr will caus a larg currnt flow in th short circuit link, so nsur th short circuit link is installd scurly. 11 Chck th circuit connction at last by two group mmbrs. 1 Ask th tchnician to chck th circuit. EEE3405/LAB03 Pag 6 of 8

7 13 St th output of th variac to zro bfor switching on th supply. 14 ncras th output voltag at th variac slowly until th full-load currnt is sn at th ammtr. Tak th radings of th wattmtr, ammtr and voltmtr immdiatly. Wattmtr Ammtr - oltmtr 15 Dcras th input voltag of th variac slowly to zro and switch off th powr supply. PART C: Calculation & Diussion 1. From th opn circuit tst rsults, calculat th paramtrs p, m, R p and X m, at ratd voltag... From th short circuit tst rsults, calculat th paramtrs R and X, rfrrd to th high voltag sid or low voltag sid. EEE3405/LAB03 Pag 7 of 8

8 3. Plot th graph of and P against th applid voltag. and commnt it EEE3405/LAB03 Pag 8 of 8