變壓(ya)(ya)(ya)器由鐵(tie)(tie)芯（或磁(ci)芯）和線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈組(zu)成，線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈有兩個(ge)(ge)或兩個(ge)(ge)以(yi)上(shang)的(de)(de)(de)(de)(de)繞組(zu)，其中(zhong)接(jie)電源的(de)(de)(de)(de)(de)繞組(zu)叫初(chu)級(ji)(ji)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈，其余的(de)(de)(de)(de)(de)繞組(zu)叫次(ci)級(ji)(ji)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈。它可以(yi)變換交流(liu)電壓(ya)(ya)(ya)、電流(liu)和阻(zu)抗。簡單的(de)(de)(de)(de)(de)鐵(tie)(tie)心(xin)(xin)(xin)變壓(ya)(ya)(ya)器由一個(ge)(ge)軟磁(ci)材料做成的(de)(de)(de)(de)(de)鐵(tie)(tie)心(xin)(xin)(xin)及套在(zai)鐵(tie)(tie)心(xin)(xin)(xin)上(shang)的(de)(de)(de)(de)(de)兩個(ge)(ge)匝數不等的(de)(de)(de)(de)(de)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈構成，鐵(tie)(tie)心(xin)(xin)(xin)的(de)(de)(de)(de)(de)作用是加強(qiang)兩個(ge)(ge)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈間的(de)(de)(de)(de)(de)磁(ci)耦合。為(wei)了(le)減少鐵(tie)(tie)內(nei)渦流(liu)和磁(ci)滯損(sun)耗，鐵(tie)(tie)心(xin)(xin)(xin)由涂(tu)漆(qi)的(de)(de)(de)(de)(de)硅(gui)鋼片(pian)疊壓(ya)(ya)(ya)而成;兩個(ge)(ge)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈之(zhi)間沒(mei)有電的(de)(de)(de)(de)(de)聯系(xi)，線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈由絕緣銅線(xian)(xian)(xian)(xian)(xian)(xian)(xian)（或鋁線(xian)(xian)(xian)(xian)(xian)(xian)(xian)）繞成。一個(ge)(ge)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈接(jie)交流(liu)電源稱為(wei)初(chu)級(ji)(ji)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈（或原(yuan)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈），另(ling)一個(ge)(ge)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈接(jie)用電器稱為(wei)次(ci)級(ji)(ji)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈（或副線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈）。實際的(de)(de)(de)(de)(de)變壓(ya)(ya)(ya)器是很(hen)復雜的(de)(de)(de)(de)(de)，不可避(bi)免地存在(zai)銅損(sun)（線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈電阻(zu)發熱(re)）、鐵(tie)(tie)損(sun)（鐵(tie)(tie)心(xin)(xin)(xin)發熱(re)）和漏(lou)磁(ci)（經空氣閉(bi)合的(de)(de)(de)(de)(de)磁(ci)感(gan)應線(xian)(xian)(xian)(xian)(xian)(xian)(xian)）等，為(wei)了(le)簡化討論這(zhe)里只(zhi)介紹理想(xiang)(xiang)變壓(ya)(ya)(ya)器。理想(xiang)(xiang)變壓(ya)(ya)(ya)器成立(li)的(de)(de)(de)(de)(de)條(tiao)件是：忽(hu)(hu)略漏(lou)磁(ci)通，忽(hu)(hu)略原(yuan)、副線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈的(de)(de)(de)(de)(de)電阻(zu)，忽(hu)(hu)略鐵(tie)(tie)心(xin)(xin)(xin)的(de)(de)(de)(de)(de)損(sun)耗，忽(hu)(hu)略空載電流(liu)（副線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈開路原(yuan)線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈中(zhong)的(de)(de)(de)(de)(de)電流(liu)）。例如電力變壓(ya)(ya)(ya)器在(zai)滿(man)載運行時（副線(xian)(xian)(xian)(xian)(xian)(xian)(xian)圈輸出額(e)定(ding)功率）即接(jie)近理想(xiang)(xiang)變壓(ya)(ya)(ya)器情況。