Lub transformer yog ib yam khoom siv uas siv lub hauv paus ntsiab lus ntawm electromagnetic induction los hloov alternating voltage. Nws yog tsim los ntawm thawj kauj, theem nrab kauj thiab ib tug tub ntxhais (magnetic core). Nws lub luag haujlwm tseem ceeb yog kom ua tiav qhov kev hloov pauv hluav taws xob, ua kom tau raws li qhov xav tau ntawm qhov tshwj xeeb ntawm cov khoom siv hluav taws xob sib txawv, thiab tseem ua lub luag haujlwm xws li kev sib cais hluav taws xob, hluav taws xob stabilization, thiab impedance txuam.
Los ntawm txoj kev xav, thaum muaj qhov hloov pauv tam sim no dhau los ntawm cov kauj ruam tseem ceeb, qhov sib txawv sib nqus flux yuav raug tsim tawm hauv cov tub ntxhais hlau. Qhov sib txawv sib nqus flux no ib txhij dhau los ntawm ob qho tib si thawj thiab theem nrab coils. Raws li txoj cai ntawm electromagnetic induction, ib qho induced electromotive quab yuam yuav raug generated nyob rau hauv txhua ntawm ob coils. Yog hais tias tus naj npawb ntawm cov lem nyob rau hauv lub thib ob kauj yog txawv los ntawm qhov uas nyob rau hauv lub thawj kauj, cov zis voltage ntawm lub thib ob kauj yuav txawv ntawm qhov input voltage ntawm thawj kauj. Qhov ratio ntawm lem txiav txim qhov ratio ntawm voltage transformation. Piv txwv li, yog hais tias tus naj npawb ntawm lem nyob rau hauv thawj kauj yog 100 thiab tus naj npawb ntawm lem nyob rau hauv lub thib ob kauj yog 200, thaum lub input voltage ntawm thawj kauj yog 220V, theoretical output voltage ntawm lub thib ob kauj yuav yog 440V.