▁is ▁slower ▁than ▁the ▁equivalent ▁rotation ▁speed ▁of ▁the ▁elect rical ▁network : ▁typical ▁rotation ▁spe eds ▁for ▁wind ▁gener ators ▁are ▁ 5 – 2 0 ▁r pm ▁while ▁a ▁directly ▁connected ▁machine ▁will ▁have ▁an ▁elect rical ▁speed ▁between ▁ 7 5 0 ▁and ▁ 3 6 0 0 ▁r pm . ▁Therefore , ▁a ▁g ear box ▁is ▁inserted ▁between ▁the ▁rot or ▁hub ▁and ▁the ▁generator . ▁This ▁also ▁reduces ▁the ▁generator ▁cost ▁and ▁weight . ▁Com mer cial ▁size ▁gener ators ▁have ▁a ▁rot or ▁carrying ▁a ▁field ▁w inding ▁so ▁that ▁a ▁rot ating ▁magnetic ▁field ▁is ▁produced ▁inside ▁a ▁set ▁of ▁wind ings ▁called ▁the ▁stat or . ▁While ▁the ▁rot ating ▁field ▁w inding ▁cons umes ▁a ▁fraction ▁of ▁a ▁percent ▁of ▁the ▁generator ▁output , ▁adjust ment ▁of ▁the ▁field ▁current ▁allows ▁good ▁control ▁over ▁the ▁generator ▁output ▁voltage . ▁ ▁Old er ▁style ▁wind ▁gener ators ▁rotate ▁at ▁a ▁constant ▁speed , ▁to ▁match ▁power ▁line ▁frequency , ▁which ▁allowed ▁the ▁use ▁of ▁less ▁cost ly ▁induction ▁gener ators . ▁Ne wer ▁wind ▁tur b ines ▁often ▁turn ▁at ▁whatever ▁speed ▁generates ▁electric ity ▁most ▁efficiently . ▁The ▁varying ▁output ▁frequency ▁and ▁voltage ▁can ▁be ▁matched ▁to ▁the ▁fixed ▁values ▁of ▁the ▁grid ▁using ▁multiple ▁techn ologies ▁such ▁as ▁doub ly ▁fed ▁induction ▁gener ators ▁or ▁full - effect ▁conver ters ▁where ▁the ▁variable ▁frequency ▁current ▁produced ▁is ▁converted ▁to ▁DC ▁and ▁then ▁back ▁to ▁AC . ▁Although ▁such ▁alternatives ▁require ▁cost ly