▁ ▁G ear box ▁ ▁In ▁conventional ▁wind ▁tur b ines , ▁the ▁bl ades ▁spin ▁a ▁sh aft ▁that ▁is ▁connected ▁through ▁a ▁g ear box ▁to ▁the ▁generator . ▁The ▁g ear box ▁converts ▁the ▁turning ▁speed ▁of ▁the ▁bl ades ▁ 1 5 ▁to ▁ 2 0 ▁rot ations ▁per ▁minute ▁for ▁a ▁large , ▁one - m eg aw att ▁tur bine ▁into ▁the ▁faster ▁ 1 , 8 0 0 ▁revolution s ▁per ▁minute ▁that ▁the ▁generator ▁needs ▁to ▁generate ▁electric ity . ▁Anal yst s ▁from ▁Global Data ▁estimate ▁that ▁g ear box ▁market ▁grows ▁from ▁$ 3 . 2 bn ▁in ▁ 2 0 0 6 ▁to ▁$ 6 . 9 bn ▁in ▁ 2 0 1 1 , ▁and ▁to ▁$ 8 . 1 bn ▁by ▁ 2 0 2 0 . ▁Market ▁leaders ▁were ▁W iner gy ▁in ▁ 2 0 1 1 . ▁The ▁use ▁of ▁magnetic ▁g ear box es ▁has ▁also ▁been ▁expl ored ▁as ▁a ▁way ▁of ▁reducing ▁wind ▁tur bine ▁maintenance ▁costs . ▁ ▁Gener ator ▁▁ ▁For ▁large , ▁commercial ▁size ▁horizontal - axis ▁wind ▁tur b ines , ▁the ▁elect rical ▁generator ▁is ▁mounted ▁in ▁a ▁nac elle ▁at ▁the ▁top ▁of ▁a ▁tower , ▁behind ▁the ▁hub ▁of ▁the ▁tur bine ▁rot or . ▁Typ ically ▁wind ▁tur b ines ▁generate ▁electric ity ▁through ▁asynchronous ▁machines ▁that ▁are ▁directly ▁connected ▁with ▁the ▁electric ity ▁grid . ▁Usually ▁the ▁rot ational ▁speed ▁of ▁the ▁wind ▁tur bine