Transformers do not change the frequency of electricity; in many regions with 100–120 V, electricity is supplied at 60 Hz, and 210–240 V regions tend to use 50 Hz.
tɹænsfˈoːɹmɚz duːnˌɑːt tʃˈeɪndʒ ðə fɹˈiːkwənsi ʌv ᵻlɛktɹˈɪsᵻɾi; ɪn mˈɛni ɹˈiːdʒənz wɪð wˈʌnhˈʌndɹɪd wˈʌnhˈʌndɹɪd twˈɛnti vˈiː, ᵻlɛktɹˈɪsᵻɾi ɪz səplˈaɪd æt sˈɪksti ˌeɪtʃzˈiː, ænd tˈuːhˈʌndɹɪd tˈɛn tˈuːhˈʌndɹɪd fˈoːɹɾi vˈiː ɹˈiːdʒənz tˈɛnd tə jˈuːz fˈɪfti ˌeɪtʃzˈiː.