https://blog.gongquiz.com/2021/05/26/%ec%a3%bc%ed%8c%8c%ec%88%98-60hz-%ec%a0%95%ec%a0%84%ec%9a%a9%eb%9f%89-%ec%9d%98-%ec%bd%98%eb%8d%b4%ec%84%9c%eb%a5%bc-%e2%96%b3%ea%b2%b0%ec%84%a0%ed%95%b4%ec%84%9c-3%ec%83%81%ec%a0%84%ec%95%95-2000/