In view of problems existing in the lagging development of rotor speed control in modern craneindustry,10 methods of speed control for asynchronous motor which could be reclassified into three major categorieswas studied,and performance analyses was conducted. New rotor-resistance speed control optimizes step speedregulation,partially solving high energy consumption and other problems. New cascade speed regulation has itsunique advantages in the problems of low power factor,high harmonics and so on. The imp...